Water-borne cryptosporidiosis: a recurring problem
J.K. Donnelly
University of Leeds and Trinity and All Saints College of the University of Leeds, UK
E.I. Stentiford
Unive rs ity o f Le e ds , UK
States that the presence of Cryptosporidium oocysts in potable water supplies con-tinues to be a high profi le problem in the UK and the USA, causing concern among public health officials and those responsible for water treatment. Reviews the current status of knowledge about this troublesome parasite and considers water treatment options.
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Introduction
Cr yp tosp or id iu m , a ga s t r oin t es t in a l p r ot o-zoa n p a r a s it e, is in cr ea sin gly b ein g id en t ifi ed a s a ca u s e of d ia r r h oea l d isea s e in h u m a n s. Du r in g t h e 1980s Cr yp tosp or id iu m em er ged a s a com m on ca u se of in fect ion in b ot h d evel-op ed a n d d evelevel-op in g n a t ion s. T h e in fect ion r a t e for in d iv id u a ls w it h d ia r r h oea l illn es s is 2 p er cen t in in d u st r ia lized cou n t r ies a n d 510 p er cen t in T h ir d Wor ld cou n t r ies. Cr y p -t os p or id ios is ca u s ed by C. p a r v u m , is -t h ou gh -t t o b e r esp on sible for u p t o 500 m illion in fec-t ion s a n n u a lly in Asia , Afr ica a n d La fec-t in Am er ica (Cu r r en t a n d Ga r cia , 1991). Wh ile cr y p t osp or id iosis m ay ca u se a cu t e, selflim it -in g d ia r r h oea -in ot h er w ise h ea lt h y p eop le, in fect ion in t h e im m u n o-com p r om ised , su ch a s t h ose w it h AIDS, m ay b e sever e a n d life-t h r ea life-t en in g. T h e in feclife-t ion r a life-t e for AIDS p a t ien t s is 10 p er cen t in t h e USA a n d 30-50 p er cen t in d evelop in g cou n t r ies (Pet er sen , 1993).
Sever a l s p ecies of t h e gen u s Cr yp tosp or id iu m a r e k n ow n , bu t C. p a r v u m is con s id er ed t o b e t h e s p ecies r es p on s ible for cr y p t os p or id iosis in h u m a n s a n d d om es t ic m a m -m a ls (Cu r r en t , 1987). T h e or ga n is-m h a s a fa eca l-or a l m od e of t r a n s m is sion . It is t r a n s-m it t ed in t h e for s-m of en v ir on s-m en t a lly s t a ble, r obu s t oocy st s w h ich h ave b een excr et ed in t h e fa eces of in fect ed h ost s (h u m a n or a n i-m a l). T h e oocy st s i-m ay b e in ges t ed in food , d r in k or t r a n s fer r ed fr om h a n d t o m ou t h .
Low levels of cr y p t osp or id ia l oocy st s a r e u b iq u it ou s in t h e w a t er en v ir on m en t . T h ey m ay b e com m on ly d et ect ed in cool, m ois t en v ir on m en t s in clu d in g p ollu t ed , p r ist in e a n d p ot a ble w a t er sou r ces (Lis le a n d Ros e, 1995), a n d ca n s u r v ive sever a l m on t h s in s oil or w a t er. Oocy st s a r e r es is t a n t t o m os t com -m on ly u sed d isin fect a n t s in clu d in g ch lor in e (Kor ich et a l., 1990). In v itr o excy st a t ion t est s on Cr yp tosp or id iu m oocy st s sh ow ed t h a t a fr ee ch lor in e level of 16,000 m g/ l w a s r eq u ir ed for zer o v ia b ilit y (Sm it h a n d Ros e, 1990). Con -ven t ion a l fi lt r a t ion of w a t er d ecr ea ses t h e level of Cr yp tosp or id iu m in w a t er s u p p lies (Kor ich et a l., 1990; Ma d or e et a l., 1987), bu t oocy st s a r e fr eq u en t ly d et ect ed a t low levels in p ot a ble w a t er (LeCh eva llier et a l., 1991a ), a s t h ey ca n over w h elm s t a n d a r d t r ea t m en t b a r r ier s. T h er e is in cr ea sin g ev id en ce t h a t w a t er -b or n e ou t b r ea k s of cr y p t os p or id ios is a r e a s s ocia t ed w it h t r a n sien t h igh loca l con -cen t r a t ion s of C. p a r v u m . Oocy st s a r e d
iffi-cu lt t o id en t ify a n d en u m er a t e u sin g r ou t in e t est in g t ech n iq u es a n d it is esp ecia lly d ifficu lt t o d et ect t r a n sien t eleva t ion s u sin g r ou -t in e m on i-t or in g p r ogr a m m es.
T h e p a u cit y of in for m a t ion on r ecen t ly id en t ifi ed p a t h ogen s su ch a s C. p a r v u m in w a t er s ou r ces h a s obv iou s p u blic h ea lt h im p lica t ion s. Wa t er su p p ly com p a n ies ca n n ot gu a r a n t ee t h a t t h eir t r ea t ed p r od u ct is Cr yp tosp or id iu m -fr ee. T h e con su m p t ion of s u fficien t v ia ble oocy st s ca n r esu lt in in fec-t ion of fec-t h e h osfec-t s. A n u m b er of d ocu m en fec-t ed w a t er -b or n e ou t b r ea k s h ave occu r r ed in t h e UK, Ca n a d a a n d t h e USA. Du r in g t h e ou t -b r ea k of cr y p t osp or id ios is in Milw a u k ee, in t h e USA in 1993, con t a m in a t ion of a la r ge m u n icip a l w a t er su p p ly led t o a p p r oxim a t ely 400,000 p eop le b ecom in g ill (Fox a n d Ly t t le, 1994; M a cKen zie et a l., 1994). T h is w a s t h e la r gest ou t b r ea k of w a t er -b or n e d isea se ever r e p or t ed in t h e USA (Ber k elm a n , 1994).
Cr yp tosp or id iu m h a s b ecom e t h e m ost im p or t a n t n ew con t a m in a n t for con t r ol in d r in k in g w a t er in t h e USA (Lisle a n d Rose, 1995). T h e or ga n is m h a s a sim ila r st a t u s in t h e UK a n d h a s a t t r a ct ed a t t en t ion w it h in t h e w a t er in d u s t r y a s t h e con seq u en ce of a n u m -b er of ou t -b r ea k s of w a t er --b or n e d isea se. Som e r esea r ch er s b elieve t h a t , a t p r esen t , m a n y sm a ll ou t b r ea k s go u n d et ect ed . It is a lle ged t h a t , in t h e UK, som e ou t b r ea k s a r e n ot in ves-t iga ves-t ed (E N DS, 1995a ).
Human cryptosporidiosis
T h e fi r st ca ses of h u m a n cr y p t osp or id iosis w er e d escr ib ed in t h e m id -1970s (N im e et a l., 1976) a lt h ou gh t h e m a jor it y h ave b een d ocu -m en t ed sin ce 1980. In t h e 1970s, t h e p a r a sit e w a s con s id er ed a n op p or t u n ist ic p a t h ogen , b eca u se t h e sy m p t om s w er e r ecogn ized on ly a m on g t h ose w h o w er e im m u n o-s u p p r essed , su ch a s AIDS p a t ien t s. By t h e m id -1980s it w a s r ea lized t h a t t h er e w er e ca s es of in fect ion in ot h er w ise h ea lt h y, im m u n o-com p et en t p eop le.
Age range for infection
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F ayer a n d Un ga r, 1986). It h a s b een r e p or t ed t h a t , in t h e UK, a p p r oxim a t ely 60 p er cen t of p osit ive fi n d in gs a r e fr om ch ild r en (Ca s e-m or e, 1990). Mos t in fect ion s a r e r e p or t ed for ch ild r en u n d er s ch ool a ge, d u e t o t h eir im m u n ologica l im m a t u r it y a n d in cr ea sed fa eca l-or a l t r a n sm is sion of t h e p a t h ogen es p ecia lly in d ay ca r e n u r s er ies. T h e su sce p -t ib ili-t y of you n g ch ild r en in d ay -ca r e cen -t r es h a s b een d em on s t r a t ed by a n u m b er of d ocu -m en t ed ou t b r ea k s (Cor d ell a n d Ad d is s, 1994; Un ga r, 1990).
Infective dose
T h e in fect ive d ose of oocy st s is low w h en com p a r ed w it h b a ct er ia l or v ir a l p a t h ogen s, bu t s im ila r t o Gia rd ia in testin a lis. T h e p ot en t ia l for w a t er b or n e or food b or n e t r a n s m is -s ion of t h e d i-sea -s e i-s con -sid er a ble. A m ed ia n in fect ive d ose of 132 oocy st s h a s b een ca lcu -la t ed for h ea lt h y in d iv id u a ls in a st u dy in volv in g volu n t eer s w it h ou t s er ologica l ev id en ce of p r ev iou s cr y p t os p or id ios is (Du p on t et a l., 1995). In t h e st u dy a n or a l d os e of on ly 30 oocy st s in it ia t ed t h e in fect ion in 20 p er cen t of t h e volu n t eer s.
