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What Caused the Yosemite Hantavirus Outbreak?

With the Yosemite tourists having died from this normally high-altitude illness, we spoke with an infectious disease expert to see if hantavirus could spread to other areas of the country
deer mouse hantavirus outbreak



Wikimedia Commons/James Gathany

Earlier this week the World Health Organization issued a global warning to travelers who might have come into contact with the deadly hantavirus while staying in Yosemite National Park in California. So far, three* people have died and at least half a dozen more have been sickened by the virus, which causes pulmonary distress. The outbreak is unusual in that cases of the virus are usually solitary and most have come from the high desert areas in or near New Mexico.

Most victims likely picked up the infection while staying in insulated tent cabins that were infested with mice, which carry the disease. The U.S. National Park Service estimates that as many as 10,000 people might be at risk of having been exposed to the disease since early June. And it has urged anyone who stayed in the "Signature Tent Cabins" in Yosemite’s Curry Village to be vigilant about looking for flulike symptoms, which can signal the onset of the illness. The virus can take as long as six weeks to incubate.

The infection was initially described in New Mexico about 20 years ago. It is exceedingly rare, with only about 600 documented cases in the U.S. But it is extremely deadly, killing more than one in three people who contract it.

The deer mice (Peromyscus) and other rodents that have been known to carry the disease, however, are by no means limited to high desert or mountainous areas and can be found throughout North America. So does that mean hantavirus could spread to other parts of the country?

To find out, we spoke with Charles Chiu, director of the Abbott Viral Diagnostics and Discovery Center and an associate professor of medicine and infectious diseases at the University of California, San Francisco. He explains what we know about hantavirus and what he and his colleagues are rushing to figure out in light of this latest outbreak.

[An edited transcript of the interview follows.]

What, exactly, is hantavirus?
Hantavirus is a very rare virus that's spread through contact with rodents. In the western U.S., it causes hantavirus pulmonary syndrome, a devastating pneumonia. There is also another disease associated with hantavirus, which is rarely found in the U.S. but is more often found in Central and South America: hantavirus hemorrhagic fever with renal syndrome.

How is the respiratory form of hantavirus spread?
It is acquired through contact with rodents, most commonly by inhaling dust that's contaminated with rodent droppings or urine. Less often, it can occur from a bite from a rodent. The main rodent that spreads this is the deer mouse.

Was hantavirus likely around before its formal description in the early 1990s?
The evolutionary data suggest that it's been around for hundreds or thousands of years. This is not a common infection, and probably a large factor in this is that there are no documented cases of human-to-human infection, so it's not contagious.

Hantavirus is usually thought of as an illness from desert areas of the Southwest, like New Mexico. Is this the first time it has sickened people in another region?
It was originally described in the Four Corners region of New Mexico in the high desert. But more than half of the states have reported cases. The cases have all been associated with wild rodents, which have been detected in every state.

The cases follow regions of the country where you're most likely to be exposed to wild rodents. Most cases are in hikers, campers and people in rural areas. There are some areas where deer mice are not found, such as Florida and some of the other states along the eastern coast, but the risk distribution map broadly covers most of North America, including areas of Canada and Mexico.

Does this mean that we should be wary of Hantavirus elsewhere in the country?
It clearly can pop up in other regions of the U.S. There have been cases in the Great Smoky Mountains and in the Sierra Nevadas. The mouse favors higher elevations. And the deer mouse prefers cool, moist forests.

How does the virus get spread to these other areas? Does it have to spread through mouse populations?
We're still figuring out how it's transmitted from mouse to mouse. The mice are reservoirs, so they can harbor the virus without any symptoms. Up to 20 percent of deer mice have evidence of hantavirus infection. In Yosemite, that level is estimated to be 18 percent.

So should people be looking out for hantavirus across the U.S.?
As opposed to watching out for hantavirus, I think it's important for people to avoid contact with rodents, which can spread other diseases, such as plague. For people who live in cabins or houses with a rodent issue, it's important that they deal with the infestation. And upon entering unused cabins, don't sweep or vacuum the dust, which can lead to aerosolization and inhalation of the dust.

This virus has such a high fatality rate—would it be lower if people were more vigilant and sought treatment for symptoms earlier?
The infection itself is quite virulent. It's not simply a matter of detecting it early. There is no vaccine or cure. The treatment we have is primarily supportive. It has been shown that patients who presentearlier in the illness tend to do better. But even patients who present early—their mortality rates are still very high.

We don't know much about how this virus causes disease. Part of the reason is that there have been so few cases. It's clear that this is a very deadly virus—especially because people getting it have been young, healthy adults. It is interesting that infections in elderly and children are not as common. Part of it might be that children and the elderly don't get the same exposure that adults do; healthy adults are the ones going out hiking and camping and doing the cleaning. We do know that when healthy children get it, the mortality rates are still high. So it's a very deadly virus. That being said, the virus is quite rare, and it really requires significant exposure to rodents.

Are there any treatments in the works for hantavirus?
There is a common antiviral that's been tried: ribavirin. It's unclear whether or not it's efficacious against the virus. It's my hope that better appreciation of this virus will help spark the development of drugs or vaccines. It would be possible to make one, but there hasn't really been a strong motivation to work on a vaccine because there have been so few cases.

Should campers be worried?
I don't think there's a reason to be alarmed. Unlike other outbreaks, this virus is not contagious.

It is unusual that there was a cluster here at Yosemite. It opens the question as to why there was a cluster. Maybe because all of the patients had the same exposure history, and as a result, it was probably by chance that they unfortunately became infected by the virus.

There is still a possibility that they were infected by a slightly different strain of the virus or that this virus has different qualities. I am working with a team now to sequence the genome of these new samples and compare them to historical strains to see: Has any change occurred in the genome of this virus to make this outbreak occur? Has the virus changed at all—has it evolved? We're actually finishing that up now, and we hope to have results in a few months. 



*Update (9/7/12): A third death was confirmed shortly after this article was posted; the figure was changed from two to three. 

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