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Northern Territory Department of Health Library Services Historical Collection

DL HIST

595.772 BOO 1988

AEDES AEGYPTI AND VECTOR MOSQUITO SURVEY

NGUIU - 1988

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CENTRAL LIBFIARV

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TERRrrORY HEAL TH SERVICES

AEDES AEGYPTI AND VECTOR MOSQUITO SURVEY NGUIU - MARCH 1988

by David Booth and Graeme Dobson Medical Entomology Branch

Dept. Health & Community Services

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Aedes aeqypti and Vector Mosquito Survey Nguiu, March 1988

1. Introduction

Nguiu, situated on the south-eastern end of Bathurst Island, is a community of approximately 1500 people.

Previous surveys ( 1982 and 1983) by the Medical Entomology branch have shown the town to be free of the vector of dengue fever; Aedes aeqypti but that other mosquito disease vectors are present.

These included the malaria vector, Anopheles farauti s. 1., and the major epidemic polyarthritis vectors, Aedes vigilax and Culex annulirostris The latter is also a vector of the potentially fatal disease, Australian encephalitis.

Two officers from the Medical Entomology branch visited Nguiu from the 29-31st March, 1988 to determine if the dengue fever vector had been introduced to the island in the interim, to determine the significance of the mosquito breeding areas found on the island and to liaise with local authorities on mosquito control. Major mosquito breeding areas were examined and mosquito control options for these are discussed.

2. Method

A random selection of 114 premises were examined. All containers which might breed mosquitoes were recorded, and these were sampled and any larvae collected were stored in 70% alcohol for later identification.

Carbon dioxide < dry ice) baited EVS mosquito traps

·were run in various localities < Map 1) from before sunset to just after sunrise. Any mosquitoes biting during the day were also collected.

The major mosquito breeding areas were visited and larvae collected and stored in 70% alcohol for later identification.

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3. Results and Discussion*

3. 1 Aedes aegypti Survey

The survey was conducted just after heavy rains at the end of March. There were more containers with water and more premises with larval breeding than in the December 1983 survey Csee Kelton & Whelan, 1987).

Nearly 30% of containers were breeding mosquitoes C Table 1

>.

The likelihood of detecting any established Aedes aegypti population was thus high. However, no Ae aegypti were collected.

Aedes notoscriptus was the most commonly collected species. It was collected from tyres, buckets, plant pots, shells, bowls, a tray, and old car bodies. This mosquito bites man, and is a suspected vector of arboviruses. It is a proven vector of dog heartworm.

Any container holding water for greater than a week is likely to be colonised by this species. The most common breeding site, old tyres in gardens, can easily be eliminated by drilling holes to allow all water to drain from the tyre. Very large numbers of mosquito larvae were found breeding in the tractor tyres in the pre-school yard.

Culex guinguefasciatus was collected from rubbish bins, an old f~idge door, and buckets. It is a vector of diseases such as Australian encephalitis, a potentially lethal disease, and other arboviruses. It was mainly found breeding in the few rubbish bins in the community which did not have draining holes in their bases. Such drainage holes would eliminate this source of this species.

Culex pullus was collected from a bucket and old tyres. In each case i t was breeding in conjunction with another species (either Cx guinguefasciatus or Ae notoscri pt us

>.

It is not a medically important species as i t is considered to mainly feed off birds.

Seven Ae funereus and one Ae vigilax were collected biting human-bait during the day (between 0700 and 1100 hrs) in the residential area. Ae funereus was also collected biting indoors at 2200hrs. This biting rate represents a minor pest problem.

*see attachment 1 mosquitoes

for notes and common names of

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3. 2 Malaria Vectors

Three Anopheles species were collected; An bancroftii An farauti s. 1., and An meraukensis The first two are known vectors of malaria and the last a potential vector.

The most important of the three is An farauti s. 1., the major vector of malaria in the western Pacific region. If more than 10 of these are collected near an imported malaria case fogging is usually undertaken to reduce the risk of transmission. Two sites on the northern side of town exceeded this number - behind the workshop, site 6 (17/trap night>, and near the main beach, site 8 (83/trap night).

