There are currently four billion people living in > 130 countries that are at significant risk of being infected with a mosquito-borne virus. Viruses such as dengue, Zika and chikungunya are responsible for the commonest human mosquito borne diseases and are transmitted between humans by the mosquitoes Aedes aegypti and Aedes albopictus, predominantly in urban environments.
Chikungunya virus is a mosquito-borne alphavirus primarily associated with acute and chronic joint pain. It also causes fever, rash and muscle ache. Chikungunya outbreaks have become increasingly common in recent years with waves of outbreaks travelling across the globe. In some Indian cities, more than half the population has been exposed to the disease whilst in the Americas, millions of cases were recorded between 2014 and 2016. In the early 2000s, a structural change in the virus appears to have allowed it to become more easily transmitted by Aedes albopictus.
Zika virus was responsible for an unprecedented epidemic in the Americas in 2016 which was declared by the WHO to be a Public Health Emergency of International Concern. Although 80% of infections are asymptomatic or lead to mild febrile illness in adults, the virus can be passed from mothers to embryos in the womb. In these neonates, the virus can cause devastating developmental and neurological effects including microcephaly and central nervous malformations. Humans are primarily infected through the bite of infected Aedes aegypti. There are currently no vaccines or cures available.
Many other mosquito borne viral diseases may only occasionally manifest in the human population. These are termed zoonoses. These viruses usually circulate within the wild vertebrate population and may be transmitted by a large number of mosquito species. In areas where these wild reservoirs are close to humans and mosquito vectors feed on both, these viruses can “spill over” into the human population and present a considerable public health threat. Two Australian examples are given below.
Ross River virus is a pathogen that circulates widely among marsupials and birds. It is transmitted to humans by mosquitoes such as Culex annulirostris and Aedes vigilax which are abundant across Australia and which commonly bite animals, birds and humans. The largest Australian Ross River fever epidemic occurred in 2015, with 9,550 reported cases. Most of these were recorded from Queensland, including metropolitan Brisbane. Symptoms include fever, headache, and pain in muscles and joints. This may be accompanied by a rash. The majority of people recover within weeks but some will experience debilitating joint pain and tiredness for many months.
Barmah Forest virus is the second most reported mosquito-borne virus in Australia. Its transmission cycles and disease symptoms are very similar to those of RRV, although disease manifestations are generally less severe than RRV. The similarity between the two viruses often results in poor clinical differentiation without laboratory confirmation.
Murray Valley encephalitis is another example of an Australian mosquito-borne zoonosis. It is maintained in a virus transmission cycle involving waterbirds and Culex annulirostris mosquitoes. The vast majority of human infections are asymptomatic, and outbreaks are rare. However when symptoms do occur, the disease has a very high case fatality rate.
Many mosquito-borne viruses are classed as “emerging pathogens” because their incidence and geographic range is increasing. This is mainly due to the increase in global trade and passenger movements which spread viraemic people and invasive mosquito vectors such as Aedes aegypti and Aedes albopictus around the world. Increasing urbanisation, climate change and the encroachment of humans in previously wild habitats with a diverse vertebrate and mosquito fauna also play a role.
There are currently no cures or preventative treatments available for most mosquito-borne viral diseases. Notable exceptions are the vaccines developed against Japanese Encephalitis and Yellow Fever, and the partially effective vaccine available for dengue. However, for the most part, the management of mosquito-borne viral diseases remains reliant on our ability to control mosquito populations and reduce the amount of contact between mosquitoes and humans. These efforts are particularly challenging for zoonotic diseases which involve many different animal and vector species, which makes it hard to deploy targeted interventions.
There is therefore a tremendous need to develop improved tools for mosquito control and surveillance, identify risky transmission pathways and environments and search for new vaccines and therapeutants. Our researchers currently work on a variety of mosquito borne viruses to better predict and control the threat that they pose to human health.