They’re small, spindly insects but their threat never dwindles – the bites of mosquitoes threaten death and disease in many parts of the world. The emergence of a little-known virus, Zika, from an African forest, is the latest to alarm the public, politicians and health authorities because of its potential link to birth defects.
What is Zika virus?
Zika virus is a mosquito-borne virus closely related to dengue and Yellow Fever viruses. Discovered almost 70 years ago in a Ugandan forest, the virus generally only causes a mild illness. Symptoms include rash, fever, joint pain and conjunctivitis.
Severe symptoms aren’t common and the illness was never thought to be fatal.
Despite detection throughout Africa and Asia, the virus rarely entered the spotlight of scientific research. It was overshadowed by the spread and impact of dengue and chikungunya viruses, which infect millions of people across the regions.
Everything changed in 2015 when Zika virus reached the Americas.
New outbreaks and severe symptoms
Since the first local Zika virus infection, cases have been reported from at least 19 countries or territories in the Americas, with more than one million suspected cases.
Rapid spread of an emerging mosquito-borne pathogen is news enough but people are also panicked by reports of more serious consequences of Zika virus infections, including post-viral Guillain-Barré Syndrome, an autoimmune condition where there person’s nerves are attacked by their own body.
Of most concern has been the rapid rise in rates of microcephaly, a birth defect which causes babies to be born with unusually small heads, in regions where Zika virus has been circulating.
While the role of Zika virus as the cause of microcephaly has not yet been confirmed, there is growing evidence of a connection between the two where pregnant women have been infected with the virus.
Babies born with microcephaly, and those who died shortly after birth, have tested positive for the virus, and there are close regional associations between clusters of birth defects and Zika virus.
There is enough concern for the Centres for Disease Control to issue health warnings to pregnant women planning to travel to these regions. Some health authorities are even advising people to postpone pregnancies.
There is no vaccine for Zika virus. Stopping mosquito bites is the only way to prevent infection.
Is Australia at risk of a Zika virus outbreak?
Mosquito-borne viruses generally aren’t spread from person to person. Only through the bite of an infected mosquito can the virus be transmitted.
In the case of Zika, there have been some unusual cases of transmission, including through sex and the bite of an infected monkey. Despite these unusual circumstances, mosquitoes will still play the most important role in any local transmission.
While dozens of mosquitoes are capable of spreading local mosquito-borne pathogens, such as Ross River virus, only one of the 300 or so mosquitoes found in Australia can transmit Zika virus: Aedes aegypti, the Yellow Fever Mosquito, which is only found in north Queensland.
For local Aedes aegypti to spread Zika virus, they must bite an infected traveller shortly after they return from a country where the virus is circulating.
While the chances of this happening are small, there is then a risk of a local outbreak occurring as the infected mosquito bites people who’ve never left the country.
This is the process that occurs in outbreaks of dengue in Far North Queensland. If we can get outbreaks of dengue, there is no reason we cannot, or won’t, get an outbreak of Zika in the future.
How to reduce the risk of transmission
Fortunately, authorities are well placed to contain an outbreak of Zika virus, as the required strategies are the same as management of dengue outbreaks.
Perhaps the real message here for Australian authorities is that they need to work diligently to keep exotic mosquitoes out of the country.
While Aedes aegypti may not become established in southern cities, even with a changing climate, there is great potential that Aedes albopictus, better known as the Asian Tiger Mosquito, could become established in southern cities. As well as a vector of Zika virus, it can spread dengue and chikungunya viruses and be a significant nuisance-biting pest. Keeping this mosquito out of our cities is critical.
Australians planning travel to South and Central America, including the Rio Olympics, should take precautions to avoid mosquito bites. Irrespective of Zika virus, mosquito-borne dengue and chikungunya viruses have infected millions of people, causing thousands of deaths, in the last few years and are reason alone to pack mosquito repellents. Be prepared to cover up with long sleeved shorts and long pants if in regions where risk is high.