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    Map of the Billiluna area in the Kimberley region of western Australia showing the location of the study site, significant waterways, and trapping locations at Billiluna.

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    Monthly rainfall at Balgo, Australia, years with extensive flooding at Billiluna, and minimum infection rates (MIRs) of Murray Valley encephalitis (MVE) virus in Culex annulirostris mosquitoes collected in the Billiluna region, July 1988 to June 2001. J N M = July, November, and March, respectively.

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    Monthly rainfall at Balgo, Australia, years with extensive flooding at Billiluna, and minimum infection rates (MIRs) of Kunjin (KUN) virus in Culex annulirostris mosquitoes collected in the Billiluna region, July 1988 to June 2001. J O J A = July, October, January, and April, respectively.

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EPIZOOTIC ACTIVITY OF MURRAY VALLEY ENCEPHALITIS AND KUNJIN VIRUSES IN AN ABORIGINAL COMMUNITY IN THE SOUTHEAST KIMBERLEY REGION OF WESTERN AUSTRALIA: RESULTS OF MOSQUITO FAUNA AND VIRUS ISOLATION STUDIES

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  • 1 Arbovirus Surveillance and Research Laboratory, Department of Microbiology, The University of Western Australia, Nedlands, Western Australia; Mosquito-Borne Disease Control Branch, Western Australian Department of Health, Perth, Western Australia; Division of Health Sciences, The Western Australian Centre for Pathology and Research, Queen Elizabeth II Medical Centre, Nedlands, Western Australia; Department of Microbiology and Parasitology, School of Molecular and Microbial Sciences, The University of Queensland, Brisbane, Queensland, Australia

We undertook annual surveys of flavivirus virus activity in the community of Billiluna of Western Australia in the southeast Kimberley region between 1989 and 2001. Culex annulirostris was the dominant mosquito species, particularly in years of above average rains and flooding. Murray Valley encephalitis (MVE) virus was isolated in 8 of the 13 years of the study from seven mosquito species, but more than 90% of the isolates were from Cx. annulirostris. The results suggest that MVE virus is epizootic in the region, with activity only apparent in years with average or above average rainfall and increased numbers of Cx. annulirostris. High levels of MVE virus activity and associated human cases were detected only once (in 1993) during the survey period. Activity of MVE virus could only be partially correlated with wet season rainfall and flooding, suggesting that a number of other factors must also be considered to accurately predict MVE virus activity at such communities.

INTRODUCTION

This paper presents results from a 13-year study of mosquito fauna and flavivirus activity in a small, remote Aboriginal community in the southeast Kimberley region in Western Australia. The results add further insight into the ecology of Murray Valley encephalitis (MVE) virus in a region of epizootic activity from those obtained during a recent serologic survey in the region.1

Murray Valley encephalitis virus, a mosquito-borne flavivirus, can cause a severe, potentially fatal disease of humans that affects the central nervous system. This disease was previously known as both Australian encephalitis and MVE,2 but the former has now been discontinued and the disease is now referred to as MVE.3 A related virus, Kunjin (KUN) virus, causes a similar but usually milder disease referred to as KUN virus disease with rare cases of non fatal encephalitis.4 Kunjin virus has recently been reclassified as a member of lineage 1 of West Nile virus.5 Several reviews of the distribution, epidemiology, and ecology of MVE and KUN viruses and clinical descriptions of MVE and KUN virus disease have been published in the last decade.6– 8 Since 1974, 68 confirmed cases of MVE and 18 of KUN virus disease have been reported.2, 9, 10 All 68 cases of MVE occurred in northern and central Australia, with the majority (41) being from the northern part of Western Australia.

