“Dengue” mosquitoes detected in Melbourne: What does it mean?

Australian media is reporting that mosquitoes that carry dengue and yellow fever viruses have been detected at Melbourne Airport. Suggestions have been made that these mosquitoes have also been collected in Perth and Adelaide. What does this mean for southern Australia?

There are over 300 mosquitoes species in Australia. Most pose very little or no public health risk. There are about 20-30 mosquitoes species that may potentially play a role in the transmission of endemic pathogens such as Ross River virus. About a dozen pose a serious risk of severe nuisance-biting and the transmission of Ross River, Barmah Forest and Murray Valley encephalitis viruses. Only one mosquito currently found in Australia can transmit dengue viruses.

Dengue and mosquitoes in Australia

Aedes aegypti, commonly known as the Yellow Fever Mosquito, is considered one of the most important species internationally with regard to the transmission of dengue. This mosquito is playing a role in serious outbreaks of dengue currently underway in Malaysia and Fiji. This mosquito is also responsible for the current outbreak of dengue in Far North Queensland. Health authorities target this species to control such outbreaks.

While the majority of Australia’s pest mosquitoes are associated with wetland and bushland habitats, Aedes aegypti is found in urban habitats. This is the “cockroach” of the mosquito world. While it may have originally evolved in association with tree holes and leaf axils, it is now almost exclusively found in water holding containers in our backyards (e.g. pot plant saucers, rainwater tanks, birds baths, buckets, watering cans, discarded plastic takeaway containers and almost anything that can hold water). This makes it hard to control and easy to move around. Movement of humans, and more specifically our belongings, has been responsible for the spread of this mosquito across the planet. They love to bite humans too.

Currently, Aedes aegpypti is limited in its distribution to Far North Queensland. It is most commonly found around Townsville and Cairns. However, historically, it has been found more widely in Australia with populations present in Brisbane and Sydney. There were even locally acquired cases of dengue in NSW as far south as Gosford. Since the 1950s, the distribution of Aedes aegypti shrunk. Why? Plenty of reasons have been suggested but nothing is definitive. Perhaps it was the availability of household insecticides, the move away from rainwater tanks as household’s primary source of water or even the decline in steam trains (possibly shutting down a regular route for southern movement of mosquitoes).

The historic and post-1990 distribution of Aedes aegypti and dengue transmission zones in Australia (Taken from Russell et al 2009 Medical Journal of Australia)

There have been suggestions that a changing climate may influence future geographic spread of Aedes aegypti in Australia. While on the surface, warmer conditions may appear conducive but climate obviously didn’t play a role in the dramatic retreat in the distribution of the mosquito and a relatively small change in temperature is unlikely to result in a dramatic invasion of southern cities by this mosquito. What else is going on? Perhaps human response to a changing climate, particularly water shortages and conservation measures (e.g. widespread use of rainwater tanks in metropolitan areas, water sensitive urban design), may facilitate future broadening of the mosquito’s distribution? If you increase potential habitat, will the mosquitoes move in?

Will there be dengue outbreaks in Melbourne?

The detection of Aedes aegypti in Melbourne is a concern. While there is a risk that populations of the mosquito may become established, the chances of that happening are relatively low. It is just too cold during winter for the mosquitoes to survive. In that regard, there is unlikely to be any established populations of the mosquito in the local area in the long term. However, that doesn’t mean that efforts shouldn’t be made to eradicate the mosquito and prevent such incursions again.

An outbreak of dengue is also unlikely. It is important to remember that these mosquitoes do not all carry dengue viruses. They must bite an infected person first before they can then pass on the virus. In FNQ, outbreaks of dengue are triggered by infected travellers arriving and being bitten by local mosquitoes. We’ve seen record numbers of travellers returning from overseas infected with dengue. The ever increasing activity of dengue in the Pacific and SE Asia is only going to increase the number of travellers bringing the pathogen back into the country so there may be a chance that one could be bitten in Melbourne and infect local mosquitoes. An unlikely scenario but not impossible.

This has been a wake up call: Look out for the tiger!

It is unlikely that the current incursion of Aedes aegypti will have direct long-term implications for public health risks. However, it has exposed the weakness in surveillance programs of local authorities to pick up exotic mosquitoes. If it is the Asian Tiger Mosquito next time, the implications may be substantially greater.

Aedes albopictus, commonly known as the Asian Tiger mosquito, is the second most important mosquito in dengue outbreaks internationally. It is also an important vector of Chikungunya virus that has caused problems for Australia travellers and is currently threatening the Americas. Fortunately, this mosquito is not currently found in Australia….but for how long?

The mosquito has been documented in Torres Strait since 2005 and authorities have put considerable effort into keeping them out of Australia. The biggest problem with Aedes albopictus is that, unlike Aedes aegypti, the mosquito is far more tolerant of temperate climates and computer modelling has indicated that populations of this mosquito could become established in cites including Brisbane, Sydney and Melbourne. Populations could become well established along the east coast of Australia. This mosquito is increasing public health risks in Europe and was likely responsible for the first locally acquired case of dengue in New York last year.

The Asian Tiger Mosquito, Aedes albopictus. (Photo CDC Public Health Image Library)

As Aedes albopictus shares a similar ecological niche to Aedes aegypti it is easily introduced to new regions of the world through the movement of humans and their belongings. The international trade in used tyres is suspected to have played a major role in the spread of the mosquito to Europe and North America. Recent studies have suggested that the mosquito was spread to the Torres Strait, not from climate change driven dispersal of mosquitoes from PNG but by Indonesian fishers. There is no reason to believe that the movement of items containing the eggs, larvae or adults of Aedes albopictus won’t “hitchhike” their way to Australia. We know it will happen because it has happened in the past. They were detected in Melbourne in December 2012.

The biggest concern associated with recent reports is that there is clearly a route of entry for exotic mosquitoes into southern Australia. If it had been Aedes albopictus, rather than Aedes aegypti that had escaped into the suburbs of Melbourne, there may have been a much more significant long-term risk to public health. Notwithstanding the potential transmission of pathogens, Aedes albopictus is considered one of the worst nuisance biting pests. Established, widespread populations of this mosquito in metropolitan areas of Brisbane, Sydney, Melbourne or Perth would pose serious problems for residents, and local authorities responsible for their control. It has been dubbed “the BBQ stopper“. The presence of this mosquito in some regions of North American has even been identified as a driving factor reducing outdoor activity of children.

While there remains much discussion about a changing climate and the role it may play in the spread of mosquito-borne disease, it is more likely to be human movement that dramatically increases public health risks in Australia. Northern Australia continues to report incursions of exotic mosquitoes and the introduction of infected mosquitoes has been identified as the source of dengue infection in individuals in NT and WA in recent years. Perhaps it is only a matter of time before local authorities are faced by greater risks of dengue and Chikungunya. Surveillance is critical but beyond adequate funding of DAFF, response plans need to be in place for when these mosquitoes are detected. We may be thinking the threat is coming from our north but these mosquitoes may just sneak in through the back door.

Photo of Aedes aegypti at the top of this piece thanks to Mr Stephen Doggett (Pathology West)