Symptoms
On ce in fect ive oocy st s a r e in ges t ed , t h e life-cycle of t h e or ga n ism ca n b e com p let ed w it h in t h e gu t of t h e h os t , oft en r es u lt in g in w id e-r a n gin g s y m p t om s of t h e d isea se cr y p t osp or id ios is. T h e d isea s e is som et im es a s y m p -t om a -t ic, bu -t in fec-t ion m ay r es u l-t in a n u n com for t a ble, a lt h ou gh s elflim it ed , d ia r -r h oea l illn es s in im m u n o-com p et en t p eop le. Sy m p t om s m ay la st for w eek s. T h e d ia r r h oea m ay b e p r ofu s e, fou l-s m ellin g a n d w a t er y (a n aver a ge t h r ee lit r es p er d ay ). In a d d it ion t o d ia r r h oea , clin ica l sy m p t om s va r y, bu t m ay in clu d e m a la b sor p t ion , a fl u -lik e illn es s, m a la is e, a b d om in a l p a in , n a u sea a n d vom it -in g, loss of a p p et it e a n d w eigh t loss (F ayer a n d Un ga r, 1986).
T h er e m ay b e r ecu r r en t b ou t s of d ia r r h oea . Du r in g t h e M ilw a u k ee e p id em ic, r ecu r r en ce w a s a com m on com p la in t of in d iv id u a ls w h o h a d la b or a t or y -con fi r m ed cr y p t os p or id iosis, 39 p er cen t , (M a cKen zie et a l., 1995). T h e b ou t s of d ia r r h oea la st ed a n aver a ge of t wo d ay s (r a n ge 1-14 d ay s) a ft er 2-14 d ay s of n or m a l s t ools.
T h e d is ea se is r e ga r d ed a s a s er iou s, even life-t h r ea t en in g in fect ion in som e im m u n o-s u p p r eo-so-s ed p a t ien t o-s, eo-sp ecia lly in t h oo-s e w it h a dva n ced -st a ge AIDS, w it h fl u id los ses of u p t o 20 lit r es p er d ay in m or e s ever e ca s es. Fol-low in g t h e Milw a u k ee e p id em ic, m a n y of t h os e w h o w er e im m u n o-com p r om is ed d ied . At p r es en t n o s p ecifi c d r u g t r ea t m en t is
licen sed for t h e p a r a sit e (Cu r r en t , 1994), a lt h ou gh sever a l st u d ies in d ica t e t h a t a n a m in oglycosid e, P a r om om ycin , m ay b e u sefu l in t r ea t m en t a n d p a llia t ion of cr y p t osp or id io-sis (F ich t en b a u m et a l., 1993; H ea ley et a l., 1995).
Life cycle
T h e life cycle in volves b ot h a sexu a l a n d sex-u a l m sex-u lt ip lica t ion a n d occsex-u r s w it h in t h e ga st r oin t es t in a l t r a ct . Follow in g excy st a t ion in t h e gu t , a n d t h e s u b s eq u en t d evelop m en t a l st a ges, m a t u r e oocy st s a p p ea r in fa eces a ft er a p p r oxim a t ely 3-11 d ay s in h u m a n s. T h er e m ay b e r e p ea t ed cycles of a u t oin fect ion .
Excystation
Oocy st s a r e sp h er ica l in sh a p e a n d 4-6µm in d ia m et er. T h ey ea ch con t a in fou r sp in d le-sh a p ed , m ot ile sp or ozoit es. On ce v ia ble oocy st s in food , w a t er or fr om t h e gen er a l en v ir on m en t a r e in ges t ed or in h a led by a su it a ble h os t , eit h er h u m a n or a n im a l, t h ey r elea se t h eir sp or ozoit es t h r ou gh a n op en in g in t h e oocy s t w a ll. T h is p r ocess is k n ow n a s excy st a t ion a n d occu r s in r esp on se t o t h e ava ila b ilit y of t h e p r ot eoly t ic en zy m es a n d b ile s a lt s w it h in t h e sm a ll in t est in e.
T h e sp or ozoit es in fect in t est in a l e p it h elia l cells, b ecom in g em b ed d ed a t t h e b a se of t h e m icr ov illi. T h e in t r a cellu la r sp or ozoit es t h en in it ia t e t h e a s exu a l m u lt ip lica t ion st a ges of t h e life cycle. Follow in g t h ese st a ges t h e sex-u a l st a ges r essex-u lt in t h e p r od sex-u ct ion of zygot es fr om t h e fu s ion of t h e ga m et es. E a ch zygot e d evelop s in t o a n oocy st w h ich b ecom es d et a ch ed fr om t h e in t est in a l e p it h elia l cell.
A p r op or t ion of t h e zygot es (20 p er cen t ) h ave a r ela t ively t h in sin gle u n it m em b r a n e su r r ou n d in g t h e sp or ozoit es. T h ey a r e r esp on sible for a u t oin fect ion . Du r in g t h eir p a ssa ge w it h in t h e in t est in e, t h es e oocy st s r u p t u r e, r elea sin g t h eir sp or ozoit es, ca u sin g r ein fect ion of t h e h ost . Desp it e t h is m ech a -n ism , t h e d is ea s e is s elf-lim it ed i-n i-n d iv id u a ls w it h n or m a l im m u n e fu n ct ion s; t h e in fect ion w ill clea r sp on t a n eou sly w it h in a few w eek s (Cu r r en t a n d Ga r cia , 1991).
Oocyst shedding
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w eek s a ft er t h e sy m p t om s h ave clea r ed (Ba r er a n d Wr igh t , 1990). AIDS p a t ien t s w it h a s s ocia t ed cr y p t os p or id ios is h ave excr et ed u p t o 1.2 ×109p er cen t oocy st s in 24 h ou r s
(Good ga m e et a l., 1993).
Few a n im a ls excr et e C. p a r v u m oocy st s a ft er w ea n in g, bu t in fect ed ca lves a n d la m b s ca n excr et e u p t o 1010p er cen t oocy st s d a ily for 14 d ay s (Ba d en och , 1990). A s t u dy by Ca sey (1991) r e p or t ed t h a t u p t o 107p er cen t oocy st s
p er gr a m of fa eces w er e sh ed by in fect ed ca lves. Oocy st s a r e im m ed ia t ely in fect ive on excr et ion in t h e fa eces.
Reservoirs and modes of
transmission
Sources of contamination
T h er e is e p id em iologica l ev id en ce for t h e cr os s -t r a n s m ission of C. p a r v u m b et w een va r iou s h ost sp ecies in clu d in g h u m a n s (Ca se-m or e, 1993). In fect ion w it h C. p a r v u se-m occu r s in lives t ock a n im a ls, esp ecia lly ca t t le a n d s h ee p. Bot h w ild a n d d om es t ic a n im a ls m ay b e im p or t a n t r es er voir s of h u m a n in fect ion , con t r ibu t in g a lso t o t h e con t a m in a t ion of t h e en v ir on m en t . T h e e p id em iology is com p lex w it h b ot h d ir ect a n d in d ir ect r ou t es of t r a n s-m is s ion . Ap a r t fr os-m d ir ect con t a ct (i.e. zoon ot ic a n d p er s on -t o-p er son sp r ea d ), w a t er, food a n d h ou seh old it em s m ay b e veh icles of t r a n sm is s ion of cr y p t os p or id ia l in fect ion .
Z oonotic transmission
Zoon ot ic t r a n sm is sion of in fect ion fr om h a n -d lin g of livest ock , esp ecia lly ca t t le a n -d s h ee p, is w ell d ocu m en t ed , bu t t h er e is n o ev id en ce t h a t livest ock a r e t h e p r im a r y m ea n s of t r a n m is s ion . T h e r ela t ive im p or t a n ce of cr os s-t r a n sm is s ion b es-t w een s p ecies h a s b een q u er ied . It is u n k n ow n w h et h er t h er e exist s a p r ed om in a n t a n im a l a s op p os ed t o h u m a n t r a n sm is s ion cycle w it h lin k a ge com in g fr om s p or a d ic zoon ot ic t r a n s m iss ion . T h e im p a ct of t h e cycles on t h e p ot en t ia l for w a t er -b or n e t r a n sm is s ion is a lso u n k n ow n .
