Scientists Investigating Ear Tumors in Catalina Island Foxes

Analysis by Dr. Karen Shaw Becker

catalina island fox

Story at-a-glance -

  • In 1999, canine distemper ravaged Catalina Island foxes, reducing their numbers by 95%
  • A conservation program that included captive breeding, vaccinating against canine distemper and monitoring of the wild fox population began
  • Their numbers quickly bounced back, and there were more than 2,000 foxes on Catalina Island at the end of 2017
  • Researchers found a “remarkably high” prevalence of the ear tumors in the Catalina Island foxes, typically found alongside ear mites and ear inflammation
  • Conservationists removed about two dozen of the surviving foxes as part of a captive breeding program, which represented only a small genetic sample; some of these foxes had ear tumors, and it’s possible they passed on a genetic susceptibility to the disease to future generations
  • Treating the foxes for ear mites has helped to reduce the infection and inflammation, and the ear tumors appear to be less prevalent than they once were

Catalina Island, one of the California Channel Islands, is home to the Catalina Island fox, a subspecies of the Island Gray fox. Known to live on the island for at least 5,400 years, it’s believed the foxes may have been brought to the island by its first human inhabitants or could have floated across the channel on a log.1

The foxes, which weigh just 4 to 6 pounds as adults, have a colorful history — one that involves overcoming near extinction in 1999 and, in even more recent years, giving insights into a certain form of cancer.

How Catalina Island Foxes Almost Went Extinct

In 1999, canine distemper, a highly contagious life-threatening virus, found its way to Catalina Island, likely via a raccoon. The disease ravaged the foxes on the island, reducing their numbers by 95%.2 While there were about 1,300 foxes on the island prior to the introduction of distemper, this dropped to about 100.

Soon after, conservation efforts began, which included moving 21 juvenile foxes to the east end of the island, where foxes were noticeably absent. This helped establish new territories for the animals, which were declared a federally endangered species in 2004.

The Catalina Island Conservancy, in partnership with the Institute for Wildlife Studies, then developed a conservation program that included captive breeding, vaccinating against canine distemper and monitoring of the wild fox population. Their numbers quickly bounced back, and it’s estimated that there were more than 2,000 foxes on Catalina Island at the end of 2017.3

However, another disease has emerged among the animals — ear tumors — which is puzzling scientists as to its origins and prevalence.

Scientists Investigate Fox Ear Tumors

In 2001, ear tumors (ceruminous gland carcinoma) were first detected in the ears of foxes on Catalina Island. Further, most of the foxes examined also suffered from ear mites, the same type that might affect dogs and cats. Researchers set out to determine just how prevalent the ear problems were and found a “remarkably high” prevalence of the ear tumors in the Catalina Island foxes.

From 2001 to 2008, 48.9% of the dead foxes the researchers examined had tumors in their ears. Tumors were also found in the ears of 52.2% of randomly selected mature foxes between 2007 and 2008. This represents “one of the highest prevalences of tumors ever documented in a wildlife population,” the researchers noted.4

Chronic inflammation in the ear and bacterial and ear mite infections were considered predisposing factors for the tumors, leading the researchers to suggest the following as for why so many foxes were affected:5

“Ear mite infections are acquired soon after birth when pups are in intimate contact with their mite-infected mother. Mite infections persist throughout life if left untreated, resulting in chronic, ongoing inflammation (otitis). Chronic inflammation leads to local tissue damage and repair, resulting in ceruminous gland hyperplasia, ectasia and dysplasia.

As these pathologic changes persist and increase in severity over time, proliferative lesions develop, and benign adenomas and malignant carcinomas arise and become more prevalent as foxes age.”

The question remains, however, why the Catalina Island foxes have so many tumors while foxes on other nearby islands, where ear mites are also common, do not. The researchers suggested it could be due to differences among the mites on the islands or exposure to other toxins or microbes, but it’s also possible that there’s a genetic component.

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Did Captive Breeding Programs Encourage Cancer?

After distemper hit Catalina Island, causing a catastrophic loss in population, the genetic diversity of the foxes living on the island was lost. Conservationists removed about two dozen of the surviving foxes as part of a captive breeding program, which represented only a small genetic sample.

Some of these foxes had ear tumors, and it’s possible they passed on a genetic susceptibility to the disease to future generations, which now populate the island. Sarah Hendricks, a geneticist who has studied the animals in depth, told the Morris Animal Foundation:6

“The repopulation of the Catalina island foxes happened quickly, but with it came the expansion of the disease. In my studies, I wanted to find out why this cancer was unique to only one subspecies of Island foxes.

Since cancer is a complex disease, we didn’t expect to necessarily find one specific genetic mutation in these animals. Instead, we found several differences in regions across the fox genome between long-lived Catalina foxes with cancer and foxes without cancer.”

The Morris Animal Foundation has worked with conservation groups to find a solution for the cancer, and it turned out that treating the foxes for ear mites helped to reduce the infection and inflammation,7 and the ear tumors appear to be less prevalent than they once were.8 Further, since most young foxes get ear mites from their mother, the trend has continued in offspring as well.

In a 2015 study, the prevalence of ear mites in Catalina Island foxes dropped from 98% to 10%, while inflammation of the ear canal and signs of developing ear tumors also decreased. “It’s rare to have a success story,” study author Dr. Megan Moriarty with the University of California Davis School of Veterinary Medicine, said in a news release, but this is one case where the foxes are gaining a strong, new lease on life.