Treating Endangered Fox’s Ear Mites Helps Recovery

Recovering from being placed on the endangered species list isn’t easy for animals such as the Catalina Island fox, especially when they face complications such as ear mites that can lead to dangerous tumors.

But researchers at the University of California, Davis, who discovered a high prevalence of tumors in the species, were able to address the issue and help with the fox’s recovery.

The Catalina Island fox was placed on the endangered species list in 2004 after populations were found suffering from canine distemper —an epidemic originating from raccoons, an invasive species on the island. The population declined dramatically in the late 1990s and early 2000s.

While assisting with their recovery effort, researchers noted that they were seeing tumors in some of the foxes’ ears. “We made an effort to examine more individuals,” said Winston Vickers, an associate veterinarian with the UC Davis Wildlife Health Center at the UC Davis School of Veterinary Medicine and a member of The Wildlife Society.

In a recent study conducted in the journal PLOS ONE, Vickers, who was lead author, and his team found that the species has in fact been suffering from a high prevalence of tumors. “The tumors were an accidental or incidental finding to help recover the population,” Vickers said. “One of the questions we asked was: Were they being hindered in recovery by the presence of tumors?”

The researchers found that roughly half of the adult foxes they studied between 2001 and 2008 had tumors in their ears and about two-thirds of the tumors were malignant. They also found that over 98 percent of the foxes were infected with ear mites, which they said appear to be a predisposing factor for ear tumors in the foxes.

Treating the Mites

Vickers and his team then thought that there might be something they could do to help treat those mites and lower the prevalence of tumors and ear mites.

Working with researchers from the Institute for Wildlife Studies and Catalina Island Conservancy, Vickers and lead author Megan Moriarty conducted another study published in PLOS ONE to see if treating ear mites helped reduce the ear changes that appeared to increase the likelihood of tumors.

The team dropped a paraciticide drug into 60 foxes’ ears, and sampled but didn’t treat a similar number of controls. “The treated animals responded exceptionally well,” Vickers said. “The percentages of ear mites in the foxes that were treated dropped dramatically over six months.” The prevalence rate dropped from 98 percent to 10 percent among treated foxes, and ear canal inflammation and other signs of ear tumors also dropped. Subsequent to the study, several hundred foxes have been treated annually by the Catalina Island Conservancy in association with trapping for monitoring and management purposes.

“We will never remove the ear mites from the equation entirely,” he said. “Thereby, there may always be some individuals that develop cancer. But the overall impact on the population is going to be dramatically diminished.”

Vickers and researchers at the University of Idaho are planning to complete a follow up study to look at how genetics might determine the reason the island’s foxes develop tumors from mites. Some foxes on other islands have mites but don’t have the tumors, he said. “There’s something different about the Catalina foxes.”

Meanwhile, the species continues to recover. “They have met the recovery goals in the recovery plan and they will likely be delisted before long,” Vickers said.