Proliferation of Mice at Park Magnifies a Mystery

A medical and environmental mystery is unfolding at Channel Islands National Park, where a succession of wet winters and a dearth of natural predators have caused a population explosion among disease-carrying mice.

Many of the rodents--up to seven in 10 on Santa Cruz and Santa Rosa islands, two of the most popular tourist destinations in the five-island chain--are infected with hantavirus. That is the disease that in 1993 killed two dozen people in the Four Corners area where Utah, New Mexico, Arizona and Colorado meet.

“If you want to see deer mice, go to the Channel Islands. The concentration is so high you can almost step on them,” said Michael Ascher, chief of the state Department of Health Services’ virus lab in Berkeley and co-author of a 1997 study on the matter. “There’s a ton of virus. It’s a pretty high risk. [Outbreaks are] all set up to happen.”

But infections among people have not occurred on the islands. Despite the fact that a higher percentage of deer mice are infected there than anywhere else in the United States, none of the 60,000 tourists who visit the scenic islands each year have died or even become ill from hantavirus. Blood tests on a dozen park employees and longtime island dwellers four years ago showed no trace of the virus.

The National Park Service is so confident that there is nothing to worry about that it has scaled back warnings to visitors. Park service officials say they believe that the type of hantavirus at the islands is not contagious for humans.

“This hantavirus is a strain that doesn’t infect human beings. Why else would no one get sick?” asked park Supt. Tim Setnicka. “Even though there’s a high percentage of mice with hantavirus, there’s no potential for [disease] incidence. There’s no risk to the public or my employees. It’s a nonissue.”

The park service’s own documents, however, contradict his statements. Files obtained through the Freedom of Information Act refer to hantavirus at the park as “life-threatening” and “potentially fatal.” A 1994 memo says, “We know that this virus has been fatal to humans and is present on the islands.”

Further, the state health department says, the hantavirus common to deer mice on the islands is known as sin nombre, the same strain blamed for the outbreak in 1993.

So what’s going on? Why is nobody getting sick? Officials cannot say.

Mice have always been abundant at the park, but these days the islands are literally crawling with the small creatures, which are about the same size as a field mouse. Mouse reproduction boomed after the drenching El Nino storms of last winter.

On San Miguel Island, the number of mice has more than doubled to an average of 300 per acre, equivalent to 11 mice roaming the space of a modest single-family house. They concentrate near campgrounds and dwellings, studies say.

“If you’re a camper, it can be a treat,” said park biologist Gary Davis. “They run right over your face when you’re in a sleeping bag. I guess that’s part of the island experience.”

The problem is so acute that mouse-proofing figures prominently in the park service’s continuing effort to upgrade ranger houses on the islands.

Hantavirus was first identified on the islands five years ago. But new concerns have arisen because the rodent population is surging at the same time that the park is becoming an increasingly popular tourist destination, lying within easy reach of 15 million Southern Californians.

Hantavirus has been detected in 28 states and typically occurs in 10% of mice. However, a recent state health department study found that 58% of the deer mice on Santa Rosa Island are infected.

People can be infected by breathing air contaminated by mice feces and urine, by being bitten by a mouse or by ingesting food or water contaminated by droppings. Hantavirus can cause a potentially fatal disease known as hantavirus pulmonary syndrome, which produces flu-like symptoms and eventually fills the lungs with fluid, suffocating the victim.

Since 1993, 205 cases of hantavirus infection have been reported in people in the United States, leading to 87 deaths. Twenty Californians have been infected, with nine dying.

Dr. Robert Levin, Ventura County’s health officer, said there is no cause for panic. He does not believe that drastic action, such as mouse eradication, is necessary to protect people.

But he said stronger public notification is needed to warn people of the risk. “I’m sure we’ll have cases sometime in the future, maybe a few, maybe a lot. It’s bound to happen,” Levin said.

A few years ago, campgrounds on all the Channel Islands were posted with hantavirus warnings, and a fact sheet on the disease was available on counter tops at the park’s mainland visitor center in Ventura. Since then, efforts to tell the public about the situation have been scaled back.

