This study provides a valuable picture of the most common morbidities and causes of death in a defined elderly canine population. It is based on data obtained from necropsy examinations, thus overcoming one of the main fragilities of previous papers on canine’s morbidity and mortality, which lay on data collected from medical or animal insurance companies’ records.
In the population included in this study, the organic systems that most frequently exhibited pathological changes were the reproductive, cardiovascular and urinary systems and, in females, also the mammary gland. These results overlap those from Fleming and collaborators [16] who described an association between increasing age and increased risk of cardiovascular and genitourinary disease in canines. The prevalence of cardiovascular and urinary disease was significantly higher in males than in females. In a previous investigation on a large group of canines up to 10 years of age, Bonnet and colleagues [13] also reported a higher frequency of cardiovascular disease in male dogs. The present results strengthen that data, demonstrating that this sexual predisposition is maintained with aging. The incidence of cardiovascular disease is also high in humans, especially in men [19, 20]. However, it is noteworthy that the specific cardiovascular diseases occurring in these species are different. Most of the cardiovascular morbidity and mortality in aging humans is related to vascular atherosclerotic and ischemic disease in the heart and brain, which is a very rare event in dogs [21]. On the other hand, age-related valvular disease (and consequent cardiac failure) is far more common in canines than in humans [22]. Whether these discrepancies represent species-related different physiological mechanisms of aging or differences in the interplay between age and other features (such as diet, medication, habits and lifestyle) remains to be clarified. Interestingly, in this study the prevalence of cardiac dilation is slightly higher than that of valvular endocardiosis. The reduced number of small dogs included, representing nearly 34% of the total population, may help to explain the lower number of cases of valvular endocardiosis (a typical condition of small breed geriatric dogs) compared to dilated cardiomyopathy (a disease more frequent in large breed animals).Urinary disease was more frequent in small breeds dogs, corroborating data from other investigators [16]. On the other hand, the prevalence of peritoneum and male genital morbidities was significantly higher in larger dogs, a finding not previously reported to the best of the author knowledge. This may be due to differences in the categorization of the organic systems; indeed, some authors bring together the genital and urinary tract under a broader designation of “urogenital system” [16].
In addition to represent an important morbidity, cardiovascular and urinary pathology also emerged as a frequent cause of death in elderly canines. However, despite the high prevalence of lesions in certain organic systems, there is not always a linear correspondence between them and the cause of death. Indeed, in spite of the high number of lesions identified in the reproductive system, their influence on the animal death is minor. Only 9.3% (7/75) of the reproductive system morbidities were determinant as cause of death. On the contrary, most of the lesions (7/9 = 77.8%) registered in the central nervous system were decisive to the animal death. Also 51.1% (24/47) of the mammary gland neoplasias and 39.3% (46/117) of the cardiovascular lesions represented a major cause of mortality.
In general, findings from this study parallel data obtained for human species, which display cancer and cardiovascular pathology as major causes of disease and death in older adults [1, 23, 24].
The main cause of death in geriatric canines population included in this study was neoplasia, which accounted for almost half of the deaths. This data is in accordance with results obtained by other authors, who described that death due to neoplastic disease increases with age [14,15,16, 25]. Moreover, larger dogs died of neoplastic disease more frequently than did smaller ones, a finding also referred by Fleming et al. [16]. Malignant neoplasms related to death mainly involved the mammary gland, followed by the hematopoietic and the digestive system. Bonnet and colleagues [13] also reported a high mortality rate attributed to mammary and hematopoietic neoplasia in a large population of dogs up to 10 years of age.
Old age and cardiac failure also represented important mortality causes, mirroring data published by Proschowsky et al. [11] and Adams et al. [14]. Neoplasia, along with cardiovascular failure and old age, were responsible for more than 80% of the deaths registered in our elderly canine population. On the other hand, traumatic lesions represented a minor cause of death in geriatric canines, which is in accordance with data published by other investigators [13, 15, 16] and parallels data obtained from humans [1].
Hyperplastic lesions (mostly registered in the prostate, liver, spleen and pancreas) and degenerative processes (especially involving the heart and the musculoskeletal system) were frequent morbidities found in this elderly dog population. Although not directly related to the animal death, they probably compromise several capabilities, contributing to the deterioration of its quality of life, leading the owners to request euthanasia for humanitarian reasons. Furthermore, animals included in the category “old age” may constitute a valuable model in the study of age-related multimorbidities, which afflicts a large proportion of human older adults, and besides of representing a challenge to clinical management, impose a serious overload on healthcare system [1, 26].
Results from this study are particularly relevant considering the One Health concept, a multidisciplinary and collaborative approach recognizing interconnections between humans, animals and their shared environment as crucial for achieving an optimal health outcome. The increase in human life expectancy observed in recent decades in developed countries, with inevitable higher demands on health services and threats to the pension systems sustainability, has justified the growing interest in understanding the mechanisms inherent to the aging process. Recently, some authors have emphasized the value of the dog as a suitable translational animal model for aging and gerontology [1, 8, 10]. Humans and canines share the same environment and are exposed to similar hazard contaminants and pollutants, carcinogenic stimuli, infectious agents and pro-inflammatory conditions [8, 10]. It is widely recognized that both develop a variety of similar spontaneous age-related morbidities and have access to similar medical and health care conditions [1, 8, 16]. Furthermore, canines’ longevity is about 6 to 7 times less than that of humans, which facilitates the collection of data in a short period of time [2]. In this sense, although there are morphofunctional differences between both species and in some diseases developed during the aging process, the dog can be regarded as a suitable translational model for human aging, meeting the concept of One Health.
Despite all the information obtained in this study, it is not exempt from some limitations. Although the age-related pathological processes categorized in this study were based on those of previous investigations, they are not entirely overlapping, which may explain some differences observed between our and other data already published. In fact, there is some variability in the definition of the pathological processes categories included in different studies. The canine population included in this study becomes from a specific geographic area (northern Portugal) and may not be entirely representative of the elderly dog population from other regions, making comparison with data from other studies difficult. Lack of information on the body condition score and on the animal age at the time of spaying/neutering did not allow the assessment of the relationship between these features and morbidity/mortality. On the other hand, some morbidities related to the central nervous and the musculoskeletal systems may have been underdiagnosed, since exhaustive assessment of these organic systems rely on the existence of clinical information suggestive of pathology (which is not always fully available). Much of the canines included in this study were euthanized upon the owner’s request, due to a deterioration in their quality of life and impairment of daily activities (such as incoordination and locomotor difficulties, inability to feed independently, incontinence and loss of vision). This was especially relevant in animals with old age or neoplasia, of which 92 and 85% were euthanized, respectively. This feature inevitably limits the value of the data obtained as an indicator of lifetime because, as an artificial shortening of lifespan, it does not measure how long the dogs could live, but still constitute very relevant information regarding the animal’s health span.
These results reinforce the potential of the domestic dog for further translational investigations on gerontology. Understanding the organic changes and morbidities inherent to aging represents an opportunity to the development of strategies for prevention, early diagnosis, and management of geriatric patients, ensuring longevity without compromising their quality of life. Insights into aging mechanisms allow the establishment of selective anti-aging interventions in the organic systems most affected by this biological process, providing an effective and more rationale use of resources. Understanding the complex mechanisms governing aging and longevity, as well as age-specific morbidity and mortality, is mandatory to retard the aging rate and extend a healthy lifespan, both in humans and animals [2, 5].
