Rare Mesothelioma Cancer Discovered in Quarter Horse

No one is immune to the effects of toxic asbestos fibers, the primary cause of mesothelioma cancer. Not even horses.

Although mesothelioma is a rare cancer in humans, it is even more rare in animals, particularly horses that spend most of their time grazing outdoors. But it still happens.

Both pleural mesothelioma and peritoneal mesothelioma cancer were identified during the autopsy of a 22-year-old gelding quarter horse that had spent much of its working life in equestrian tourism activities.

Animals, an international peer-reviewed journal devoted to veterinary sciences, detailed the rare mesothelioma case report in its September issue.

“Mesothelioma can affect different species, including dogs, cats, cattle, sheep, goats and horses. The onset in horses is very rare,” the authors wrote. “Ours is a rare case of mesothelioma in the abdominal and thoracic cavities.”

Other Malignancies More Common in Horses  

The horse was from the Lazio region of Italy that includes the capital city of Rome. The autopsy was conducted by doctors from the Department of Veterinary Medicine, University of Bari.

According to an earlier necropsy study done by the University of Kentucky Veterinary Diagnostic Laboratory, malignancies occur in 8% of horses between the ages of 15 and 19, with an increase to 17% in horses aged 30 and above. 

The most common tumors are squamous cell carcinoma, pituitary adenoma, melanoma, sarcomas and abdominal lipoma.

Rarely, though, is mesothelioma cancer ever seen. According to the authors, there have been only 11 reports of mesothelioma in horses since 1976.

“In most cases of equine mesothelioma, the diagnosis is made post-mortem at the slaughterhouse or during a clinical necropsy,” they wrote.

In humans, mesothelioma most often stems from long-ago occupational exposure to asbestos. Otherwise, it mostly is traced to heavily urbanized areas, where aging residential and commercial buildings are filled with legacy asbestos. Neither was the case with this horse.

Veterinarians Were Expecting Colic Syndrome

The horse was brought initially to the veterinary clinic for what was believed to be colic syndrome. It appeared emaciated, with congested mucous membranes and an abdominal region that was enlarged and distended. 

Blood counts and ultrasound examinations were performed, revealing abdominal masses and abundant accumulations of abdominal and pleural fluids. 

After five days of failing to respond to treatment, the horse was euthanized. The autopsy exam revealed abundant effusion and nodular masses throughout the abdomen and thoracic cavity. 

A diagnosis of the epithelioid type of mesothelioma, the most common type in humans, was made through histopathology and immunohistochemistry, according to the authors. They speculated that the horse’s exposure to asbestos was environmental contamination, and not from nearby aging construction.

More Horses Will Be Checked for Mesothelioma

The authors also pointed out that the horse lived in an area with a very high incidence rate of human mesothelioma, suggesting that other horses nearby should be monitored to evaluate the potential presence of this cancer and the biological indicator of the disease.

The staff at the veterinary clinic had an earlier case that involved both a horse and owner suffering from lymphoma.

“Our study has led us to hypothesize that horses, in the geographical area, could be considered excellent sentinel animals,” the authors wrote, including for air pollutants and asbestos.

Lead study author Dr. Giuseppe Passantino did not respond to a request from The Mesothelioma Center at Asbestos.com to provide additional insight into the findings.

Studying Mesothelioma in Animals

Although rare, the horse was not alone in recent research involving mesothelioma in animals.

In 2021, the Journal of Zoo and Wildlife Medicine published a case report in which a male, wild-caught tiger rat snake in Canada was diagnosed with pericardial mesothelioma, the first documented case of its kind.

It was treated with therapeutic pericardiocentesis, and returned to normal behavior within two weeks. Six months later it was treated again, but it died four days after the second procedure.

Earlier this year, the journal Veterinary and Comparative Oncology published a study from the United Kingdom involving the benefits of chemotherapy in treating dogs diagnosed with mesothelioma cancer.

There were 34 dogs in the study, including 25 that were treated with chemotherapy. Those treated had a median survival of 234 days. Those not treated with chemotherapy had a median survival of just 29 days.

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