Introduction

The incidence of cancer continues to increase world wide both in humans and companion animals (^{Pinho et al 2012}). Despite the seriousness of this process and the large number of studies performed, current medical research in this area continues to be hampered by the complex biology of cancer (Pinho et al 2012).

The studies of spontaneous neoplasia in companion animals, particularly in canine species, provides useful information in the research on comparative epidemiology, pathogenesis, and therapeutics, since dogs have a shorter lifespan and develop tumors similar to those diagnosed in humans regarding both morphological and biological behaviour (^{Marconato et al 2009}). Although rodents have been used for the development of highly standardised cancer models, they are often limited to very specific aspects of spontaneous human cancer such as latency period, recu rrence, and metastasis (^{Thamm and Dow 2009}, ^{Ranieri et al 2013}). Canine species share a greater proportion of its genome with humans than rodents representing a powerful model to study human mammary tumors (^{Pinho et al 2012}).

Mammary tumors are one of the most common neo plasms in female dogs (^{Rivera and von Euler 2011}). These processes are very similar to those that occur in humans, although the incidence in the dog is three times higher (^{Moe 2001}, Rivera and von Euler 2011). Daily, veterinarians assess patients with mammary tumors establishing their clinical stage, prognosis and treatment (^{Perez Alenza et al 2000}). Recently, an increase in the diagnoses of malignant mammary tumors in female dogs has been observed (^{Salas et al 2015}), probably linked to the increased exposure to oncogenic substances such as pyrethroids, that represent an important environmental pollution agent (^{Andrade et al 2010}).

The aim of this study was to describe the incidence of mammary tumors in female dogs in the region of Extremadura, Spain, and to determine the epidemiological characteristics (age, breed and size) and their relationship with tumor incidence and malignancy.

Material and methods

Results and discussion

In this study, we analysed the relevance of mammary tumors in female dogs and the influence of 4 variables (breed, age, size, and tumor malignancy). Neutering status and spay age were not analysed in this study because only 11.69% of the patients were spayed at the time of diag nosis and none of them before the fourth estrous. There are not many records about the incidence of mammary tumors in dogs (Evengall et al 2005). Our results support the importance of oncological pathology in female dogs with a prevalence of 43.8% (95 percent of confidence interval, CI₉₅%: 40.48-47.12%). In addition, the preva lence of mammary tumors within tumor-bearing patients (Figure 1) was 60.37% (CI₉₅%: 55.42-65.31%). These re sults agree with other studies in which the most common tumor type in the female dog corresponded to neoplasms of the mammary gland (^{Lana et al 2007}, ^{Merlo et al 2008}, ^{Choi et al 2016}). As several authors point out, the fact that mammary tumors together with skin tumors represent the most frequent veterinary consultations might be because both locations are easily observed and accessed by both the owner and the veterinarian (^{Vascellari et al 2009}, ^{Dobson 2013}, ^{Gruntzig et al 2015}). Thus, in this study, skin tumors are the second most frequent type of tumors diagnosed in female dogs (17.02%) as shown in (Figure 1).

Figure 1 Distribution of oncological patients (group II) according to the type of tumor. 

The incidence of mammary tumors according to their histological type and stage of the patient is shown in (Tables 1 and 2), respectively. We observed a high impact of malignant mammary tumors (88.13%) (CI₉₅%: 83.92%, 92.34%). Our results do not agree with those observed by other authors, who detected much lower percentages of histological malignancy, with studies showing an incidence between 30% (^{Salas et al 2015}) and 50% (^{Gilbertson et al 1983}). The reason for this might be that our referral hospital is the only one in the entire region of Extremadura and patients with clinical signs of malignancy are habitua lly attended at the hospital, whereas those patients with small tumors, with slow growth ratio and with no visible signs of malignant disease are frequently treated by local veterinarians. As reported by ^{Olivera et al (2003}), we cannot exclude that neutering status is related to the high percentage of malignancy observed in our study with only 11.69% of spayed females; in addition, ^{Vascellari et al (2016}) observed a significantly proportion of complex carcinoma in intact females. However, we cannot exclude the role of other factors such as environmental pollution (e.g. pesticides and fertilizer) as the responsible for the high incidence of malignant tumors in the region. Thus, 64.74% of our patients were classified in stages II to V (Table 2).

