As if West Nile virus (search) wasn't bad enough, now U.S. health officials are on the lookout for another mosquito-borne disease, fearing it could become a permanent part of the American landscape if it entered the country.

Rift Valley fever (search), which originated in Africa and hasn't been seen outside that continent in three years, is the only disease at the top of both human health and agriculture lists of dangerous diseases.

The virus can kill people, with a near 1 percent mortality rate, making it deadlier than West Nile. But Rift Valley poses a greater threat to cattle and sheep.

It kills up to 30 percent of the livestock it infects and if it were found in animals here, it would probably prompt livestock bans by other countries.

"This is not a disease that occurs here now, but we want to make sure people are aware of the signs and symptoms," said Dr. Thomas Ksiazek, chief of the special pathogens branch of the U.S. Centers for Disease Control and Prevention (search). "The medical and public health community need to be mindful of it."

Most people get a flu-like illness when infected. Some may develop serious symptoms, including liver or kidney disease, Ksiazek said.

About 14 percent of those seriously ill with Rift Valley fever in previous outbreaks died. West Nile kills about 10 percent of those with serious complications, such as brain inflammation.

The virus is worrisome because at least 30 species of mosquitoes are capable of carrying it from cattle or sheep to humans, far more than the kind of mosquitoes that can carry West Nile. People also can catch it by handling the blood or fluids of an infected animal.

Scientists said that Rift Valley fever was being researched as a possible weapon during the Cold War and showed promise because of its stability in an aerosol form.

Despite the concern, health and agricultural officials have been slow to prepare for Rift Valley, said Dr. Corrie Brown, a professor of veterinary pathology at the University of Georgia and a member of the U.S. Secretary of Agriculture's advisory committee for animal and poultry diseases.

"I think people weren't very worried about it until we started to think about agri-terror," Brown said.

The disease could appear here as mysteriously as West Nile, which first showed up as the culprit in the unexplained deaths of birds in New York in 1999.

The virus was first identified in a 1930 sheep outbreak in Kenya's Rift Valley in eastern Africa. For the next 70 years, it remained on the continent, emerging for the first time outside of Africa in outbreaks in Saudi Arabia and Yemen in 2000. In those cases, about 100 people died and 800 became ill.

Luckily, the virus seems to have disappeared, and there have been no new cases outside of Africa since 2001.

Rift Valley fever is one of several emerging viruses being studied by the University of Texas Medical Branch in Galveston, Texas. One focus is better vaccines, since there is no approved vaccine for people or livestock in case of an outbreak, Ksiazek said. The military developed a vaccine that has been approved for testing in people.

"It would really be hard to control this without a vaccine," said Dr. C.J. Peters, director of the biodefense center at UTMB in Galveston.