Person-person transmission
Up t o a q u a r t er of ca s es of in fect ion m ay b e d ir ect ly zoon ot ic, w it h t h e r es t m a in ly d u e t o p er son p er s on t r a n s m is sion a n d t h e w a t er b or n e r ou t e (P u blic H ea lt h La b or a t or y Ser -v ice St u dy Gr ou p, 1990). La ck of p er s on a l h ygien e a n d fa eca l in con t in en ce a r e im p or -t a n -t fa c-t or s in p er s on -p er son -t r a n s m is sion , w h ich is t h e m ost fr eq u en t m od e of t r a n s m is-s ion a m on g h u m a n is-s (P ozio et a l., 1992).
In fect ion m ay b e d er ived fr om a n a n im a l s ou r ce a n d t h en p er son -p er son sp r ea d ca n r es u lt in a n ou t b r ea k in t h e com m u n it y, p a r -t icu la r ly in fa m ilies or p laygr ou p s (Sim m on s, 1991). It h a s b een r e p or t ed t h a t in E n gla n d a n d Wa les u p t o a q u a r t er of t h e ca ses of
cr y p t osp or id iosis a r e follow ed by ca ses of ga st r oen t er it is a m on g ot h er m em b er s of t h e h ou s eh old (P u blic H ea lt h La b or a t or y Ser v ice St u dy Gr ou p, 1990).
Follow in g t h e Milw a u k ee e p id em ic, t h e r isk of secon d a r y t r a n s m is sion t o h ou seh old m em b er s fr om a d u lt v isit or s t o t h e a r ea w h o h a d d evelop ed cr y p t os p or id iosis w a s in vest i-ga t ed (Ma cKen zie et a l., 1995). A 5 p er cen t r a t e of secon d a r y t r a n sm ission w a s r e p or t ed . T h e r a t e is low er t h a n t h a t w h ich m ay b e a ssocia t ed w it h ou t b r ea k s in d ay -ca r e cen t r es for you n g ch ild r en . T h e in vest iga t or s in d i-ca t ed t h a t t h e a ge of t h e p r im a r y i-ca se m ay in fl u en ce t h e r a t e.
Water-borne transmission
Wa t er w h ich h a s b een con t a m in a t ed by h u m a n or a n im a l fa eces r e p r es en t s a sign ifi -ca n t r ou t e of t r a n sm ission (Rob er t son a n d Sm it h , 1992). Sever a l com m u n it y ou t b r ea k s of cr y p t osp or id iosis in t h e UK a n d t h e USA h ave b een lin k ed w it h m a in s w a t er su p p ly. Du r in g t h e su m m er of 1995 a m a jor ou t b r ea k of t h e d isea se w it h 355 con fi r m ed ca ses occu r r ed in sou t h -w est E n gla n d . T h e ou t b r ea k w a s su s-p ect ed t o b e w a t er -b or n e by t h e con t r ol t ea m (E N DS, 1995b ).
T h e h igh ou t p u t of oocy st s fr om in fect ed you n g lives t ock , t oget h er w it h t h e b r oa d h ost r a n ge, en su r es a h igh level of con t a m in a t ion in t h e en v ir on m en t . T h is sit u a t ion m ay favou r b ot h w a t er -b or n e t r a n sm iss ion of t h e d isea se t h r ou gh con t a m in a t ion of t h e w a t er -sh ed (Sm it h a n d Rose, 1990) a n d zoon ot ic t r a n sm iss ion in r u r a l a r ea s (Cu r r en t et a l., 1983). Also, t h er e is som e ev id en ce t h a t t h e oocy st s w h ich ca u se w a t er -b or n e ou t b r ea k s oft en com e fr om fa iled sew a ge t r ea t m en t sy st em s. T h e fi r s t d ocu m en t ed w a t er -b or n e ou t b r ea k of t h e d isea s e, w h ich occu r r ed in Sa n An t on io, Texa s, w a s lin k ed t o sew a ge lea k a ge in t o w ell w a t er w h ich w a s t r ea t ed on ly by ch lor in a t ion (D’An t on io et a l., 1985).
Food-borne transmission
Ou t b r ea k s a t t r ibu t ed t o con t a m in a t ion of food by in fect ed food h a n d ler s h ave n ot b een r e p or t ed . Un less t h e h ygien e of food h a n d ler s is p oor, in fect ion t h r ou gh t h is seem s u n lik ely, a s t h e m ost p r eva len t r ou t e is fa eca l-or a l.
An im a l food p r od u ct s a n d p ot en t ia lly con t -a m in -a t ed food s w h ich h -ave n ot b een
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w a s a n on -a lcoh olic cid er m a d e fr om a p p les, fr es h ly p r es s ed d u r in g a s ch ool a gr icu lt u r a l fa ir in t h e USA. It w a s su gges t ed t h a t t h e fa r m -collect ed a p p les h a d b een con t a m in a t ed by ca lf fa eces on t h e gr ou n d b efor e or d u r in g t h eir h a r ves t . T h e a p p les h a d b een in a d e-q u a t ely w a sh ed p r ior t o p r es s in g.
Immunity in the population
St r ict con t r ol of C. p a r v u m in w a t er s u p p lies cou ld r ed u ce t h e com m u n it y r es er voir of in fect ion a n d it s con seq u en t s p r ea d (St a n -w ell-Sm it h , 1991). H o-w ever, t h e p r es en ce of oocy st s in p ot a ble w a t er m ay n ot n eces sa r ily m ea n t h a t t h e exp osed p op u la t ion is a t r isk , b eca u se low con cen t r a t ion s m ay fr eq u en t ly b e d et ect ed in fi n a l w a t er s w it h n o ev id en ce of d is ea s e in t h e com m u n it y (a lt h ou gh t h e w a t er -b or n e r ou t e m ay b e im p lica t ed w h en clu s t er s of cr y p t os p or id ios is ca s es a r e a s s oci-a t ed w it h p ot oci-a ble w oci-a t er.)
Risk a s s ess m en t m od els su gges t t h a t con t a -m in a t ion of w a t er s ou r ces -m ay con t r ibu t e t o a n en d em ic level of d isea se (Lis le a n d Ros e, 1995), or a n in cr ea sed level of im m u n it y in t h e exp os ed p op u la t ion , or b ot h (Gr im a son et a l., 1990). M a n y m em b er s of a com m u n it y m ay b e im m u n e t o in fect ion a n d low levels of oocy s t s m ay b e p r es en t in p ot a ble s u p p lies w it h ou t a su b seq u en t s ign ifi ca n t eleva t ion in t h e in cid en ce of d isea se, i.e. t h er e a r e in su ffi-cien t s u s ce p t ible com m u n it y m em b er s t o ca u s e a n ou t b r ea k . T h er e is m or e lik elih ood of a n ou t b r ea k w h en a com m u n it y h a s few w it h im m u n it y a n d m a n y s u s ce p t ibles. Du r in g a s u s p ect ed w a t er b or n e ou t b r ea k of cr y p -t os p or id ios is in a r es or -t a r ea in sou -t h -w es -t E n gla n d d u r in g 1995, t h e gr ou p s r e p or t ed t o b e m os t a ffect ed w er e t h os e t oo you n g t o h ave b een a ffect ed by ea r lier ou t b r ea k s a n d t h ose w h o h ave r ecen t ly m oved t o t h e a r ea t o r et ir e (E N DS, 1995b ).
Wit h r e ga r d t o food -b or n e in fect ion , it seem s t h a t r e gu la r con s u m er s of r aw m ilk m a in t a in t h eir im m u n it y, p r even t in g sy m p t om a t ic in fect ion (Ba d en och , 1990).
Occurrence in the water
environment
Surveys of water sources Cr yp tosp or id iu m con t a m in a t ion is
w id es p r ea d in t h e w a t er en v ir on m en t a n d it seem s t h a t Cr yp tosp or id iu m oocy st s m ay b e fou n d in w a t er s q u it e com m on ly, in clu d in g t r ea t ed d r in k in g w a t er. An Am er ica n s u r vey (Ros e et a l., 1991) fou n d t h a t 28 p er cen t of t r ea t ed d r in k in g w a t er s a m p les con t a in ed oocy s t s, a lt h ou gh levels w er e a ls o q u it e low (0.002-0.009 / lit r e).
Oocy s t s a r e n ot n or m a lly d et ect ed in gr ou n dw a t er s u n less t h es e a r e u n d er t h e d ir ect in fl u en ce of s u r fa ce w a t er. F r om ou r
cu r r en t k n ow led ge of t h e or ga n ism a n d it s close a s socia t ion w it h livest ock , it s eem s p r ob a ble t h a t m ost of t h e oocy s t s fou n d in b ot h su r fa ce a n d gr ou n d w a t er s d er ive fr om fa r m in g p r a ct ices w h ich r esu lt in oocy st s fr om fa eces en t er in g w a t er cou r s es.
