Q&A with the CDC's hantavirus expert

The fear mongers strike; what's the real danger?
Are the mice in my house dangerous?
Should we blame ourselves for killing all the predators?
Are we safe in Texas and Oklahoma?
Does hantavirus like the frozen northland?
Are pet store mice a concern?

The fear mongers strike; what's the real danger?
Q: I am researching a story on a possible Hantavirus case in Whistler, B.C. Canada. On July 10, 1995 a 48-year-old West Vancouver teacher died, apparently after handling a dead racoon in his cabin. The news spread through the valley faster than the virus itself, even though the cause of death has yet to be released.

Now local exterminators are advertising pest control, using "mice" and "virus" in the ads. Whistler is lousy with deer mice; they are everywhere. What precautions do you recommend for hikers, and for that matter, people whose homes are full of deer mice? Should the local medical clinic be prepared? As a four-season resort, there are over 1 million visitors to Whistler every summer. Are the odds high a case could pop up here again?

Kevin Damaskie
Senior editor, Pique Newsmagazine

Dr. Mills: Thanks for your query. This first question touches on some important basic aspects of hantavirus risk reduction, so I'll give some background information and answer in some detail. I hope not too much!

First, it is unlikely that a person could develop hantavirus pulmonary syndrome (HPS) from exposure to a raccoon. We have found no evidence of hantavirus infection in this species. However, even if they were infected, it is unlikely that raccoons would be an important source of infection for humans. All known hantaviruses cause a chronic infection in one or a few species of rodent hosts with which they have coevolved for many years. In the U.S., only the deer mouse, the white-footed mouse, the cotton rat and, perhaps, the rice rat are known to carry hantaviruses which may cause HPS in humans.

In spite of the national attention it has received, HPS is an extremely rare disease. Human infection results from exposure to infected rodents or their urine, saliva, or droppings. The primary method of transmission is probably via inhalation of virus particles suspended in the air, but the disease also may be acquired by direct contact with broken skin or mucous membranes, or by the bite of an infected rodent. Hikers, and others who take part in activities might be more likely to be exposed to rodents and their contaminated excretions and secretions. Hikers should avoid activities which would bring them into contact with wild rodents:

• Open up abandoned or unused cabins and let them air out for an hour or more before occupying them; if the cabin has signs of rodent infestation, don't use it, and, especially, don't create aerosols by sweeping the cabin.
• If you are sleeping outdoors, pick a campsite that does not have evidence of rodent burrows, nests, or droppings; don't sleep near woodpiles or garbage areas which might attract rodents; don't sleep directly on the ground; use, at least, a groundsheet and air mattress or foam pad.
• Don't disturb rodents, their burrows, or nests.
• Store your food in rodent-proof containers; keep your campsite clean; don't leave food or trash out to attract rodents.
• The CDC has published a pamphlet on hantavirus risk reduction directed at hikers and campers. The pamphlet, and additional information, is available by calling (404) 639-1510.
• Risk of exposure in the home can be decreased by taking three important steps:
• Seal up the home to keep mice out: repair screens, holes, cracks, or any opening as small as a dime; use caulking, metal screen (1/4 inch mesh), or steel wool.
• Remove mice already in the home by setting snap traps. Place the trap on newspaper; when a mouse is captured, put on rubber gloves, soak the mouse and trap with disinfectant (like Lysol or 10% household bleach); roll up the trap and mouse in the newspaper and place it in a double plastic bag; place the bag in the trash; wash gloved hands in soap and water, then remove the gloves and wash your hands in soap and water.
• Clean up any nests or droppings found in the home by putting on rubber gloves and soaking the material with disinfectant; then pick up the soaked material with paper towels, place the material and towels in a double plastic bag and place it in the trash. Wash gloved hands and then wash bare hands in soap and water.

Greater detail concerning "rodent proofing" and cleanup procedures can be found in the CDC publication "Hantavirus Infection -- Southwestern United States: Interim Recommendations for Risk Reduction," available by calling (404) 639-1510. Additional general information on hantaviruses and HPS can be obtained from your local or state health department, or by calling the CDC hantavirus information line at (800) 532-9929.

Physicians at medical clinics throughout North America should already be prepared and alert to the symptoms of HPS. The chances of campers, hikers, or homeowners acquiring the disease are very low under normal circumstances, and they can be decreased even more by following these simple precautions.

Eek! We have mice in our house
Q: We live in a log cabin in Piedmont, N.C., and despite the seven cats, we do have mice. We keep the house about as clean as most people keep houses. Are we at risk of contracting the hantavirus?

Allen Weed

Dr. Mills: Allen, you are not alone. Most of us do have mice in our homes from time to time. I listed some detailed recommendations for "rodent proofing" and trapping in the previous answer. With the seven cats, you have another factor to consider -- I'll bet they go hunting and bring you presents from time to time! When one of your cats shows up with a dead mouse at the doorstep, you should don your rubber gloves, soak the mouse in disinfectant, place it in a double plastic bag and put it out in the trash. Wash your gloved hands with soap and water; then remove the gloves and wash your hands in soap and water.

