Steadily spreading westward across the United States, the mosquito-borne West Nile virus has already claimed at least 14 lives, infected almost 300 people and has been detected in all but seven of the contiguous 48 states.
The year's nationwide death toll could rise to 16 if the Centers for Disease Control and Prevention confirm two deaths from the virus reported Thursday in Georgia.
The victims were a 51-year-old Atlanta man and a 77-year-old man from Columbus, but state officials did not say when they died.
At least 33 people have died since the virus appeared in the United States in 1999. There is no cure for humans.
The Food and Drug Administration approved the first national trial of a drug this week, clearing the way for James Rahal of the New York Hospital Queens to see whether interferon can lessen the symptoms and duration of the illness in infected patients.
Transmitted to humans by mosquito bites, West Nile virus can cause fever, body aches, brain swelling, coma, paralysis or death.
Most of this year's deaths and the 296 reported human cases have occurred in the South. Louisiana alone accounts for 147 illnesses and eight fatalities.
Dr. Lyle Petersen, a CDC expert on the virus, said it probably will always be worse in the South because warm weather means more mosquitoes.
"But outbreaks could eventually happen anywhere," he added.
Montana health officials on Thursday confirmed finding West Nile in a horse. New Mexico officials said two horses there were infected and birds were being tested.
"I'm sure if we keep looking hard enough we'll find some more," said Thurman Reitz, assistant New Mexico state veterinarian. "I don't have any reason to think it's going to quit at the Texas border."
Arid New Mexico is not as mosquito-prone as some other states, and especially not in this period of drought, but the insects are around, state officials said.
The virus is most dangerous for children, the elderly and people with weak immune systems. It can cause flu-like symptoms and encephalitis, a potentially fatal brain infection. Most people bitten by an infected bug never get sick.
West Nile is common in Africa and the Middle East. Since 1999, when the virus reached New York, mosquitoes have spread the virus south and west.
"We fully expect that over time the virus will make it to the West. But the timing of that is unknown," Petersen said. "Whether it's this year or next year or the year after that is just a matter of conjecture."