A su r vey of su r fa ce a n d gr ou n dw a t er s by t h e N a t ion a l Cr yp tosp or id iu m Su r vey Gr ou p (N CSG) (1992) in d ica t ed t h e occu r r en ce of oocy st s in Br it ish w a t er sou r ces. Levels of en v ir on m en t a l oocy st s r e p or t ed fr om UK su r vey s a r e oft en low er t h a n t h ose r e p or t ed by Am er ica n wor k er s, esp ecia lly in st u d ies in volv in g la r ge in d u st r ia lized r iver s (N CSG, 1992). T h is su r vey gr ou p r e p or t ed t h e r esu lt s of sa m p lin g of t h r ee r iver s a n d six b or eh oles ca r r ied ou t over sever a l m on t h s. Sa m p lin g of r iver w a t er r evea led t h a t , w h ile oocy st con -cen t r a t ion s w er e ver y low, 30 p er -cen t of sa m p les w er e p osit ive (aver a ge 0.25/ lit r e m a xim u m 4.0/ lit r e). T h er e w a s a n ot a ble a b sen ce of oocy s t s in t wo of t h e t h r ee r iver s. An a ly sis of t h e b or eh ole s a m p le r es u lt s r evea led t h a t on ly eigh t of 138 sa m p les fr om on e r e gion w er e p osit ive for oocy st s (aver a ge 0.25 oocy st s/ lit r e), a n d a ll t h e sa m p les fr om t h r ee b or eh oles in a n ot h er r e gion w er e n e ga t ive.
T h e r esu lt s of a s u r vey in Scot la n d (Sm it h et a l., 1991) in d ica t ed t h a t 40 p er cen t of Scot -t ish r aw w a -t er s sa m p led a n d 40 p er cen -t of t r ea t ed w a t er sa m p led con t a in ed oocy st s. Ma xim u m levels of oocy st s w er e low er in t h e t r ea t ed w a t er com p a r ed w it h t h e u n t r ea t ed w a t er (0.007-0.72/ lit r e a n d 0.006-2.3/ lit r e r esp ect ively ). P ea k levels of oocy s t s in u n t r ea t ed w a t er sa m p les d id n ot cor r ela t e w it h p ea k s in t h e t r ea t ed w a t er sa m p les.
Sin ce t h e ou t b r ea k of cr y p t osp or id iosis in Sw in d on a n d Oxfor d s h ir e in 1988/ 89, T h a m es Wa t er h a s r ou t in ely m on it or ed r aw w a t er a t a b st r a ct ion p oin t s on t h e T h a m es (P ou lt on a n d Colb ou r n e, 1991). Oocy st s w er e d et ect ed in fr eq u en t ly fr om t h is sou r ce; on ly 6.7 p er cen t of s a m p les w er e p os it ive, w it h gr ea t er fr eq u en cy in m or e r u r a l a r ea s. Dr in k in g w a t er w a s a lso m on it or ed in t h e T h a m es Wa t er su r vey, b et w een Feb r u a r y 1989 a n d Ap r il 1991, a n d oocy st s w er e d et ect ed in on ly 13 sa m p les. It h a s b een su ggest ed t h a t en v ir on m en t a l levels of oocy s t s a r e low er n ow t h a n t h ey w er e in t h e la t e 1980s in t h e UK, a lt h ou gh t h is is n ot r efl ect ed in t h e in cid en ce of cr y p t osp or id iosis.
Sewage and agricultural run-off
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of con t a m in a t ion of w a t er cou r ses a n d r es er voir feed er st r ea m s, s o p ollu t in g w a t er s u p -p lies a n d food cr o-p s w h ich a r e ir r iga t ed (Ca s em or e, 1993).
Up t o 13,700 oocy st s/ lit r e h ave b een d et ect ed in r aw sew a ge (Ma d or e et a l., 1987). Levels of u p t o 149,100 oocy st s/ lit r e in u n t r ea t ed la ir a ge efflu en t of on e sla u gh t er h ou se h ave b een r e p or t ed (Sm it h , 1991). In t h r ee US su r vey s in wh ich a gr icu lt u r a l con t a m in a t ion cou ld b e d ist in gu ish ed fr om h u m a n sew a ge d isch a r ge, t h e con cen t r a t ion of oocy st s w a s a lm ost t w ice a s gr ea t in w a t er s in fl u en ced by a gr icu lt u r a l a ct iv it ies (Rose, 1990).
Viability studies
T h er e a r e few r e p or t s of v ia b ilit y a s s es sm en t of oocy s t s d et ect ed in en v ir on m en t a l sa m p les. H ow ever, on e s u ch st u dy fr om t h e USA (LeCh eva llier et a l., 1991b ) r e p or t ed t h e p r esen ce of oocy st s in d r in k in g w a t er (oocy s t s w er e ob ser ved in 27 p er cen t of s a m p les ), a n d con clu d ed t h a t d esp it e t h eir fr eq u en t d et ect ion , m ost of t h e or ga n ism s in t h e s a m p les w er e n ot v ia ble: on ly 32 p er cen t of t h e Cr yp tosp or id iu m ob ser ved in t h e r aw w a t er s a m p les in t h is su r vey con t a in ed s p or ozoit es w it h in t h e oocy st , s o t h e m a jor it y of oocy s t s w er e n ot v ia ble.
In on e st u dy it w a s r e p or t ed t h a t on occa -s ion -s w h en t h er e w er e d et ect a ble level-s of Cr yp tosp or id iu m in on e Scot t is h w a t er s ou r ce (0.002 t o 0.024 oocy st s p er lit r e), t h er e w a s n o in cr ea s e in ca ses of cr y p t os p or id ios is a b ove b a ck gr ou n d level (Sm it h et a l., 1993). In t h is w a t er s ou r ce, v ia ble oocy st s occu r r ed on ly r a r ely d u r in g t h e p er iod w h en a n a ly sis w a s con d u ct ed ; t h e m a jor it y of s a m p les w h ich con t a in ed s u fficien t oocy st s for r ecover y a n d v ia b ilit y a ss es sm en t d id n ot con t a in v ia ble oocy st s. Un for t u n a t ely s t u d ies in d ica t in g t h e v ia b ilit y a s w ell a s t h e fr eq u en cy a n d level of d et ect ed oocy st s a r e less com m on .
Alt h ou gh oocy st s a r e en v ir on m en t a lly r obu s t , fr om t h e few r e p or t ed st u d ies, it s eem s t h a t on ly a s m a ll p r op or t ion of t h ose d et ect ed in w a t er s ou r ces a r e v ia ble. Via b ilit y of w a t er -b or n e oocy st s is d e p en d en t on a n u m b er of fa ct or s in clu d in g a ge a n d t h e d est r u ct ive effect s of slu r r y, s ew a ge a n d w a t er t r ea t m en t p r ocess es (Sm it h et a l., 1993). It is b elieved t h a t sew a ge d is ch a r ge a n d t h e r u n off fr om ca t t le gr a zin g la n d a r e s ign ifi -ca n t s ou r ces of v ia ble oocy st s. F a e-ca l m a t t er m ay p r ot ect a ga in st d es sica t ion , bu t on ce in t h e r eceiv in g w a t er, oocy st s u r v iva l r a t es d ecr ea se (Ca m p b ell et a l., 1992).
Conventional treatment for
potable water supplies
Alt h ou gh Cr yp tosp or id iu m oocy st s m ay b e u b iq u it ou s a t a low level in t h e w a t er
en v ir on m en t (Lisle a n d Rose, 1995), it seem s t h a t a sign ifi ca n t h ea lt h h a za r d a r ises on ly w h en a h igh loca l con cen t r a t ion of oocy st s a p p ea r s in t h e r aw w a t er sou r ce (Ives, 1990). Ou t b r ea k s of cr y p t os p or id iosis m ay r esu lt fr om a fa ilu r e of w a t er t r ea t m en t if su fficien t n u m b er s p en et r a t e t h e p r oces s. T h is m ay occu r b eca u s e of exces sive ch a llen ge, e.g. t h e r u n off of la r ge a m ou n t s of w a t er fr om fa r m la n d follow in g h eav y r a in or d elib er a t e or a ccid en t a l d u m p in g of slu r r y in w a t er cou r ses. Ca t ch m en t con t r ol m ay t h er efor e r ed u ce t h e a s socia t ed p ot en t ia l h ea lt h r isk s.
Risk assessment
In t h e USA t h e in t r od u ct ion of t h e Su r fa ce Wa t er T r ea t m en t Ru le (SWTR) r eq u ir es fi lt r a -t ion a n d d isin fec-t ion of a ll su r fa ce w a -t er s u p p lies (cr it er ia a r e a lso sp ecifi ed t o d et er m in e if a s y s t em cou ld avoid fi lt r a t ion t ech -n ology ). T h is r u le w a s d evelop ed t o co-n t r ol Gia rd ia sp ecies a n d en t er ic v ir u ses
(LeCh eva llier et a l., 1991a ). T h e USE PA u sed r is k m od els t o d et er m in e t r ea t m en t goa ls. For gia r d ia sis con t r a ct ed fr om w a t er, on e ca se p er yea r p er 10,000 p eop le is t h e w a t er t r ea t -m en t goa l in t h e SWTR.
