A comparison of Aedes vigilax larval population densities and associated vegetation categories in a coastal wetland, Northern Territory, Australia.

Darwin's northern suburbs border an extensive coastal reed and upper mangrove wetland recognized as an important larval habitat for Aedes vigilax (Skuse), the northern salt marsh mosquito, an established vector for Ross River and Barmah Forest viruses and an appreciable pest species. We sought to identify the most important vegetation categories associated with Ae. vigilax breeding to maximize the efficiency of mosquito control efforts. Using a generalized linear model with negative binominal distribution and log link, this study compares larval densities, determined by focused dipping, between 13 discernable vegetation categories. The incidence rate ratios (RR) generated can be used to compare the magnitude of larval densities for each vegetation category, compared with the reference category. Aedes vigilax larval densities were almost ten times greater in artificial drainage areas (RR=9.82), followed by tide-affected reticulate (Sporobolus/Xerochloa) areas (RR=8.15), then Schoenoplectus/mangroves (RR=2.29), compared with the reference vegetation category "lower mangroves." Furthermore, larval densities were highest in May, due to tidal inundation, for drainage areas and tide-affected reticulates (RR=12.2, 11.7, respectively) compared with March, the reference month. Thus, to maximize the efficiency of aerial salt marsh mosquito control operations in this wetland, larval control is best accomplished by concentrating on drains, Schoenoplectus/mangroves, and tide-affected reticulate areas, commencing early after the wet season. These results should apply to other areas of salt marsh mosquito breeding across northern Australia.

A geospatial evaluation of Aedes vigilax larval control efforts across a coastal wetland, Northern Territory, Australia.

Adjacent to the northern suburbs of Darwin is a coastal wetland that contains important larval habitats for Aedes vigilax (Skuse), the northern salt marsh mosquito. This species is a vector for Ross River virus and Barmah Forest virus, as well as an appreciable human pest. In order to improve aerial larval control efforts, we sought to identify the most important vegetation categories and climatic/seasonal aspects associated with control operations in these wetlands. By using a generalized linear model to compare aerial control for each vegetation category, we found that Schoenoplectus/mangrove areas require the greatest amount of control for tide-only events (30.1%), and also extensive control for tide and rain events coinciding (18.2%). Our results further indicate that tide-affected reticulate vegetation indicated by the marsh grasses Sporobolus virginicus and Xerochloa imberbis require extensive control for Ae. vigilax larvae after rain-only events (44.7%), and tide and rain events coinciding (38.0%). The analyses of vector control efforts by month indicated that September to January, with a peak in November and December, required the most control. A companion paper identifies the vegetation categories most associated with Aedes vigilax larvae population densities in the coastal wetland. To maximize the efficiency of aerial salt marsh mosquito control operations in northern Australia, aerial control efforts should concentrate on the vegetation categories with high larval densities between September and January.

Murray Valley encephalitis virus surveillance and control initiatives in Australia. National Arbovirus Advisory Committee of the Communicable Diseases Network Australia.

Mechanisms for monitoring Murray Valley encephalitis (MVE) virus activity include surveillance of human cases, surveillance for activity in sentinel animals, monitoring of mosquito vectors and monitoring of weather conditions. The monitoring of human cases is only one possible trigger for public health action and the additional surveillance systems are used in concert to signal the risk of human disease, often before the appearance of human cases. Mosquito vector surveillance includes mosquito trapping for speciation and enumeration of mosquitoes to monitor population sizes and relative composition. Virus isolation from mosquitoes can also be undertaken. Monitoring of weather conditions and vector surveillance determines whether there is a potential for MVE activity to occur. Virus isolation from trapped mosquitoes is necessary to define whether MVE is actually present, but is difficult to deliver in a timely fashion in some jurisdictions. Monitoring of sentinel animals indicates whether MVE transmission to vertebrates is actually occurring. Meteorological surveillance can assist in the prediction of potential MVE virus activity by signalling conditions that have been associated with outbreaks of Murray Valley encephalitis in humans in the past. Predictive models of MVE virus activity for south-eastern Australia have been developed, but due to the infrequency of outbreaks, are yet to be demonstrated as useful for the forecasting of major outbreaks. Surveillance mechanisms vary across the jurisdictions. Surveillance of human disease occurs in all States and Territories by reporting of cases to health authorities. Sentinel flocks of chickens are maintained in 4 jurisdictions (Western Australia, the Northern Territory, Victoria and New South Wales) with collaborations between Western Australia and the Northern Territory. Mosquito monitoring complements the surveillance of sentinel animals in these jurisdictions. In addition, other mosquito monitoring programs exist in other States (including South Australia and Queensland). Public health control measures may include advice to the general public and mosquito management programs to reduce the numbers of both mosquito larvae and adult vectors. Strategic plans for public health action in the event of MVE virus activity are currently developed or being developed in New South Wales, the Northern Territory, South Australia, Western Australia and Victoria. A southern tri-State agreement exists between health departments of New South Wales, Victoria and South Australia and the Commonwealth Department of Health and Aged Care. All partners have agreed to co-operate and provide assistance in predicting and combatting outbreaks of mosquito-borne disease in south-eastern Australia. The newly formed National Arbovirus Advisory Committee is a working party providing advice to the Communicable Diseases Network Australia on arbovirus surveillance and control. Recommendations for further enhancement of national surveillance for Murray Valley encephalitis are described.

