Mesothelioma in dogs – Rare Tumor

Mesotheliomas are rare tumors derived from the cellular tissue that lines the cavities and interior structures of the body. These linings are called the epithelial linings, specifically the mesothelium. 1 Healthy mesothelium is composed of a flattened monolayer of mesothelial cells that line the pleural, pericardial, and peritoneal cavities. These cells also cover the tunica albuginea of the testes. Mesothelial cells are characterized by the presence of microvilli and desmosomes. They also have the potential to phagocytose cells and particulate matter. When mesothelial cells are perturbed following inflammation or irritation, they may increase in size and number. Following fluid accumulation within the body cavities, mesothelial cells may exfoliate and implant on serosal surfaces. Mesotheliomas are malignant neoplasms.

Mesotheliomas are the result of abnormal division and replication of mesothelial cells, and their migration to other sites in the body. This cellular behavior can occur in the thoracic cavity, the abdominal cavity, the pericardial sac around the heart, and for male dogs, in the scrotum. The resultant tumors will often displace internal organs, causing gastrointestinal or cardiac symptoms. Mesotheliomas also produce a lot of fluid, making a microscopic examination of fluid samples an extremely relevant diagnostic tool.

There are also several breeds that are more likely to develop canine mesothelioma:

• Irish Setter
• Bouvier des Flandres
• German Shepherd

Symptoms 

• Trouble breathing
• Muffled heart, lung, and abdominal (ventral) sounds
• Abdominal enlargement/swelling with fluid build-up
• Large scrotum
• Exercise intolerance
• Fatigue
• Vomiting

A few months after exposure to the fiber, the dog will develop the first symptoms such as a cough and vomiting, fatigue and inactiveness, swollen abdomen and large scrotum. Other symptoms include muffled lungs and heart, abdominal sounds and breathing difficulties.

Causes 

Asbestos is the main cause of mesothelioma in dogs. Experts also suggest that pentachlorophenol – a compound in disinfectants and pesticides – can cause mesothelioma. Unlike humans who get mesothelioma after ingesting or inhaling asbestos, dogs get the substance through inhaling or licking it off surfaces. If the owner of the dog works at the construction site, he can bring asbestos home and the dog licks it from his clothes. Dogs that play at the construction site or those that wander in the neighborhood can come across items containing asbestos fiber.

Clinical Signs

Mesotheliomas usually occur in older dogs, with an average onset at 8 years of age. However, documented cases of mesothelioma have been reported in patients from as young as 7 weeks to as old as 15 years. Extremely early age of onset (7 weeks old) suggests that congenital mesothelioma may occur infrequently in the dog. Bouvier des Flandres, Irish Setters, and German Shepherd Dogs appear to be at greater risk for tumor development, and mesotheliomas are more common in male than in female dogs. The greatest incidence of mesotheliomas occurs in the pleural cavity, followed by the peritoneal and pericardial cavities. More than one body cavity may be involved in the neoplastic process. Clinical signs of disease usually are evident for up to one month prior to diagnosis. The major clinical sign of mesothelioma in the dog is dyspnea secondary to body cavity effusions or a large, space-occupying mass. Effusion is the result of fluid exudation from the surface of the tumor or from blocked lymphatic channels. Effusion often results in abdominal discomfort, respiratory distress, cough, tachypnea, and exercise intolerance.

Diagnosis

A definitive diagnosis of mesothelioma can be challenging, especially in the early stages of the disease. If the effusion is detected upon physical examination, a complete blood cell count, biochemical profile, urinalysis, radiographs, and ultrasound can provide critical staging criteria. Ultrasound is often used to evaluate any visceral involvement in the neoplastic process. However, mesotheliomas not visible with ultrasound or CT because they often fail to penetrate the surface of the viscera and may not form discrete masses. In such instances, mesotheliomas may appear only as a diffusely thickened surface.

In diagnosing mesotheliomas, it must be demonstrated that the neoplasm has originated in the coelomic cavity and has metastasized via transcoelomic implantation. Whenever the majority of neoplastic tissue is located on the coelomic surface, mesothelioma must be considered in the differential diagnosis.

Treatment

Most pets can be treated on an outpatient basis. If your dog is having trouble breathing, it should be given a quiet place to rest, safe from activity and anything else that would be an exertion. If your dog has an excess of fluid in any of its body cavities as a result of the mesothelioma, such as in the chest or abdomen, your veterinarian will need to hospitalize it for a short period of time in order to drain these cavities. If the fluid has collected in the pericardial sac, surgery to relieve the pressure will be required.

Canine mesotheliomas are similar to human mesotheliomas in their clinical and morphologic appearance. Humans with mesotheliomas usually die from secondary complications of mesothelioma rather than the tumor per se. However, mesothelioma in humans and dogs usually is rapidly progressive and has a grave prognosis.