A few mice caused havoc in the United States during the summer of 2012, as officials from Yosemite National Park warned up to 10,000 park visitors that they might have been exposed to a deadly form of hantavirus (hanta virus). As of September 1, 2012, at least six people had fallen ill from infection with the rodent-borne virus. Two of those people, both young, previously healthy adults, have died. Additional suspected cases have been identified but have not been confirmed.

Officials from Yosemite have been notifying people that they may be at risk of hantavirus infection if they visited the park and stayed in the “signature tent cabins” in Yosemite’s Curry Village between mid-June and late August 2012.

The outbreak serves as a grim reminder that hantavirus remains a poorly understood and difficult to control infectious agent – and that we have but few tools to combat it.

What is Hantavirus Pulmonary Syndrome?

Infection with hantavirus causes a serious and often fatal disease called hantavirus pulmonary syndrome (HPS). The condition leads to acute respiratory distress syndrome (ARDS), a serious and often fatal lung condition. In ARDS, fluid builds up in the tiny sacs (alveoli) that contain the even tinier blood vessels (capillaries) where oxygen and carbon dioxide are usually exchanged. As a result, air cannot enter the fluid-filled alveoli — and no oxygen/carbon dioxide exchange can take place. Oxygen levels in the blood fall and carbon dioxide levels build. Without adequate oxgyen, organs begin to fail.

Symptoms of HPS can occur from one to six weeks after exposure to hantavirus. Early symptoms of HPS could be mistaken for flu and include fever, muscle aches, fatigue, and other flu-like symptoms. However, hantavirus pulmonary syndrome can progress rapidly to life-threatening illness.

Although doctors have gotten better at recognizing and treating hantavirus pulmonary syndrome, the mortality rate (i.e. the percent of people who die after developing a disease) remains above 36 percent.

Hantavirus at Yosemite

Yosemite officials warned on the park’s website: “Individuals who stayed in Curry Village ‘Signature Series Cabins’ (which are numbered in the 900s) between June 10 and August 24 should seek immediate medical attention if they exhibit any symptoms of hantavirus pulmonary syndrome (HPS).”

Park officials advised all people who have any flu-like symptoms and are concerned that they might have been exposed to the virus to contact their local health-care providers immediately. There is no treatment for hantavirus infection, nor is a vaccine available to prevent the disease. However, people who fall ill may do better if they receive intensive medical care as soon as symptoms appear. In intensive care units, people are intubated and given supplemental oxygen, which may help them through the period of extreme difficulty breathing.

Yosemite National Park has set up a general, non-emergency phone line for all questions and concerns related to hantavirus in Yosemite National Park. The phone number is (209) 372-0822 and it will be staffed from 9 am to 5 pm daily.”

What is Hantavirus?

Hantavirus was first identified in the United States when a mysterious cluster of illness occurred in 1993 in the Four Corners area of the American Southwest. A young Navajo man who was having difficulty breathing was rushed to a hospital in northern New Mexico. He died very quickly. The medical team that explored his death soon found out that the young man’s fiancée had died only a few days earlier after experiencing similar symptoms.

Epidemiologists from the New Mexico Office of Medical Investigations (OMI) and Indian Health Services (IHS) launched an investigation. Within hours, they discovered that five other healthy young people in the region had died suddenly after experiencing acute respiratory failure.

But officials were still puzzled. Laboratory tests did not identify any typical culprits. New Mexico health officials called in the CDC Special Pathogens Branch and alerted health departments in the neighboring states of Colorado, Arizona, and Utah. Health experts from the University of New Mexico joined the investigation.

More cases were identified over the next few weeks, eventually pinpointing the culprit as a previously unknown type of hantavirus, carried primarily by deer mice. Originally, officials dubbed it the Muerto Canyon (“canyon of death”) virus. Later, the name was changed to the Sin Nombre (“without a name”) virus (SNV).

Subsequent investigations showed that this “new” virus had actually been around for a long time. After identifying the virus, investigators began looking at lung tissue samples from other people who had died of unexplained adult respiratory distress syndrome. Eventually, officials confirmed that the 1959 death of a 33-year-old man had been the result of SNV infection – the earliest confirmed case on record.

However, even that may be relatively recent. As investigators conversed with Navajo Indians, they learned that their medical traditions included ways to respond to a similar disease – and that disease was linked with mice in Navajo traditions. Researchers were also stunned by another discovery: The Navajo tribe’s traditional methods of preventing the disease matched CDC’s public health recommendations for prevention of hantavirus infection.

Since the discovery of SNV, at least 25 additional hantaviruses have been identified in the Americas. Researchers have linked several of these newly-identified hantaviruses to hantavirus pulmonary syndrome. These cases of hantavirus pulmonary syndrome occurred in Texas, Louisiana, New York, and Florida. Investigators identified at least 14 additional hantaviruses in Central and South America.

How does Hantavirus Spread?