Envenomation by the billygoat plum stinging caterpillar (Thosea penthima).

We report the first case of envenomation by the billygoat plum stinging caterpillar, Thosea penthima Turner (Limacodidae). The sting, on the forearm, caused immediate burning pain and local wheal formation. Pain radiated up the arm and there was severe "crushing" chest pain lasting four hours. The local pain persisted for 10 hours and required opiate analgesia.

Australian encephalitis in the Northern Territory: clinical and epidemiological features, 1987-1996.

BACKGROUND: The last epidemic of Australian encephalitis occurred in 1974. Since then, cases have been reported from the Kimberley of Western Australia (WA). AIMS: To describe the epidemiology and clinical features of Australian encephalitis in the Northern Territory (NT) of Australia. METHODS: Review of cases of Australian encephalitis presenting to Royal Darwin Hospital from 1987-1996 and review of sentinel chicken surveillance for Australian encephalitis viruses. RESULTS: Sixteen patients were identified; ten from the NT and six from WA. Cases occurred in the years 1987, 1988, 1991 and 1993. Infection was acquired throughout northern NT below latitude 20 degrees S in the months March to July. All infections were due to Murray Valley encephalitis (MVE) virus. Eleven of the patients were children. Distinguishing features were spinal cord and brainstem involvement and the absence of seizures in adults. CT scanning was normal and EEG showed no focal activity. Five died (31%) and four (25%) have residual neurological disability. Sentinel chicken surveillance since 1992 shows yearly seroconversion to MVE virus throughout northern NT; human cases occurred simultaneously with chicken seroconversion in 1993. CONCLUSIONS: Australian encephalitis is endemic in the NT; the areas at risk are north of Tennant Creek. Outbreaks are seasonal and occur every few years. Young children are most at risk. Mortality and morbidity are high. Prevention of disease is by avoidance of mosquito exposure and vector control measures.

Communicable diseases surveillance

Communicable diseases surveillance highlights (vaccine preventable diseases, arboviruses, respiratory viruses), and surveillance outcomes from several reporting series as follows: National Notifiable Diseases Surveillance System tables, 14 October to 10 November 1998; Laboratory Serology and Virology Reporting Scheme tables, 8 October to 4 November 1998; Australian Sentinel Practice Research Network report tables, weeks 40 to 43, 1999; Gonococcal surveillance, reporting period 1 April to 30 June 1998; Sentinel chicken surveillance program, reporting period September 1998; HIV and AIDS surveillance, reporting period 1 to 30 June 1998, assessed as at 30 September 1998; Australian childhood immunisation coverage, 1 April to 30 June 1998 cohort, assessed as at 30 June 1998.

An outbreak of epidemic polyarthritis (Ross River virus disease) in the Northern Territory during the 1990-1991 wet season.

OBJECTIVE: To describe the epidemiology of a large outbreak of epidemic polyarthritis in the Northern Territory during the wet season of 1990-1991. DESIGN, SETTING AND PARTICIPANTS: Arbovirus cases notified to the Northern Territory Department of Health and Community Services by general practitioners and local laboratories between 1 July 1990 and 30 June 1991. MAIN OUTCOME MEASURES: Date and place of infection, age, sex and symptoms. RESULTS: Doctors in the Northern Territory notified 368 cases; another 14 were infected interstate. The epidemic started in September, peaked in January and tailed off in April. The highest attack rates occurred in the rural areas of Jabiru, Litchfield Shire and Katherine. Those most affected were 30-34 year olds. Children, the elderly and Aboriginal people were under-represented. CONCLUSIONS: Epidemic polyarthritis is a wet season problem in the Northern Territory, affecting the rural towns and districts more than the cities. Pre-planned mosquito control measures (effective water drainage and larval control) limited the extent of the 1990-1991 epidemic in Darwin City and Palmerston. The low attack rate in children reflects asymptomatic and less clinically severe infections. The under-representation of Aboriginal people may be the result of infection occurring earlier in life. A related cross-sectional seroprevalence survey has shown that rural Aboriginal people across all age groups have a significantly higher seropositive rate than urban non-Aboriginal residents.

Is Aedes albopictus in Australia?

In Brisbane during October 1988 one larva of the exotic dengue vector Aedes albopictus (Skuse) was collected by quarantine officers from a consignment of used vehicle tyres imported from Asia. Although this is not the first report of this mosquito in Australia, the finding was of sufficient importance to change quarantine procedures. Subsequently in Darwin during June 1989, two Ae. albopictus larvae were recovered from an ovitrap located near the wharves. This article reviews the global expansion of this species and, on the basis of previous imports of tyres into Australia, suggests that Ae. albopictus may already be established in this country.