Canine Cancers

Cancer is among the top fatal diseases in domestic and feral dogs and cats. Incidence of canine or feline cancer ranges from 1% to 2% and cancer currently accounts for about half of the deaths of domestic animals older than 10 years. The most common forms of cancer in dogs and cats are skin, lymphoma, mammary, bone, connective tissue, and oral cancers. The significant incidence and mortality associated with canine and feline cancers continues to challenge modern veterinary medicine to develop more reliable therapies. One of the most promising new cancer therapies is oncolytic virotherapy. This method is based on the capability of OVs to preferentially infect and lyse cancer cells and to initiate tumor-specific immunity. Several oncolytic viruses including human and canine adenoviruses, canine distemper virus (CDV), reovirus and vaccinia virus strains have been tested with convincing results in preclinical studies.

Most people associate the word tumor with a growth occurring on the skin or somewhere in the body. However, any sort of lump, bump, growth or swelling, such as an abscess, is a tumor. Those which are true growths are called neoplasms. Benign neoplasms are growths which do not invade and destroy, nor do they spread. They are cured by surgical removal. Malignant neoplasms, also called cancers, carcinomas, sarcomas, or lymphomas (depending on the cell type), invade, destroy, and tend to spread via the bloodstream and lymphatic system to distant parts of the body. This is called metastasizing. Although much has been learned, the exact cause of cancer is unknown.

Long-standing irritants to tissues are a definite cause of some cancers. The irritant agent appears to speed up tissue repair (and therefore the rate of cell reduplication, or mitosis) and interferes with the immune mechanism which destroys newborn cancer cells. Examples of agents known to increase the risk of cancer in people are: ultra-violet rays (skin cancer), x-rays (thyroid cancer); nuclear radiation (leukemia); chemicals (aniline dyes causing bladder cancer); cigarettes and coal tars (causing lung and skin cancer); viruses (causing experimental cancer in laboratory animals); and parasites (a cause of bladder cancer). Some benign tumors, such as warts and oral papillomas, are clearly due to a virus infection.



Cancers are approached in the following manner: Suppose a female dog has a lump in her breast. Since it is solid, it is probably a neoplasm. It could be benign or malignant. The decision is made to biopsy the lump. This is a surgical operation during which the lump, or a part of the lump, is removed and sent to the pathologist who will make a diagnosis by visual inspection of tissue under microscope. About half the cancers occurring in dogs are visible as growths or sores on the outer surface of the body (on or beneath the skin, in the perianal area, in the mouth, and in breast tissue). Signs that a tumor can be a cancer are visible growth, ulceration of the skin with bleeding, and a sore which does not heal. One other sign is a lump or knot in a place where none should be (the breast). If you observe any of these signs, be sure to discuss them with your veterinarian. Some tumors occur internally where you would be unlikely to detect them until they were quite large. When your dog has difficulty eating and digesting his food, or when he has an unexplained bowel disturbance, such as constipation or the passage of blood, you should consider the possibility of gastrointestinal cancer. Cancer in the reproductive tract of females causes few signs, but you should look for vaginal discharge and bleeding Canine conjunctival tumors (hemangiomas and hemangiosarcomas) are also common, but under-reported. Risk of conjunctival tumors is seen more often in dogs with increased outdoor activity. Early surgical therapy is recommended and may be curative; however, recurrence is possible and more likely with hemangiosarcomas. Recently, the first clinical studies with vaccinia and adenovirus for canine cancer therapy are underway.

The Bernese mountain dog breed is prediposed to histiocytic sarcoma
The Bernese mountain dog breed is prediposed to histiocytic sarcoma

Breed Predisposition

The mean mortality rate to cancer for all breeds was 27%, thus those breeds recording greater than 30% deaths from cancer might be considered to be at greater risk or predisposed and those less than 25% cancer deaths, at reduced risk, although that is not to say that these latter breeds do not suffer particular types of cancer.