An farauti represents a species complex - two members are known to breed in freshwater, and the third in brackish water. There was an abundance of both types of breeding sites near the traps, but larval Anopheles were only collected at site 5 (see map 1), a freshwater dam not likely to be the source of the large numbers trapped at sites 6 and 7.

However, as the breeding sites are almost certainly to the north of the town Cthe traps on the south and west sides of town caught few Anopheles mosquitoes at this time) a suitable fogging route is available, which would be effective in the predominantly south-easterly winds Csee fogging route marked on map 1) . This fogging route is i nterupted by housing near the coast CKelantumama and Puantulura streets).

Further development should leave the fogging route intact as far as possible. A fogging machine could be landed by barge in the event of an imported malaria case. Alternatively, a backpack ULV fogger could be transferred to Nguiu by plane Cif the barge timetable was not timely).

Although the major breeding was occurring to the north of the town in late March, there were potential breeding areas to the south of the town, near the old airstrip, that may be important later in the dry season as the water in the swamps and borrow pits recedes. Highest numbers of Anopheles mosquitoes usually occur in May and June in other coastal areas of the Top End. Traps operated on the island as necessary Cie in response to public complaints) could determine the relative importance of any suspected breeding areas. Engineering is unlikely to be feasible as a means of reducing the breeding areas,

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and as the health risk is likely to be severe only in the event of an introduced malaria case, fogging is probably the only practical immediate adult mosquito control method here.

3. 3 Other Disease Vectors

Aedes vigilax the salt marsh mosquito, is a major vector of arboviruses, particularly of the causative agent of epidemic polyarthritis. It was not common in any traps in this survey <maximum of 3 at coastal site 8), but in the Dec.ember 1983 survey was collected at all trap sites, varying from 29/trap night C site 5) to 1 52 I trap night C adjacent to low level sewage pump station, site 7). Suitable breeding areas for this species close to town were not large, but this species is capable of flying great distances.

This species probably is of importance in the weeks following spring tides in the late dry season.

Cx annulirostris , the common banded mosquito, was collected in all traps (maximum 54/ trap night at site 4 near the sewage ponds). It is a vector of the potentially lethal Australian encephalitis, and of other arbovirus diseases including epidemic polyarthritis. It breeds in a wide range of habitats, from shallow grassy flooded areas to sewage ponds and brackish swamps. Larvae were collected from goundwater at the rubbish dump, and from the sewage ponds. This is discussed in the next section.

Ae funereus is known to breed in slightly brackish swamps, usually in shaded areas. It was collected from grassy pools adjacent to mangrove areas (site 8), from a freswater paperbark swamp near the old airstrip

C see Kap 1

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It was the most common species collected bitin~ man in the residential area, biting at any time of the day, both indoors and outdoors. It has been shown to be a potential vector of arboviruses.

No other mosquito species were collected in high numbers during this survey or the December 1983 survey. The other mosquito species collected (Tables 2 and 3) are not likely to represent major disease threats or nuisance species judging from the population sizes reflected in trap catches.

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Biting midges were collected at most traps sites, occurring at more than a 100/trap night at sites 4 and 9; and more than 1 000/ trap night at site 3. The major species at all trap sites was Culicoides ornatus an avid man- biter but not known as a vector of disease.

This species breeds in the neap tide zone, a zone often marked by the presence of the · mangrove Rhizophora stylosa ~ ornatus can be expected to occur in greatest numbers a couple of days after the neap tide. It can occur in high numbers up to 2 km from its breeding site. It bites mainly in crepuscular hours (dawn and dusk>, though in shaded areas may bite at any time oC the day. The biggest catches were at trap sites relatively remote from residential areas. Any town expansion should avoid the zones where biting midges occur in high numbers. Personal protection (long sleeved clothing, footwear, insect repellent>, avoiding areas where the midges are active, and using double mesh insect screens are the only methods currently available to reduce the effects of this species.

