"The supreme end of education is expert discernment in all things – the power to tell the good from the bad, the genuine from the counterfeit, and to prefer the good and the genuine to the bad and the counterfeit."
Samuel Johnson, 18th Century English Writer
“Yosemite National Park provides hantavirus education materials to all overnight park visitors regardless of where they stay. ”
Yosemite National Park Hantavirus Webpage
The last thing guests hear when completing their check-in at Housekeeping Camp is a request to sign a document acknowledging that a staff member reviewed the Camp rules and policies with them. This includes confirming receipt of information about the risk of contracting hantavirus from deer mice that live in Yosemite.
Deer mice inhabit areas in Yosemite from Tuolumne Meadows to Yosemite Valley. About 12% of the deer mice carry the Sin Nombre Virus (SNV). Humans who come in contact with this hantavirus by breathing in microscopic dust particles that contain feces or urine from infected deer mice may suffer from Hantavirus Pulmonary Syndrome (HPS), a serious respiratory illness.
Although a relatively rare disease, HPS has a mortality rate in California of about 30%. So, the seriousness of the illness cannot be downplayed despite its rare occurrence. Consequently, assisting Yosemite visitors to be aware of the risks serves as a first step in prevention and to better manage any outbreak, such as the one that occurred in the summer of 2012 at Curry Village which gained national headlines.
At that time, Yosemite had just completed building new, warmer “signature” tent cabins with foam insulation enclosed between the outside canvas wall and the interior wallboard (most tent cabins have only the outer canvas covering). Unfortunately, the insulation provided the perfect nesting location for deer mice to burrow in and raise their young (See image below). The new “housing opportunities” allowed the deer mice a comfy environment to propagate and to establish residence in most of the walls of the signature tent cabins.
Image Courtesy of US Center for Disease Control (Click on image for article)
The increase in deer mice droppings and urine particulate matter infected nine Park visitors with HRS. All of those who became ill had lodged in the signature tent cabins. A tenth person staying in a regular tent cabin in Tuolumne Meadows also became infected with HPS. In line with established mortality rates, sadly, three of the infected Park guests died according to a report by the U.S. Center for Disease Control (CDC).
That same CDC report indicates that strict measures were taken immediately, which included tearing down all signature tent cabins, “increasing staff and visitor education for HPS prevention, enhancing mouse control measures in and around human-made structures, and applying rodent exclusion measures to other buildings.” The swift actions resulted in a “substantially lower” trap rate (51% In August vs. 14% in September 2012), and “no (0/10) deer mice were positive for SNV [in September].”
That CDC report also indicates that annual and ongoing trappings and analysis of deer mice presence and seropositive blood samples by the California Department of Public Health (CDPH) and The National Park Service (NPS) have shown no repeat threat of a hantavirus outbreak since the elimination of the signature tent cabins in Curry Village.
Why, then, is the warning still given to guests prior to lodging at Housekeeping Camp, located about a half mile from Curry Village? What really are the risks of being infected by the hantavirus there? If you listen to one reviewer who lodged at Housekeeping Camp in October 2023, you might be misled to believe the risks are high. Note the following review by 0John1 on TripAdvisor.com:
“When you check in you sign a waiver acknowledging the danger of acquiring the Hanta virus, which is spread through the droppings of deer mice. I arrived at my filthy unit and within five minutes had two deer mice run right past my feet. The camp host stopped by my unit shortly thereafter. I pointed out the filthy condition and he said, not his department, but the units were rarely cleaned and, unless I was sleeping in a pile of deer mouse feces, I shouldn’t worry about the Hanta virus.”
On closer examination, this experience sounds dubious. Having a negative attitude about being educated about the risks of contracting any illness stretches the limits of reason. The only known hantavirus outbreak in Yosemite occurred in 2012, eleven years before this person’s visit. Plus, the 2012 outbreak was unequivocally linked to the high concentration of deer mice nesting in the foam insulation of the signature tent cabins (one report indicated that up to 15 deer mice can live in a nest). All of those tent cabins were removed from the Park and no outbreak has occurred since (See California HPS Infection Rate Chart to the right).