In Western Australia, MVE and KUN virus activity is restricted mainly to the northern half of the state, with virus activity occurring in the tropical Kimberley region in most years. Culex annulirostris (Skuse) is the main vector of both MVE and KUN viruses. Various species of waterbirds, particularly members of the Order Ciconiiformes are thought to be the major vertebrate hosts.11 The only confirmed enzootic focus of MVE virus in Australia is in the Ord River region near the northeast Kimberley town of Kununurra, although the virus is probably also enzootic in other parts of the Kimberley and in the Northern Territory. The virus is thought to be epizootic in the Pilbara, Gascoyne, Murchison, and Midwest regions of Western Australia, particularly during the mid-to-late wet season (February to May). Murray Valley encephalitis virus is most active in years with very heavy wet season rains and extensive flooding. Consequently, the risk of human infection increases in these years. Most cases of MVE in Western Australia occur in the Kimberley region, although some cases also occur further south in the Pilbara, Gascoyne, Murchison, and Midwest regions of this state.2, 12

Billiluna and Balgo are two small, remote Aboriginal communities located on the northern edge of the Great Sandy Desert in the southeast Kimberley region of Western Australia. Three of 12 serologically confirmed cases of MVE in 1978 and 1981 were from these two communities.2 All three cases were young Aboriginal children. Since these communities account for less than 2% of the total Kimberley population, these cases represent a high case attack rate compared with the remainder of the population of the Kimberley region.

Large numbers of Cx. annulirostris mosquitoes were trapped at Billiluna and Balgo in 1981 soon after the cases were diagnosed, and 88 isolates of MVE virus were obtained from this species. The minimum infection rate (MIR) of Cx. annulirostris mosquitoes (8.3/1,000 at Balgo and 10.9/1,000 at Billiluna) was the highest ever recorded in the Kimberley.13 The high case attack rate and associated high infection rates of Culex mosquitoes in 1981 prompted our laboratory to undertake a prospective study in 1989 to monitor MVE virus activity at Billiluna. The first aim of this study was to determine whether MVE virus persists (is enzootic) in the region or disappears and is reintroduced when conditions favor dispersal from enzootic foci further north. A second aim of the study was to further define the environmental conditions that predispose to high levels of MVE virus activity, with a subsequent increase in the risk of human infection, in the region.

MATERIALS AND METHODS

Ethical review.

This long-term study of arbovirus activity in the region study was carried out with the full consent of the Traditional Owners of the community. In addition, we obtained permission from the members of the Billiluna community to allow us to use the name of their community in this article.

Study site and climate.

General descriptions of the study site, its location, and climate have been published previously.1 The locations of the study site, significant waterways, and mosquito trapping locations are shown in Figure 1.

The main water course in the region is Sturt Creek, which flows southwest from its initial catchment in the west central part of Northern Territory approximately 250 km into an inland salt lake (Lake Gregory), about 80 km southwest of Billiluna. For most of the year, Sturt Creek consists of a series of intermittent, permanent water holes along the main channel of the creek itself. However, during most wet seasons it flows, and in wetter years spreads out across its entire floodplain, which varies in width between 5 and 8 km along most of its length. Lake Stretch is the main permanent body of water in the Billiluna area and is located 20 km southwest of the community. The lagoon is approximately 700 meters long, but the amount of water in the lagoon varies considerably each wet season. Culex annulirostris, the major vector of flaviviruses in the region, generally breeds in shallow fresh water sites14 and is trapped in plague numbers in the areas around Sturt Creek and Lake Stretch after heavy wet season rains and flooding. This mosquito species has been shown to have a flight range of several kilometres15, 16 suggesting that mosquitoes trapped at Billiluna may also have originated from breeding sites away from the community. Vegetation on the Sturt Creek floodplain is dominated by eucalypts, especially Eucalyptus camaldulensis, the northern river gum. Many of these river gums are very old and their rot holes provide breeding sites for tree hole mosquitoes, especially species in the Macleaya subgenus within the genus Ochlerotatus. (Some mosquito subgenera previously classified within the genus Aedes were recently moved into the new genus Ochlerotatus.) Ground level vegetation on the floodplain is intermittent and consists predominantly of annual grasses. Away from the floodplain, trees and shrubs are very sparse and consist mainly of various Acacia spp. Ground level vegetation is predominantly spinifex (Triodia intermedia), a tough, and perennial clumping grass. The countryside is largely flat, interspersed with occasional sand ridges up to eight meters high.

Weather data was obtained from the Western Australian Bureau of Meteorology in Perth, Western Australia.

Sampling regimen and protocols.

