Human pathogens associated with on-farm practices - Implications for control and
3. Selected Human Pathogens - Spread and Transmission
Approximately 150 diseases are considered to have a zoonotic character (Teufel, 1998).
Table 1 displays a selection of those agents we consider to be of main interest for on-farm practices including reservoirs and the frequency they are reported. It is obvious that this chapter cannot describe all zoonotic agents exhaustively so we will focus on some diseases - emerging and re-emerging, classical and ‘new’ - to show current trends.
Bacillus anthracis is a very good example for a zoonotic agent which has a reservoir in soil.
Herbivores get infected when taking up the spores with their feed. Humans usually get infected directly when the bacteria or spores are incorporated via contaminated food or when handling contaminated material during processing. Anthrax is therefore a direct and indirect (via vector or vehicle) zoonosis (Table 2).
In developed countries anthrax in animals and man is rare. Effective veterinary countermeasures like notification, culling and destruction of infected animals, treatment of suspected cases, and vaccination of livestock in endemic areas reduced the incidence of the disease. Industrial cutaneous and pulmonary anthrax in man was controlled by an Andreas Hensel and Heinrich Neubauer
Andreas Hensel and Heinrich Neubauer
Table 1. Important human pathogens of animal or environmental origin associated with on farm practices.
AGENT RESERVOIRS FREQUENCY
Bacteria
Mycobacterium spec.* Wildlife, livestock, man Frequent, world wide
Brucella spec. * Domestic and wildlife Regional, world wide
Campylobacter spec.+ Domestic and wildlife, fowl Frequent, world wide
Clostridium perfringens+ Soil, water, ingesta of animals Sporadic to frequent, world wide Eschericha coli (EHEC/STEC)+ Man, animals, (cattle) Frequent, world wide
Erysipelothrix rhusiopathiae* Livestock Sporadic or group wise, world wide Leptospira interrogans subsp.* Domestic animals and pets, rodents Regional, world wide
Listeria monocytogenes+ Soil, feed, milk products Sporadic, group wise, world wide Bacillus anthracis* Soil (abbatoirs), ruminants, pigs Sporadic, world wide, regional Staphylococcus aureus / toxins+ Man, pets Sporadic frequent, world wide Salmonella-serovars+, # Domestic mammals, fowl, reptiles, Frequent, group wise to endemic,
water, waste water world wide
Shigella spec.+ Man, feces, waste water Frequent world wide Clostridium botulinum# Environment, animals Sporadic, world wide Clostridium tetani# Soil, waste water, man, animals Sporadic, world wide
Yersinia spec.+ Livestock, rodents (?) Sporadic, world wide
Pasteurella spec.*+ Livestock, rodents Sporadic
Chlamydia psittaci* Fowl Sporadic to group wise
Viruses
Influenza virus* Fowl, pigs Sporadic to endemic, world wide
West-Nile fever virus* Fowl, horses, cattle, sheep, man, Sporadic, world wide biting midges
Rift-Valley fever virus* Domestic and wildlife, man, Sporadic to endemic biting midges
Orthopox viruses* Livestock, cats, monkeys, wildlife Sporadic, world wide Equine encephalitis viruses* Rodents, fowl, horses, snakes, Sporadic
(Eastern, Western, Venezuelan) biting midges
Rabies virus* Domestic and wildlife, bats Sporadic, world wide
Fungi or Algae
Microsporum spec. * Livestock ?, worldwide
Trichophyton spec. * Livestock ?, worldwide
Aspergillus spec. *, # Livestock, soil ?, worldwide
Candida spec. * Man, livestock ?, worldwide
Prototheca spec.+, # Soil, water ?, worldwide
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Table 2. Transmission of various zoonoses.
