Central Queensland University

02/11/2019 | News release | Distributed by Public on 02/11/2019 04:21

Do plagues of mozzies pose an increased risk of arbovirus transmission post Townsville flood?

Do plagues of mozzies pose an increased risk of arbovirus transmission post Townsville flood?

Do plagues of mozzies pose an increased risk of arbovirus transmission post Townsville flood?

Published:11 February 2019

The mosquito is responsible for the spread of the dengue fever virus in many tropical and subtropical parts of the world.

In the wake of the recent flooding in Townsville, we asked CQUniversity's resident expert on mosquito-transmitted diseases, Professor Andrew Taylor-Robinson, for his views on the risk of virus infection brought about by increased numbers of mosquitoes.

Mosquitoes need water in which to lay their eggs. Therefore, it stands to reason that mozzies will be the inadvertent beneficiary of the flooding in and around Townsville. A population explosion of local mosquito species can be anticipated. However, while flood waters will likely bring more mosquitoes this does not necessarily immediately equate to an increased threat of mosquito-transmitted diseases in that region. It is possible, but it is not inevitable.

Indeed, the risk to an individual of contracting infection with a so-called arthropod-borne (arbo) virus is complex and multifactorial. The (ease of) availability of water into which female mosquitoes may lay eggs is just one variable, albeit an obvious and important one. It is thus fair to assume that the threat of arbovirus infection following a flood, as in north eastern Queensland, is far greater than that after a bush fire, as is concurrently occurring in Tasmania and Victoria.

The escalation in risk after a flood event, however, is not as apparent as it may at first appear. Mosquitoes very much prefer still water in which to breed, so flood waters should have ceased to flow, which may take both time and distance from the event source. It is likely that urban locations will drain more quickly than rural settings.

Arboviruses that are indigenous to Australia, such as Ross River virus and Murray Valley encephalitis virus, are maintained in the environment by passage between the mosquito vector (of which there are many species nationwide) and one or more reservoir host species. In this zoonotic transmission cycle, humans may be considered as an atypical host in which the virus does not replicate very well. Hence, levels of the virus in human blood are lower than those detected in other mammals. These preferred hosts may include native Australian wildlife such as kangaroos, wallabies and possums, as well as horses and other domesticated animals. In a non-urban setting, an emerging adult female mosquito on the hunt for its first blood meal is more likely to alight upon a marsupial or a water bird, another common reservoir of these viruses, than it is a human.

Nevertheless, in the days and weeks that the post-flood 'mopping up' process will take the explosion in mosquito numbers that is already starting to be reported will lead to more human bites than would otherwise be expected towards the end of summer. It is axiomatic that with each bite there comes the risk of viral infection, however low. The more a person is bitten, the greater the chance that one of those bites is from an infectious mosquito that will then pass on, for instance, Ross River virus, when it takes a blood feed.

Just from the perspective of being an irritant, vigilance should be adopted to reduce the mozzie threat - through personal protection and reducing potential breeding sites in close proximity to domestic settings. Fortunately, the population of Townsville is well versed in taking such anti-mosquito preventive measures while the local health authorities will be on high alert to correctly diagnose the fever-related symptoms of arboviral infection.

Most people infected with an arbovirus experience no symptoms (asymptomatic) or mild symptoms of a slight fever, headache, muscle or joint pain, and/or a skin rash, which resolve with no serious health problems. However, if symptoms persist or more serious and debilitating complications occur, expert medical advice should be sought. There are no specific treatments available for any Australian arbovirus disease; patients are standardly provided supportive care and prescribed general analgesics and anti-inflammatory agents to treat symptoms.