This summer, people in rural areas--especially in the western and southwestern United States--may be at higher than usual risk of contracting hantavirus, a rare but often fatal infection caused by a virus carried by deer mice.

Federal and state health officials are warning visitors to national parks and people in rural areas not to feed or handle wild rodents and to take extra precautions when cleaning up rodent-infested cabins and other buildings. In addition to the danger of hantavirus, ticks and fleas harbored by rodents can transmit Lyme disease, bubonic plague and other infections.

At least four types of hantaviruses found in the United States can cause a severe lung infection known as hantavirus pulmonary syndrome (HPS), which kills 43 percent of its victims. Although only 217 U.S. cases have been confirmed since the syndrome was discovered in 1993, early evidence suggests that 1999 may be a bad year for the disease.

From January through May, seven confirmed and 11 suspected cases of HPS have occurred in nine states, according to the federal Centers for Disease Control and Prevention (CDC). Eight of those cases came from Arizona, Colorado and New Mexico, a region that has averaged only two cases during the same five-month period in each of the past four years.

Hantaviruses frequently made news in 1993, when a mysterious epidemic of lung disease killed at least 26 people in the "Four Corners" region of the southwestern United States and CDC scientists implicated a new species of hantavirus (a group of organisms known to cause other illnesses in Asia and Europe) as the cause.

CDC researchers are still investigating what factors led to that disease outbreak, according to James N. Mills, chief of the medical ecology unit in the CDC's division of viral and rickettsial diseases. "The short, simple answer is, we don't know," Mills told members of the House subcommittee on national parks and public lands at a hearing on the risks of hantavirus last week. "We know there are local environmental conditions . . . that lead to high density of rodent populations and high rates of infection."

Some of those conditions appear to be recurring this year. Mills said deer mouse populations are up because of a rodent baby boom last year, which occurred after El Nino-related rains in 1997 led to luxuriant growth of the plants that deer mice eat. Scientific surveys this spring found evidence of high rates of hantavirus infection--35 to 45 percent--in deer mice in New Mexico and Colorado, greater than the 30 percent rates seen during the 1993 epidemic. Deer mice are found everywhere in the United States except for certain parts of the East and Southeast. The animals do not get sick when infected with hantavirus. Most of the time, infection rates in deer mice average 10 to 15 percent.

The type of hantavirus that infects deer mice, called Sin Nombre, has produced most human cases of HPS in the United States, but three other hantavirus types, each carried by a different rodent species, can also cause the illness, Mills said. Since 1993, cases of the disease have been reported from 30 states, including Virginia, West Virginia and Pennsylvania. New Mexico has had the most cases, 41, followed by Arizona with 24 and California with 22.

"Though there are more cases in the West, it's still a national issue," said Rep. Elton Gallegly (R-Calif.) at last week's hearing.

Hantavirus infection is transmitted by contact with rodent urine, feces or saliva. Most people become infected by inhaling virus particles suspended in the air, usually while they're inside a building that has been infested with deer mice. It's rare to contract the virus outdoors, and there is no evidence that it can be transmitted from person to person, Mills said.

Symptoms of illness usually begin about two weeks after someone becomes infected. At first, HPS causes headache, fatigue and muscle aches; then sufferers develop severe respiratory problems. There is no vaccine or cure, although chances of survival are better if patients are hospitalized and aggressively treated as soon as possible.

In videotaped interviews played at the hearing, survivors of HPS described their ordeal. "I went to bed for about three days and then it started in my lungs and I couldn't breathe," said a middle-aged woman identified as Gloria. "I just felt like I was being suffocated, that somebody was putting a band around my chest and putting a pillow over my face."

Last week's hearing was prompted by concerns that the National Park Service might not have done enough to warn park visitors of the possibility of encountering rodents harboring the virus. Over Memorial Day weekend, a family sailed to a remote beach on Santa Rosa Island, part of Channel Islands National Park in southern California. Researchers testing deer mice on these islands have sometimes found infection rates as high as 71 percent, among the highest rates anywhere in the United States. However, no cases of human infection have been found among Channel Islands residents, visitors or park employees.

After several hours at the beach, the mother realized that her 7-year-old son had been playing with a deer mouse, according to testimony by Maureen Finnerty, associate director of park operations and education for the Park Service. The family caught a deer mouse on the beach the next morning and it was sent for testing by the California Department of Health Services. The mouse tested positive for antibodies to hantavirus, indicating current or past infection. The boy was observed for several weeks. Although he briefly developed a fever and cough during the first week after exposure, a blood test for hantavirus antibodies was negative.

Finnerty and other officials testified at the hearing that the Park Service has intensified efforts to warn visitors about hantavirus, including trying to reach those, such as hikers and boaters, who may enter parks by less traveled routes. Finnerty said the campaign includes brochures, signs, orientation talks by rangers and information posted on the Internet.

"In comparison to other diseases which may have a higher incidence, the vigilance for hantavirus pulmonary syndrome, I think, is very high," said the CDC's Mills.

A CDC brochure on HPS advises vacationers that it's safe to travel within areas where the disease has been reported but suggests taking certain precautions. Among them:

* Before occupying unused cabins, open them up to air out. Don't use cabins if there are signs of rodent infestation.

* Before sleeping outdoors, check campsites for rodent droppings or burrows.

* Don't disturb rodents, their burrows or dens.

* Don't sleep near woodpiles or garbage areas.

* Use a mat or cot rather than sleeping on bare ground.

* Store food in rodent-proof containers; discard, bury or burn garbage.

Mills said the rodents that often inhabit city dwellings--house mice, black rats and Norway rats--don't carry hantavirus, but added that people cleaning out buildings in rural areas where deer mice live should be especially careful. Sweeping, vacuuming or dusting could stir up mouse droppings and suspend virus particles in the air.

"Let the building air out before you go in," he said. "Don't sweep, but wet-mop." Use household disinfectant or a 10 percent bleach solution to wet areas where rodent droppings may be present, he suggested, and wear rubber gloves. If you find dead rodents, wet them thoroughly with bleach or disinfectant before handling, use gloves, and dispose of them in plastic bags.

In another vacation-season health advisory, the CDC and Health Canada warned travelers to Alaska last week about an ongoing outbreak of influenza in Alaska and the Yukon Territory. A total of 388 cases, mostly influenza A, were reported from May 22 through June 21. The CDC said most infections have occurred among tourists during the land portion of combination land and sea tours, and among workers in the tourism industry. People aged 65 or older and those with chronic illnesses are at greater risk of serious complications from influenza, which is a viral respiratory infection. Antiviral drugs can shorten the duration of the illness and reduce its severity if given within the first 48 hours.

CAPTION: Hantavirus Infections in the United States (This chart was not available)