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Department of Medical Entomology |  |
Recent Research
Projects
Infectivity of various strains of Ross River virus in Culex
annulirostris and Aedes
vigilax.
At the 6th Australian Arbovirus Symposium in
1992, we presented preliminary results which appeared to support the hypothesis
that arboviruses (particularly Ross
River virus) and mosquitoes from the same region are mutually better adapted
in terms of smaller infective virus dose and higher virus yield than
combinations of viruses and vectors from different regions. Since then, further
groups of field-collected Aedes
vigilax and Culex
annulirostris mosquitoes have been fed field strains of Ross
River (RR) virus in homologous and heterologous locality combinations.
Contrary to the preliminary findings, further testing indicated there was no
correlation between a low Mosquito ID50 and matching geographical
source of mosquito and virus. The virus strains varied considerably in
infectivity, but the infectivity of any particular strain was reasonably
consistent for mosquitoes collected from all localities.
Competence of the domestic container breeding mosquito Aedes notoscriptus to transmit inland and coastal strains of Ross River and Barmah Forest viruses.
Aedes
notoscriptus is a mosquito that breeds in containers in close
association with human environments and is widely distributed in residential
areas throughout Australia. In the laboratory, it has been infected with Murray
Valley encephalitis (MVE) virus and Ross
River (RR) virus. In the field, RR has been isolated from Ae.
notoscriptus collected in Darwin, NT, and Tamworth, NSW and Brisbane,
Qld. Following an isolation of Barmah
Forest (BF) virus in urban Sydney from Aedes vigilax
during the recent outbreak of 1995 in NSW, the potential for the common
urban species, Ae.
notoscriptus, to transmit inland and coastal NSW isolates of RR and BF
viruses was investigated. Ae.
notoscriptus was found to be capable of transmitting both RR and BF.
Thus with the recent evidence of field infections with RR in various parts of
Australia, Ae.
notoscriptus must be considered a potentially important vector of these
viruses.
The impact of microsporidia
on arbovirus transmission in mosquitoes.
Microsporidia are
obligate intracellular parasites and have been investigated for use as mosquito
biocontrol agents. To date no work has been undertaken to investigate possible
indirect public health risks in relation to coinfections of microsporidia and
arboviruses in mosquitoes. Microsporidia are transmitted transovarially;
therefore one risk is the potential increase in the level of arboviral
transovarial transmission which may lead to greater arbovirus activity. Other
hazards include the possibility that microsporidia infected mosquitoes may be
more susceptible to infection with one or more arboviruses, and that virus
growth may be enhanced. These possible risks were examined in the mosquito Culex
annulirostris infected with the microsporidia Amblyospora
dyxenoides and the arbovirus Ross
River (RR) virus. Testing found no evidence for any enhanced acquisition or
growth of RR virus in Cx.
annulirostris when infected with A. dyxenoides. Likewise, there
was no evidence for any increased level of transovarial transmission of RR
virus. However, evidence for increased levels of vertical transmission requires
further investigation. Thus, current evidence suggests that there are no
indirect public health threats associated with the use of A. dyxenoides
as a biocontrol agent against Cx.
annulirostris.
Development of a quality control procedure for a fixed-cell ELISA used
for identifying arboviruses isolated from mosquitoes.
The Medical Entomology laboratory undertakes arbovirus surveillance and mosquito monitoring in NSW to provide a system of early warning of flavivirus activity. A fixed-cell ELISA (FCE), developed by colleagues at the arbovirus laboratory in the Department of Microbiology, University of Western Australia, was introduced to screen field collected mosquitoes for flaviviruses. There was a need to monitor the performance of the FCE and, therefore, a quality control (QC) system was developed. This system has allowed our laboratory to recognise and minimise analytical errors which may arise in the FCE. Furthermore, the immediate charting of the QC graphs, via the computer program, has allowed rapid remedial action when required.
Development of a multi-sample device for grinding mosquitoes for
arbovirus extraction.
To monitor field rates of arboviruses in
mosquitoes, or to examine the vector competence of a
species, mosquitoes have to be ground to allow the release and
detection of an infecting arbovirus. Traditional methods of grinding used mortar
and pestles (M&P), or tissue grinders, however, these methods are highly
labour intensive, both in preparation and operation. To overcome these problems,
our laboratory designed and built "MOSAVEX" (MOSquito ArboVirus Extractor) to
mechanically grind mosquitoes. MOSAVEX is a shaker powered by a 240v DC motor,
with speed adjusted via a variable speed controller. The motor is connected
indirectly to two arms that oscillate at high speeds. Mosquitoes to be ground
are placed into 5x1cm tubes containing five 5mm glass beads and diluent. Six of
these tubes are placed into one 11x5cm urine jar. Six urine jars are then fixed
into holders at the end of each of the two arms. Thus 72 grinding tubes are held
in total, however MOSAVEX can be modified to hold 12 urine jars and thus
doubling the capacity to 144 grinding tubes. Testing of MOSAVEX found that
labour costs were 1/10th that of using mortar & pestles, and less
disinfectants and cleaning agents were required. MOSAVEX was also as efficient,
if not slightly better, at extracting virus from mosquitoes as mortar &
pestles.
Past Research
Projects
Investigations into
Lyme disease in
Australia.
Lyme disease (LD) is a tick-borne syndrome which was
first recognised in the mid-1970s from the United States, although in Europe
various syndromes, now attributed to LD, have long been recognised. A wide
spectrum of symptoms can occur including rashes, fevers and arthritis. In
Australia, the disease was first described from the Hunter Valley region of New
South Wales in 1982. From 1990-92, the Department of Medical Entomology
undertook investigations to identify the causative organism of LD in southeast
Australia from possible tick vectors. Ticks were collected in coastal areas of
NSW, from localities associated with putative human infections. The ticks were
dissected; a portion of the gut content was examined for spirochaetes by
microscopy, the remaining portion inoculated into culture media. The detection
of spirochaetes in culture was performed using microscopy, and immunochemical
and molecular (PCR) techniques. Additionally whole ticks were tested with PCR
for spirochaetes. Approximately 12,000 ticks were processed. No evidence of
Borrelia burgdorferi (the causative agent of LD) nor any other
spirochaete was recovered from or detected in likely tick vectors. Thus there is
no definitive evidence for the existence in Australia of B. burgdorferi,
or any other tick-borne spirochaete that may be responsible for a local syndrome
being reported as LD. (For more information see
fact sheet on Lyme
Disease).
Over the past 15 years, Assoc. Prof. Richard Russell and colleagues, have undertaken intermittent investigations into the relative importance of different mosquitoes as vectors of Dirofilaria immitis, the dog heartworm. Field and laboratory studies have been used to compare the potential of various species for heartworm transmission, and to estimate their relative efficiency under various circumstances. Throughout this series of investigations, Aedes notoscriptus (a common domestic breeding mosquito), has been shown to consistently be the "best vector", but its survival is much reduced when it ingests higher worm loads. In localities where dogs have high levels of parasitaemia, it may be that Culex annulirostris (which breeds in freshwater marshes) is the more effective vector. (For more information see fact sheet on Dog Heartworm).
