The most well-documented neoplastic diseases in marine mammals include urogenital carcinoma in California sea lions and mixed neoplastic disorders in beluga whales in the St. Lawrence estuary.
Urogenital carcinoma in California sea lions has been documented for decades and is associated with gammaherpesvirus Otariid herpesvirus 1. Genetic factors (internal relatedness) and higher levels of polychlorinated biphenyls and DDT have also been found in California sea lions with urogenital carcinoma. The cancer originates in the cervix or vagina of females and penis or prepuce of males and is often widely metastasized in wild stranded animals. In one California stranding center, a disease prevalence of 26% was found among of adult and subadult California sea lions. Additionally, it has been reported in some captive California sea lions that initially were free-ranging.
Clinical signs include hind limb paresis, hind flipper and perineal or vulvar edema, prolapse, vaginal or prepucial discharge, chronic wasting, lethargy, and anorexia. Abdominal ultrasonography may show masses within the abdomen, enlarged sublumbar lymph node, and hydroureter and hydronephrosis in severe cases. No treatment has been attempted.
Other herpesviruses as well as papillomaviruses have been associated with mixed neoplastic diseases including oral squamous cell carcinoma in dolphins and cutaneous lesions in manatees. Mammary carcinoma has been documented in older captive California sea lions, and various tumor types have been documented in mixed marine mammal species. Various types of neoplasia have been documented in the resident beluga whales in the St. Lawrence estuary, although no single type of neoplasia is routinely noted. An association with toxic pollutants (polyaromatic hydrocarbons) in local waters has been suggested. Biliary and hepatocellular adenocarcinomas have been documented in older polar bears.
Other tumors in marine mammals are infrequent, although a wide variety has been reported. They are of little consequence except for malignant lymphoma in harbor seals, in which horizontal transmission can occur in a closed population.