A feral cat population on the western slopes of Mauna Kea is carrying multiple diseases including toxoplasmosis, which can cause severe illnesses in fetuses as well as adults with immune system problems, new research shows.

The cats, however, are not a direct threat to humans since most of them are in uninhabited terrain at elevations from 6,500 to 9,000 feet.

They do pose a serious problem for native birds in the region and could be a threat to humans if the cats blend into rural and urban feral cat populations.

More than a third of the Mauna Kea cat population shows signs of having been infected with toxoplasmosis, a protozoan parasite that needs cats to go through its full life cycle, but whose cysts can cause severe illness in the human fetus and in humans with suppressed immune systems.

"Cats are the primary host," said Dr. Arlene Buchholz, a veterinarian with the state Department of Health. "A mouse gets it, and then the cat eats the mouse, and that's how the cycle goes on."

The U.S. Centers for Disease Control estimate roughly 60 million people nationwide may be infected, but most are without symptoms. The state Department of Health does not have an estimate for how many people in the Islands may have been exposed to the disease.

A healthy immune system controls the parasites quickly. But for women who may be pregnant or people with a challenged immune system, it's best to keep the cats indoors and not let them hunt, where they could eat contaminated flesh and be exposed. Regular hand-washing and avoiding places where cat feces may be found — like gardens and catboxes — also are important precautions, Buchholz said.

MULTIPLE DISEASES

Wildlife biologist Paul Banko of the U.S. Geological Survey's Pacific Island Ecosystems Research Center at Kilauea on the Big Island was part of a team of researchers that has studied the feral cats. Others involved include Steven Hess, Daniel Goltz, Raymond Danner and Kevin Brinck.

Buchholz said the prevalence of toxoplasmosis in cats nationwide ranges from about 16 percent to 43 percent, so the 37 percent tallied by Banko and associates is "about average for feral or domestic cats."

A significant part of the wild cat population also suffers from feline leukemia virus, Banko said. Many of the males suffer from feline immunodeficiency virus, the cat form of HIV.

The larger threat of the feral cats of Mauna Kea is for wildlife.

One of the reasons the last few native Hawaiian crows, or 'alala, were taken out of the wild is that they were found sickly and infected with toxoplasmosis. Crows presumably became infected eating infected carrion.

Cats in general — diseased or not — also prey directly on native birds. Banko said cameras aimed at the nests of the yellow forest bird known as the palila have shown cats killing the chicks. He said that about 10 percent of each year's chicks are killed by feral cats.

They also kill the Hawaiian goose or nene, ground-nesting seabirds like the Hawaiian petrel and many other species, including game birds.

"They're killing just about every species that's up there," Banko said of the forest lands between one and two miles high.

"A lot of species are sitting ducks," he said.

The cats have been particularly damaging to the burrow-nesting petrels of Mauna Loa, said wildlife biologist Heidi Hansen, formerly with the U.S. Geological Survey and now with the state Division of Forestry and Wildlife.

"It's really easy for a cat to get into a burrow and kill the adult while it's sitting on a chick," she said. Both the Hawaiian petrel and band-rumped petrel are affected, she said.

Banko said genetic studies show that the Mauna Kea population is dispersing across the uplands of Hawai'i Island. Cats closely related to Mauna Kea animals have been found at Kilauea, for example, he said. And they will occasionally go as high as 10,000 feet in elevation.

"They're right up to the tree line and sometimes beyond," he said.

ALSO FEEDS ON INSECTS

During times when there are few rodents or birds to be found, they eat insects — mainly larger crickets and grasshoppers.

"They may rely on those insects during tough times," Banko said.

Cats were brought to the island by early European voyagers, the offspring of the ships' cats that were kept to control rodents. It did not take long for them to get into the wild. Feral cats were reported far from human habitation in the Hilina Pali area of the Big Island as early as 1842, Banko said.

Today's feral cats, mostly gray and gray-black, have been away from domestication for dozens of generations. "My guess is that these cats trace their lineage quite far back," Banko said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.