Cancer is a common problem in dogs and although all breeds of dog and crossbred dogs may be affected, it is notable that some breeds of pedigree dogs appear to be at increased risk of certain types of cancer suggesting underlying genetic predisposition to cancer susceptibility. The breeds with the highest proportional mortality for cancer in the Kennel Club/BSAVA study included the following, in descending order: Irish water spaniel, flat-coated retriever, Hungarian wirehaired vizsla, Bernese mountain dog, rottweiler, Italian spinone, leonberger, Staffordshire bull terrier, Welsh terrier, and giant Schnauzer. Bernese mountain dog, Irish wolfhound, flat-coated retriever, boxer, and Saint Bernard were the five breeds of dog with the highest mortality from tumor-related death.

Osteosarcoma of the long bones is the most common malignant tumor of bone in dogs accounting for 85 −90% of primary bone tumors and almost exclusively affects the large and giant breeds such as rottweiler, great Dane, Irish wolfhound, greyhound, Saint Bernard. More recently hemangiosarcoma appears to have become a significant problem in golden retrievers in North America with an estimated life-time risk of 1 in 5 reported by the Golden Retriever Club of America. Mast cell tumors (MCTs) are common tumors of the canine skin, estimated to represent 7–21 percent of all skin tumors in this species. The boxer and bull dog breeds including bullmastiffs, Boston terriers, and Staffordshire bull terriers are reported to show an increased risk of developing MCT. Rhodesian ridgebacks, pugs, weimaraners, Labrador retrievers, beagles, and golden retrievers have also been reported to be at increased risk. Lymphoma is the most common hematopoietic malignancy in the dog. Dog breed has been shown to play a role in the epidemiology of lymphoma with several studies showing a significantly higher relative risk for boxers, bullmastiff and bulldog breeds compared to other breeds. Small breeds especially cocker spaniels and poodles and dogs with heavily pigmented oral mucosa are reported to be at greater risk of oral melanoma. A more recent study of canine oral melanomas showed the Chow Chow, golden retriever, and Pekingese/Poodle mix breeds to be overrepresented. Cutaneous melanoma occurs more commonly in dogs with heavily pigmented skin, with Schnauzers (both miniature and standard) and Scottish terriers at increased risk. Poodles (toy and miniature), spaniels (English springer, cocker, and Brittany), Puli, English setter, pointers, German shepherd, Maltese terrier, Yorkshire terrier, and dachshund have all been reported to be predisposed to mammary tumors. Golden retrievers and boxers were at increased risk to develop brain tumors.

In Bernese mountain dogs the disseminated form of histiocytic sarcoma accounts for up to 25% of deaths in the breed. The flat-coated retriever accounts for approximately 25% of all tumors in the breed, and up to 50% of all cancers.3

Cancer Treatment Options

The traditional and established methods for pet cancer treatment include surgery, radiation therapy, chemotherapy, hyperthermia and photodynamic therapy. However, the available treatment options for pet patients with advanced-stage disease are limited and the prognosis for such patients is very poor. Virotherapy using oncolytic viruses is one promising new strategy for cancer therapy. Oncolytic viruses (OVs) preferentially infect and lyse cancer cells, without causing excessive damage to surrounding healthy tissue, and initiate tumor-specific immunity. While only a few clinical trials of oncolytic virotherapy in canine or feline cancer patients have been carried out, evidence for the lack of toxicity of OVs in pets is emerging. Intravenous administration of canine adenovirus-2 in normal dogs and AdCD40L (adenovector expressing the CD40 ligand) in canine malignant melanoma cases did not show any virus associated toxicity 4

References

  1. Delbert G. Carlson, D.V.M. and James M. Giffin, M.D. Dog Owner's Home Veterinary Handbook
  2. Pirie CG, Knollinger AM, Thomas CB, Dubielzig RR. Canine conjunctival hemangioma and hemangiosarcoma: a retrospective evaluation of 108 cases (1989-2004).
  3. Breed-Predispositions to Cancer in Pedigree Dogs
  4. Oncolytic Virotherapy of Canine and Feline Cancer





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