Neoplasia is common in cats and dogs but care is needed when interstudies which include statistical analyses relating to prevalence and incidence because for companion animals these studies often contain a relatively low number of animals, and invariably they have been conducted using the case load presented to second opinion referral centres and not those in first opinion practices, or in the population as a whole. Geo-graphical variation in cancer incidence is also an important consideration when comparing reports from different parts of the world.

Dorn et al. (1968b) reported the annual incidence rate of neoplasia in dogs to be 381.2 per 100000 population, and Priester & Mantel 1971 reported an annual incidence rate of 687 per 100000 population. Between 34% (Dorn et al. 1968a) and 40% (Priester & Mantel 1971) were reported to be malignant neoplasms in dogs. It has been estimated that 50% of dogs aged over 10 years die of neoplasia (Kitchell 1988).

The estimated rate of occurrence for feline neoplasia is 264.3 per 100000 (Hodgkins 1980).

The commonest neoplasms reported to occur in dogs and cats are shown in Table 6.2, 6.3, 6.4, 6.5. For tumours of the skin, for example, the mean age for almost all types of tumour is over 8 years, and only canine cutaneous histiocytoma and mast cell tumours are commonly seen in animals less than 5 years of age (Dobson and Gorman 1988).

Most of the papers published on the incidence of neoplasia are generated by academic institutions and are based upon referral cases. This presents us with a serious weakness in interpretation because common easily rectified cases are dealt with by the first opinion practice and are not referred on. For example, testicular tumours, e.g. sertoli cell tumours are common in male dogs and yet they do not feature in Table 6.2.

After thyroid adenomas, lymphomas are the most common neoplasms in cats and advancing age is an important factor in some forms but not others.

Factors that are known to influence the occurrence of neoplasia are given in Table 6.6

The majority of neoplasms occur in older dogs and the relative risk of neoplasia increases with age there being a peak occurrence at an average age of 8 years, however some neoplasms are also common in younger animals (Table 6.7). The precise relationship between advancing age and the occurrence of neoplasia has not been determined but with advancing time:

  1. The likelihood of exposure to influencing factors increases.
  2. The duration of exposure to influencing factors present in the envirincreases.
  3. The more time there has been for oncogenes to express themselves.