Outbreaks of a serious mosquito-borne disease have exploded in WA this year, with five times more people contracting Ross River virus than three years ago.
The virus, which can leave victims with a lifetime of debilitating symptoms and side-effects, infected 1570 people across the State in 2011-12.
Public health officials in Mandurah branded the 2011-12 mosquito season the worst on record.
WA cases of Ross River virus reached 332 in 2009-10 and the number more than doubled to 770 in 2010-11.
Department of Health entomologist Peter Neville said there had been more mosquitoes over the past two years.
"It's largely to do with weather events," he said. "Over the last two years we have been under La Nina weather conditions."
Those conditions meant more rain and higher minimum temperatures, leading to more mosquitoes. He said there was a spike in Ross River virus cases every three to four years.
Infected people get a fever, headaches, rashes and painful, swollen joints.
"In some cases it can last up to 12 months," Dr Neville said. "In some people, it can be quite devastating. The virus can reduce people's capacity to work.
"It's quite debilitating."
A report to the City of Mandurah this week revealed the council has struggled against mosquitoes.
"The continuation of the La Nina weather event resulted in local weather and tide behaviour that made mosquito management very difficult due to consistent inundation of breeding sites and the frequent hatching of salt marsh mosquito larvae," environmental health officer Brendan Ingle wrote.
Ross River virus cases in the Peel region soared from 68 in 2009-10 to 206 in 2011-12.
Mandurah residents complained this week that swarms of mosquitoes make it impossible for them to go outside and warned the city's reputation was being harmed.
Mandurah mayor Paddi Creevey said the council had quadrupled the amount of insecticide sprayed to kill mosquito larvae. "What we can't control is the El Nino, La Nina effect and when those tides stay up and they inundate the breeding areas, no amount of spraying will kill them," she said.
People are urged to be especially vigilant about mosquitoes at dawn and dusk, wear long, loose clothing and apply insect repellent.