Mosquito survey, Nguiu, Bathurst Island : 28-30 November 1991

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Northern Territory Department of Health Library Services Historical Collection

MOSQUITO SURVEY NGUIU, BATHURST ISLAND

28-30 NOVEMBER, 1991

Wendy Kelton and Paul Donaldson Medical Entomology Branch

DEPARTMENT OF HEALTH AND COMMUNITY SERVICES

DL HIST 595.772 KEL 1991

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MOSQUITO SURVEY

NGUIU, BATHURST ISLAND, 1991 28-30 November, 1991

1.0 Introduction

Nguiu is the major community on Bathurst Island, which is situated approximately eighty kilometres northwest of Darwin.

Following the diagnosis of a malaria case in the community, an entomological investigation was made to determine the likelihood of infected mosquitoes and subsequent malaria transmission.

The patient had recently returned from a sojourn in Papua - New Guinea, and had been in residence at Forest Street, Nguiu from 23/10/91 until diagnosed as suffering from malaria on 23/11/91, when she was transferred to Royal Darwin Hospital. The blood slide taken on 24/11/91 indicated Plasmodium vivax, with all stages present including gametocytes.

Previous mosquito collections in Ngui u have shown high numbers of Anopheles farauti s.l. to be present within the community in March 1988 (Booth & Dobson, 1988), low numbers in December 1983 (Kelton ^& Whelan, 1983) and in moderate numbers in October 1981 (Davis, 1981) thus rendering Nguiu vulnerable to the introduction of malaria. Malaria transmission has occurred on neighbouring Melville Island at Fort Dundas in 1827 (Powell, 1982), but no records exist of cases from Bathurst Island, although the Tiwi people suffered with malaria, thought to have been introduced by Macassan trepangers, during the latter part of the 19th century.

During the visit, a brief survey for container breeding mosquitoes was carried out, and an inspection of the sewage ponds and excess effluent discharge was also made.

2.0 Methods

2.1 Adult mosquitoes

Eight carbon dioxide baited, modified EVS traps were set at locations close to the community and the patient's residence overnight on the 29th November, 1991. Trap locations are shown in Fig 1. The mosquitoes were collected the following morning, killed by freezing, and a preliminary check made by microscope for the presence of malaria vectors. The catches were then packaged and returned to the laboratory for complete identification.

2.2 Container breeding mosquitoes

Containers with water, likely to be breeding mosquitoes, were searched for in key places such as the dry dump and council and mechanics yards. Any larvae located were collected and preserved in labelled vials of 70% alcohol, for later identification in the laboratory.

2.3 Larval survevs

An inspection at the sewage ponds was made using a standard dipper to sample at random points along the margin of the ponds.

The larva collected was preserved in a labelled vial of 7096 alcohol and returned to the laboratory for identification.

3.0 Results

3.1 Adult Collections

The results of the eight traps are shown in Table 1. Anopheles farauti was recorded in low numbers in the four traps set closest to the community. It was not collected in either of the two traps set at Forest Street.

Aedes vigilax was present in all traps, and in very high numbers along the margin of the community.

Culex annulirostris was present in low numbers in all except one trap, with the highest number occurring at the sewage ponds.

3.2 Container breeding mosquitoes.

A summary of the container breeding survey is shown in Table 2.

A total of fifty-eight containers with water were examined, and of these five were found to be breeding mosquitoes. The majority of these were Cul ex quinquef asciatus 1 a common container breederr which wasr in two instances, sharing the habitat with another species. In both cases the containers supporting two species were half 44 gal drums, and the other species being Cx. halifaxii and an as yet unidentified Uranotaenia species.

The most commonly occurring containers were from the rubbish category, and recreational i terns. The former occurring mainly at the dry dump area adjacent to the wet dump, and a vehicle dump by the road to the sewage ponds. The latter category consisted mainly of tyres used for swings and climbing equipment in the school grounds.

3.3 Larval survey_

The inspection of the sewage ponds and adjacent outfall located one single Cx. annulirostris larva in a small clump of grass at the margin of the ponds. The, outfall does not present a mosquito breeding problem.

