Experts suspect dengue mosquito is resistant to pesticides

With the number of confirmed dengue fever cases in north Queensland now thought to be in excess of 200, experts are concerned that the mosquitoes responsible for spreading the disease may have developed a resistance to pesticides.

Dr. Scott Ritchie from the Tropical Health Unit in Cairns says alternative pesticides to address is the state's worst dengue outbreak since before World War II, are now being explored, but concern remains that some residents in the affected areas still do not recognise the public health risk associated with neglecting potential mosquito breeding grounds on their properties.

According to Dr. Ritchie dengue mosquitoes are very clever at finding water in a wide variety of containers, are secretive, indoor, daytime biters, which rest in dark areas under furniture and in dark corners, and even one or two mosquitoes in a house are enough to maintain the transmission cycle.

Dr. Ritchie says to date, 13 suburbs in Cairns have now been affected and as many as 190 dengue fever cases have been confirmed - in Townsville 20 people have been infected.

A study published in the current issue of the British Ecological Society's journal Functional Ecology raises fears as it says that the revival in popularity of rainwater tanks in Australian back yards could encourage the dengue mosquito to areas as far south as Sydney.

Dr. Michael Kearney of the University of Melbourne, says due to climate change, the backyard rainwater tank is making a come-back and rainwater tanks and smaller storage tubs such as modified wheelie bins are potential breeding sites for the Aedes aegypti mosquito which carries the disease.

Dr. Kearney says in the late 19th century, infected water barrels introduced the spread of mosquitoes from Queensland to Sydney and west to Perth but by the late 1960s it had been restricted to the northern half of Queensland partly due to the removal of old galvanised rainwater tanks, the installation of piped water, insecticides and new lawnmowing equipment that helped keep yards tidy and he warns the dengue mozzie could spread again in the same way.

Dr. Kearney says a major impact of climate change is reduced rainfall and that results in a dramatic increase in domestic rainwater storage and other forms of water hoarding and unless care is taken with water storage hygiene, the mosquito's current range could dramatically re-expand.

In Cairns, the hotspot of the current dengue outbreak, health inspectors and teams of trained staff are targeting suspect areas in attempts to curb the spread of the outbreak, checking on potential breeding sites, spraying yards and the interiors of homes and setting dengue mosquito traps.

As the dengue mosquito favours the colours red and black, the traps are usually black containers two-thirds full of water with a strip of red fabric permeated with an insecticide which are placed in dark areas attractive to dengue mosquitoes.

There are few commercially available traps and those which are successful target the female dengue mosquito using spectrum light and sound to attract them.