Eradication of Island Mice Called Impossible

There is virtually no way to stop an infestation of virus-infected mice at Channel Islands National Park that has prompted one congressman to call for the park to be closed, a top medical expert said Thursday.

“The eradication of mice in a large area is near impossible. It’s impractical and not as easy as it seems. We’ll be stuck with this for many years to come,” said James Mills, chief of the medical ecology unit for special pathogens at the U.S. Centers for Disease Control and Prevention in Atlanta.

Not much bigger than golf balls, deer mice can easily hide in the nooks and crannies around the islands, thwarting any attempt to eliminate them. Mice are prolific breeders, Mills explained, and under the right conditions, even a few can repopulate a large area.

Mills’ comments followed a controversy that began when an Oxnard family returned from a visit to Santa Rosa Island last week and reported that their 7-year-old son had been playing with a mouse, which later tested positive for hantavirus antibodies. The boy is being monitored for the potentially deadly disease, which can spread to people through bites and excrement-tainted dust.

In response to the incident, Rep. Elton Gallegly (R-Simi Valley) called this week for an immediate closure of the park until federal land managers devise better methods to notify the public of the risk. A congressional subcommittee will explore the matter during a hearing June 29. Among the topics before the subcommittee are how to eradicate the mice, said Gallegly’s spokesman, Tom Pfeifer.

Other experts shared Mills’ view that eradicating the mice could be impossible.

“Just look at the difficulty the park service has removing the sheep from the islands,” said Richard Davis, public health biologist for the state Department of Health Services. “It would be impossible to get rid of the mice, even if you put all the money in the world into it. It’s not even a consideration.”

Carol Spears, spokeswoman for the Channel Islands park, said eradicating mice is not a serious option for the park service. Deer mice, even though they carry diseases, are endemic to the islands and an important part of the environment.

The 1916 Organic Act, which created the National Park Service, mandates that the agency conserve plants, animals, wildlife and scenic landscapes for future generations.

“They [deer mice] are a native species, and it’s our mission to preserve native species and native landscapes that were here,” said park service spokeswoman Elaine Sevy. “That mouse is part of that ecosystem. If you destroy all those mice, then what are you going to do to the ecosystem? Will the bug population explode? What impact are you going to have on bird species that feed on the mice? It’s all a web of life and it’s all interconnected.”

Wet winters have sent the islands’ mice populations skyrocketing over the past two years, although their numbers appear to have peaked in spring, according to park service officials. Studies by scientists at the University of California and the state health department published in 1997 show up to 71% of the mice at three park islands--Santa Rosa, Santa Cruz and San Miguel--are infected with hantavirus. That rate is seven times higher than what is typically found in mice populations on the mainland and the highest disease prevalence over such a large area ever reported in the nation, experts say.

At the time mouse populations soared, numbers of island fox, which eat mice, declined by about 90% across the islands. Efforts are underway to recover the foxes through captive breeding and other measures, but it is unclear whether more foxes will mean fewer mice.

UCLA biologist Gary Roemer, who works on the fox recovery program, said that although studies in other places show foxes affect populations of prey species, information for the Channel Islands is limited.

“Predators do influence rodent populations, but whether island foxes are the key predator in controlling deer mice populations is currently unknown,” Roemer said. “Lots of things affect rodent populations. You can have lots of foxes and still have lots of mice.”

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The best strategy, Mills said, the one commonly used around western states to reduce exposure to hantavirus, is public awareness and avoidance of the rodents. Reasonable preventive measures include posting signs, keeping houses mouse-free, distributing public information, training employees thoroughly and using traps and poisons on a limited basis around houses and sheds.

Those techniques form the backbone of the strategy used at Channel Islands National Park since March 23, when James Stratton, state health officer, recommended park managers redouble efforts to protect visitors and employees from infected rodents. Before that, Channel Islands park officials denied that hantavirus at the islands was a human contagion and had all but eliminated public notification of the threat.

Indeed, a Thursday meeting of local, state and federal health officials who discussed the park’s response to the problem resulted in no calls to close the park and only minor recommendations, which are under development, to better notify private boaters of the risk, said Davis of the state health department.

“We couldn’t come up with anything new or different in terms of precautions they could take or things they could do to better inform the public. As far as I can tell, they are doing everything they should,” Davis said.

Hantavirus causes flu-like symptoms that can result in hantavirus pulmonary syndrome, a rare, but often fatal disease that causes the lungs to fill with fluid until the victim drowns. Since 1993, at least 207 Americans have become infected with hantavirus and 43% of them have died, according to the Centers for Disease Control. The viral strain found at the Channel Islands is the same one that killed two dozen people six years ago in the Four Corners area, where Utah, Arizona, New Mexico and Colorado meet.

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However, National Park Service officials assure that no one is believed to have contracted the disease, become ill or died from contact with hantavirus-infected rodents at the Channel Islands park. Consequently, park managers have cautioned against overreaction and have rejected calls to temporarily close the park.

“There are no documented cases of people contracting hantavirus at the islands. People have overreacted on this issue of hantavirus at the Channel Islands,” said Paul Collins, senior associate curator for the Santa Barbara Museum of Natural History.