4.0 DISCUSSION

4.1 Adult mosquito collections and insecticide application.

4.1.1 Adult mosquito collections

Ae. vigilax was present in very high numbers at the time of the survey. These high numbers would have resulted from the spring tides experienced in the week prior to our visit. Ae. vigil ax

00'-\~4

4

is a vector of Ross River virus, as well as being a vicious biter both by day and night. The presence of so many of this species within the community would pose a significant heal th threat due to Ross River virus, as well as biting discomfort. A Council employee stated in conversation that occasionally the breeding areas are treated with dieseline following high tides to reduce Ae. vigilax breeding. This practice is not recommended by the Health Department as environmental damage can occur. A more suitable control method would be to apply the biological agent Bacillus thuringiensis var israelensis (Bti) which is most effective against mosquito larvae, and has no effect on non-target organisms. Bti is a liquid formulation, diluted with water prior to application, and can be applied by pressure sprayer. Information about this product can be supplied by the Medical Entomology Branch.

Of the other species collected ex. annulirostris is the only mosquito of major concern and numbers of this species were low in all traps. This species breeds in freshwater swamps and drains and is likely to be more prevalent when areas are flooded by rains in the wet season. It is a vector of several arboviruses and could constitute a health threat at certain times.

Cx. si ti ens and Ae. funereus breed in brackish water areas and do not fly far from the breeding site, so that these species would not cause a problem away from the mangrove areas. They are not known to carry disease.

Ae. kochi breeds in Pandanus species axils, and does not travel far from the breeding sites. It is not associated with arbovirus transmission.

Ae. notoscriptus breeds in containers around populated areas and occurred in the traps closest to the Council yard and close to the drain from the Social Club. This area had numerous containers and rubbish which would be suitable breeding sites.

An. farauti was caught in low numbers in four of the traps.

This species ·is recognised as a very effective vector of malaria in Australia. An. f arauti is a species complex, with several sibling species known from the Northern Territory, and to date determination of the species is not possible by taxonomic means.

Identification of sibling· species can be determined by the use of a DNA probe which has been developed by a joint project involving Menzies School of Health and Department of Health and Community Services Medical Entomology Branch. All An. farauti s .1. collected during the survey will be forwarded to Menzies for identification to species level.

4.1.2. Insecticide application

Due to the presence of low numbers of An. f arau ti s .1. in the four traps closest to the community, coupled with the presence of gametocytes in the patient's blood slide, a decision was made to apply an adulticide by ULV method into the harbouring areas adjacent to northwestern margin of the community on the evening of 28th November. Approximately seven hundred and fifty

millilitres of Maldison ULV Insecticide was applied by means of a Porta-Pak model 98600 Ultra Low Volume sprayer. The operator and sprayer machine were driven along the fogging route on the back of a Toyota Landcruiser utility ^r at a speed slightly less than 4km per hour. The distance covered was approximately 1. 4 kilometres I and a distance of approximately 400 metres at the eastern end of the run was repeated. This area had recorded the highest numbers of An. farauti s.l. in previous surveys, and was the locale of numerous houses closest to the harbouring area.

Thus a linear distance of 1. 8 km was treated, and estimating a swathe width of 15 metres ^r an area of 27 000 square metres or 2.7 ha. This is an application rate of approximately 277 millilitres per hectare, with the recommended rate being within the range of 200 to 280 ml/ha.

The ULV insecticide application was commenced at 1910 hours.

(sunset at 1900) and completed at 1945. The route taken was along the margin of the open forest and bushland verging on the mud flat mangrove area to the northwest of the community. The direction of travel was north-easterly, so that the most populated area was treated at the end of the run ^r when more mosquitoes were likely to be active. See Fig 2 for fogging route details. The breeze was minimal at the time of the insecticide application ^r being slightly from the north-east if any.

