SALT LAKE CITY — State health officials have been tracking West Nile virus since the first human case was detected in Utah in 2003.

The Utah Department of Health confirmed Thursday that two human cases have been reported in Salt Lake County, and one horse is infected in Carbon County.

The mosquito-borne illness is transmitted by the noxious bugs who have bit an infected bird. In addition to humans, the virus can be found in wild birds, horses and sentinel chickens — at least those are the species the state typically follows.

At least 67 infected mosquito populations have been found throughout the state — in Salt Lake, Davis, Utah, Box Elder, Cache, Duchesne, Millard, Uintah and Weber counties, according to the health department.

The agency asks residents to protect themselves from mosquito bites whenever spending time outdoors. The infection can be deadly for horses, but a vaccine exists to protect them from illness.

Not all mosquitoes carry the virus, but those that do tend to be out from dusk to dawn, according to health department epidemiologist Keegan McCaffrey.

"There is no vaccine for humans, so taking simple precautions to avoid mosquito bites is the key to reducing your risk for infection," he said. McCaffrey encourages use of mosquito repellents containing DEET, picaridin or oil of lemon eucalyptus. He said repellents with up 30 percent DEET are safe for anyone older than 2 months.

He also said people should wear long-sleeved shirts and pants while outdoors.

Both cases of West Nile virus contracted in Utah were people age 40 to 64 and both developed a fever, but they steered clear of more serious and neuroinvasive symptoms of meningitis and encephalitis, according to health department statistics.

Since West Nile virus was first detected in Utah, 2006 has been the most active season, resulting in five human deaths. Last year, the illness infected 13 people and seven horses, and one person over age 65 died from the symptoms.

Nationally, more than 1,600 people were infected with the West Nile virus last year, resulting in 84 deaths, according to the United States Centers for Disease Control and Prevention.

While most people infected by the virus won't notice any symptoms, they can include flu-like symptoms such as fever and body aches, or worse, according to the health department. Symptoms can develop within three and 15 days after a bite from an infected mosquito, and sickness can last a few days to weeks.

More severe cases include high fever, headache and stiff neck, disorientation and confusion. People at highest risk of serious infection include those age 50 and older, and people with compromised immune systems.

Though protection from infection is important for everyone, said state health educator Becky Ward.

"While many people who get West Nile virus don't have neuroinvasive disease, it can occur and can result in death," she said.

The department will continue virus surveillance into the fall. For more information, visit health.utah.gov/wnv.

While mosquito abatement districts have been busy trying to manage mosquito populations, homeowners are asked to help.

Mosquitoes breed in standing water, so removing puddles or buckets, even birdbaths and plant containers, where water can pool, is imperative to control populations of the unpleasant insects.