The Diagnosis of Canine Infectious Diseases
Amy K. Swinford, DVM, MS, ACVM
Microbiology Branch Chief, Texas A&M Veterinary Medical Diagnostic Laboratory
Our canine companions can become infected by a wide variety of bacterial, fungal, parasitic, and viral pathogens. These organisms are capable of causing many different clinical manifestations of disease, from mild skin lesions to devastating systemic illnesses that can result in death. The traditional methods of diagnosing disease, such as microscopic examination of tissues and culture of specimens, remain among the most reliable and widely used techniques available to veterinary diagnosticians. In recent years, newer technologies such as polymerase chain reaction (PCR) detection of pathogens have been developed in an effort to improve the speed, sensitivity, and specificity of diagnosing canine infectious disease.
Pathogens responsible for canine diseases fall into the broad categories of bacteria, viruses, fungi, and protozoal as well as multi-cellular parasites. There are a variety of methods to detect the presence of these organisms. Some are large enough to be visualized with the naked eye. An example of this would be multi-cellular parasites like tapeworms. But most are microorganisms that require some form of microscopy for visualization. Microorganisms that can be seen under the light microscope include bacteria, fungi, and protozoal parasites; viruses can be seen using electron microscopy that gives very high magnification. While microscopy can give a definitive diagnosis and is still very widely used in the diagnostic lab, it is laborious, requires skilled and experienced personnel, and is a relatively insensitive method (i.e. has low probability of detecting a microorganism).
Because of these issues with microscopy, other methods to “look” for infectious agents have been developed. Some involve looking for direct evidence of microorganisms by growing and identifying them, such as is done in the Bacteriology and Virology testing sections at TVMDL. Other methods rely on indirect evidence of infection by using the detection of antibodies against microorganisms, as is done in the Serology section. Techniques that detect the genetic material (DNA/RNA) or the antigenic composition of infectious agents are gaining acceptance and are increasingly being developed.
In this presentation, both the conventional as well as some of the more cutting-edge tests used by veterinary diagnosticians to diagnose infectious diseases of dogs will be reviewed. Several recent TVMDL submissions illustrating how diagnostic testing is selected and performed to arrive at a diagnosis will be presented. Finally, some tips on sample submission to maximize the return of diagnostically useful test results will be discussed.