A r isk m od el for Cr yp tosp or id iu m w a s u sed t o a n a ly se a d a t a set fr om a fi n is h ed w a t er su r vey (Lisle a n d Rose, 1995): t h e m in im u m con cen t r a t ion of oocy st s w h ich cou ld b e p r e-sen t in t h e fi n is h ed w a t er t o m eet a n a n n u a l r isk sa fet y level of 1:10,000 w a s ca lcu la t ed t o b e < on e oocy st p er 34,000 lit r es. If a p la n t w er e t o con d u ct m on it or in g on a d a ily b a sis, it w a s est im a t ed t h a t t h e occu r r en ce of on e oocy st p er 34,000 lit r es for < 8-10 d ay s wou ld b e w it h in t h e a n n u a l goa l lim it . T h e E PA d id n ot in clu d e Cr yp tosp or id iu m in t h e r u le b eca u se of t h e p a u cit y of u sefu l d a t a , bu t it a im s t o in clu d e Cr yp tosp or id iu m in t h e n ext ser ies of r e gu la t ion s, t h e E SWTR.
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in cid en ce of in fect ion w h er e oocy st s en t er ed t h e w a t er s u p p ly a ft er it h a d b een t r ea t ed (Sm it h a n d Rose, 1990).
Chemical disinfection
E v id en ce fr om in ves t iga t ion s follow in g ou t -b r ea k s in d ica t es t h a t oocy st s ca n p a ss t h r ou gh con ven t ion a l t r ea t m en t p r ocess es a n d b e r esist a n t t o com m on ly u sed d isin fec-t a n fec-t s su ch a s ch lor in e a fec-t con cen fec-t r a fec-t ion s w h ich a r e effect ive for ot h er w a t er -b or n e p a t h ogen s (see Ta ble I). Oocy st s exp os ed t o u n d ilu t ed h ou seh old blea ch (5 p er cen t N a OCl) for sever a l h ou r s w er e st ill ca p a ble of in d u cin g in fect ion (Da n iel et a l., 1994) a n d ch lor in e d ioxid e a p p ea r s t o b e in effect ive b elow a con cen t r a t ion of s ever a l m g/ lit r e for 15 m in u t es con t a ct t im e.
Flocculation and filtration
E ffect ive fl occu la t ion a n d fi lt r a t ion d u r in g con ven t ion a l w a t er t r ea t m en t w ill r em ove oocy st s, bu t n o sin gle con ven t ion a l t r ea t m en t p r oces s is con sid er ed a ble t o r em ove oocy st s a b s olu t ely. T h e Secon d Re p or t of t h e Gr ou p of E xp er t s on Cr yp tosp or id iu m in Wa t er Su p p lies (Ba d en och , 1995) in d ica t es t h e va lu e of s low s a n d fi lt r a t ion a n d s u gges t s t h a t coa g-u la t ion con t r ol, t h e m a n n er in w h ich fi lt er s a r e r ein t r od u ced a ft er b a ck w a s h in g a n d t h e m on it or in g of fi lt r a t e a r e im p or t a n t fa ct or s in oocy s t r em ova l in con ven t ion a l ch em ica l t r ea t m en t p la n t s. An a p p r op r ia t e s eq u en ce of u n it p r ocess es op er a t ed in a n in t e gr a t ed fa s h ion (w it h good coa gu la t ion con t r ol a n d effect ive fi lt r a t ion ) ca n r ed u ce t h eir n u m b er s ign ifi ca n t ly t o a low level of h a za r d (Ives, 1990). Mem b r a n e fi lt r a t ion offer s a p h y sica l b a r r ier s olu t ion t o t h e p r oblem a s oocy st s a r e la r ger t h a n t h e p or es in a m icr ofi lt r a t ion s y s t em . H ow ever, m em b r a n e fou lin g is a n in h er en t p r oblem w h ich is n ot ea sily over -com e for lon g-t er m p er for m a n ce a t t r ea t m en t p la n t s.
Ou t b r ea k s of cr y p t os p or id ios is s u ch a s t h a t w h ich occu r r ed in J a ck s on Cou n t y, Or e gon (Reid , 1992), d em on st r a t e t h e n eed for op t i-m u i-m p er for i-m a n ce of fi lt r a t ion t r ea t i-m en t for s u r fa ce w a t er sou r ces. At p r es en t t h er e a r e n o fi lt er p er for m a n ce cr it er ia for
cr y p t osp or id ia l oocy st s a n d lit t le d a t a t o u s e
t o m a k e d ecision s r e ga r d in g op t im u m w a t er t r ea t m en t p r a ct ices for t h e or ga n ism (LeCh eva llier et a l., 1991b ), bu t va r iou s st u d ies h ave sh ow n t h a t r a p id gr av it y sa n d fi lt er s m ay b e 91 p er cen t efficien t a n d slow sa n d fi lt er s m or e t h a n 92 p er cen t efficien t a t r et a in in g oocy s t s (Rob er t s on a n d Sm it h , 1992). Wh ile fi lt r a t ion efficien cies ca n b e ver y h igh t h e sm a ll p er cen t a ge of oocy st s w h ich ca n n ot b e r em oved m ay p r esen t a p r oblem ; u n less t h ey h ave b een r en d er ed n on -v ia ble d u r in g t h e t r ea t m en t p r ocess (Rob er t son a n d Sm it h , 1992). T h e d ifficu lt y w it h con ven t ion a l fi lt er m ed ia is t h a t t h ey ca n n ot p r ov id e a n a b solu t e b a r r ier t o p a r t icles in t h e oocy st s’ size r a n ge so, d esp it e t h e ca p t u r e efficien cy, t h er e r em a in s a fi n it e ch a n ce of p en et r a t ion (Gr e gor y, 1994). Cu r r en t ly it is con sid er ed t h a t a r em ova l of a t lea s t 99.99 p er cen t (4 log) sh ou ld b e a ch ieved (Gr e gor y, 1994), bu t t h e m a jor it y of t r ea t m en t p la n t s a r e cu r r en t ly a ch iev in g on ly a b ou t 99.7 p er cen t (2.7 log) r em ova l (LeCh eva llier, 1994).
Turbidity surrogate
LeCh eva llier et a l. (1991b ) ob ser ved t h a t of t r ea t m en t p la n t s in clu d ed in t h eir USA st u dy t h a t w er e p os it ive for Cr yp tosp or id iu m , m ost w er e a ble t o m eet t h e SWTR t u r b id it y r eq u ir em en t (aver a ge t u r b id it y for s it es t h a t w er e “p a r a s it e p osit ive” w a s 0.19 N TU cf. 0.18 N TU for t h ose w h er e n o p a r a sit e w a s
d et ect ed ). T h e p r od u ct ion of low t u r b id it y w a t er d id n ot en su r e t h a t t h e p la n t efflu en t wou ld b e oocy s t -fr ee. H ow ever, t h e
r esea r ch er s d em on s t r a t ed a s ign ifi ca n t cor r e-la t ion b et w een t h e r em ova l of t u r b id it y a n d t h e r em ova l of Cr yp tosp or id iu m ( p < 0.01), w it h a n im p r oved r ela t ion s h ip if d a t a for a p a r t icu la r sit e w er e p lot t ed . As a
con seq u en ce of su ccessfu l r esea r ch , a n a p p r o-p r ia t e su r r oga t e, su ch a s t u r b id it y, m ay b e u sed in fu t u r e t o r elia b ly p r ed ict t r ea t m en t p la n t p er for m a n ce in oocy st r em ova l fr om d r in k in g w a t er (LeCh eva llier et a l., 1991b ).
Water-borne outbreaks
Sever a l w a t er b or n e ou t b r ea k s of cr y p t osp or id ios is h ave b een a s socia t ed w it h p u b -lic w a t er su p p lies a n d d ocu m en t ed in t h e UK (Ba d en och , 1990; J ose p h et a l., 1991; Sm it h et a l., 1989) a n d t h e USA (D’An t on io et a l., 1985; H ayes et a l., 1989; Lela n d et a l., 1993; Reid , 1992) d u r in g t h e 1980s a n d 1990s. T h e sp or a d ic n a t u r e of t h e ou t b r ea k s con t in u es t o con cer n b ot h p ot a ble w a t er su p p lier s a n d t h ose in volved in com m u n it y h ea lt h ca r e.
