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Mesothelioma & Asbestos Exposure in Pets

Although humans are most commonly diagnosed with mesothelioma, animals are also prone to the cancer. Most animals that contract mesothelioma are the pets of an asbestos worker, yet rare congenital cases of mesothelioma have been diagnosed in animals younger than seven weeks of age.

Asbestos Exposure in Pets

While breeding practices have caused certain animals to be genetically predisposed to cancer, environmental toxins are the major factor in the development of mesothelioma. Human incidences of the illness are almost exclusively caused by asbestos exposure, and this fiber is also thought to play a major role in the development of the disease in animals.

While asbestos does occur naturally and animals may have inhaled it if they played outdoors in areas where it was mined, secondhand exposure is the most likely source of contact with the mineral. Asbestos often got stuck on workers’ clothing and skin, and once it was transported home, family members and pets frequently inhaled it. Research has shown that the incidence of mesothelioma in dogs is related to their owners’ exposure, with the pets of asbestos workers making up the largest percentage of animals diagnosed with the disease.

Pets living in homes where asbestos had been released into the air were prone to inhaling or ingesting asbestos fibers. Additionally, animals that spent time in contaminated workplaces were just as susceptible to inhaling asbestos as humans. A 1931 study by N.H. Schuster reported a case of asbestosis in a terrier who had served as “ratter” for 10 years at a London asbestos factory. Animal studies, which rose in prevalence at the turn of the 20th century, continued to illustrate the correlation between fibrosis and other asbestos-related diseases in experimental animals that were exposed to asbestos-containing dust.

Mesothelioma in Pets

Mesothelioma typically occurs in older animals. One report on mesothelioma in dogs found that the average onset of the disease in canines was eight years of age, although animals between seven weeks and 15 years had been diagnosed. Additionally, certain breeds of dogs are typically more prone to tumor development. German Shepherds, Irish Setters and Bouvier des Flandres are at an elevated risk for the illness, while male dogs are more commonly diagnosed with mesothelioma than females.

Animals with mesothelioma display similar symptoms to human mesothelioma patients. Dyspnea, or shortness of breath, is the most common symptom in animals. Pet owners should also watch for muffled heart and lung sounds, abdominal enlargement or protrusions, lethargy and weakened pulse.

Diagnostic and treatment methods for animals with mesothelioma are similar to those for people, yet should only be performed by a licensed veterinarian or veterinary oncologist. During diagnosis, a vet typically looks for any pleural effusions and a biopsy or radiograph may also be ordered for confirmation. While surgery is impractical for most animal mesotheliomas, a chemotherapy regimen has been shown to yield complete remission in some dogs and can also quickly reduce fluid accumulation and other symptoms. Surgical debulking procedures may also offer some relief and comfort, yet any treatment for animal mesothelioma can be very costly.

While domestic dogs and cats are the animals that are most commonly affected by mesothelioma, farm animals such as sheep and cows have also been diagnosed with the disease. A couple studies have found mesothelioma tumors in sheep, while a 1998 article explored malignant peritoneal mesotheliomas of the biphasic subtype in two Holstein cows.

Mesothelioma Studies in Animals

Two studies of 21 and 11 rats illustrated the abilities of chrysotile asbestos to cause pleural mesothelioma or lung papillomas in animals. Six rats from each study developed lung tumors or mesothelioma within nine and 28 months of exposure via intra-tracheal injection.  Another project conducted in 1976 injected rats with either chrysotile asbestos dust or talc; 18 of the rats injected with asbestos developed mesothelioma while none from the talc control group developed the disease. Interestingly, in studies where the rats were fed asbestos (rather than being directly injected with it,) there was no significant increase in tumor incidence.

A 1983 study confirmed the correlation between asbestos and mesothelioma in animals. The fiber, which is primarily responsible for human development of mesothelioma, was found in the bodies of three of five dogs affected by mesothelioma but not in dogs from the healthy control group.  All of the dogs with mesothelioma also had pleural plaques on their mesothelial surfaces.

One 2010 case report of a nine year old female dog indicated that in animals, mesothelioma may be the underlying cause of cranial vena cava syndrome. Cranial vena cava syndrome is typically caused by a neoplasm, and in this case, it was a resulting complication of a 10-cm diameter mass of cancerous mesothelial cells. Despite a pleural effusion and respiratory distress, the dog was not diagnosed with mesothelioma until a post-mortem examination.

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