There are no visible notices at the mainland center and no mention of hantavirus on the park’s Web page. The campground at Santa Cruz Island, which attracts the most visitors, is not posted with warnings. Park brochures do not mention the issue, but a bulletin the park service says it sends to prospective campers does, although campers do not always receive it. Some tourists who visit the islands say they have never heard about hantavirus.

Conditions at the national park today are similar to those in 1993, when a major hantavirus outbreak occurred in the Southwest. Heavy rainfall during the winter of 1991-92 produced an abundance of food and cover and an explosion of deer mice the next year, according to the Centers for Disease Control and Prevention.

Mice are at least 10 times more abundant and hantavirus twice as widespread on the Channel Islands as during the outbreak at Four Corners, studies say. Consequently, the CDC is urging people to pay special attention to preventive measures to reduce exposure to rodents.

At the outset of the current mouse population explosion, hantavirus, which does not harm mice, probably dipped because older animals had not yet spread it to juveniles. But as age groups mingle, the disease tends to rip through the expanded population like wildfire, said James N. Mills, an official at the CDC.

That may portend a bad year for hantavirus around the Southwest and on the Channel Islands. “It could mean you have high prevalence and high numbers of [mice] that survive winter, which would represent a high risk to human populations,” Mills said.

Hantavirus-related fatalities occurred a few years ago in Santa Barbara and in the Sierra Nevada, where a researcher working at the UC Valentine Eastern Sierra Reserve near Mammoth became infected. Since that incident, UC field research stations have cut the hantavirus risk through good housekeeping, including reducing trash around buildings, setting traps and warning people to avoid mice. Those practices are observed at the UC Santa Cruz Island Reserve at Channel Islands National Park.

“We’re not paranoid about this, but we ask people to take reasonable precautions,” said reserve Director Lyndal Laughrin. The mice “populations explode more out here than on the mainland. There’s more public access. There’s more interaction and more public risk, so that’s a concern. There’s no reason why [visitors] shouldn’t be informed.”

But that does not always happen.

“What’s hantavirus?” asked Roberta Hodson as she returned from a January trip to the islands.

Nancy Eyraud, who camped on Santa Cruz Island in November, said: “I think they should tell people. That would be optimal. The public has a right to know.”

Supt. Setnicka said hantavirus is too rare to be a significant concern for visitors. “[There is] a much greater chance of getting burned on a campfire or falling off a cliff than getting hantavirus,” he said.

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Hantavirus Found on the Channel Islands

The wet weather of recent years has spurred a population explosion of deer mice at Channel Islands National Park. Some health officials are concerned because many of the mice are infected with hantavirus, which can be transmitted to humans. Though an outbreak of the illness caused by the virus killed 24 people in the Southwest in 1993, so far no tourist has been infected. The park service believes this proves there is no danger. Though difficult to contract, people can get the virus from exposure to infected mouse droppings, urine or saliva.

How the virus is given to humans

1. Mouse droppings, urine and saliva containing the virus collect in areas where humans and mice coexist.

2. Particles containing the virus become airborne when disturbed.

3. Particles are inhaled and person becomes ill with the hantavirus pulmonary syndrome.

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Clinical Symptoms

* Fever (usually ranging from 101 to 104 degrees and unresponsive to medication)

* Muscle and body aches

* Chills

* Dry cough

* Nausea, vomiting, diarrhea

* Fatigue

Symptoms usually last for a few hours to several days. The lungs then begin to fill with fluid, making breathing difficult. Shortness of breath is a major indication of problems. Anyone with these symptoms should seek medical care immediately.

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Treatment

* Respiratory therapy

* Drugs that inhibit viral reproduction

* Routine hospital care

Precautions

* Avoid contact with mice

* Seal openings to keep mice out

* Keep pet food covered

* Wet floor before sweeping when rodent droppings are present

* Set mousetraps

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Deer Mouse

Peromyscus maniculatus

* Identification: Head and body 2 to 4 inches; tail 2 to 5 inches. Color ranges from pale gray to deep reddish brown. Tail always sharply bicolored, white below, dark above.

* Habitat: Nearly every dry-land habitat within its range is occupied by this species.

* Habits: Nests in burrows in ground, in trees, stumps and buildings. Feeds on seeds, nuts, acorns, insects; stores food.

Source: Centers for Disease Control and Prevention; California Department of Health Services