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Catalina's endangered foxes are plagued by tumors, but there's hope, study says

A group of foxes on a road in Catalina Island.
Catalina Island Conservancy
A group of foxes on a road in Catalina Island.

Endangered foxes on Santa Catalina Island have recovered from the brink of extinction since the late 1990s, when their numbers totaled roughly 150. Now the population is estimated to exceed 1,700, and conservation efforts for the species have been hailed as a wildly successful implementation of active management strategies.

Though some talk of delisting the species has arisen, active management will need to continue in at least one form, according to researchers from the University of California, Davis. A pair of studies released within the past week describes the prevalence of cancerous ear tumors in Santa Catalina Island foxes.

The tumors were initially discovered in the early 2000s during research into the 90 percent population decline. The decline was eventually attributed to an outbreak of a disease called distemper. During the course of investigation, however, researchers also discovered widespread ear tumors among the island foxes.

“As time went by, and we examined more and more animals, we found more and more of these tumors, and so we began looking at trying to determine why that was and whether they occurred on any of the other islands,” said Winston Vickers, a wildlife veterinarian with the Wildlife Health Center at UC Davis.

Though the tumors have not hindered the species’ recovery as a whole, they do need to be addressed as a matter of keeping the population healthy. The study found about half of mature foxes on the island had the tumors, two-thirds of which were cancerous.

Vickers said the tumors range in size from microscopic to the equivalent of a walnut, some of which can be seen protruding out of an affected fox’s ear.

“They can be quite ugly when they really get large,” he said.

The tumors can lead to death by causing bacterial infection, invasion into skull bones or cancerous spread to other parts of the body.

Cause and treatment

Vickers and colleagues were able to link the tumors to infestations by ear mites. The species of fox that lives on Santa Catalina Island seems uniquely predisposed to developing tumors in response to mite infection. Genetically distinct populations of foxes on the other Channel Islands that have mites do not develop the tumors.

Fortunately for the Santa Catalina Island foxes, treatments were able to remove about 90 percent of the mites in studied foxes. Subsequent research has shown a dramatic decrease in mite infestation in treated foxes and suggests a reduction in the prevalence of the tumors.

“Our hope and our expectation now is that the tumor prevalence will go down and is going down already with the much lower mite prevalence,” Vickers said.

Studies also show greatly reduced transference of mites from parents to their young after treatment.

Vickers said conservationists and managers will need to continue to provide treatments in order to keep the mites in check.

“It will be ongoing in order to stay effective in the population, and that’s a challenge,” Vickers said.

However, he said the active management and close monitoring necessitated by the recent near extinction allowed his team to obtain findings that clearly identified the source of the tumors, as well as a cancer-prevention strategy for a wild population.

“You just don’t find that in wildlife medicine or wildlife ecology very often at all, so it’s a pretty unique set of studies,” Vickers said.