This study allowed (i) estimation of the canine and feline population in two provinces of Veneto region (northern Italy), (ii) characterization of the population and (iii) estimation of spontaneous neoplasm incidence in dogs and cats living in the catchment areas.
To estimate the owned-pet dog and cat populations in the Vicenza and Venice provinces, we carried out a household telephone survey. The random-digit dial method is widely used and has been proven to be valid and reliable for many purposes [12]. Notwithstanding, people may not be listed in the phone book directory and may have a mobile phone only. As a consequence the sample selected based on a random sample of telephone numbers as well as individual habits, e.g. the time spent at home, could have introduced some bias. However, telephone lines that were busy or not answered were recalled a minimum of 4 times on different days and hours.
No previous data were available about the cat population in the Veneto region. The estimated dog population resulted to be higher than that reported in the demographic registry. Despite the fact that the registration of dogs is a legal requirement the combined efforts of both the official veterinary services and the veterinary practitioners, seem to be insufficient to assure a complete and unbiased dog registration. Availability of accurate estimates of pet populations is essential to conduct any population based epidemiological study, to evaluate the incidence of infectious and neoplastic diseases, to highlight geographical differences providing insight into the aetiology of the diseases and the possible environmental risk factors related.
In fact, it has been claimed that cancer occurrence in pets may warn of possible environmental causes of cancers in humans for specific geographic areas. Several noteworthy examples of environmental exposures that increase risk of cancer in companion animals have been e.g. a significant association with urban air pollutants was noted for dogs with tonsillar carcinoma [13]; bladder cancer in dogs may be associated with exposures to environmental contaminants, particularly herbicide and insecticide products, and some of these agents may also be associated with human cancer risk [14].
In our survey, the most frequently diagnosed tumours in dogs affected the mammary gland, skin and soft tissue, genital tract and oral cavity. These findings are in accordance with those reported by the animal tumour registry of Genoa (Italy) [6], and by the Norway canine cancer registry [15]. Cancers of the skin and mammary gland were by far the most common type of tumour encountered in these registries, followed by oral and testicular cancers [6, 15]. Mammary gland, genital, and skin tumours are easier to recognize by physical examination, as opposed to other tumours affecting internal organs that require specific investigations such as X-ray, computed tomography scanning (CT), magnetic resonance imaging (MRI) and ultrasound examination. In our registry, cases diagnosed by cytology and diagnostic imaging were also included, to minimise the possible underestimation of tumours affecting internal organs. Notwithstanding, due to the fact that CT and MRI are not routinely used as diagnostic tools, and post mortem investigations are not always required by owners, IRs are likely to have been underestimated.
In our registry, lymphomas rise particular concern, accounting for 3% of all canine tumours, and the IR increased constantly with age and in purebred dogs. These findings are in agreement with data from the Alameda Registry (1968) [3] and with the age specific incidence reported by Edwards and coll. (2003) in the UK population of insured dogs [16]. In the Genoa registry, non-Hodgkin's lymphoma was one of the most frequently diagnosed tumours, particularly in male dogs (20%) [6], and showed the highest IR in the age group of > 7-9 years dogs, decreasing in older dogs.
The estimated IR for all cancers in dogs was 143 in all the catchment area. Published IRs for approximately all cancers ranged between 310 and 958 per 100,000 dogs [6]. In the canine Genoa registry, IRs for all cancers increased significantly in the calendar period 2000-2002 compared with the period 1985-1989 showing a monotonic increase across the entire study period [6]. Considering the difference between the crude IRs of the two periods of our study (table 3) a possible underestimation of our rates can be hypothesized, due to the relatively short life of this registry and the increasing trend of sample submission observed during the registration period. Continuing sample collection may provide more stable and refined incidence data. Furthermore, differences in the estimated IRs between registries, could be attributable to the different methods applied to estimate the population, as well as to the completeness of the reported cases.
IRs of malignant tumours in dogs increased with age, peaking at age > 12 years. A similar pattern was observed in the canine Genoa registry, with a peak of IRs in dogs > 9-11 years [6].
In our survey, purebred dogs and cats seem to have a higher risk to develop malignant tumours than crossbred. Notwithstanding, it is noteworthy the wide breed-variation because of the genetic diversity between breeds. More data are necessary to make further investigation about IRs in different purebreds. Preliminary data from the Danish dog registry show increased relative risk in breeds such as Bull terrier, American Bull dogs and Boxers and the most frequently occurring neoplasm is shown to be mammary tumours [1].
It is difficult to make a direct comparison between our and other cancer registries because of differences in the estimated at risk populations, inclusion criteria and tumour diagnosis and classification. Within the veterinary community, the agreement to use a specific international classification system is strongly desirable, in order to increase the comparability of the information gathered in the existing veterinary cancer registries.