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 cuttingedge 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.