An initial mosquito collection from Billiluna had previously been made in March 1981 after a case of MVE was reported from the community. Some unpublished results from this initial survey are included to enable comparison between this survey and our long-term study. Our annual surveys were initiated in 1989, with mosquito collections at Billiluna from 1989 to 2001 inclusive. From 1989 to 1993, the collections were timed to coincide with the end of each wet season to sample peak numbers of Cx. annulirostris (the principal vectors of MVE and KUN viruses), and to maximize the possibility of virus isolations from that species. However, in later years, field collections at Billiluna were carried out as part of a wider survey of flavivirus activity in all major towns and some Aboriginal communities in the Kimberley. The timing of the Billiluna collections from 1994 to 2001 was therefore not always optimal for Cx. annulirostris due to considerable variation in the amount and location of wet season rainfall throughout the vast Kimberley region. The dates of each field collection and years when flooding occurred around the Billiluna community between 1989 and 2001 and also from the collection in 1981 are shown in Table 1.

Mosquito collections, virus isolation, and virus identification.

EVS/CO2 mosquito traps,17 modified for local conditions,18 were set at between 15 and 24 sites in the vicinity of Sturt Creek, along a 7-km transect from Myarra Pool to Sturt Creek, on the west side of Lake Stretch and at the community during each visit (Figure 1). The same trap sites were used each year, except when seasonal flooding prevented access to sites at the time of the collection.

The EVS/CO2 traps were set before sunset and collected soon after sunrise the following day to minimize mortality. Collections were then stored on dry ice, transported to the laboratory in Perth, and stored at −70°C. Each sample was sorted on the basis of date, site, species, sex, and whether or not they were blood fed, then processed for virus isolation, as described elsewhere.19 A maximum of 500 mosquitoes per trap was processed for virus isolation when larger collections of mosquitoes were obtained. In addition, a number of mosquitoes were retained as reference specimens. Care was taken when identifying, macerating, and processing mosquitoes for virus isolation to ensure that mosquito suspensions were not contaminated with virus from other mosquito pools.18 Suspected virus isolates were harvested when a cytopathic effect was observed in indicator (porcine stable-equine kidney and Vero) cell lines.

Many isolates of alphaviruses were obtained, particularly Ross River, Barmah Forest, and Sindbis viruses. However, these results will be presented elsewhere because the focus of this study was activity of flaviviruses. Flavivirus isolates obtained from mosquitoes collected in 1981 were identified by a micro-neutralization test using a modification of the method of Rosenbaum and others.20 All flavivirus isolates obtained from mosquitoes collected from 1989 onwards were typed using a fixed-cell enzyme-linked immunosorbent assay with virus-specific monoclonal antibodies.21

RESULTS

Mosquito fauna and virus isolations.

Mosquito species collected and processed for virus isolation during the entire study are shown in Table 2. Overall, Cx. annulirostris was the most abundant species, comprising 52.4% of the total catch. However, the number of mosquitoes collected and species composition varied considerably from year to year, depending on the timing of the collecting trip, amount of rain, and extent of flooding of Sturt Creek each wet season. A number of specimens of Ochlerotatus vigilax (Skuse), a salt marsh-breeding species, were obtained in the EVS/CO2 traps.

Results of virus isolation studies, infection rates of Cx. annulirostris with MVE and KUN viruses, and annual abundance of Cx. annulirostris (mean/trap/year) between 1989 and 2001 and also from the previous survey in 1981 are shown in Table 3. A total of 164 isolates of MVE virus were obtained from mosquitoes processed for virus isolation between 1989 and 2001. Murray Valley encephalitis virus was isolated in eight of the 13 annual surveys (Table 3). Culex annulirostris yielded more than 90% (152 of 164) of all isolates. Minimum infection rates of MVE virus for this species (calculated per 1,000 mosquitoes processed) varied markedly from year to year (Figure 2). This species comprised more than 50% of the catch in 1991, 1993, 1994, 1995, 1997, and 2000. Major flooding occurred in the region in 1981, 1991, 1993, 1997, 2000, and 2001 (Table 1). In 1981 and 1993, high rains and flooding correlated with high numbers of MVE virus isolates and extremely high MIRs in Cx. annulirostris mosquitoes. A case of MVE was reported from Billiluna in both of these years. Other years with heavy rainfall, with or without flooding, such as 1994, 1997, 1999, and 2001, had absent or low levels of MVE virus in trapped mosquitoes. Isolates of MVE virus were also obtained from Oc. normanensis (Taylor) [2], Oc. eidsvoldensis (Mackerras) [1], Oc. Marks sp. No. 160 [1] (this species is one of several in Western Australia recognized as a separate species by the late Dr. E. N. Marks, a leading Culicid taxonomist in Australia, but is yet undescribed due to inadequate study of the immature stages of the species), Cx. quinquefasciatus Say [2], Anopheles amictus Edwards [4], and An. annulipes Walker [2]. This is the first time that MVE virus had been isolated from Oc. E.N. Marks sp. No. 160.

