Every wet season Northern Australia faces the possibility of a surge in mosquito-borne virus infections. But the actual numbers of cases vary considerably. Some of that variation can be blamed on fluctuations in rainfall, the wetter years being worse for specific arboviruses like Kunjin virus and Murray Valley Encephalitis virus.
You might think Murray Valley Encephalitis should be in the border territory between Victoria and New South Wales, where most of the early cases were described. But in recent years, MVEV has relocated in the Northwest where conditions appear to favour its mosquito vector and reservoir hosts more than the Murray Valley.
It is fortunate that MVEV only rarely causes encephalitis, since long term complications of nervous system infection often follow this extreme form of the disease. Milder forms of infection are more common, judging from seroepidemiology studies. In the states where chicken flocks are monitored routinely, evidence of MVEV exposure usually precedes the first cases of human infection.
This year, there have already been several cases of encephalitis and many more cases of milder disease. While most of these have occurred in the Northwest, there have been a scattering of MVEV infections in the Southeast. In Western Australia the earliest cases occurred a bit later than usual despite heavy rainfall early in the wet season. Satellite sensing is now being used to predict MVEV risk. However there are likely to be other drivers of infection risk, most likely affecting the complex interactions between virus, vector and reservoir host.
[Notes by the MicroGnome, subject matter expertise courtesy DW Smith, Division of Microbiology & Infectious Diseases, PathWest, Nedlands, WA, 21-APR-11.]