Health authorities are concerned there will be a big increase in the number of people contracting mosquito-borne viruses in Western Australia this summer.

The Health Department has already detected mosquitos carrying Ross River and Barmah Forest viruses around the Perth and South West regions.

The department's senior scientific officer Andrew Jardine said current conditions were conducive to mosquito breeding.

"We had an El Nino year last year, which generally means you get those drier conditions and less tidal activity," he said.

"Now we've moved towards the La Nina end of the spectrum and that's exactly what we're seeing — rainfall that's still occurring in mid to late November and increased tidal activity.

"The virus is out there and it's active so people really need to be taking care to avoid being bitten my mosquitos."

Mr Jardine said the 2011–2012 summer in particular was rife with Ross River cases and he is warning this summer could be similar.

He said he expected mosquito numbers to drop off as the weather warms up.

"But tides can still breed mosquitos all throughout summer in the coastal regions, they're not going to go away entirely," he said.

The department said there was no vaccine or specific treatment for both Ross River and Barmah Forest.

Symptoms included painful or swollen joints, sore muscles, skin rashes, fever, fatigue and headaches.

Virus spike expected within weeks

Authorities urging people to make sure they cover up and protect themselves from mosquito bites.

Mr Jardine said there were no statistics available at this point for the number of people that have contracted either Ross River or Barmah Forest so far this year.

"We expect the case numbers will start increasing in the next few weeks but it hasn't actually occurred yet," he said.

"That's why we do this testing of the mosquitos because it gives us a quicker heads up to know there is virus activity out there and there is a risk present."

Authorities recommend people take the following precautions: