Climate change, urbanisation and globalisation are changing the risk of mosquito-borne diseases around the world. So how do we keep track of mosquitoes in Australia?
Thursday, February 9th, 2017
Standing in the middle of the Newington wetland, on an island of beaded glasswort, you really have to stretch your ears to hear the faint hum of traffic — it reminds you that you are smack-bang in the middle of greater Sydney. The Parramatta River, hidden by a row of mangroves only a hundred metres to the north, is ferrying commuters into the city, and the M4 freeway is masked by bushland to the south.
I’m with Dr Cameron Webb, the mozzie guy for NSW Health Pathology, and we’re surveying the estuarine wetlands that run through Sydney Olympic Park. A specialist in medical entomology, Webb studies mosquitoes that can impact human health and the viruses they carry.
In Australia, Ross River virus is the most commonly reported mosquito-borne disease. It is carried through coastal wetlands, urban environments and freshwater floodplains by multiple mosquito species. Several reservoir hosts, like kangaroos, also play a role: the virus can hide out in mammals before being picked up again by mosquitoes. There has been a spike in cases across New South Wales this summer, and the virus is encroaching on Melbourne too.
To report on the risk posed by resident mosquitoes to Sydney-siders, Dr Webb monitors mosquito numbers using traps set routinely at different locations across the wetlands in Sydney Olympic Park. After being left out overnight, the traps are taken back to the lab where numbers are tallied and the diversity of species noted, particularly those capable of carrying viruses. If any females have recently enjoyed a blood meal, the blood is analysed to look for host DNA, to work out who is getting bitten, and to detect viral pathogens.
The day I head out into the field with Webb follows a week of rain and precedes the February heatwave. The sun beats down, cicadas are buzzing, and I’m sweating underneath the layers of insect repellent and sunscreen. Webb visits these wetlands a few times a week across the summer months to survey mosquito populations in the area. He strides across the marsh; I’m trying to keep up, dodging spider webs and knee-deep puddles.
We hop across the beaded glasswort (Sarcoconia quinqueflora) that blankets the saltmarsh. The succulent forms a dappled cover of red, orange, purple and green and looks like Neptune’s necklace seaweed transplanted from a rock pool. With high tides, salt water floods the marsh and pools between patches of the glasswort; the water is brassy and warm. It’s here that we find mosquito larvae squirming and after recent rain; there are lots of them — most likely Aedes vigilax and no more than 48 hours old.
To study mosquitoes, you need to keep one eye on the weather and the tides, so Webb is always checking the forecast. Mosquitoes need an influx of water to breed. Warm, stagnant water is a depository for their eggs, then an incubator for their larvae. After a week or two, the adults emerge and with humid heat, mosquito populations flourish. In their short life, less than a month, a female mosquito can lay hundreds of eggs. When the weather is in sync with their life cycle — that is, rainfall or high tides once a month that flood the wetlands but cannot drain quickly away — booming mozzie populations can cross over from a pest to a public health risk.
Monitoring mosquito breeding grounds is key to managing the risk because Webb and his team can respond to emerging populations. As we leave Newington wetland, Webb is straight on the phone to arrange for the area to be treated with a bacterial spray that kills mosquito larvae when ingested. Proteins expelled by the bacteria react with the gut lining of the hungry larvae — it’s a only small window in the mozzie life cycle and a precise target to hit. Webb will return a few days after the treatment has been applied to check if there are any live larvae left to assess the effectiveness of these control programs.
We visit another site on the banks of the Duck River. This tributary of the Parramatta River, flanked by industrial land, is one of the most polluted waterways in Sydney. The site is not actively managed like the wetlands in Sydney Olympic Park. Of the sites that Webb monitors, it has the greatest diversity of species and routinely, the largest mosquito populations. Duck River serves as a reference point — an indication of mosquito populations without control measures.
Stepping over the kerb of the concrete carpark, we walk down a slope to the edge of the mangroves and they are teeming with mosquitoes. We set a trap and in less than 10 seconds we’ve caught one little gal — overnight we will trap around 5,000 mosquitoes at this site. Here, nuisance mosquito populations are a symptom of a degraded urban wetland, neglected by its industrial neighbours.
The swarms at Duck River, the buzz at my ear, brought back fierce memories of Asseradores, a small township on the west coast of Nicaragua, and its mosquitoes. Asseradores is a magnet for surfers: my fiancé and I took four buses and a taxi, a 12-hour journey all up, to reach the home of a wave called ‘The Boom’. It is also home to a soggy mangrove forest, through which we had to run to avoid being bitten on our way to the surf each morning. There had been reports of chikungunya in the area, a mosquito-borne disease we had never heard of.
Chikungunya is new to the Americas as well. It was first described in 1952 in eastern Africa, then identified in countries in the Indian Ocean and Asia. An epidemic on Reunion Island in 2005-2006 was attributed to a genetic change in the virus that made a prolific nuisance-biting mosquito, Aedes albopictus, more effective as a vector — and this carried the virus into new territory. Chikungunya had not been documented in the Americas until the first case was confirmed in the Caribbean in late 2013, likely introduced by infected travellers. It has since spread north to the United States and south to Argentina.
Closer to home, in the Torres Strait islands to Australia’s north, Aedes albopictus is knocking at our door. Better known as the Asian tiger mosquito, this species is known to carry dengue and Zika viruses in addition to chikungunya. It has been held at bay in the Torres Strait by a strict program to eradicate breeding in urban areas.
As for the wetlands, a new species has moved into the saltmarshes at Sydney Olympic Park. Coquillettidia xanthogaster, a distinctive mosquito with an orange body that is a major pest in northern Australia, started appearing in small numbers in Webb’s wetlands about five years ago. Changes to Sydney’s climate may have enticed these mozzies south, but it is more likely that larvae of this species, which attach themselves to submerged mangrove roots, were transplanted during the rehabilitation of the Sydney Olympic Park wetlands. These mozzies are susceptible to Ross River viral infection in the lab but their vector status in the wild is unknown. While it’s unlikely to pose a health threat, it is yet another example of humans moving mosquitoes to new locations.
Stepping out of the wetlands and back into the concrete jungle, I got swept up in the 5 o’clock traffic on my journey home. Humans will continue to sculpt the environment, either directly through urbanisation or incrementally by climate change — and mosquitoes are coming along for the ride. Mozzies may look like delicate creatures under the microscope with their translucent wings and spindly legs but en masse, they are resilient and capable of adapting to niche environments. Their presence is one of countless ecological indicators of our changing world. Rather than swatting them away, we should be paying attention.
With 35 kilometres of bike trails, walking paths and boardwalks, you can explore the Sydney Olympic Park wetlands for yourself. Keep an eye out for birds, frogs and mushrooms after March was the wettest month for Sydney in over 3 decades! You can also follow Dr Cameron Webb on Twitter for regular updates, photos and stories from his field work in the wetlands.
Edited by Jack Scanlan