The outbreak of Zika virus has refocused the attention of health authorities on mosquito-borne disease. The virus has now been reported from almost 40 countries and the list is growing with imported cases of the disease popping up across the globe, from Norway to north Queensland.
Notwithstanding the millions of cases of disease predicted to occur this year, the ever strengthening link between Zika virus and birth defects is cause for ongoing concern.
Zika virus may be in the headlines but the burden of other mosquito-borne diseases, such as dengue fever, should not be overlooked.
Mosquito bites are the critically important route of transmission of Zika and other viruses but when it comes to spreading these pathogens, not all mosquitoes are equal. The secret is in the spit.
Why do mosquitoes suck?
Only female mosquitoes bite. Blood provides a perfect nutritional boost for egg development but who or what mosquitoes bite varies with species. Many mosquitoes opportunistically bite whatever warm blooded creature is about. That includes people. Some people more than others.
Mosquitoes aren’t “dirty syringes”. They don’t pass on pathogens through infected droplets of blood. When mosquitoes insert their mouthparts into our skin, it’s not like they’re sticking in a drinking straw. There are some tubes that suck and some tubes that spit. The spit they inject contains a mix of chemicals, some help get the blood flowing and some make their bite a little less noticeable. Unfortunately, that spit may also contain virus.
If a mosquito sucks up a virus-filled blood meal, that virus must then escape the gut and spread throughout the body of the mosquito until the salivary glands are infected. This process can take from a few days to over a week. But time isn’t all that matters.
There is no malicious intent in mosquitoes transmitting viruses. The viruses are simply taking advantage of the mosquitoes’ evolutionary initiative to exploit vertebrate blood. The viruses don’t make the mosquito sick, that wouldn’t make sense as the virus wants to make sure it makes it to the next host. Unfortunately, humans can fall ill following the injection of a mouthful of virus filled mozzie spit.
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Many different mosquitoes, many different risks
There are dozens of mosquitoes in Australia that can transmit local pathogens such as Ross River virus. Most of these mosquitoes are found in our local wetlands. But when it comes to Zika virus, there is only one mosquito that poses a threat of local transmission. That mosquito is Aedes aegypti, commonly known as the Yellow Fever mosquito, and it loves water-holding containers in suburban backyards in northern Queensland.
There is little doubt Aedes aegypti is playing the critical role in the spread of Zika virus in South America. Currently, it poses the biggest threat of an outbreak in Australia too. Not just for Zika but other viruses, including chikungunya.
The absence of Aedes aegypti from Australia’s major metropolitan regions greatly reduces the risk of a major outbreak but a risk still exists where this mosquito is present. Local authorities have been on alert following the detection of infected travellers in Townsville and Rockhampton but there have still not been any confirmed cases of local transmission in Australia.