Hantaviruses, including SNV, are carried by rodents and spread by contaminated urine, saliva, and feces. Most infections occur when people unwittingly touch dust contaminated by rodent droppings or touch rodent urine, then touch their eyes, noses, or mouths.

During the 1993 outbreak, investigators found that about 30 percent of deer mice in the region were infected with SNV. About 18 percent of deer mice in Yosemite carry hantavirus. In 2000 and in 2010 nonfatal hantavirus infections had been linked to mice in Tuolumne Meadows, a popular hiking area in the park.

The 2012 cases were linked to a group of 91 double-walled insulated cabins at Curry Village, the park’s high-end accommodations. The California Department of Public Health ordered the cabins shut down Tuesday, August 27. When park employees took the cabins apart for cleaning, they found evidence of mouse nests in the insulation.

Hantavirus outbreaks tend to occur during years after periods of heavy rain and especially after a mild winter, when deer mouse population is on the increase.

Health officials who tracked SNV in 1993 initially were concerned about the potential for hantavirus to spread from human to human, like the common cold. However, no human-to-human transmission of the disease has been reported in the United States, even among health-care workers exposed to infected persons. That’s not to say it’s impossible: During an outbreak of a different strain hantavirus in Argentina in 1996, researchers gathered evidence of apparent person-to-person spread of the disease.

How to Prevent Hantavirus Infection: What to Do

Rodents are everywhere. No matter how good a housekeeper you are, there’s still a chance that you’ll open up an unused cupboard to discover mouse droppings or a cabinet in the garage.

The primary way to prevent hantavirus infection is to get rid of rodents – which may be harder to do than it sounds. The Centers for Disease Control and Prevention (CDC) recommends a “three up” approach to preventing rodents from infiltrating your living space:

Seal Up: Seal off all holes in your garage, home, and outbuildings. Mice can squeeze through a hole the size of a nickel; rats can squish themselves through a hole the size of a half-dollar. Look both inside and outside the building. Inside, look in the attic, basement or crawl space, laundry room drains, around fireplaces, kitchen cabinets, behind refrigerators and stoves, around doors, and any other nooks and crannies. Outside, look in the roof, around windows, doors, and the foundation, and in attic and crawl space vents. To seal small holes, the CDC recommends that you use steel wool and hold in place with caulk. Larger holes can be sealed using lath screen or lath metal, cement, hardware cloth, or metal sheeting.
Trap Up: Choose an appropriate trap – different traps for different rodents. Place traps next to walls. The CDC does not recommend the use of glue traps or live traps. Mice that are caught live tend to urinate from fear – and the urine may contain hantavirus and other germs, increasing your risk of being exposed to other diseases.
Clean Up: Keep anything that might attract rodents in tightly sealed plastic or metal containers with tight-fitting lids. That includes human food, bird seed, pet food, grains, and animal feed. Wash dishes soon after use and put them away. Use cans with tight-fitting lids for garbage and recycling, both inside and outside the home. Wash all garbage containers regularly. Outdoors, keep bird feeders and woodpiles away from the home; keep woodpiles, hay, and garbage cans at least one foot off the ground. Haul off old trucks, cars, tires, and other potential nesting places. Mow your lawn regularly and trim shrubbery within 100 feet of your house.

After Rodents Move In

So if you do find a mouse in the house, what do you do? If you do find any signs of rodent activity in your home, garage, or shed, it’s time to clean up – safely.

While you clean, follow a few rules of thumb, starting with a big DO NOT:

Do not vacuum or sweep. Vacuuming or sweeping only stirs up potentially infected dust and increases your risk of infection.
Seal and trap rodents first. There’s no point in cleaning up, only to have rodents move back into your nicely disinfected space.
Give it a week. Generally, hantavirus in a rodent’s dropping or urine is no longer infectious about a week.
Ventilate. Ventilate the space for at least 30 minutes before starting any clean-up project. Open windows and doors. Your goal is to get a cross-breeze going through the space. Leave the area while you ventilate it.
Disinfect. The CDC recommends a solution of one part bleach to 10 parts water or a commercial disinfectant. Spray surfaces thoroughly and let sit for 5 minutes before you start scrubbing.
Wear protective gear. At the very least, wear protective latex, vinyl, or rubber gloves and a mask with a high-efficiency particulate air (HEPA) filter. If rodent infestation is heavy, done gloves, a mask, coveralls, goggles, and rubber boots or disposable shoe covers. Disinfect or dispose of protective gear at the end of each day’s cleanup project.
Clean and disinfect. Using your 1:10 bleach solution, mop floors and wipe down countertops. Steam-clean carpets and upholstery. Wash bedding and clothing in hot water with laundry detergent. Either dry in a hot dryer or hang bedding and clothing outside to dry in the sunlight after laundering.
Use sunlight. Any materials that can’t be washed – for example, books and papers – can be disinfected by leaving them outside in the sunlight. The virus is deactivated by ultraviolet rays in sunlight.