A review article explores possible reasons that might explain why cats and dogs are not susceptible to all of the same types of diseases.
Dogs and cats may be the two most common small animal species that people keep as pets, but they are not equally susceptible to all of the same types of diseases.
Certainly, a search of the literature seems to suggest that cats are less affected by arthropod-borne diseases. According to Michael J. Day, BVMS, PhD, from the University of Bristol, United Kingdom, between 1997 and May 2016, a search of the Thomson Reuters Web of Science database revealed 496 publications about these diseases for the dog and 175 for the cat.
In a review article published in Parasites and Vectors, Dr Day discusses some of the possible reasons for these differences.
Other simple possibilities have been suggested for the lower prevalence of arthropod-borne infections in cats, including the behavior and lifestyle of cats. “In some countries, many more cats have an indoor only lifestyle that of course minimizes the risk of exposure to arthropods,” the author writes. Can outdoor cats also avoid ticks better than dogs can avoid them? Does their grooming behavior possibly dislodge ticks more efficiently?
Owners may also take cats for veterinary care less frequently than they take dogs, and may spend less money on veterinary attention for cats, Dr Day adds.
With respect to infectious diseases in general, conditions such as leptospirosis and non-invasive upper respiratory tract (URT) fungal infections tend to affect dogs more than they affect cats. In contrast, mycobacterial infections, viral infections, and invasive URT fungal infections affect cats more. However, cats tend to be relatively more resistant to natural influenza virus infection than dogs are.
The Immune System
Both dogs and cats have the same range of lymphoid subsets and antigen presenting cells; both also express the same range of cytokines, key molecules, and pattern recognition receptors.
“Although dogs and cats largely share equivalent immune systems, there are clear differences between the species as to how the elements of the immune system interact—creating species diversity in susceptibility to, and clinicopathological expression of, immune-mediated, neoplastic, and infectious diseases,” writes Dr Day.
However, he adds that subtle differences may exist in canine and feline immunoglobulins (Igs). “The dog has four IgG subclasses which are functionally equivalent to those of man. In contrast, only three IgG subclasses are recognized in the cat,” he explains. Although both dogs and cats have IgM and IgE antibodies, IgD has been formally identified only in the dog. The dog is known to have four genetic variants of IgA, but feline IgA has not been similarly investigated.
What could explain these differences? Because dogs are also relatively more susceptible than cats to allergic diseases and autoantibody-mediated diseases, Dr Day raises the question about whether dogs’ immune responses are Th2 dominated, involving antibody production. In contrast, these diseases could be less common in cats because they generate a Th1 immune response, involving cell-mediated immunity with cytotoxic responses.
However, this background of Th1 immunity would, theoretically, be more protective against viral infections in cats—when, in fact, cats are actually more susceptible to viral infections than dogs are.
But the Th1 versus Th2 model could help explain the difference in prevalence of arthropod-borne infectious diseases between cats and dogs, says Dr Day. “If dogs truly were a Th2-dominated species they might logically have greater susceptibility to vector-borne pathogens that often require a Th1 immune response to control or sterilize the infection.”
Could a genetically-controlled immunological resistance to arthropods help explain cats’ reduced susceptibility to arthropod-borne diseases?
Genes of the major histocompatibility complex (MHC) are the most important genes regulating immune responsiveness. Although in both dogs and cat, this gene complex is spread over two chromosomes, cats lack one locus within the MHC class II gene cluster (the DQ gene), which could reduce possibilities for antigen presentation.
Dr Day also noted that inbreeding has reduced genetic diversity within the different breeds of dog. Within dog breeds, high linkage disequilibrium and restricted MHC type also exist, whereas much less linkage disequilibrium exists in cat breeds.
“Such restricted genetic diversity might help explain the susceptibility of dogs to certain diseases, including potentially, the arthropod-borne infectious diseases,” explains Dr Day.
Could differences in canine and feline immunity be explained by differences in the microbiome, which is known to regulate immune development, immune function, and disease susceptibility?
The canine and feline microbiomes differ in the type of organisms they contain and their metabolic profiles. Despite some similarity in the major families of bacteria involved, the fungal components of the microbiome are much more diverse in cats than in dogs. The microbiome of the two species also differs in disease. In inflammatory bowel disease, for example, intestinal dysbiosis is characterized by increased numbers of Clostridium perfringens in dogs, but not in cats.
Overall, Dr Day emphasizes that the reasons for the reduced susceptibility of cats to arthropod-borne infectious diseases are probably complex and unable to be explained by some simple immunological concept. “[M]uch work is still required to characterize more accurately the true prevalence and clinical significance of these infections in the cat,” he concludes.
Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.