Hantavirus in Yosemite: Why It Surfaced, and How We Can Stop It

(This post was first published on TakePart.com)

Seeing the news of a hantavirus outbreak in Yosemite, I was immediately transported back to work I did nearly 20 years ago, when a “mystery illness” killed three out of four of its victims in the Four Corners region of the United States in May 1993.

I was a member of the Arizona team of the hantavirus investigation group that included public health experts from Colorado, New Mexico, Utah, and the U.S. Centers for Disease Control and Prevention. Back then the headlines did not include the name of the pathogen. It took us weeks to determine the cause: a new hantavirus we eventually called Sin Nombre Virus—that which has no name.

Now, knowing hantavirus is the cause of the illness that has so far killed two people allows for quick action, including outreach to those potentially exposed. That’s what the Yosemite folks, assisted by health professionals and public health authorities, are doing.

The current list of preventive measures for hantavirus is nearly identical to the list we created 20 years ago. Hantavirus risk reduction still depends on avoiding contact with rodents or their urine and feces, and includes simple actions to reduce that likelihood. So while some things have not changed, the ability to rapidly communicate to people who have been exposed has certainly improved in 20 years.

If this particular campground in Yosemite bears out as a common point of exposure (as suggested in the early reports), then this will be a great stroke of luck, uncovering the outbreak’s origin when the total known cases can be counted on one hand.

Acting on this possible link early in the investigation will, at a minimum, inform a few thousand people who can be vigilant for symptoms, and, ideally, prevent others from future exposure through improved practices and awareness at the campground.

A bigger challenge is getting the word out to healthcare providers who may be seeing single cases of disease elsewhere in the United States or in other countries where travelers to this campground may have returned.

For epidemiologists, a cluster of illness or common exposure to a known pathogen is of much higher concern when the disease is able to spread from human to human. Thankfully, hantavirus has never spread from human to human, so this gives a little breathing room in tracking down those who might have been exposed. Since we know the source of hantavirus is the deer mouse, a cluster of illness from a common source like what we’re likely seeing in Yosemite begs the questions: why there and why now?

In 1993, months of substantial rainfall in the Southwestern part of the United States led to increased local vegetation that produced pinion nuts—a food source for the local mice. Increased mice meant more mice droppings that filled cabins closed for the winter, empty seasonal camp grounds, and houseboats left in storage.

Normal activities like sweeping out a cabin may have aerosolized the invisible microbes from the mice urine and feces on the floor and led to human illness and death. Epidemiologists will no doubt be looking at similar factors to try to understand the Yosemite outbreak.

Zoonotic diseases are those spread from animals to humans. Hantavirus, though rare, is just one of many infections that can be transmitted from rodents. Another zoonitic disease, West Nile virus, is currently in the headlines as well. Introduced to the United States in 1999 (many speculate it came via mosquitoes in stagnant water in wheel wells of aircraft or as a “passenger” inside the cabin), it is now endemic throughout the country. We can expect periodic disease outbreaks when vectors like mosquitoes are part of our ecosystem—just like the mice that carry hantavirus.

Insects, rodents, bats, birds, and animals in general each carry disease risks. Sometimes these manifest in ways that give us warning. We can get signals like bird deaths from West Nile virus, or bird flu in chickens and ducks that can be a warning to humans. But often a microbe lives silently and benignly within another creature, like hantavirus in mice, and we only learn of its presence when we see outbreaks in humans, like the one in Yosemite.

Visitors to rural areas, national parks, and wildlife preserves should understand that contact with any wildlife carries some risk. In many cases, including hantavirus, risks can be reduced, as long as we’re aware there’s a threat. Keep informed, protect yourself and others by following the recommendations of public health and medical experts, and remember that we are all connected together from a health perspective—human, animals, and the complex ecosystem of our planet.

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