A total of 27 KUN virus isolates were obtained from 1989 to 2001. Kunjin virus was isolated in seven of the 13 study years and, as with MVE virus, the majority (88.5%) of the KUN virus isolations were from Cx. annulirostris. In addition, three isolates were obtained from pools of tree hole-breeding Ochlerotatus species in the subgenus Macleaya, and one from Oc. vigilax. Minimum infection rates for KUN virus were lower than those for MVE virus (0.7–1.77/1,000). The relationship between MIRs of KUN virus in Cx. annulirostris and rainfall is shown in Figure 3.

Weather.

Monthly rainfall between July 1988 and June 2001 at Balgo Hills (the closest weather station with complete data for the study period) is shown in Figures 2 and 3. Balgo Hills was chosen because it is an Aboriginal community located 85 km southeast of Billiluna and cases of MVE have also been reported from this community. Above average wet season rain was recorded at Balgo in seven years during the study period (1989, 1993, 1994, 1997, 1999, 2000, and 2001) and extensive flooding of Sturt Creek occurred five times (1991, 1993, 1997, 2000, and 2001).

Human cases.

Single cases of MVE were reported from Billiluna in 1981 and 1993. Both were young children. The 1981 case was designated moderate by Mackenzie and others,2 and the 1993 case was fatal. One symptomatic and one asymptomatic case of KUN virus disease were reported in 1995.

DISCUSSION

The study was undertaken to determine whether MVE virus is enzootic or otherwise in the Billiluna region. Results presented here suggest that the virus is not enzootic in the region, but occurs in occasional epizootics. One year of the study (1993) had levels of activity comparable to that recorded in 1981. Extensive flooding of Sturt Creek in both of these years apparently led to sufficient virus activity in and around the community, resulting in cases of encephalitis.

There were seven other wet seasons with average or above average rainfall when high levels of MVE virus activity did not occur. The mean number of Cx. annulirostris per trap was higher than in 1993 (Table 3) in four of these seven years. There were five wet seasons with below average rainfall and in four of these (1990, 1992, 1996, and 1998) mosquito numbers were very low and no MVE virus isolates were obtained from mosquitoes collected. In 1991, below average rainfall was recorded, but Sturt Creek did flood and MVE activity (virus isolations and human seroconversions1) did occur. High levels of MVE virus activity occurred in other areas of the Kimberley in 1991 (Broom AK, unpublished data) and it is possible that the virus could have been introduced by infected waterbirds moving from the northeast Kimberley region to flooded areas around Sturt Creek.22

These results suggest that although MVE virus activity is less likely to occur in years with below average rainfall, the combination of high rainfall and high vector numbers is not sufficient in itself to produce high levels of MVE virus activity in the region. Therefore, the limiting factor for activity of MVE virus may be a lack of sufficient non-immune hosts (migratory waterbirds) to allow amplification in a mosquito-bird cycle, or non-reintroduction of the virus from enzootic areas or a combination of both. Low numbers of non-immune waterbirds may occur because they have not migrated to that area during the particular season, or that environmental conditions, such as food and nest availability, do not favor overt breeding. Little is known about migratory pathways or bird populations in this area. The one formal study at Billiluna in 1993 found that Ciconiiformes (the major hosts of MVE virus11) were present in the region in late March of this year and comprised 11% of the population (Halse S, unpublished data). That is consistent with the known high activity that year, but does not contribute to understanding the lack of activity in other years with high rainfall and vector numbers. Alternatively it is possible that even in years when they are present in large numbers, that relatively few of then are non-immune. In 1993, it is known that very high levels of MVE virus activity led to infection of nearly 50% of the susceptible human population,1 and it is likely that infection rates would have been much higher in animal hosts. Therefore, the low levels of MVE activity in the following year, despite high levels of rainfall, may be because the majority of birds had been infected in the previous season and were immune to further infection. However, the non-immune population should gradually have increased because there was little MVE virus activity at Billiluna or elsewhere in the Kimberley from 1994 to 2001.23, 24 In those later years, the lack of activity at Billiluna may have been caused by the absence of opportunities for reintroduction of MVE virus due to the low activity in the endemic areas.

