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

Rift Valley fever, which originated in Africa, 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. “The medical and public health community need to be mindful of it.”

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

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.