A la r ge w a t er -b or n e ou t b r ea k of cr y p t osp or id iosis occu r r ed in Ca r r ollt on , Geor gia , in 1987 a n d a b ou t 13,000 p eop le w er e a ffect ed . Follow in g t h eir in vest iga t ion of t h e ou t b r ea k , H ayes et a l. (1989) a t t r ibu t ed t h e Table I
Example s o f dis infe c tants te s te d o n o o c ys ts to ac hie ve 9 0 pe r c e nt inac tivatio n
mg/ min
Chlorine 7 ,2 0 0
M onochloramine 7 ,2 0 0
Chlorine dioxide 7 8
Ozone at 25°C 3 -5
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p a s sa ge of oocy st s t h r ou gh t h e w a t er t r ea t -m en t p la n t t o t h r ee fa ct or s :
1 M ech a n ica l a git a t or s t h a t w er e sch ed u led for r e p la cem en t h a d b een r em oved p r ior t o d eliver y of t h e n ew on es. T h is r ed u ced t h e efficien cy of fl occu la t ion a n d im p a ir ed p a r t icu la t e r em ova l.
2 T h e efficien cy of fi lt r a t ion w a s im p a ir ed by t h e eq u ip m en t a n d p r oced u r es u s ed t o con t r ol w a t er fl ow t h r ou gh t h e fi lt er s a n d m on it or t u r b id it y.
3 T h e fi lt er s w er e som et im es r es t a r t ed w it h ou t b ein g b a ck w a s h ed . Des p it e t h e s u b op -t im a l fl occu la -t ion a n d fi l-t r a -t ion , -t h e t r ea t ed w a t er m et t h e r eq u ir ed st a n d a r d s for colifor m s a n d t u r b id it y.
As in t h e Ca r r ollt on ou t b r ea k , su b op t im a l p er for m a n ce d u r in g t r ea t m en t w a s bla m ed for a n ou t b r ea k in J a ck s on Cou n t y, Or e gon , in 1992 (Lela n d et a l., 1993). T h e ou t b r ea k a ffect ed a b ou t 15,000 p eop le.
The Milwaukee outbreak
Ap p r oxim a t ely 400,000 p eop le in M ilw a u k ee, Wiscon s in , w er e a ffect ed by a n ou t b r ea k in ea r ly Ap r il 1993, w it h m ost illn es s a s s ocia t ed w it h a n a r ea s er ved by on e of t h e t wo w a t er t r ea t m en t p la n t s in t h e cit y. H igh t u r b id it y fi n is h ed w a t er a t t h e w a t er wor k s w a s bla m ed , in d ica t in g a b r ea k d ow n in t h e fi lt r a t ion p r ocess a lt h ou gh t h e t r ea t m en t m et exist in g q u a lit y s t a n d a r d s (Fox a n d Ly t t le, 1994; Ma cKen zie et a l., 1994).
T r ea t m en t a t t h e p la n t in volves coa gu la -t ion , fl occu la -t ion , sed im en -t a -t ion , r a p id gr av it y s a n d fi lt r a t ion a n d d is in fect ion w it h ch lor a m in es. Six m on t h s p r ior t o t h e ou t -b r ea k t h e p la n t h a d ch a n ged t h e coa gu la n t u sed fr om a lu m t o p olya lu m in iu m ch lor id e t o r a is e t h e p H of t h e t r ea t ed w a t er. H ow ever, fi lt er efflu en t t u r b id it y h a d ver y h igh p ea k s (u p t o 1.7 N TU), a n d a t t h e b e gin n in g of Ap r il a lu m w a s r ein st a t ed a s t h e coa gu la n t . T h e t u r b id it y of t h e t r ea t ed w a t er p ea k ed a ga in on t h e 5 Ap r il 1993 t o 1.5 N TU a s t h e coa gu la n t d os a ge w a s n ot a d ju s t ed by t h e cu r r en t s t r ea m in g m on it or, d u e t o im p r op er in s t a lla -t ion . Als o, -t h e -t u r b id i-t y m on i-t or s for fi l-t er ed w a t er on t h e in d iv id u a l fi lt er s w er e n ot u sed . Tu r b id it y m ea su r em en t s on w a t er leav in g t h e p la n t w er e t a k en on ly ever y eigh t h ou r s.
M ilw a u k ee d r aw s it s r aw w a t er fr om La k e M ich iga n . T h r ee r iver s t h a t join in
M ilw a u k ee em p t y in t o t h e la k e a n d t h e p lu m e, t oget h er w it h s p r in g r u n off, cr os ses t h e in t a k e for t h e t r ea t m en t p la n t . In ves t iga -t or s w er e u n a ble -t o im p lica -t e a n y s ou r ce for t h e con t a m in a t ion in cid en t , bu t ca t t le a lon g t h e r iver s, s la u gh t er h ou s e d isch a r ge a n d h u m a n s ew a ge a r e a m on g t h e p os sib ilit ies. N o oocy s t s w er e d et ect a ble in t h e t r ea t ed
w a t er sa m p led d u r in g t h e in vest iga t ion (J ou r n a l of th e A m er ica n Wa ter W ork s A ssoci-a tion Up d ssoci-a te, 1993ssoci-a ), ssoci-a lt h ou gh ice-block s m a d e d u r in g t h e p er t in en t p er iod w er e a n a ly sed s u b s eq u en t ly a n d fou n d t o con t a in oocy st s (Ma cKen zie et a l., 1994). Follow in g t h e Milw a u k ee cr y p t osp or id iosis ou t b r ea k , t h e t r ea t m en t p la n t w a s sh u t d ow n (b eca u se it w a s t h e su sp ect ed sou r ce of t h e ou t b r ea k ) a n d w a s d is in fect ed . St r ea m in g cu r r en t m on it or -in g a n d con t -in u ou s t u r b id it y m on it or -in g a t ea ch fi lt er w er e in st it u t ed . T h er e w a s n o r eu se of t h e b a ck w a s h w a t er a t t h e p la n t follow in g t h e ou t b r ea k in vest iga t ion . A n ew goa l of < 0.5 N TU w a s set for t h e fi n is h ed w a t er (J ou r n a l of th e A m er ica n Wa ter W ork s A ssocia tion Up d a te, 1993b ).
Common deficiencies
Most of t h e w a t er -b or n e ou t b r ea k s h ave b een a ssocia t ed w it h op er a t ion a l d ifficu lt ies. Lisle a n d Ros e (1995) h ave p u blish ed a r ev iew of ou t b r ea k s in t h e UK a n d t h e USA. T h ey n ot ed t h a t t h e J a ck s on , Ca r r ollt on a n d Milw a u k ee ou t b r ea k s in t h e USA h a d on e or m or e of a n u m b er of d efi cien cies in com m on : 1 La ck of u se of m on it or in g eq u ip m en t . 2 La ck of r esp on se t o p r oblem s in t r ea t m en t
m on it or in g.
3 Use of r ecycled b a ck w a sh w a t er.
4 Sou r ce of h igh con cen t r a t ion of oocy st s in t h e v icin it y of t h e r aw in t a k e, bu t n o r et en -t ion p on d s.
5 Sou r ces a n d con cen t r a t ion s of oocy st s in w a t er sh ed u n k n ow n p r ior t o t h e in cid en t . 6 Oocy st s m ay h ave b een fl u sh ed in t o w a t er u p st r ea m by sn ow m elt or h eav y r a in p r ior t o t h e in cid en t .
7 In cr ea sed levels of t u r b id it y w er e n ot ed in t h e fi n ish ed w a t er, follow in g in cr ea ses in t h e t u r b id it y of t h e in com in g w a t er a t a p er iod w h en t h er e w a s a lik elih ood of oocy st fl u s h in g fr om la n d t o w a t er sou r ce. T h e fi lt r a t ion p r ocesses w er e eit h er a lt er ed or fou n d t o b e su b op t im a l.
Swimming pools
It is in t er est in g t o n ot e t h a t sever a l ou t b r ea k s of cr y p t osp or id iosis h ave b een a t t r ibu t ed t o m u n icip a l sw im m in g p ools in t h e UK (J oce et a l., 1991) a n d t h e USA (McAn u lt y et a l., 1994; Sm it h , 1991). T h ese in cid en t s h ave b een a t t r ibu t ed t o h u m a n fa eca l p ollu t ion .
Alternative treatments for
elimination of Cryptosporidium
Effect of temperature
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oocy s t s w a s h ea t ed t o > 72.4°C or h igh er w it h in on e m in u t e, or w h en t h e t em p er a t u r e w a s h eld a t > 64.2°C for t wo m in u t es of a fi ve-m in u t e h ea t in g cycle, in fect iv it y w a s los t , in d ica t in g t h a t C. p a r v u m oocy st s in w a t er ca n b e r en d er ed n on -in fect iou s w h en h eld for a r ela t ively sh or t t im e, for exa m p le, a t t em -p er a t u r es w h ich m igh t b e n or m a lly u sed t o a ch ieve p a st eu r iza t ion of food s.