Horizontal direct transmission of zoonoses
Disease Sheddding by Entry Mode of transmission
Anthrax Carcass, blood Skin Contact
Brucellosis Genital tract Skin (lesions) Contact
Orthopox viruses Crusts Skin (lesions) Contact
Mycoses Skin Skin surface Contact
Helminthiases Faeces Mouth Contact
Horizontal indirect transmission of zoonoses
Disease Shedding by Entry Mode of transmission Vehicle
Anthrax Meat Mouth Alimentary Food
Brucellosis Milk Mouth Alimentary Food
Tuberculosis Milk Mouth Alimentary Food
Psittacosis Faeces Respiratory tract Aerogenic Air, dust
West Nile fever Blood Skin Via vectors Insects
Helminthiases Faeces Mouth Alimentary Food
Prion-induced disease ‘Risk material’ ? Mouth Alimentary Food
Table 1. continued
AGENT RESERVOIRS FREQUENCY
Parasites
Protocoa: Amoebia spec.+
Toxoplasma Lifestock, cats, rodents Giradia spec.+
Trematodes: Fasciola+ Cestodes: Taenia+
Echinococcus+ Dogs, cats, foxes Sporadic, world wide Nematodes: Ascaris+
Trichinella+ Lifestock, rodents Sporadic to group wise, world wide Anisarkis+ Fresh water fish
Unknown origin: prions (?)+ Cattle, sheep, food Sporadic to endemic
*: zoonosis mainly transmitted by contact
+: foodborne zoonosis
#: environmental contamination
improved factory hygiene when producing wool, hides, meat, and bones. Quality requirements forced the industry to start up processing already in the country of origin of animal products reducing the import of raw materials which could be contaminated with spores. On the other hand, the prosperity of many industrial countries lead to a rise in the import of bone meal for fertilizers or feed. Raw meat of exotic animals, exotic fruits, spices, and vegetables are easily available and are consumed increasingly not only from people who are allergic to domestic food. This is a possible reason why anthrax cases occur in non-endemic areas. Re-introduction with products which are spread on fields and meadows might even lead to the establishment of new local foci with an increased risk for livestock (Turnbull, 1998). Anthrax highlights also the problems of pest control in developing or undeveloped countries of endemic areas in Africa and Asia. Public health and veterinary regulations are often not existing or are ignored due to poverty or customs and traditions.
A short review on the PROMED mails (a program for monitoring emerging diseases of the International Society for Infectious Diseases) of the last year can teach us the shortcomings hygienists face at the pre-harvest stage in such countries. In Kazakhstan (August 2000) various human anthrax cases were reported. Governmental countermeasures like police controls and sanitary checkpoints were of little value as cattle breeders and sellers could avoid these controls by using hidden paths. Criminal machinations can also be assumed when animals with a certificate of vaccination get ill with B. anthracis. Hence, inadequate handling of the vaccine by a veterinarian might also be possible. A lack of infrastructure or of interest from governmental personnel involved can cause life threatening situations. Although a case of animal anthrax was suspected correctly a time delay of several days in the final diagnosis occurred. Meanwhile the meat was processed to food. In this outbreak a case of cutaneous anthrax was reported which turned into a fatal septic form when treated unsufficiently by the patient on its own. A dramatic rise in number of human cases in an outbreak can be observed in very poor countries like Zimbabwe (January 2001) when people are forced to beef cadavers. In this outbreak at least 980 patients were involved. As it is not possible to control anthrax in wildlife outbreaks can be observed in countries with a high number of free-living herbivors like South Africa involving man and livestock (Januar 2001). In India where rendering plants for carcass disposal are rare and vultures still ‘dispose’ most of the carcasses of animals, a die-off of the vulture population raised the fear that anthrax can emerge in future (February 2001). In South Africa most of the infected animals are game which cannot be vaccinated. Infections in man are again due to eating the carcasses of died cattle (February 2001). It is clear that many of these mails which are reported on a private basis do not provide the exact facts but they can be used to show actual trends in zoonoses (http://www.promedmail.org).
An interesting distribution of brucellosis outbreaks caused by Brucella species is reported for Europe (WHO, 2000). In Mediterranean countries like Italy, Portugal and Spain as well as in the eastern European countries of the former Soviet Union brucellosis is still an important cause for human disease with more than 1000 outbreaks every year. In contrast, Germany like most of the northern European countries is considered to be free of brucellosis. However, in 1999 20 cases of human brucellosis have been notified. About 15 of these cases have been imported, 9 from Turkey. Sources of infections were the contact to sheep and goats and the consumption of raw milk or products made from raw milk Andreas Hensel and Heinrich Neubauer
(Anonymous, 2000). An unintentional import of Brucella via the living animals but also through imigrants should always be considered (Rasch et al., 1997). Further efforts have to be made in the future to control this zoonosis already at the farm level with strengthened veterinary countermeasures like vaccination, serological survey, and re-imbursement of the animal owner whose life stock has been destroyed to get acceptance from the farmers.
