Avian Flu Diary: MMWR: Yosemite Hantavirus

Credit CDC

# 6731

Late last August reports emerged of a handful of Hantavirus infections among recent visitors to the Yosemite National Park campgrounds (see Hantaviruses Revisited) in California.

Hantaviruses’ are a collective term for a group of viruses carried by various types of rodents - that vary in distribution, symptomology, and severity around the world.

In the United States the most common form is the `Sin Nombre’ (nameless) virus, which can cause a severe form of pneumonia called HPS (Hantavirus Pulmonary Syndrome) and is fatal in about 30% of the cases.

Hantaviruses are not transmissible from person to person.

By the end of August we saw Yosemite Hantavirus Cases Increase To Six, and tens of thousands of summer visitors to the park were urged to seek medical care if they developed signs of respiratory illness.

While most of those infected had stayed at one of the 91 "signature tent cabins" in Curry Village, by mid September we learned of at least 1 `outlier’ who had backpacked and camped along park’s high country called the High Sierra Loop, which provides pre-setup tent camps along the route.

This week the CDC’s MMWR carries a `Notes from the Field’ report on the Hantavirus outbreak at Yosemite, which as of October 10th, encompassed 10 cases and 3 fatalities.

Notes from the Field: Hantavirus Pulmonary Syndrome in Visitors to a National Park — Yosemite Valley, California, 2012

Weekly
November 23, 2012 / 61(46);952-952
On August 16, 2012, the California Department of Public Health announced two confirmed cases of hantavirus pulmonary syndrome (HPS) in California residents who had stayed overnight in Yosemite National Park, launching an investigation by the National Park Service, California Department of Public Health, and CDC. On August 27, Yosemite National Park announced two additional cases, and by October 30, 10 cases had been confirmed.

For this outbreak, a confirmed case was defined as detection of 1) a febrile illness and hantavirus (Sin Nombre virus) specific antibodies in serum, or 2) virus antigen in postmortem tissue using immunohistochemistry, in a person who had stayed overnight in Yosemite National Park during June 1–August 28, 2012. CDC notified public health officials and clinical providers in the United States and internationally. The National Park Service notified by e-mail, telephone, or mail all registered overnight Yosemite National Park visitors (approximately 260,000 guests) who had stayed at the park during June 1–September 17, 2012.

The 10 confirmed patients came from three states: California (eight), West Virginia (one), and Pennsylvania (one). Ages ranged from 12 years to 56 years; four were female. Nine patients had typical symptoms of HPS, and one lacked respiratory symptoms; three died.

Nine patients stayed in Curry Village "signature" cabins, which have insulation between the canvas exterior and interior hard walls. Rodent infestations were detected in the insulation, and all 91 signature cabins were closed indefinitely on August 28. In addition, educational interventions were enhanced for staff members and visitors parkwide, and multifaceted rodent control measures, including trapping throughout Curry Village, were implemented.

HPS is a nationally notifiable disease caused in the United States most commonly by Sin Nombre virus. The deer mouse (Peromyscus maniculatus) is the reservoir. Infected mice shed virus in urine, feces, and saliva. Humans become infected through inhalation of aerosolized virus from rodent excreta and via direct contact from rodent bites. The incubation period ranges from 1 to 6 weeks. Early symptoms include fever, chills, myalgia, headache, and gastrointestinal symptoms for 1–7 days, progressing rapidly to respiratory distress and shock (1). Most patients require hospitalization, supplemental oxygen, and intubation. The case-fatality rate is approximately 36% (2). There is no specific treatment for HPS, but early supportive care can reduce mortality (2). Before this outbreak, 58 cases of HPS had been reported among California residents since 1994; two had been visitors to Yosemite National Park before 2012 (California Department of Public Health, unpublished data, 2012).

Clinicians are reminded to consider the diagnosis of hantavirus infection in all persons with febrile illness and sudden onset of respiratory symptoms with a history of rodent exposure. Because HPS is a reportable disease in the United States, clinicians suspecting HPS should notify and consult their state health department about confirmatory testing. More information is available from CDC regarding hantavirus clinical assessment, treatment and diagnostics (3). Park visitors and the public are advised to avoid contact with rodents and their urine, droppings, and nesting materials.

You don’t have to trek the high country of Yosemite National Park, or stay at Curry Village, to risk exposure to the Hantavirus. As the chart below shows, while rare, this rodent borne disease has a wide range in the United States.

Although Colorado, New Mexico, and Arizona are the nation’s hot spots for the virus, a few cases have been reported as far east as Florida and the Eastern Seaboard.

In the United States, fewer than 600 cases of HCPS have been identified over the past 19 years, most of which have occurred in the Southwest. Exposure to mice or rodents, and their droppings, has been established as the primary vector for this virus.

The range of susceptible rodents (predominantly the deer mouse, but also the white footed mouse in the Northeast, and the cotton rat in the south) is such that some risk of exposure is possible practically anywhere in the United States.

While the odds of contracting Hantavirus are slim -given the high mortality rate - it is worth heeding the following advice from the CDC.

Preventing Hantavirus Pulmonary Syndrome (HPS)

Eliminate or minimize contact with rodents in your home, workplace, or campsite. If rodents don't find that where you are is a good place for them to be, then you're less likely to come into contact with them. Seal up holes and gaps in your home or garage. Place traps in and around your home to decrease rodent infestation. Clean up any easy-to-get food.

For more information on how you can prevent rodent infestations, the following information is available on the CDC Rodents site:

Got Mice?

Seal Up!

Seal up holes inside and outside the home to keep rodents out.

Trap Up!

Trap rodents around the home to help reduce the population.

Clean Up!

Avoid illness: Take precautions before and while cleaning rodent-infested areas.