OKLAHOMA CITY (AP) _ The West Nile virus has returned to the state, and the Oklahoma State Department of Health is bracing itself for the unknown.

``Last year we had 22 cases,'' said Dr. Kristy Bradley, interim state epidemiologist. ``Is that going to be our typical number of cases we're going to have each season, or are we going to have a year when we have several hundreds or thousands of cases?''

Because the virus is relatively new to the United States, it is too early to project how significant of a health threat it will be this season, or in seasons to come, Bradley said.

``That's why we continue to maintain a high level of surveillance of this virus,'' she said.

The health department confirmed Thursday that two horses in southwestern Oklahoma tested positive for West Nile, but has not confirmed any human cases this year, Bradley said.

The bites of infected mosquitoes transmit the virus, which affects the nervous system and can result in symptoms including fever, muscle weakness, dizziness and extreme headaches and tiredness. Adults over the age of 50 are at the greatest risk of developing encephalitis, or swelling of the brain, which can be fatal.

The virus was first reported in the United States in New York City in 1999, and has spread across the entire country in five years. Since the virus came to Oklahoma in 2002, 122 people have been diagnosed with the virus and five have died, Bradley said. The fatalities have all been adults over the age of 60, she said.

This year there have been 13 reports of the West Nile virus in humans across the nation in Indiana, California, Arizona, New Mexico, Texas, South Dakota, Kansas and Missouri, according to the Centers for Disease Control and Prevention Web site. One person has died this year.

The virus has also left some people with disabilities, such as paralysis, persistent pain and chronic headaches, Bradley said.

However, not everyone who contracts the disease is seriously affected, Bradley said.

About 80 percent of the people who contract the virus do not suffer any symptoms at all, and up to 20 percent of those who contract the disease experience only mild symptoms that go away within days or weeks, according to the Centers for Disease Control and Prevention. About one in 150 people who get the virus will develop severe illness.

Bradley said that doesn't mean people should become complacent about the virus because the disease is potentially fatal and has no treatment.

``This is a disease that you do not want to get,'' she said.

To protect against the virus, Bradley said people should avoid outdoor activities at dusk and dawn, wear long pants and sleeves, use insect repellent and drain items with stagnant water, which is where immature mosquitoes live until they become adults.

The health department has a public education campaign to inform the public about the risks of West Nile, and is working with local communities to aid in mosquito control interventions.

In past years, the health department tracked the virus by conducting tests on dead birds _ a very visible sign of the virus, Bradley said.

The tests helped to identify what the peak months of West Nile are _ July through October _ and which areas of the state are at the highest risk. The dead bird testing was discontinued this year because of funding issues, but the department is continuing surveillance of the virus.

Oklahoma State University's Department of Entomology and Plant Pathology received a $15,000 grant from the health department to trap and test mosquitoes for the virus, said Lisa Coburn, senior agriculturist.

Coburn, who has tested mosquitoes for West Nile for three years, said last year the trapping occurred in 10 different Oklahoma counties. The trapping is limited to Stillwater this year due to a shortage of finances, she said.