Exotic Vectors

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Overall, Australia has six exotic mosquito species (Culex quinquefasciatus, Aedes aegypti, Ae. albopictus, Ae. vexans, Cx. gelidus, Cx. molestus) established amongst its native fauna, and more than 10 exotic species have been recorded arriving at ports in Australia but have not established: Aedes aegypti and Ae. albopictus at ports other than in Queensland (QLD) where these two mosquitoes are established (albeit Ae. albopictus is currently not on mainland QLD), and Ae. malayensis/scutellaris, Ae. quasirubrithorax, Ae. papuensis, Ae. dasyorrhus, Ae. novalbitarsis/subalbitarsis, Culex fragilis, Cx. spathifurca, Cx. sitiens gp. species (Russell and Kay 2004). Aedes aegypti continues to be imported to Australian seaports from nearby infested regions, with regular cargo vessels but also illegal fishing vessels and refugee boats (Whelan et al. 2001). In the fiscal year 2005/06, there was a total of 9 interceptions in the Northern Territory (NT) (8 Ae. aegypti and 1 Ae. albopictus), in 2004/05 there were 22 interceptions comprising 22 Ae. aegypti and 1 Ae. albopictus, and in 2003/04 there were 6 Ae. aegypti and 1 Ae. albopictus interceptions (P. Whelan personal communication).

In Australia, Ae. aegypti remains confined generally to northern QLD, although in early 2004 an established population was detected in the NT town of Tennant Creek (seemingly introduced by road transport from QLD) and an eradication program was undertaken; the species apparently did not spread to other NT communities and the eradication efforts were proclaimed as successful in 2007. Control was based on repeated rounds of treating all potential receptacles with bifenthrin or deltamethrin residual sprays, methoprene pellets or chlorine solution; with treatments of all vacant urban and rural residential and industrial properties, waste storage and disposal areas, storm water systems, telecommunication inspection pits and manholes, and any other area found to be potentially breeding or harbouring mosquitoes. Rainwater tanks were sealed and treated with methoprene briquettes (18 g/kg S-methoprene) and pellets (40 g/kg S-methoprene) and storm water systems were cleared of underground soil obstructions. (Whelan et al. 2004, 2005; P. Whelan pers.comm.). An established infestation was also detected on Groote Eylandt in the NT in 2006 and is currently subject to a similar eradication program (Whelan 2006).

Aedes albopictus was first detected on Yorke Island in the Torres Strait between Australia and Papua New Guinea (PNG) in April 2005 (Russell et al. 2005; Ritchie et al. 2006). A delimiting survey throughout 22 communities on 17 islands in the Torres Strait and in the Northern Peninsula Area (NPA) of the mainland, with collection of larvae and pupae from containers and adults attracted to human bait, detected

Aedes albopictus in 10 island communities but not in any mainland sites. Immature forms of Aedes albopictus are difficult to distinguish morphologically from the local Aedes scutellaris but we are able to confirm identification using Polymerase Chain Reaction (PCR) methods (Beebe et al. 2007). Retrospective genetic analysis of larvae collected in April 2004 and April 2005 on Yorke Is. indicated that Ae. albopictus was present in low densities in 2004, and that there were three genetically distinct mitochondrial haplotypes on Yorke Is. in April 2005, suggesting one significant or multiple incursions (Ritchie et al. 2006).

An eradication program was established in early 2006 and has been progressing under supervision of an expert Technical Advisory Group (TAG).

The program has reduced Ae. albopictus populations on five of the ten islands, but the species has spread to an additional three islands although not to the mainland (Davis 2007).

Control was based around removing those breeding sites that could be disposed of and treating those sites that couldn't be removed, with treatments of residential properties, waste storage areas, and any other area found to be breeding or harbouring mosquitoes. Briquettes (18 g/kg S-methoprene) and pellets (40 g/kg S-methoprene) were used as insecticide, and supplemented with spray (80 g/L Bifenthrin).

Control efforts continue to target positive communities and adjacent vegetated areas. The current proposal is to fund the program until 2009. The goal is to eliminate Ae. albopictus from the Torres Strait, prevent future incursions and work with community councils to develop and maintain a mosquito management program (Davis 2007). The potential for the species to spread to southern states has been investigated and there is concern that it may provide a vector of dengue where none currently exists (Russell et al. 2005).

Aedes vexans (Australia) was first detected in northern Western Australia (WA) in 1996 and has been found regularly in the region since 2001 (Johansen et al. 2005a). It is thought to have been introduced by wind currents from Indonesia or possibly by the occasional light aircraft that arrives from nearby islands. This species is of interest for a number of reasons, not the least being its extraordinary global distribution through Europe, Asia and North America; it is found also in parts of Africa, Central America and the Pacific (where it has long been called Ae. nocturnus), indicating it has the capacity to spread widely in Australia. On the basis of northern hemisphere experience, it presents as a potential nuisance pest and possible vector of pathogens, although its competence for local arboviruses is completely unknown.

Culex gelidus, a secondary vector of Japanese encephalitis virus in Southeast Asia, was first detected in QLD in 1999. However, a check of museum specimens revealed it had been collected previously in QLD in 1994 (Ritchie et al. 2001) and also in the NT in 1996 (Whelan et al. 2001). It is thought to have arrived in the NT from QLD, and more recently moved to northern WA via cattle trucks or other vehicles from near sewage ponds, cattle yards or abattoirs.

Vector competence experiments with a NT strain of Cx. gelidus have shown that it was a highly efficient vector of Japanese encephalitis virus (JEV), Murray Valley encephalitis virus (MVEV) and Kunjin virus (KUNV), a moderately good vector for

Ross River virus (RRV), but was refractory to Barmah Forest virus (BFV) (Andrew van den Hurk pers. comm.). In QLD, JEV (Ritchie et al. 2007) and RRV (Ritchie pers. comm.) have been detected in Cx. gelidus by PCR analysis, from Torres Strait islands and Cairns, respectively (Scott Ritchie pers. comm.).

It still seems to be relatively rare in WA, and in QLD and NT it appears to have not created pest problems, with high numbers of adults being found only near high organic aquatic habitats where there are high numbers of larvae, so it may be that it presents little appreciable risk as a vector to humans in most situations. Williams et al. (2005) published a CLIMEX projection of the distribution of Cx. gelidus in Australia, revealing a potentially wide distribution throughout coastal Australia, particularly in tropical and subtropical areas.

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