4.2 Container breeding survev

Numerous receptacles capable of holding water and breeding mosquitoes were observed during the brief survey. The number of mosquitoes resulting from container breeding could be expected to increase once the wet season starts, and more of these containers are filled with water. Cx. quinquefasciatus was the most encountered mosquito in containers. This is a hardy species and can survive in quite polluted water. Ae.

notoscriptus, the most common container breeding mosquito in the Northern Territory was not found as larvae, but adults were present. It is most likely that this population will increase once the wet season starts.

Ae. aegypti, the dengue fever mosquito, widely distributed throughout Queensland, was not located during this survey. Its absence can not be positively claimed for Ngui u, due to the failure to find Ae. notoscriptus, which inhabits similar types of breeding sites. A further survey is required when containers are filled ^r to be certain tha,t Ae. aegypti has not been introduced to the area.

The dry dump area adjacent to the wet dump is very widespread and haphazard, and there are many containers that would support mosquito breeding during the wetter times of the year. The dump is located nearly three kilometres from the community, and because of this distance is unlikely to be a suitable site for Ae. aegypti breeding, as this species is usually found in close proximity to places of human occupation.

6

A number of tyres are utilized in the school grounds as swings and climbing equipment. Two of approximately 20 tyres were found to be breeding mosquitoes, and this number could be expected to increase when the wet season begins. These breeding sites could be eliminated simply by drilling holes to allow water to drain out. Garbage bins around the school yard were also filled with water, and should either be emptied regularly or have holes drilled in the bottom to allow water to drain out.

4.3 Larval survey

The sewage ponds were inspected for mosquito breeding, and found to be in very good condition. Some small clumps of grasses were found at the margins, but the overall maintenance of the ponds was good. One

ex .

annulirostris larva was found and many aquatic predators were sighted, indicating that biological control of mosquito breeding in the ponds is effective.

The excess effluent from the ponds is piped underground to a tidal section of a creek flowing northwest from the community.

The creek is well defined at the effluent inflow point, and a large fish population was observed feeding in the sewage flow.

No mosquito problems are associated with the excess effluent disposal.

5.0 Conclusions and Recommendations

5.1 Risk of malaria transmission was present at the time of the survey, and should be precluded as a result of the adulticiding prompted by the findings of the survey.

5.2 Aedes aegypti but further absence.

was not present at the time of investigation is required to

the survey ¹ verify its

5.3 Aedes vigilax presents both a nuisance and a health risk to the community. Its breeding sites need to be located, larval surveys undertaken by community personnel following spring tides, and larviciding perf orrned as required. The Medical Entomology Branch would be available to advise on this recommendation.

5.4 Tyres and rubbish bins in the school and pre-school grounds should have holes drilled to ensure that they are not breeding sites for mosquitoes during the wet season.

6.0 Acknowledgements

Father Telhoy and Nguiu Council leaders and staff are gratefully thanked for their help and co-operation during the survey.

Special thanks to Town Clerk Herman Timaspapua for his assistance in obtaining the necessary permits and the use of a vehicle for the duration of the survey.

7.0 References

Booth, D & Dobson, G. (1988) Aedes aegypti and vector mosquito survey, Nguiu March 1988 Unpublished report of the

Medical Entomology Branch, Department of Heal th and . ~

Community Services.

(1983) 1981.

Davis, G. P., October, Entomology Services.

Vector mosquito survey, Nguiu Community, Unpublished report of the Medical Department of Health and Community Branch,

Kelton, W and Whelan P. (1984) Aedes aegypti and vector mosquito survey, Bathurst and Melville Islands, 12th to 16th December, 1983. Unpublished report of the Medical Entomology Branch, Department of Health and Community Services.

Powell, A. (1982) Far country a short history of the Northern Territory. Melbourne University Press.

Wendy Kelton

Technical Officer

Paul Donaldson

Technical Assistant

Table l. MOSQUITO SURVEY - NGUIU, BATHURST ISLAND NOVEMBER, 1991

Number of female mosquitoes collected in CO2 baited traps overnight on 29/11/91 Medical Entomology Branch

N. T. DEPARTMENT OF HEALTH & COMMUNITY SERVICES

!TRAP LOCATION . j SITE NO

I .