Ongoing monitoring by the CDPH and the CDC has resulted in no concerns about another outbreak. One seasonal Yosemite worker/vlogger reported being diagnosed in 2023 with HPS while living in an employee tent cabin causing her a fair amount of distress. She reports that her suffering was confounded by receiving minimal assistance from the NPS medical clinic and little attention to her housing conditions by Aramark even though hantavirus is a CDC reportable illness. Additionally, ongoing trapping and monitoring have resulted in no evidence of deer mice taking up residence in and around Housekeeping Camp.
Consequently, the hantavirus acknowledgement merely serves as ongoing education to heighten awareness and to provide a measure of protection. Does anyone object to the “no swimming” signs near Vernal Falls restricting access to the Fall? Even though some ignore the warning (and some have been swept away and have died), it still serves as practical information even when Vernal Fall becomes a trickle during the late summer. As Samuel Johnson once said, “The supreme end of education is expert discernment in all things.” That would include discerning how information can prepare and protect us rather than promote disdain.
The thought of seeing two “deer mice” run by the reviewer’s feet also intrigues me for several reasons. First, how does the reviewer know the mice he saw were deer mice (Peromyscus maniculatus)? During the bi-annual trappings in Yosemite, brush mice (Peromyscus boylii) were also discovered to be plentiful in Curry Village and the “peridomestic” areas. As one report on deer mice notes, “These [multiple Peromyscus] species are difficult to distinguish from one another, as almost all of them have characteristic white undersides, legs, and feet and dark to light brown backs.” Plus, brush mice do not carry the hantavirus; only deer mice do.
Additionally, most mice are nocturnal, which is true of both the brush mice and deer mice. The reviewer does not state the time he checked in at Housekeeping Camp. Nevertheless, check-in time is 4:00 p.m. and sunsets in October occur about 6:30 p.m. It would be unusual to see either mouse species out in the daylight searching for food at that time, especially around people! Generally speaking, deer mice avoid humans!
Although deer mice do predominate in Yosemite Valley compared to brush mice, even if the reviewer arrived in the dark, that would make it even more unlikely he would be able to distinguish between the different species of mice! Given their proclivity to be wary of humans, deer mice move as fast as any mouse. That is why they are called deer mice!
How can a person rapidly identify a mouse species in relative darkness when “loitering” remains atypical of most mice? Undoubtedly, the majority of visitors to Yosemite would be unable to distinguish one mouse type from another. Why would that be any different from the reviewer noted above? The “two deer mice” that ran by his feet could even have been two fast-moving chipmunks or even two small squirrels chasing each other (typical behaviors among chipmunks and squirrels at Housekeeping Camp!).
California Ground Squirrel
Perhaps the reviewer fell under the influence of the “recency effect”! The recency effect, according to the American Psychological Association, is a “memory phenomenon in which the most recently presented facts, impressions, or items are learned or remembered better than material presented earlier.” This experience can also create an internal bias that may lead to inaccurate ratings of a person’s abilities due to the inordinate influence of the most recent information received about that person.”
What was the last thing the reviewer heard? ‘Watch out for deer mice; they carry the hantavirus . . . now sign here!’ The power of the recency effect could even persuade a Yosemite visitor to attribute the hantavirus to a rock badger if that was the first thing they saw and were anxious enough! “Oh, no, look there’s a R.O.U.S.* with the hantavirus!” (*Rodent Of Unusual Size)
Finally, the Housekeeping Camp units do not provide nesting opportunities for deer mice. Six-inch cement walls prohibit burrowing; quite a contrast to the insulated “condos” commandeered in 2012 by the deer mice in the signature tent cabins at Curry Village. Wildlife does come and go in the open-air units of Housekeeping Camp, but they don’t move in. As one pest control website states, “Mice like to live in warm cozy areas.”
The data suggests that deer mice do not find Housekeeping Camp as pleasing a place to lodge as do humans, unless, however, you are a human under the influence of the recency effect. Clearly, the last thing heard can “create an internal bias” that may “lead to inaccurate ratings” about the presence of all sorts of things, including deer mice and the hantavirus at Housekeeping Camp.
The reviewer noted above “ended up sleeping in my car that night and got a hotel outside the park for my second night.” That turn of events had nothing to do with the presence of deer mice or the hantavirus in Housekeeping Camp, but, more than likely, it had everything to do with the power of the recency effect.
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