While U.S. health officials battle West Nile virus, they also are on the lookout for another mosquito-borne disease — not yet in this country — that could be deadlier to humans and livestock.
Known as Rift Valley fever, or RVF, the disorder was first reported among livestock in Kenya early last century. It is the only condition at the top of both human health and agricultural lists of dangerous diseases kept by the U.S. Public Health Service and the U.S. Department of Agriculture.
RVF holds that ominous position even though no new cases of the illness have been reported anywhere in the world since 2001.
“This is not a disease that occurs here now, but we want to make sure people are aware of its signs and symptoms. The medical and public health community need to be mindful of it,” said Dr. Thomas Ksiazek, chief of the special pathogens branch of the federal Centers for Disease Control and Prevention.
Most sufferers have flulike symptoms and some may develop more serious problems, such as eye, liver or kidney disease, he said. In the 1930s, RVF killed tens of thousands of sheep and also spread to humans. In 2000, it killed about 100 people and sickened another 800 in Saudi Arabia and Yemen.
RVF is most commonly associated with mosquito-borne epidemics during years of unusually heavy rainfall.
Dr. Ksiazek recalls how “another explosive disease,” West Nile virus, appeared in the United States for the first time in New York in 1999, where it killed countless crows and other birds. The virus then spread rapidly down the East Coast before moving westward.
Sixty-two human cases of West Nile and seven human deaths were reported in the United States in 1999. Last year, 9,858 human cases and 262 deaths were reported.
Dr. Ksiazek said RVF, like West Nile, is primarily a disease of livestock. “People who get infected are secondary” victims, he said.
Elmer Gray, public health extension specialist at the University of Georgia in Athens, said “Rift Valley is more virulent than West Nile virus. So it would be more serious than West Nile if it were to arrive in the States.”
He said it is viewed as the most realistic threat to this country of any of the mosquito-borne diseases still outside the United States.
The reason? Rift Valley kills up to 30 percent of livestock it infects. It threatens both cattle and sheep, which are not affected by West Nile virus, Mr. Gray said. Goats also are susceptible.
Rift Valley is potentially more fatal to humans than West Nile. Its overall mortality rate is 1 percent, the CDC said. By comparison, one in 150 people will develop an illness after being infected with West Nile, Mr. Gray said.
Particularly problematic, health professionals say, is that as many as 30 species of mosquitoes can carry the virus that causes RVF to spread from cattle, sheep and goats to humans.
In contrast, five or six species of mosquitoes can transmit West Nile, Mr. Gray said.
Mr. Gray said funding for mosquito surveillance has increased significantly as a result of West Nile.
“And that surveillance is picking up most of the other viruses carried by mosquitoes. So the infrastructure is out there. And that’s important, since there will be other mosquito-borne diseases that will be introduced in this nation, be it RVF or something else.”