Health officials detected West Nile virus infections in birds in three counties where the disease had not previously been found this year.

The Maine Center for Disease Control and Prevention announced Monday that four birds in Piscataquis, Sagadahoc and Washington counties had tested positive for the disease. An alpaca in Lincoln County also tested positive, following a case there in a bird earlier this summer.

Mosquito-borne illnesses have been found in 13 of Maine’s 16 counties.

Mosquito pools in Waldo County tested positive for West Nile virus, and pools in Penobscot County tested positive for eastern equine encephalitis – another mosquito-borne illness, the Maine CDC said. Mosquito pools are groups of up to 50 mosquitos collected for routine testing. Monday’s announcement marked the first time either disease was found in a pool this year.

Mosquito-borne diseases have now been found in 13 of Maine’s 16 counties, the Maine CDC said.

Last week, a Mainer with no history of travel tested positive for West Nile virus. Officials said they believe that person acquired the disease locally.

“With 13 out of our 16 counties showing the presence of mosquito-borne disease activity, Maine people and visitors should be extra vigilant about taking protections whenever you go outside,” Puthiery Va, director of the Maine CDC, said in a statement. “We urge you to take simple steps to prevent mosquito bites, wherever you are in Maine, especially during this high mosquito-activity, high-risk time.”

Many people who contract West Nile virus are asymptomatic or have fever and flu-like symptoms, the Maine CDC said. In rare cases, the disease can be fatal.

The Maine CDC recommends individuals, especially those in high-risk regions, cover their skin when outdoors and use insect repellent. Residents also can avoid being outdoors during dawn and dusk, when mosquitos are most active, the agency said.