In summary, the presence of high rainfall and large numbers of mosquitoes is not sufficient, in itself, to lead to MVE virus activity in the Billiluna area. It may also depend on whether waterbird hosts enter the area and breed, and on whether those waterbirds are susceptible to infection. The latter will be determined by MVE virus activity at Billiluna or other areas of the Kimberley in the immediately preceding years and the proportion of birds infected in those years.

Kunjin virus was the only other flavivirus isolated from mosquitoes trapped at Billiluna during the annual surveys. This virus was detected in seven of the 13 years of the study period (Figure 3) and, as with MVE virus, the majority of the isolations were from Cx. annulirostris. Infection rates with KUN virus were generally lower than those with MVE virus (0.3–1.77/1,000 mosquitoes for KUN virus compared with 0.22–8.99/1,000 for MVE virus) (Table 3). Two documented human infections did occur in 1995, when the MIR was highest. No KUN virus isolates were obtained in 1981 or 1993 when MVE virus infection rates were highest and conditions therefore appeared to be suited to increased flavivirus activity. The observation that the activity of MVE and KUN viruses varied independently in the community over the study period is interesting given that both viruses are thought to share similar vectors and hosts.6 This could be explained by differences in vector competence as suggested by Kay and others,25 differences in timing of peak activity following rainfall (which was not determined in this study), or differences in the relative importance of the various potential amplifying hosts and their levels of susceptibility to the two viruses.

The finding that MVE is epizootic in Billiluna raises the question of how virus activity is reactivated in the region. Possible mechanisms of MVE virus movement in the Kimberley region of Western Australia have been described elsewhere.1, 26 The mechanism of virus movement is not known for Billiluna, but it is proposed that infected birds or mosquitoes or a combination of both could facilitate the introduction of MVE virus into the region. In support of this, the movement of waterbirds from the Ord River area near Kununurra (northeast Kimberley), where the virus is enzootic, to both Sturt Creek and nearby Lake Gregory (southeast Kimberley) has been observed, particularly in wet years (Johnstone R, Museum WA, unpublished data). In addition, extensive waterbird breeding occurs in this region27 (Halse S, unpublished data). This mechanism of dispersal of MVE virus was also proposed for Halls Creek, a similarly arid area in eastern Kimberley.22

It has also been suggested that wind-blown mosquitoes may play an important role in arbovirus dispersal,28 and dispersal of infected mosquitoes from areas where MVE virus is enzootic is another possible means by which MVE virus could be introduced into Billiluna. The occurrence of mosquito species such as Oc. vigilax in remote areas like Billiluna, several hundreds of kilometers from the nearest coastal salt marsh, could occur by wind dispersal (Whelan P, personal communication). Similarly, this dispersal mechanism could bring infected mosquitoes into the region.

Alternatively, MVE virus may be reactivated in arid regions following vertical transmission in desiccation-resistant eggs of Ochlerotatus and Aedes mosquitoes after heavy rain, as was proposed for Halls Creek.22 The isolation of MVE virus from pools of Ochlerotatus mosquitoes in 1981 and during this study indicates that these species can become infected with the virus. If vertical transmission is a mechanism of persistence for MVE virus in Billiluna, the eggs must be able to survive between wet seasons (up to nine months) and the virus must remain viable in the eggs during this period. No studies to determine survival times for mosquito eggs or the duration of virus activity in eggs have been carried out with Australian species. However, limited studies in the United States using similar mosquito species with drought-resistant eggs have shown that this is a possible mechanism of persistence for arboviruses in arid areas,29– 31 although it is not clear whether this is sufficient for very long periods between seasons conducive to vector breeding, such as the 1994 to 1997 period at Billiluna. Independent of the mechanism of virus entry into the area, the presence of waterbird hosts is likely to be essential, either as a vehicle of entry or as amplifying hosts for virus that is already present.

While this study has determined that MVE virus is epizootic in Billiluna, questions remain about the exact environmental triggers for activity. The effect of different environmental factors on the breeding of amplifying hosts and virus survival or reintroduction into the region are still not known. Further studies are required to address these issues.

Table 1

Timing of annual (1989–2001) and 1981 mosquito collections at Billiluna, Australia and environmental conditions preceding each collection

YearMonth(s) of main rainfall*Amount of rainfall And presence/absence of floodingDate of tripTiming of trip after rain (weeks)
* Month (or months) when majority of rainfall was recorded.
These results are only estimates of the weather conditions at Billiluna. Rainfall figures were from the nearby community of Balgo and there is no reliable record of the extent of flooding around the community from 1981 to 1988.
AA = above average wet season rainfall; F = flooding; B = below average wet season rainfall; A = average wet season rainfall.
1981FebAA + F3/30/814
1982AA
1983AA
1984B
1985B
1986A
1987B
1988B
1989MarAA4/21/893
1990Jan/FebB4/29/908
1991Jan/FebB + F4/16/916
1992Jan/FebB5/6/9212
1993FebAA + F4/20/93, 5/6/937, 9
1994FebAA4/18/946
1995Feb/MarA4/1/953
1996Feb–AprB6/7/966–7
1997FebAA + F3/30/976
1998JanB3/29/984–8
1999Jan/FebAA4/13/996
2000Mar/AprAA + F4/1/000
2001MarAA + F3/29/011
Table 2

Mosquito species collected at Billiluna, Australia and environs from 1989 to 2001

Species or category*Total% of total
* Ad. = Adeomyia; Ae. = Aedes; An. = Anopheles; Cx. = Culex; Oc. = Ochlerotatus.
Includes females, bloodfed females, and males of each species.
This species is one of several in Western Australia recognized as a separate species by the late Dr. E. N. Marks, a leading Culicid taxonomist in Australia, but is yet undescribed due to inadequate study of the immature stages of the species.
§ Specimens identified as Oc. (Macleaya) species were not identified to species level but include Oc. tremulus, Ae. E. N. Marks species No. 121, and possibly other species in this subgenus.
Ad. (Aedeomyia) catasticta8<0.01
Ae. E.N. Marks’ species No. 1601270.03
An. (Anopheles) bancroftii9<0.01
An. (Cellia) amictus31,6827.95
An. (Cellia) annulipes s.l.68,09717.08
An. (Cellia) hilli5910.15
An. (Cellia) meraukensis17<0.01
An. (Cellia) novaguinensis17<0.01
Cx. (Culex) annulirostris209,37652.52
Cx. (Culex) bitaeniorhynchus550.01
Cx. (Culex) crinicauda310.01
Cx. (Culex) palpalis5860.15
Cx. (Culex) quinquefasciatus1,2620.32
Cx. (Culex) sitiens650.02
Cx. (Culex) squamosus12<0.01
Cx. (Culex) starckeae2,7300.68
Cx. (Culex) vicinus4<0.01
Cx. (Culiciomyia) pullus8<0.01
Oc. (Macleaya) species§4,0491.02
Oc. (Mucidus) alternans2740.07
Oc. (Ochlerotatus) E.N. Marks’ species No. 711890.05
Oc. (Ochlerotatus) E.N. Marks’ species No. 8514<0.01
Oc. (Ochlerotatus) eidsvoldensis6,5711.65
Oc. (Ochlerotatus) normanensis69,76617.50
Oc. (Ochlerotatus) pseudonormanensis9000.23
Oc. (Ochlerotatus) vigilax3620.09
Oc. (Pseudoskusea) bancroftianus1,2400.31
Tripteroides (Polylepidomyia) punctolateralis5<0.01
Unidentifiable6000.15
    Total398,647100
Table 3

Details of numbers of mosquitoes collected, flaviviruses isolated, human cases of flavivirus disease, and infection rates (MIRs) of Culex annulirostris at Billiluna, Australia during each annual survey (1989–2001) and a preliminary investigation in 1981*