Use of active oxygen species
Disin fect ion st u d ies in volv in g t r ea t m en t by ozon a t ion or ch lor in a t ion a t t h e d os es cu r -r en t ly u s ed in d ica t e t h a t som e oocy st s a -r e a ble t o w it h s t a n d su ch t r ea t m en t (Ta ble I). M ost ch em ica l d isin fect a n t s cu r r en t ly in u s e a r e n ot effect ive a ga in st oocy st s (Ba d en och , 1990; J os e p h et a l., 1991; Ros e, 1990; Sm it h a n d Ros e, 1990). As 100 p er cen t r em ova l of p a r t i-cles is n ot p oss ible u sin g gr a n u la r m ed ia fi lt r a t ion , com p let e t r ea t m en t for r em ova l of v ia b le oocy st s w ill p r ob a b ly r eq u ir e effect ive d isin fect ion t ech n iq u es. T h ose m ed ia t ed by fr ee r a d ica l a t t a ck m ay b e t h e m os t fea sible, bu t h ave n ot yet b een d em on st r a t ed a t a la r ge-s ca le p la n t .
On e of t h e m a jor d r aw b a ck s t o t h e p oss ible u se of .OH gen er a t ion for d es t r u ct ion of oocy st s is t h a t h yd r oxy l r a d ica ls a r e s o r ea c-t ive c-t h a c-t c-t h ey m ay n oc-t c-t a r gec-t c-t h e oocy sc-t s u n les s t h ey a r e clos e t o t h e sit e of gen er a t ion (i.e. t h e .OH r a d ica ls m ay b e “u s ed u p ” b efor e t h ey r ea ch t h e d esir ed t a r get ). .OH r ea ct iv it y is so gr ea t t h a t t h ese r a d ica ls r ea ct w it h w h a t ever or ga n ic m olecu les t h ey com e in t o con -t a c-t w i-t h , p r od u cin g s econ d a r y r a d ica ls of va r ia ble, bu t lesser r ea ct iv it y. Ot h er fr ee r a d ica ls, s u ch a s su p er oxid e a r e less r ea ct ive a n d m or e lik ely t o d iffu s e fu r t h er fr om t h e sit e of p r od u ct ion b efor e ca u sin g d a m a ge. Su p er oxid e r a d ica ls a r e good r ed u cin g a gen t s bu t r ela t ively w ea k oxid a n t s. H ow ever, in t h e p r es en ce of t r a n s it ion s m et a l ion s t h ey m ay gen er a t e .OH r a d ica ls t h r ou gh t h e H a b er -Weis s Rea ct ion (H a lliw ell a n d Gu t t er id ge, 1989).
Suitable methods of free radical generation
Met h od s w h ich h ave b een con s id er ed for w a t er t r ea t m en t sy s t em s in clu d e t h e u s e of UV ligh t , h yd r ogen p er oxid e, p h ot
osen sit izer s, u lt r a sou n d a n d ozon e. T h e p ot en -t ia l -t o in d u ce oxid a -t ive d a m a ge d u r in g -t h e w a t er t r ea t m en t p r ocess t h r ou gh t h e in t en -t ion a l gen er a -t ion of r ea c-t ive oxy -s p ecies w h ich in fl ict fa t a l d a m a ge t o m icr o-or ga n ism s t h r ou gh r a d ica l-m ed ia t ed a t t a ck on b iop oly m er s is t r em en d ou s; in d eed t h e su ccess of est a blis h ed a gen t s of d is in fect ion su ch a s ozon e is d e p en d en t on t h e r ea ct iv it y of u n s t a ble fr ee r a d ica ls (Don n elly et a l., 1994).
Ozone
Ozon e (O3)is a p ow er fu l oxid izin g a gen t , e.g. ca u sin g p er oxid a t ion of lip id s, m u ch of t h e d a m a ge in d u ced b ein g fr ee r a d ica l-m ed ia t ed . Ozon e is m u ch les s r ea ct ive a n d t h er efor e m or e sen sit ive t h a n t h e .OH r a d ica l; it r ea ct s r a p id ly w it h on ly a few w a t er p ollu t a n t s (Son -n t a g et a l., 1993). Ap p r oxim a t ely h a lf t h e ozon e gen er a t ed b ecom es .OH r a d ica ls w h ich a r e in volved in t h e in d ir ect a t t a ck s on or ga n ic com p ou n d s. In a q u eou s solu t ion m u ch of t h e d a m a ge in d u ced by O3occu r s a s t h e r esu lt of .OH for m a t ion , esp ecia lly a t a lk a lin e p H va lu es, a lt h ou gh s om e d a m a ge m ay a lso occu r in t h e r a n ge p H 6-8. O3r ea ct s r a p id ly w it h a va r iet y of or ga n ic m olecu les, in clu d in g m em b r a n e lip id s t o p r od u ce r a d i-ca l sp ecies a n d it i-ca n t h u s s t im u la t e lip id p er oxid a t ion . It a ls o r es u lt s in in a ct iva t ion of m em b r a n e-b ou n d en zy m es a n d t h e for m a t ion of p r ot ein a ggr e ga t es.
T h e com p osit ion of t h e w a t er a n d ot h er fa ct or s w h ich a r e k n ow n t o in fl u en ce t h e d isin fect ion m ech a n is m sh ou ld b e t a k en in t o a ccou n t w h en a p p ly in g t h es e d a t a . T h e m et h od of ozon e d osin g a n d t h e t em p er a t u r e of t h e w a t er a r e lik ely t o b e p er t in en t fa ct or s w h en con sid er in g t h e a p p lica t ion of ozon e (P a r k er et a l., 1993). At w a r m er t em p er a t u r es, > 25°C, ozon e is m or e effect ive in oocy st d es t r u ct ion (Ta b le II), p r ob a b ly b eca u se t h e oocy st w a ll b ecom es m or e p er m ea ble a n d t h er efor e p er m it s in gr ess of t h e d isin fect ion a gen t .
Ultraviolet
Wh en u sin g UV t r ea t m en t a lon e, w avelen gt h s of a p p r oxim a t ely 260n m a r e b elieved t o b e t h e m os t d est r u ct ive. In con ven t ion a l UV d isin -fect ion sy st em s, ligh t s op er a t in g a t 254 n m a r e oft en u sed for b iocid a l p u r p oses. H ow ever, t h e UV in t en sit y in con ven t ion a l sy st em s is n ot su fficien t t o ca u se p h ot och em ica l effect s, i.e. t h e for m a t ion of a ct ive oxygen sp ecies fr om m olecu la r oxygen . For t h e in a ct iva t ion of r esist a n t oocy s t s of Cr yp tosp or id iu m sp. m or e in t en s ive t r ea t m en t is n ecessa r y. It w a s d em on st r a t ed t h a t a n in t en sit y of 80 m w.s/ cm2
p er cen t w a s in su fficien t for in a ct iva t ion of oocy st s (Ra n s om e et a l., 1991). An ot h er st u dy, a st u dy by Lor en zo-Lor en zo et a l. (1993),
Table II
Inac tivatio n o f o o c ys ts by o zo ne
Ct mg/ 1-min Ct mg/ 1-min
90 per cent 99 per cent
Temperature (˚C) inactivation inactivation
5 2 4 -6 5 4 7 -1 2 8
1 0 9 -3 1 1 8 -6 2
2 5 3 -5 5 -1 0
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con clu d ed t h a t exp os u r e of oocy st con t a m i-n a t ed d r ii-n k ii-n g w a t er t o UV ligh t (15,000 m W/ s ) a t a d is t a n ce of 22cm r eq u ir ed a t lea st 150 m in u t es t o en s u r e zer o in fect iv it y in m ice follow in g in ocu la t ion w it h t h e con t a m in a t ed w a t er. Sou r ces ca p a ble of op er a t in g a t low er w avelen gt h s in t h e UV r a n ge m ay b e m or e effect ive a s fr ee r a d ica ls ca n b e gen er a t ed . Ligh t of w avelen gt h s b et w een 150n m a n d 210n m is effect ive in ca u sin g p h ot och em ica l effect s, i.e. in it ia t in g ch em ica l r ea ct ion s a s t h e low er w avelen gt h s excit e con t a m in a n t s t o a s t a t e w h er e t h ey a r e m or e ea sily oxid ized a n d t h ey a r e m or e su sce p t ible t o b on d b r ea k a ge. UV m ay con t r ibu t e t o a dva n ced oxid a -t ion p r oces ses in volv in g com b in a -t ion s of oxid a n t s. For exa m p le, t h e effect of UV is en h a n ced by H2O2(i.e. t h er e is a sy n er gis t ic effect ), s in ce UV ca u ses h om oly t ic fi ss ion of H2O2p r od u cin g .OH r a d ica ls. Alt er n a t ive com b in a t ion s in clu d e O3/ UV, O3/ H2O2a n d UV/ O2(p h ot oca t a ly sis, e.g. u sin g t it a n iu m d ioxid e). Wh en t it a n iu m d ioxid e (e.g. a s a w h it e p a in t ) is illu m in a t ed w it h UV ligh t it oxid izes h yd r oxy l ion s t o ext r em ely r ea ct ive h yd r oxy l r a d ica ls. T h ese .OH r a d ica ls a t t a ck a ll or ga n ic com p ou n d s a n d r es u lt in sever e d a m a ge. T h e com b in a t ion of UV a n d a s em i-con d u ct or (t it a n iu m d ioxid e) cou ld p r eclu d e ozon e gen er a t ion a n d t h e p r ov ision of H2O2, a lt h ou gh p r ocess a n d sca le-u p p r oblem s n eed t o b e over com e (Fos t er et a l., 1991). Tit a n iu m d ioxid e is n on -t oxic, in solu ble (it m ay b e im m ob ilized on s a n d ) a n d it is com p a r a t ively ch ea p. It is ava ila ble in for m s w h ich exh ib it h igh p h ot oa ct iv it y.