3. 5 Mosquito Breeding Areas

A number of mosquito areas close to the community were identified these were the freshwater to brackish swamp near the sewage pump station (Map 1, site 7);

depressions behind the beach mangroves (site 8); a paperbark swamp and borrow pits on the south-east side of the old airstrip; a large freshwater swamp at the western end of the old airstrip C site 3); the sewage ponds ^C site 4); the rubbish dump; borrow pi ts at the north western end of town; and some parts of the stormwater drainage system.

None of these breeding areas were producing high numbers of mosquitoes <Table 3) at the time of the survey. However, larger numbers could be produced at other times of the year. Should these areas produce increased mosquito numbers mosquito control options for each breeding site varies and are discussed below.

The largest mosquito breeding areas were those at sites 3 and 7. The swamp near site 7 had freswater areas from which the minor species Uranotaenia lateralis and Culex ^C Loph. > sp. were collected.

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The presence of Eleocharis reeds indicate this is at least a semi-permanent swamp. The swamp was continuous with the mangrove area and contained brackish water breeding areas.

It was thus a suitable breeding area for a wide range of species, including the malaria vector, Anopheles farauti s.l .. This species is most likely to breed in the north-western section of the swamp, where the stormwater drain discharges over a wide area of mangrove. Adult trap catches (traps 6 and 7, Table 3) indicate breeding was occurring there at the time of the survey, but extensive sampling failed to produce any Anopheles larvae there.

Mosquito control could be achieved by larviciding (using Teknar a biological insecticide which does not harm organisms other than mosquitoes, or similar larvicide) from a backpack sprayer. Such spray operations would need to be conducted weekly until mosquito numbers were at acceptable levels. At the time of this survey there was less than 100 man biting mosquitoes per trap night and thus control would not normally be necessary.

However, in the event of an imported malaria case i t would be necessary to fog the Anopheles mosquito harbourage areas around the swamp. A suitable fogging route exists and this is marked on Map 1.

The large grassy fringed freshwater swamp at the western end of the old airstrip <which probably developed from old borrow pits> could produce very large numbers of mosquitoes in the early months of the dry. Culex sp. 32 and Cx C Loph. > sp. were collected .from the grassy edges, but many species are likely to breed there. The adult trap <site 3) yielded An farauti s. 1., An bancroftii and Cx annulirostris as well as other less important species.

Larvivorous fish Ceg rainbow fish> should be introduced to the deeper pits. Backpack applied larvicide could be used to control mosquitoes here late in the dry, but access in the wet and early dry would be difficult. The swamp is surrounded by trees so aerial application would be difficult. A suitable fogging route exists between the town and the swamp but i t could only be used when there is little or a northerly wind (Map 1).

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paperbark swamp on the north eastern side of the old airstrip. Most of the borrow pits were not breeding mosquitoes at the time of the survey. Ae funereus and Cx pullus were collected from the paperbark swamp, which could also be a source of Anopheles species. It could easily be controlled by the addition of larvivorous fish or backpack applied larvicide. The fogging route shown in Map 1 could be effective here in light or northerly winds.

Ae funereus a potential arbovirus vector and the main species collected by human-bait collection, was also found breeding in the grassy depressions behind the mangroves near the main beach <site 8). Many of these depressions could be drained <either into the stormwater drain or the mangrove area directly).

Small numbers of Cx annulirostris were found breeding in the sewage ponds. Mosquito breeding can be a problem in such ponds if the sludge builds up, if the edges become degraded through vegetation growth

<this results in less wave action across the pond - the wave action disrupts mosquito breathing), and if vegetation on the degraded margins prevents predator access to mos qui to larvae. The weeds on the edge of the pond were showing the effects of recent he rbi ci ding.

A freshwater dam (site 5) was breeding An farauti s. 1., An bancroftii An meraukensis < all potential malaria vectors) and Cx < Loph. > sp.. This dam should be drained as i t would provide a near continuous source of mosquitoes.

