Another nasty mosquito-borne disease brought by the Indo-Papuan conveyor. S Richie, Cairns, QLD
[notes on a lecture at the Centenary of tropical medicine meeting, Townsville, QLD, 12-13th June, 2010]
In 1995 cases of fatal encephalitis appeared for the first time in the Torres Strait. These were initially thought to be Murray Valley Encephalitis, but on further investigation turned out to be Japanese encephalitis. Conditions in the Torres Strait Islands (TSI) were suitable for JE transmission due to the presence of wading birds, pigs and receptive mosquito vectors, notably Culex annulirostris. There was concern that the combination of herons, egrets and feral pigs might lead to the spread of JE south through much of the Eastern Australia.
By April 2005 Aedes albopictus was present in the TSI and both Dengue and Chikungunya viruses could be detected. Some of the movement of this mosquito vector was blamed on the movement of heavy mining equipment around the region. Expansion of its geographic distribution now included areas as far afield as SE USA, S Europe and the W Pacific. As a potential Dengue virus vector there was concern that it might spread into Eastern Australia. Its ability to breed easily in small quantities of still water included in the TSI beached boats and discarded coconut shells.
Much of the fauna of the regional has been divided into distinct biogeographic regions separated by the Wallace line (named after the naturalist and co-discoverer of natural selection, Alfred Russell Wallace). This neat separation of fauna and flora has been blurred recently by anthropogenic factors such as transmigration. There have also been climate-mediated effects such as the Indo-Pacific region drought caused by a pronounced El niño effect in 1997/8. Modelling of the movement of an air packet during Cyclone Sid showed the plausibility of propulsion of Cu annulirostris from Papua New Guinea to the northern Cape York Peninsula during a cyclone.
Further mosquito breeding opportunities have been created by the well-intentioned installation of rain water tanks in the TSI by aid agencies. The legal entitlement to permit traditional owners’ family visits adds another means of assisting arbovirus migration. So a cordon sanitaire was established against Aedes albopictus. Other Culicine species have been observed on the Australian mainland, including Culex gelidus close to Brisbane airport. This species is a potential vector of JE in SE Asia.
The mechanism of arbovirus movement from SE Asia to Australia is less a conduit and more a conveyor belt. The question is which arbovirus will be next? Could it be Chikungunya virus? And what impact will the global movement of the human population have on this process?