Cats who come into contact with infected mice may become infected themselves. Five percent of the domestic cats we have tested at CDC have antibody to hantavirus. This does not mean that you can become infected from your cat. Studies with hantaviruses and cats in Europe have shown that, unlike rodents, cats do not become chronically infected and shed virus in secretions and excretions. They appear to develop antibody and quickly clear the infection.

Should we blame ourselves for killing the predators?
Q: Is it a fair assumption that the recent prevalance of hantavirus could be caused, at least in part, by the reduction of rodent predators, such as coyotes and wolves, and that we are just now seeing the effects after all of these years of "predator control?" If so, could this be documented and possibly extrapolated into a bigger picture of the alteration of ecosystems and the effects on human health? What is CDC doing about this approach?

Sinjin Eberle

Dr. Mills: Sinjin, first, I noticed you used the word "recent" when referring to hantavirus disease. Certainly, it has only recently been discovered in North America. Yet we have confirmed cases in the U.S. back as far as 1959, and almost certainly, there were outbreaks which were never identified. The Navajos have ancient legends which associate increases in rodent densities with episodes of disease. The Russians were studying what was probably hantaviral disease as early as 1913, and a Chinese medical text refers to a disease which might well have been hantaviral in 960 A.D.!

On a local scale, predator control practices may influence the population dynamics of some rodent species. Nevertheless many rodent species are subject to natural periodic fluctuations in spite of the presence of healthy predator populations. Predators may have a damping effect, but I believe that climatic conditions and food supply have a larger effect on rodent populations. On the Argentine pampas a common mouse carries a virus that causes Argentine hemorrhagic fever. After particularly favorable weather conditions, I watched rodent populations on the pampas reach record levels. Sure enough, that year, there were more cases of hemorrhagic fever than there had been for 20 years. In the American Southwest, a long drought ended in the spring of 1992 with abundant rainfall. Robert Parmenter and colleagues at the University of New Mexico showed that rodent populations increased tenfold during the one-year period ending in May 1993. This resulted in greatly increased contact between deer mice and humans in the southwestern U.S. This, in turn, led to the cluster of HPS cases in the Four Corners area which brought the disease to our attention.

The CDC has initiated long-term studies of rodent populations in the southwestern U. S. From these studies we hope to learn what specific factors are associated with changes in rodent population densities, changes in numbers of infected rodents, and changes in incidence of human disease. These studies will take many years, but it is the only way to answer questions like yours. We're working on it!

Are we safe in Texas and Oklahoma?
Q: I hike and camp alot in the state parks in Texas and Oklahama. Is there anyway to protect yourself from the virus?

Dr. Mills: Robert, actually, there is a very low prevalence of human hantaviral disease in Texas and Oklahoma. We are not sure why; there are infected rodents there, too. Nevertheless avoid contact with rodents and their excretions/secretions by following the precautions which I outlined in the first question. I love hiking and backpacking. Knowing about hantavirus hasn't made me enjoy it any less -- I just follow those common- sense precautions and keep having a good time.

Does the virus like the frozen northland?
Q: My question concerns the spread of the hantavirus into Canada. Of course a disease like that knows no boundaries as indicated by its appearance in the U.S. But due to lower average temperatures to the north, would it be as large a threat up here. What is the furthest north location of a documented occurance of the disease (e.g. any reported cases in Alaska?)

Greg Unrau

Dr. Mills: Greg, good question. Judging from the data currently available, there does seem to be a pattern of decreased disease in the far north. Even though the range of the deer mouse extends into central Alaska, we do not have any cases from Alaska. Furthermore, we have checked a large sample of deer mice from that state and have found no evidence of infection with hantavirus.

The explanation is unclear. It may have to do with climate or population densities. It may be that the "subspecies" of deer mouse in Alaska doesn't support the infection. There have been about seven cases from southern Canada but I don't know the specific localities (You would have to contact Canadian authorities for that information).

Our studies in the southwestern U. S. have shown that the prevalence of infection in deer mice is lowest at the extremes of altitude and climate (e.g. alpine tundra and desert ). These are also areas where the population density of deer mice is lowest. That means that deer mice would be less likely to come into contact with one another and pass on the infection.

We hope that our long-term studies of rodent populations in the southwestern U. S. and our continuing national surveillance program will help provide more definitive answers to these complex questions.

Are pet store mice a concern?
Q: Should we be concerned about pet store rats and mice carrying the virus?

Dr. Mills:We have no evidence that pet store rats and mice carry the virus which causes hantavirus pulmonary syndrome. We have tested hundreds of wild rats and house mice captured in the wild (the same species as the white mice and rats commonly sold in pet stores) and have found the prevalence of infection in these species to be extremely low. So even in the wild, these species are unlikely to be important in the epidemiology of HPS.