Cancer in dogs
Cancer is the leading cause of death in dogs. It is estimated that 1 in 3 domestic dogs will develop cancer, which is the same incidence of cancer among men. Dogs can develop a variety of cancers and most are very similar to those found in humans. Dogs can develop carcinomas of epithelial cells and organs, sarcomas of connective tissues and bones, and lymphomas or leukemias of the circulatory system. Certain breeds are known to be at high-risk for specific cancers as a result of selective breeding. Veterinary oncology is the medical study of cancer in animals. Veterinarians that specialize in cancer diagnosis and treatment are called veterinary oncologists.
Cancer is a complex, multifactorial disease. Carcinogenesis is linked with DNA mutations, chromosomal translocations, dysfunctional proteins, and aberrant cell cycle regulators. Cancer alters the DNA of cells and the mutated genetic material is passed on to daughter cells, resulting in neoplasms. The mutated DNA effects genes involved with the cell cycle, classified as either oncogenes or tumor supressor genes. Oncogenes are responsible for cell proliferation and differentiation. Oncogenes responsible for cell growth are overexpressed in cancerous cells. Tumor supressor genes prevent cells with erroneous cell cycles from replicating. Cancer cells ignore cell cycle regulators that control cell growth, division, and death.
The selective breeding techniques used with domestic dogs causes certain breeds to be at high risk for specific cancers. Selection for specific phenotypes in dog breeding causes long-range linkage disequilibrium in their DNA. Certain areas of alleles have the tendency to separate less frequently than normal random segregation, which leads to long ranges of repeated DNA sequences. These repeated sequences caused by decreased genetic diversity within breeds, can lead to a high prevalence of certain diseases and especially cancer in breeds.
Symptoms of cancer in dogs may include:
- Lumps (which are not always malignant, but should always be examined by a vet)
- Persistent sores
- Abnormal discharge from any part of the body
- Bad breath
- Rapid, often unexplained weight loss
- Sudden lameness
- Offensive odor
- Black, tarry stools (a symptom of ulcers, which can be caused by mast cell tumors)
- Decreased or loss of appetite
- Difficulty breathing, urinating or defecating
Types of Cancer
- Mast cell tumors are the most common type of skin cancer in canines.
- Prostate cancer
- Brain cancer
Hemangiosarcoma is a type of cancer that develops almost exclusively in dogs, the only two other species known to have it are cats and horses. Hemangiosarcomas are tumors that form on the blood vessels, and can occur all over the body. These tumors can develop on the skin, subcutaneously, or on a blood vessel within an organ and are highly malignant. The tumors are most fatal when they rupture, causing the dog to suffer from severe loss of blood.
Dogs are one of three mammalian species that are known to suffer from a transmissible cancer. Canine transmissible venereal tumor (CTVT) is species specific and highly contagious. The cancerous cell lines are transmitted between individuals that are in close contact with eachother through acts of intercourse, biting, scratching, or licking. The cancer is prevalent in populations of stray dogs or environments of uncontrolled copulation. The tumors occur around the area of external genitalia and can grow up to 15 cm in area. Canine transmissable venereal tumors can often be infected, ulcerated, and hemorrhagic.
Cancer prevalence in dogs increases with age and certain breeds are more susceptible to specific kinds of cancers. Millions of dogs develop spontaneous tumors each year. Boxers, Boston Terriers and Golden Retrievers are among the breeds that most commonly develop mast cell tumors. Large and giant breeds, like Great Danes, Rottweilers, Greyhound and Saint Bernards, are much more likely to develop bone cancer than smaller breeds. Lymphoma occurs at increased rates in Bernese Mountain dogs, bulldogs, and boxers. It is important for the owner to be familiar with the diseases to which their specific breed of dog might have a breed predisposition.
Treatment options vary and depend on the type and stage of cancer. Common treatments include surgery, chemotherapy, radiation therapy, amputation, and immunotherapy. A combination of therapies may be used. Knowledge and treatment of cancer have increased significantly in the past three decades. Survival rates have also increased due to the increase prevalence of canine cancer treatment centers and breakthroughs in targeted drug development. Canine cancer treatment has become an accepted clinical practice and access to treatment for owners has widely expanded recently. Cancer-targeting drugs most commonly function to inhibit excessive cell proliferation by attacking the replicating cells. However, there is still a prevalent pharmacy gap in veterinary oncology.
There is one canine tumor vaccine approved by the USDA, for preventing canine melanoma. The Oncept vaccine activates T-cell responses and antibodies against tumor-specific tyrosinase proteins. There is limited information about canine tumor antigens, which is the reason for the lack of tumor-specific vaccines and immunotherapy treatment plans for dogs.
Success of treatment depends on the form and extent of the cancer and the aggressiveness of the therapy. Early detection offers the best chance for successful treatment. The heterogeneity of tumors makes drug development increasingly complex, especially as new causes are discovered. No cure for cancer in canines exist.
Some dog owners opt for no treatment of the cancer at all, in which case palliative care, including pain relief, may be offered. Regardless of how treatment proceeds following a diagnosis, the quality of life of the pet is an important consideration. Some forms of treatment can eradicate the cancer from the patient's body, and almost all patients receive at least some benefit from treatment. In cases where the cancer is not curable, there are still many things which can be done to alleviate the dog's pain. Good nutrition and care from the dog's owner can greatly enhance quality of life.
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