Common Australian mosquitoes can't spread Zika

New research from Australian scientists shows common local mosquitoes aren’t able to spread Zika virus. This means Australia is unlikely to see a major outbreak of the disease. But a risk remains in northern Queensland.

Since the World Health Organisation’s declaration of a Public Health Emergency of International Concern in February, due to the effects of Zika on pregnant women, authorities have been on high alert.

The concern was not just for travellers heading to regions experiencing outbreaks but for the potential for local mosquitoes to spread the disease when those travellers returned home.

How do mosquitoes spread viruses?

Mosquitoes are not like dirty syringes, spreading viruses in droplets of blood from person to person. When a mosquito bites, she injects saliva to assist blood-sucking. Unfortunately, mixed in with that spit can be a virus.

For the saliva to become infected, the mosquito must first bite a person or animal infected with a virus. As the mosquito becomes infected, the virus spreads throughout the body of the mosquito; from the legs to the head.

Inside the head are the mosquito’s salivary glands. When they become infected, they can pass on the virus to the next unsuspecting host.

From acquiring to passing on the virus, the process can take anywhere from a few days to a couple of weeks.

Working out which mosquitoes spread which viruses is critical to developing strategic responses to outbreaks of disease.

Which mosquitoes currently spread Zika virus?

Back in Uganda, where Zika virus was first discovered, it was spread by African forest-dwelling mosquitoes such as Aedes africanus.

Decades later, when the first substantial outbreak was reported on Yap Island in the Federated States of Micronesia, it was Aedes hensilii.

The relationship between mosquitoes and Zika virus hasn’t been well studied but there is general consensus that, internationally, the mosquito of greatest concern is likely Aedes aegypti.

Aedes aegypti is the cockroach of the mosquito world. It loves water-filled containers around the home and it spreads yellow fever, dengue and chikungunya among others.

It loves biting people – lots of people. Wherever Aedes aegypti is found, outbreaks of disease keep popping up.

Of the 300 or so mosquitoes found in Australia, only Aedes aegypti can spread exotic viruses and it was considered the most likely to possibly spread Zika virus locally.

Testing Aussie mozzies

Australian mosquitoes have been tested against a range of exotic viruses including dengue, chikungunya, West Nile, Rift Valley fever, Japanese encephalitis and yellow fever viruses.

But no-one had tested the ability of local mosquitoes to spread Zika virus – until now.

Scientists from Queensland Health, the University of Queensland and James Cook University studied the potential for Australian mosquitoes to spread Zika virus and found Aedes aegypti does indeed pose the greatest risk. The results are published today in the journal PLoS Neglected Tropical Diseases.

The study tested seven types of mosquito implicated in the spread of disease in Australia, including Aedes notoscriptus, Aedes vigilax, Culex annulirostris, Culex quinquefasciatus and Aedes aegypti. These mosquitoes are frequent biters, are found close to local suburbs and have been shown to transmit other pathogens.

The researchers exposed groups of mosquitoes in the laboratory to blood infected with one of two strains of Zika virus: one originating from a traveller returning to Australia from the Pacific; another derived from the original strain isolated in Uganda.

Once the mosquitoes fed on the blood and virus mixture, they were tested to see if they become infected with the virus and, most importantly, whether the virus was being transmitted in their spit.

The only mosquito found to transmit Zika virus was, unsurprisingly, Aedes aegypti.

However, what was surprising was the relatively low rate of transmission. Fourteen days after taking a Zika virus-infected blood meal, only 67% of infected mosquitoes were found with virus in their saliva. Based on other mosquito-borne viruses, such as chikungunya, it would be expected to be much higher. The result, however, echoes the results of another study from the United States.

Aedes aegypti is playing an important role in outbreaks of Zika virus but perhaps it is due to more than just their ability to transmit the virus. They have a propensity to bite humans, and to bite many different people.

Mosquito-control efforts should therefore remain focused on Aedes aegypti.

Is Australia safe from Zika?

These results have important implications for predicting where in Australia we may see an outbreak of Zika virus.

Aedes aegypti is generally only found in tropical regions of Queensland. The mosquito isn’t in the major urban area of Southeast Queensland such as Brisbane, Sunshine Coast or Gold Coast. The areas at greatest risk are likely to be those around Cairns, Townsville as well as the Torres Strait. There is a moderate risk around Rockhampton and Gladstone.

The absence of Aedes aegypti from any of our major metropolitan regions means the risk of a major outbreak is extremely low.

But there is a risk of a minor disease outbreak where the Aedes aegypti is present. Every year, there are small local outbreaks of dengue in tropical Queensland; there’s a chance we’ll see similar outbreaks of Zika.

The good news is that authorities in tropical Queensland are well-equipped to deal with outbreaks of mosquito-borne disease. They have been responding to local outbreaks of dengue for decades and are prepared for potential outbreaks of chikungunya. The response to the threat of Zika virus remains essentially the same.

The potential for human to human sexual transmission of Zika virus remains a small but not insignificant risk for all regions of Australia. As international health authorities recommend, travellers returning from countries experiencing outbreaks of Zika virus must use condoms or abstain from sex for six months.

If you’re planning to travel to a Zika-affected area, consult the Smart Traveller website for the most up-to-date information about the best ways to avoid mosquito bites.