Cx. annulirostrisMVE virusKUN virus
Cx. annulirostrisCx. annulirostris
YearTraps setTotal Mosquitoes collectedMean/ trapMajor floodingTotalMean/ trap/year% of catchHuman casesIsolates (all species)IsolatesMIRHuman casesIsolates (all species)IsolatesMIR
* MIR = minimum infection rate per 1,000 Cx. annulirostris mosquitoes processed; MVE = Murray Valley encephalitis; KUN = Kunjin.
Includes both Cx. annulirostris and Cx. palpalis.
Cases at Billiluna.
Initial survey
    198183,997500Yes1,006126251131110.9300
Annual surveys
    19892723,090855No3,02213113531.00110.33
    1990232,04389No27110000
    19911812,996722Yes7,5654215814121.93220.32
    1992191297No372290000
    19933034,2681,142Yes26,4128807711211158.9900
    19941947,7692,514No29,9681,57763660.82660.82
    19951922,2101,169No13,30270060110.221981.77
    19961815,606867No6563640000
    19972049,8572,493Yes36,3481,81773760.82420.27
    19982160829No50280000
    1999224,333197No2,1079649210.49420.97
    200016108,2016,763Yes67,9234,24563881.5600
    20011177,5377,049Yes22,0142,001280010
Totals (1989–2001)263398,6471,516209,4315311641523.00127210.41
Figure 1.
Figure 1.

Map of the Billiluna area in the Kimberley region of western Australia showing the location of the study site, significant waterways, and trapping locations at Billiluna.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 3; 10.4269/ajtmh.2003.69.277

Figure 2.
Figure 2.

Monthly rainfall at Balgo, Australia, years with extensive flooding at Billiluna, and minimum infection rates (MIRs) of Murray Valley encephalitis (MVE) virus in Culex annulirostris mosquitoes collected in the Billiluna region, July 1988 to June 2001. J N M = July, November, and March, respectively.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 3; 10.4269/ajtmh.2003.69.277

Figure 3.
Figure 3.

Monthly rainfall at Balgo, Australia, years with extensive flooding at Billiluna, and minimum infection rates (MIRs) of Kunjin (KUN) virus in Culex annulirostris mosquitoes collected in the Billiluna region, July 1988 to June 2001. J O J A = July, October, January, and April, respectively.

Citation: The American Journal of Tropical Medicine and Hygiene Am J Trop Med Hyg 69, 3; 10.4269/ajtmh.2003.69.277

Authors’ addresses: Annette K. Broom, Arbovirus Surveillance and Research Laboratory, Department of Microbiology, University of Western Australia, Nedlands, Western Australia 6907, Australia, Telephone: 61-8-9346-2213, Fax: 61-8-9346-2912, E-mail: abroom@cyllene.uwa.edu.au. Michael D. A. Lindsay, Mosquito-Borne Disease Control, Western Australian Department of Health, PO Box 8172, Perth Business Center, Perth, Western Australia 6849, Australia, Telephone: 61-8-9385-6001, Fax: 61-8-9383-1819. Anthony E. Wright, 139 Chowan Creek Road, Uki, New South Wales 2484, Australia, Telephone: 61-2-6679-4224. David W. Smith, Division of Microbiology and Infectious Diseases, The Western Australian Center for Pathology and Medical Research, The Queen Elizabeth II Medical Center, Hospital Avenue, Nedlands, Western Australia 6009, Australia, Telephone: 61-8-9346-2164, Fax: 61-9-8346-3960. John S. Mackenzie, Department of Microbiology and Parasitology, School of Molecular and Microbial Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia, Telephone: 61-7-3365-4648, Fax: 61-7-3365-6265.

Acknowledgments: We thank the Traditional Owners of the Billiluna community who allowed us to visit their community during the study period. We acknowledge the community for their ongoing support of the Arbovirus program in Western Australia and for kindly allowing us to use the name of their community in this article. We also thank Steven Crocker, Rosa Duthie, Brenda van Huezen, Sarah Vetten, and Kerryn Sturrock for their technical assistance, Cheryl Johansen for producing the map (Figure 1), and Professor Richard Russell for comments on the manuscript.

Financial support: This study was supported by the Western Australian Department of Health.

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Author Notes

Reprint requests: Annette K. Broom, Arbovirus Surveillance and Research Laboratory, Department of Microbiology, University of Western Australia, L Block, Queen Elizabeth II Medical Centre, Nedlands, Western Australia 6907, Australia.
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