Groundwater pools at the rubbish dump were breeding Cx annulirostris These pools were not extensive or deep and are not expected to present an ongoing problem. Cx pullus and Cx halifaxi were collected from a drum. Despite the recent rains there were few rubbish items holding water, and the adult trap <trap 9) collected few container-breeding species.

The heavy monsoonal rains in the days before and after our arrival allowed evaluation of the stormwater drains. Little water remained in the town, and water was flowing through to discharge points with minimal impedence. There was ·some buildup of

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vegetation in the drain along Puti drive <behind the beach) and in the drain behind~ the council building. Such vegetation impedes flow and causes pooling in which mosquitoes breed.

4. Conclusions

1. No Aedes aegypti were detected from the larval surveys or adult trap catches in Nguiu. It is therefore considered free of this species. No vector of dengue fever is present in the community.

2. Container breeding mos qui toes were quite common in Nguiu. The major breeding areas could be eliminated by drilling holes in the old tyres (particularly at the pre-school), and punching holes in the rubbish bins to allow all water to drain from them.

3. Many Anopheles farauti s. 1. were collected from adult traps near the residential areas on the north side of town. The major breeding areas of this malaria vector are likely to be the brackish water area at the interface of the stormwater drain and the mangroves <near site 5 on Map 1), the swamp near the sewage pump station, and the swampy areas near the main beach. The swamps on the western edge of the old airstrip may be a major source of potential malaria vectors later in the dry season.

4. Anopheles mosquitoes of various species were found breeding in the stormwater dam <site 5). This could be drained to prevent further breeding.

5. As Nguiu is receptive to malaria, i t is important to maintain a suitable fogging route to the main breeding areas. Potential routes are marked on Map 1.

Residents visiting overseas malarious areas should take prophylactics, avoid mosquito bites when there, and take malaria eradication drugs on their return.

6. Other disease vectors that may be important in the area are Culex annulirostris Aedes vigilax and Aedes funereus The former was collected from the sewage ponds and highlight the need to regularly maintain the ponds to avoid weed buildup on the sides or sludge buildup. There was evidence of recent weediciding and only low-scale larval breeding was detected. The effluent discharge from the ponds was well situated and would not create a mosquito breeding area.

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7. There are many deep borrow pits on the island, particularly around the old airstrip. Should mosquito breeding become a problem in these, larvivorous fish Ceg rainbow fish) should be added. Such fish should be added to the swamp at site 3.

5. Acknowledgements

We thank Tony Neeson of Catholic Missions for organising a vehicle for this survey, Charles Lindsay and Jimmy Tipungwuti from tha Nguiu Town Council for their help in facilitating the survey, and John Baptiste for assistance in our initial inspection of mosquito breeding areas. This survey has been assisted by Commonwealth funding under the National Disease Control Program .

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Table 1. Aedes aegypti Survey - Nguiu - March 1988

Larvae and containers recorded in survey 28-29/3/88

Type of container with

water

rubbish bin bucket

No. of times container recorded

29 1 3 ice cream bucket 8 laundary bucket 4

bin lid 1

old tyre 1 7 glass jar 1 plant pot 3

shell 4

tin 1

pool 2

pots 3

bowl 2

tray 4

boat 1

swings 2

fridge door 1 wheel rim 1

Total 97

No. premises surveyed: 114 No. premises with larvae:

No. of times larvae present

5 3

12

1 1

2 1

1

26

1 3 No. containers with larvae: 26

No. of containers positive for each species

Ae notoscriptus

1

1 2

1 1

2 1

1 8

Cx quinquefasciatus Cx pullus

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Table 2. Larvae collected from local breeding areas, Nguiu, 28 - 30 March, 1988

Map 1

"Aet. •

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_L1 Sewage Ponds

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L2 Stormwater drain dam (freshwater, paperbark)

L3 Sewage Pump Station

L4

(freshwater swamp, Eliocharis) Behind beach mangroves C grassy depressions,

slightly brackish) LS South of old airstrip

(freshwater in borrow pits, paperbark)

L6 West of old airstrip

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(freshwater swamp, grassy, with water hyacinth>

Rubbish dump C groundwater) Cold drum>

Truck dump

Cold car bodies)

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age of larvae collected Cas instars>

Species

4* Cx annulirostris 2* An farauti s. ^1.