Regarding the risk factors that favour the appearance of the different types of tumors, it was possible to observe that the onset age of the disease does not have an effect on the type of oncological process developed (mammary tumor versus other tumors; P=0.073). In group II (on cological patients) age is directly correlated with tumor malignancy (P=0.012). In mammary tumors our results confirm previous reports suggesting a remarkable increase in mammary neoplasms in females between 6 and 10 years, with an average age of 10 years. The highest peak of presentation is reached at 10 years old which is followed by a marked decrease in the incidence of tumors of the mammary tissue (^{Misdorp 2002}, Evengall et al 2005, ^{Lana et al 2007}, ^{Salas et al 2015}). There was a trend between a more advanced age at diagnosis and malignancy of mammary tumors (P=0.065), in agreement with ^{Sorenmo et al (2009}) and ^{Vascellari et al (2016}), who indicate that patients with benign tumors have a mean age of 8.5 years while those with malignant tumors are older (9.5 years of age), with statistically significant differences. However, these observations differ from those reported by Salas et al (2015), who did not observe a relationship between these variables. These discrepancies may be due to the smaller size of our sample and the low percentage of benign mammary tumors observed in our study (Table 3).

Table 1 Distribution of patients with mammary tumors according to histological type. 

Table 2 Distribution of patients with malignant mammary tumors according to their staging. 

Table 3 Distribution of patients with benign and malignant mammary tumors according to their age, breed and size. 

The relationship between the age of onset of canine mammary tumors and the animal size was analysed in this study, observing a trend between the size of the patient and the age at diagnosis of mammary tumors (P=0.058). This is in line with the results of ^{Sonnenschein et al (1991}) who suggested that animals of larger breeds have, by their very nature, a shorter lifespan, aging faster than animals of smaller breeds and, therefore, present neoplasms at an earlier age.

In this study a high incidence of tumors in purebred dogs was observed, which is likely to reflect the large number of purebreds attended at this particular Hospital. The evidence for the increased frequency of purebred dogs that suffer from mammary tumors (64.32%) is consistent with the findings of several studies (Perez Alenza 2012, Perez Alenza et al 2000, Evengall et al 2005, ^{Vascellari et al 2016}). However, ^{Karayannopoulou et al (1989}) observed the contrary and these discrepancies were attributed to the differences in the populations analysed (Perez Alenza et al 2000). Moreover, it was found that breed affects the type of tumor developed (P=0.000), with approximately 1.32 times (OR) being more likely to have a mammary tumor than any other oncological process if the patients are mixed-breeds. It was also observed that breed affects tumor malignancy in group II (P=0.000) and III (P=0.000), concluding that the likelihood to develop a malignant tumor in mixed-breeds compared to purebred patients is approximately 1.47 and 1.76 times, respectively. There is no explanation for this observation; further studies are required to identify those factors influencing tumor malignancy in mixed-breeds compared to purebred dogs. Otherwise, age at diagnosis of any type of tumor (group II) and mammary tumors (group III) in this study did not differ between mixed-breed or purebred dogs (P=0.092 and P=0.436, respectively).

In this study, the group of breeds with the highest incidence of mammary tumors is Retrievers- Flushing Dogs-Water Dogs (breed distribution of female dogs with mammary tumors is shown in Figure 2). These results are similar to those observed by ^{Perez Alenza et al (2000}). However, other studies describe an increased risk for Teckels and Molossoides (^{Moe 2001}), Grazing breeds (^{Sorenmo et al 2009}, ^{Andrade et al 2010}) and Companion Dog group (^{Lana et al 2007}). Our results could be explained to a great extent by the outstanding presence of Retrievers-Flushing Dogs-Water Dogs in our region due to the fact that hunting plays an important role in the regional economy. Thus, 14.6% of all patients treated in the Surgery Service at the Veterinary Clinical Hospital of the UEx belong to this group, supporting our assertion.

Figure 2 Distribution of female dogs with mammary tumors (group III) classified according to their breed. 

On the other hand, the majority (66.1%) (CI₉₅%: 59.94%, 72.26%) of female dogs with mammary tumors were medium-small sized; furthermore, the size of ani mals influenced the development of a particular type of neoplasia (P=0.000), being 0.49 times (OR) more likely to have a mammary tumor if the female dog is large than if it is medium-small sized dog.

Finally, animal size affected the tumor malignancy in groups II and III (P=0.000), with approximately 0.85 times (OR) more likely to have a malignant tumor (group II) and 2.34 times more likely to be a malignant mammary tumor (group III) when the patients are large sized dogs. These results agree with the findings of ^{Itoh et al (2005}) in mammary tumors and differ with ^{Salas et al (2015}) who found no association when the occurrence of malignant tumors was compared to the size category, suggesting that there might be an effect caused by the different classification system used to group patients according to their size.

It is concluded that oncological pathology is the most frequent clinical condition observed in female dogs at tended at the Surgery Service of the Veterinary Clinical Hospital of the UEx and mammary tumors represent the most common neoplasia in female dogs. Mixed-breed and large size represent risk factors for malignancy. Likewise, older age should be considered as a risk factor for the development of malignant neoplasia.