Ozon e d ecom p osed by UV ligh t is p r im a r ily con ver t ed in t o h yd r ogen p er oxid e wh ich is a ver y wea k oxid a n t . Hyd r ogen p er oxid e for m ed in it ia lly fr om UV p h ot oly sis m ay in it ia t e fu r t h er d ecom p osit ion of ozon e in t o .OH r adicals, especially at h igh er pH valu es, wh er e it r ea ct s fa st er w it h ozon e a n d a t lower UV in t en sit ies, wh er e p h ot oly sis b ecom es slower, so t h a t r esid u a l ozon e is exp osed t o t h e a ct ion of t h e h yd r ogen p er oxid e d u r in g a n ext en d ed p r ocess t im e (Hoign e a n d Ba d er, 1987). Bot h ozon e/ UV a n d ozon e/ h yd r ogen p er oxid e p r ocesses r esu lt in t h e for m a t ion of r esid u a l h yd r ogen p er oxid e wh ich m ay a ct on la b ile ch em ica ls p r esen t in t h e w a t er a n d in t er fer e w it h a n y for t h com in g d isin fect ion p r ocess sin ce it r ed u ces ch lor in e a n d sh or t en s t h e lifet im e of ozon e (Hoign e a n d Ba d er, 1987).
Ultrasound
Son oly s is ca n b e u s ed t o d is r u p t b iologica l con t a m in a n t s in w a t er a s m icr ob ia l cell w a lls a r e s u s ce p t ible t o t h e h igh -s p eed p en et r a t ion of sm a ll cav it a t ion m icr obu bbles, r es u lt in g in t h e lea k a ge of t h e cell con t en t s (M a son a n d Lor im er, 1988). If t h e u lt r a sou n d is a p p lied t o w a t er a t a su fficien t in t en s it y t h en a gr ea t
d ea l of d a m a ge in or ga n ic m a t er ia ls ca n b e in d u ced v ia t h e gen er a t ion of fr ee r a d ica l sp ecies, in clu d in g .OH r a d ica ls. T h ese r a d i-ca ls a r e p r od u ced a s a con seq u en ce of t h e en or m ou s in st a n t a n eou s in cr ea ses in loca l t em p er a t u r es a n d p r es su r es a s t h e cav it a t ion bu bbles colla p s e. T h u s, exp osu r e t o t h e exces-sive h ea t or for m a t ion of r a d ica ls t h r ou gh t h e u se of p ow er u lt r a sou n d ca n m ed ia t e ext en sive d est r u ct ion of or ga n ic m a t er ia l, in clu d -in g b iop oly m er s, -in t h e w a t er.
T h e u lt r a son ic in t en sit y a t w h ich a cou st ic cav it a t ion w ill occu r in a liq u id d e p en d s on it s p u r it y, t h e ga s con t en t , p r ev iou s u lt r a -son ic a n d p r essu r iza t ion h ist or y, a m b ien t t em p er a t u r e a n d p r es su r e, u lt r a son ic fr eq u en cy a n d p u lsin g ch a r a ct er ist ics a n d t h e con fi gu r a t ion of t h e u lt r a son ic fi eld . As t h e u lt r a son ic fr eq u en cy is in cr ea s ed , a h igh er in t en sit y is r eq u ir ed t o p r od u ce cav i-t a i-t ion . T h e a cou si-t ic p r essu r e r eq u ir ed i-t o p r od u ce cav it a t ion in w a t er d ecr ea ses a s t h e ga s con t en t is in cr ea sed . St u d ies in ou r la b o-r a t oo-r y u sin g a p ilot -sca le p ow eo-r u lt o-r a sou n d d ev ice in d ica t e t h a t a t h r ee or fou r log r ed u c-t ion in v ia ble oocy s c-t s ca n b e ea sily a ch ieved u sin g son ica t ion , bu t p ow er cost s a r e h igh (gr ea t er t h a n 50 p en ce p er cu b ic m et r e of w a t er ).
T h e u se of u lt r a sou n d p lu s ozon e (son ozon e) for w a t er t r ea t m en t h a s b een con sid -er ed (Da h i, 1982) p r e-t r ea t m en t , a s w ell a s sim u lt a n eou s s on ica t ion t r ea t m en t , is a ble t o b r in g a b ou t a s y n er gist ic a ct ion w it h ozon e; t h u s t h e ozon e d osa ge r eq u ir ed m ay b e effect ively r ed u ced , a leffect h ou gh in cer effect a in cir cu m -st a n ces t h e com b in a t or y effect s m ay b e on ly a d d it ive or even a n t a gon ist ic. T h e sy n er gism a r ises a s t h e u lt r a son ic ir r a d ia t ion of d is-solved ozon e in cr ea s es t h e a u t od ecom p osi-t ion of ozon e sign ifi ca n osi-t ly, by in cr ea sin g p r od u ct ion of fr ee r a d ica ls. Also, ozon e r ea c-t ion s m ay b e a cceler a c-t ed b eca u se of c-t h e loca l h ea t a n d p r ess u r e effect s of cav it a t ion (Sier k a a n d Am y, 1985). Seven t y p er cen t of t h e ozon e d osa ge r eq u ir ed for t h r ee log in a ct iva t ion , a n d 80 p er cen t of t h e ozon e d osa ge r eq u ir ed for fou r log in a ct iva t ion by ga seou s ozon e a lon e cou ld b e saved by t h e sim u lt a n eou s a p p lica t ion of u lt r a sou n d .
Concluding remarks
F r om cu r r en t in for m a t ion a b ou t
J.K. Do nne lly and E.I. Ste ntifo rd Wate r-bo rne c rypto spo ridio sis: a re c urring pro ble m
Enviro nme ntal Manage me nt and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
r a t e of in fect ion less t h a n 10–4, it h a s b een
es t im a t ed t h a t , in t h e USA a t lea st , s u p p lier s w ill n eed t o p r ov id e 5 or even 6 log of t r ea t -m en t (LeCh eva llier, 1994).
Du r in g t h e p a s t few yea r s, t h e w a t er in d u s-t r y h a s wor k ed s-t ow a r d s op s-t im izin g con d i-t ion s b oi-t h for i-t h e u se of b oi-t h coa gu la i-t in g a n d fl occu la t in g a gen t s a n d for fi lt r a t ion m et h od s t o a id oocy s t r em ova l. Rev iew of t h e d ocu -m en t ed ou t b r ea k s in d ica t es t h a t -m a jor im p r ovem en t s ca n b e m a d e in s u b op t im a l t r ea t m en t p r ocesses t o r ed u ce t h e n u m b er s of v ia ble oocy s t s in d r in k in g w a t er s u p p lies, p a r t icu la r ly by m a xim izin g fi lt r a t ion effi-cien cy. It is im p or t a n t t o a ss es s t h e va lu e of a lt er in g in d iv id u a l p r ocess t r ea t m en t s a n d p a r a m et er s in t h e ligh t of in for m a t ion fr om r isk a s s es s m en t .
Wa t er s u p p lier s con t in u e t o r eceive a dver se p u blicit y a s ou t b r ea k s in t h e com m u n it y or t h e is s u e of n ot ices t o b oil w a t er con t in u e t o ca u s e d ifficu lt ies. T h er e is a n eed t o con d u ct fu r t h er r es ea r ch w h ich r es olves t h e p r oblem of Cr yp tosp or id iu m r em ova l fr om s u p p lies d u r in g t r ea t m en t . We sh ou ld con sid er t h a t ot h er les s w ell k n ow n w a t er -b or n e p r ot ozoa n p a r a s it es m ay a lso b ecom e a focu s of a t t en -t ion in fu -t u r e yea r s, s o p er sever a n ce a n d s u cces s w it h t ech n iq u es for Cr yp tosp or id iu m elim in a t ion fr om w a t er s u p p lies is lik ely t o h ave fu r t h er r ew a r d .
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