3* An meraukensis 4* An bancroftii 3* Cx C Loph. > sp.

2* Ur. lateral is 2* Cx C Loph. > sp.

2* Ae funereus

4* Ae funereus 4* ex pullus

2*

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3* Cx sp 32

3* Cx C Loph. > sp.

4* Cx annulirostris 4* Cx pullus

4* Cx halifaxii

2,3,4* Ae notoscriptus

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traps at Ngui u, 28-30 March, 1988.

Trap sites shown on Map 1.

Species Trap Site

1 2 3 4 5 6 6* 7 8* 9

An bancroftii 1 1

An farauti s. 1. 1 5 1 7 1 6 2 83

An meraukensis 1 1 2 1 1

Ae alternans 1

Ae funereus 3 2 6 1 7

Ae kochi 2 1 1

Ae notoscriptus 9 1 16 24 1 2

Ae reesi 1

Ae sp 76

Ae tremulus 3

Ae vigilax 1 2 3 2

Cx annulirostris 16 8 31 54 24 16 7 26 25

Cx

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Cx sitiens 1

Cx sp 32 2

Cx sp 167 1

Cq xanthogaster 1 1

Total 20 9 48 57 35 57 48 1 0 122 30

* Traps run 29-30/3/88, all others run 28-29/3/88

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33 53

MOSQUITO SPECIES

BROWN HOUSE MOSQUITO (Culex quinquefasciatus).

SALT WATER MOSQUITO (Culex si tiens)

COMMON BANDED MOSQUITO (Culex annulirostris)

CONTAINER BREEDING MOSQUITO (Aedes notoscriptus)

SALT MARSH MOSQUITO (Aedes vigilax)

WATER HYACINTH MOSQUITO (Mansonia uniformis)

SALTWATER ANOPHELES

MOSQUITO (Anopheles hilli) MALARIA ANOPHELES

MOSQUITO (Anopheles farauti)

COMMON ANOPHELES MOSQUITO (Anopheles annulipes)

BLACK AUSTRALIAN ANOPHELES MOSQUITO

(Anopheles bancrofti)

BREEDING PLACE

Polluted water close to human habitation septic tanks, drains and water filled containers.

Brackish to salt pools, along the coast left by high tides.

Freshwater swamp edges. Low Lying grassy areas after rain. Storm drains Sewage ponds with grassy edges.

Tree holes, artificial containers such as old tyres, drums and tanks.

Salt to brackish swamps filled by highest tides. Disturbed mangrove edges.

Permanent and semi-permanent freshwater swamps and lagoons, associated with aquatic plants.

Brackish to salt water swamps and ground pools.

Sunlit shallow swamp margins and ground pools, usually brackish.

Temporary and permanent ground pools, stream edges.

Shaded freshwater swamps.

ADULT HABITS

Night biter. Will enter houses and remain inside.

Bites at night.

Bites at night. Very common around Darwin, Nhulunbuy, Jabiru and Alice Springs.

Bites by night or by day in shade.

Bites by day and night.

Occurs in plagues.

Bites by night and by day near breeding areas.

Bites at night. Will enter houses.

Bites at. night. Bites people readily.

Bites at night. wiil enter houses.

Bites readily at night and by day in shade near breeding grounds.

1. 1

POTENTIAL DISEASE CARRIED

Heartworm of dogs.

Murray Valley Encephalitis Virus Filariasis in.other countries.

No record.

Murray Valley Encephalitis, Ross River Virus and many other viruses. Heartworm of dog.

Heartworm of dogs.

Ross River Virus and other viruses. Heartworm of dogs.

Filariasis in .other countries.

Malaria.

Malaria. A confirmed vector in Australia

Malaria

Malaria.