Glanders caused by the bacterium Burkholderia (Pseudomonas) mallei is a chronic disease in solipeds. It leads to disastrous losses in horse populations in the past and is a rare but often fatal infection in humans (Neubauer et al., 1997). It can be transmitted to humans by contact or by the consumption of contaminated meat. Glanders is one of the first zoonoses which was distributed all over the world with its host, the war animal horse. With the decline of the horse population due to motorization and strict controls in trade, the number of glanders infections in man and horses has dropped. For many years it was not reported to the OIE and was considered to be eradicated. However, in a survey in the European part of Turkey evidence was found that it is still widely distributed in the domestic horse population but being ignored by the veterinary administration (Arun et al., 1999). The infection was distributed by commonly used bridles, drinking troughs, and mangers. Transport, crowding and bad keeping conditions maintained the infection. In the north-east of Brazil, namely in the States of Alagoas, Ceara, Maranhao, Pernambuco, Piaui and Sergipe it was recognized by the local authorities in 1999 and 2000 again and notified to the OIE. Glanders is still a threat to solipeds and humans demanding the need for keeping up the strict, expensive and ‘old-fashioned’ countermeasures which are in action in many countries for more than a century.
Since the eradication of variola virus - the causative agent of smallpox in humans - new threats have emerged from two closely related orthopox virus species. In Africa, a growing number of smallpox like infections has been recognized in humans. Monkey pox virus was isolated from infected humans and monkeys during various outbreaks (Jezek and Fenner, 1988). The declining immunity in the human population after cessation of smallpox vaccination, hunting and eating of monkeys, deforestation due to overpopulation and therefore closer contact to monkeys or reservoir animals like squirrels were identified to be the main reason for infections in humans (Mukinda et al, 1997; Neubauer et al., 1998).
Due to their impact on human health pox viruses were always under special investigation including sophisticated epidemiological studies. Monkey pox virus infection and the reasons for its emergence could be considered as the ‘index case’ for a long list of ‘new’
zoonoses like Nippah virus, Meanangle virus or even Ebola virus infections namely the increasing human intrusion into previously un-populated areas. Although the animal ancestor of vaccinia virus is unknown it was commonly supposed that the vaccinia virus strains which have been used in the campaign against smallpox would not be able to establish new natural reservoirs. However, Buffalopox virus in India and Cantagalo virus in Brazilia may have derived from the locally used smallpox vaccine strains infecting nowadays cattle and humans. Cantalago virus infection is even considered to be a emerging disease (Damaso et al., 2000). Therefore, these two agents have an extraordinary position in the class of zoonoses; they are indeed man made.
Andreas Hensel and Heinrich Neubauer
A special problem in controlling human pathogens on the farm level is the spreading of an agent by wildlife vectors which can cover far distances in short times. Migrating birds can carry biological agents along their travelling routes but also in areas far away from their normal range by accident for example when displaced by storms. The outbreaks of West Nile fever in Europe and Northern America in the last five years are well documented examples for this phenomenon. In the Old World the virus is introduced by viraemic migratory birds into European countries like Italy, France or Romania from the African continent.
Ornithophilic mosquitoes like Culex pipiens are then the local vectors and birds are again the amplifying hosts. Humans and horses get diseased mainly in or near wetlands where the birds have contact to large numbers of biting midges. The disease can take a severe course in humans with fatal outcome. In contrast to Europe, West Nile fever is a new zoonosis for the United States starting with a die-off of birds and several human cases in New York City. Normal migration between Europe and America, displacement by storms or legal/illegal imports have been discussed as the source of the outbreaks. Whether West Nile fever virus can establish a steady reservoir in the temperate areas of the Northern hemisphere in the light of a steadily global warming up will be experienced in the future (Work et al., 1955; Hubálek and Halouza, 1999; Rappole et al., 2000). In the year 2000 another emerging arthropod-borne virus, Rift Valley fever virus, spread into Saudi Arabia and Yemen on the Arabian peninsula. But not only viruses can be transported by migrating animals over thousands of miles. The appearance of Mycobacterium africanum in cattle in northern Bavaria, Germany (Weber et al., 1998) gave rise to the assumption that this bacterium was imported into a tuberculosis free country by a swallow.
Intoxications caused by toxins of Staphylococcus aureus, Clostridium perfringens, or Bacillus cereus have to be considered as important sources of human and animal disease. Helminth infections or infestations with protozoa play an important role as human pathogens in Asian countries. Dermatomycoses are a threat to animal keepers all over the world.
Although these diseases cause enormous economic losses they will not be discussed as this review can focus only on some outstanding highlights.