SPECIES COLLECTED i TOTALSj

I I

^IAn.farauti Ae.notoscriptus Cx.sitiens I

I

' J (Fig. 1) j Ae.funereus Ae. vigil ax j

I

I

^! Ae.kochi Cx.annulirostris I

I

I I I I l

! I i I ⁱ

···--·-·---··----·-·-·---... ---·---·---r---.l...--- --- --·-- - - ---·--- - - ---·---·- ---1

!Rear Jones residence,

I

1

I

2 15 1 1

I

19j

!Forest Street. 1 1

I

I

!

I I I

i • ^I

J:Vestern end of Forest street,

I

2 13 2 ^1·

Im open scrubland. I

I I I

! I I

I

!sewage Ponds l 3 i 53 13 i 66

I I I I

^I

IPandanus thicket at rear

I

4 11 1 14 3 253 2

l

²⁷⁴¹

I

of Council yard. ) ^!j

I ,

1 :

l I ^I

' I

I Adjacent to drain to mud f] at,

l

⁵

1 3

^{1 O 1 2 1 8 O 1 ' 2 O}

6 1

I

opposite Social Club l I ¹

k ow level sewage pump ! 6

l 2

^{4 504 3 5131}

I

I i i

!Bridge over drain, north 1 7 jl 12 112 1 1261

(of community ¹

1

/

I

' I I I

. I

I I I

[Low lying, sand-mined area i 8 j 113 ^J 1131

~_5_0 om .. __ sou th ... of c_o mm unity. _________

J _________ Lj --- --- -- --- --- --- -- · - - --- + --- - 1

iTOTALS

1 1

^{1 24 33 1243 23 1 I 13321}

1 .... ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ,, _____ ,,,,,,,,,,,.,,,,,,,,,,,,,,,,,,,,,,,,, ____ ,, __ ,,_,,,, _ __________ __

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NGUIUl .XLS

TYPF~S OF CONTAINERS FROM WHICH LARVAE WERE COLLECTED

Medical Entomology Branch

N. T. DEPARTMENT OF HEALTH AND COMMUNITY SERVICES ----r---·---.-,.-,---

1

TIMES OF

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/TYPE OF CONTAINER i TIMES

!WITH WATER i RECORDED

i l

POSITIVE /SPECIES FOR LARVAE !LARVAE

i

I

~--- - - - -- ~ · I

I

I )Garden acoutrements

I

(saucers under pots, etc.)

!

!water storage containers

I

(rainwater tanks, drums, wells.etc)

!Discarded household

!

\Rubbish

I (tins, bottles, tyres, car bodies.etc) I

jused commercial_ & domestic

i

(still in use: tubs, recyclable drink

I

cans, animal water dishes, etc) l

!Recreational items

!

i (boats, swimming pools, tyre swings & play equipment, etc)

!Building fixtures

I

(roof gutters, drains, etc)

i

:Natural habitats

i (Tree holes, shells, plant axils, etc)

i ---

iTOT AL CONTAINERS

0

0

6

22

8

22

0

0

1

1 1

2

--·---·---!

- - - 1

I

i

I I

! I

i

I

i I

!

!j

·;cx.quinquefasciatus

1\Cx.halifaxii

I

1

1cx. quinq uefascia tus /Cx.quinquefasciatus I\Uranotaenia species

I

lex. quinquefasciatus

i

l

I

I

I

I

I

!

I

I

I

I

I I

_i

I I

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_.

I

_I

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i

^!

I

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+----

·-·---1

! ____________________ _

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58

I

5 !

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" " ' '' "_W _ _ _ W _ _ _ , _ , , _ , _ _ w_l, ____________ .! __________ ! """"""'"w _____________ , _________ , _______ ,""'"" i

NGUIU2.XLS

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11

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LEGEND

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(BA.T\-\URST

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gc.,A LE

Sewage Ponds

Department of Health & Community Services

LEGEND NGUIU

(BATHURST ISLAND)

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Road Creek Mangrove

Mud flat/salt marsh Trap Location

Potential breeding/

harbouring site, Fogging route,

direction taken, direction of fog

SCALE

::2.oa.... 400,,,..

N

r

Apsley Strait

•