Reaching a definitive diagnosis in veterinary medicine is the ultimate achievement for any practitioner. Treatment, prognosis, and expectations can be specifically tailored to provide our patients the best chance at improving their current state.
The reality is that achieving a true diagnosis within veterinary microbiology is often a multi-step process with the common need for anesthetic procedures to procure samples of small intestine or liver tissue for evaluation. Despite improving morbidity and mortality rates of patients under anesthesia, it’s often a common barrier where clients decline further investigation due to cost, perceived effects of anesthesia, or worries that all of the work involved will produce a non-diagnostic sample. This situation leaves us with a big question:
What kind of assay can we consider that is less invasive but still has the potential to provide an accurate diagnosis?
One option to consider may be veterinary PCR testing.
PCR is referred to as polymerase chain reaction and is a process where a single DNA biosample from feces, fluid, blood, and more is amplified or copied a million times. First, the section of DNA of interest is identified via denaturation. The annealing process then follows where the DNA sections are bound with a primer which acts as the complementary DNA strand.
The extension process then finalizes this connection. This process creates a new, exact copy of the section and is then repeated millions of times.
What this means for diagnostic testing is that if the DNA of the virus or bacteria in question is present, then that segment will be amplified, and once processed, the test will yield a positive sample. If the DNA is not present, the PCR test will be negative. Some PCR tests provide quantitative results as well, and the most common type of sample diagnostic laboratories receive include nasal swabs, oral swabs, conjunctival swabs, tissue biopsies, choanal swabs, and cloacal swabs.
As stated previously, primers are used to connect to a desired section of DNA in a sample. Their molecular characterization almost acts like a name tag, but not all name tags are as clear as others, ie: is it Jeff or Geoff? Therefore, there is a chance that PCR tests for the same disease are not of equivalent sensitivity and specificity.
Now that we know how PCR testing works, when is it appropriate to use? Well, it depends.
Essentially, a PCR test can be performed on any tissue or fluid at any time. But to make this test work in the way we want, we must understand the pathophysiology of the virus or bacteria we are trying to identify.
Let’s take for instance disease A. We know that this disease lives primarily in the bloodstream for the entirety of its existence in the body. Therefore, it would be fair to submit a blood sample for PCR testing to understand if disease A is present. We would feel very confident that submitting a blood sample would rule in or out this disease.
However, what about disease B? This disease sometimes lives in the bloodstream but will then harbor in liver tissue as well depending on the stage of disease. Disease B is similar to how Chlamydia pssitaci acts in avian species. We could in theory submit a blood sample, and a positive test would confidently diagnose this disease. However, a negative test does not definitively rule out the disease as it could solely be in the liver at the time of testing. In this instance, PCR testing is a fair preliminary diagnostic, but further testing such as serology or even a biopsy may still be required.
Now take disease C: this infectious process has a respiratory manifestation but also a gastrointestinal one that may be mutually exclusive. This would be like Mycobacterium genevense for instance. If a PCR test for disease C states it can be run on a fecal sample or a sample from the respiratory tract, careful consideration must be taken when choosing the type of sample tested. For example, if the respiratory form of disease C is present, then sending a fecal sample for PCR testing would tell you very little about what is going on in the respiratory system. Essentially, keeping in mind the stages of infection is important for any diagnostic assay.
Finally, PCR testing does not differentiate between viable and non-viable organisms, so the DNA of a pathogen can be found anywhere on any animal. Therefore, it is important to know what species the disease is actually able to infect. Let's take a positive result on a PCR screening for Macrorhabdus ornithogaster, a common infectious agent in small parrots but yet to be reported as an infectious agent in medium to large parrots: If this disease was noted in a sick budgie or cockatiel, it’s likely to be clinically relevant. However, if a positive test was reported in a macaw, further inquiry into this bird's environment is warranted as the potential for contamination from another household bird may be more likely than an active infection in this species.
What makes PCR testing so great is that diagnostic laboratories can take a single DNA molecule and quickly turn it into millions of copies. Turnaround time can generally take two to five business days.
Veterinary PCR testing gives clinicians a viable assay technique to achieve a definitive diagnosis in a potentially minimally invasive way so long as the right sample is provided.
In case you didn’t know, Moichor helps you access these tests! Here is a list of PCRs currently offered at our laboratory. For more information on sample requirements please refer to our test menu.
Circovirus (Beak & Feather) PCR
Macrorhabdus (AGY) PCR
Spironucleus genus PCR
Mycobacterium species PCR
Mycobacterium genavense PCR
Mycobacterium avium PCR
E. Cuniculi PCR
Bearded Dragon Atadenovirus pCR
Arenavirus (IBD) PCR
Feline Upper Respiratory Panel: Bordetella bronchiseptica, Mycoplasma felis, Chlamydia sp., Pneumovirus, Influenza A virus (all subtypes), Mycoplasma cynos, Streptococcus zooepidemicus, Feline herpesvirus (viral rhinotracheitis), Feline calicivirusa
Canine Upper Respiratory Panel: Canine Adenovirus Virus, Canine Distemper Virus, Canine Parainfluenza Virus, Canine Pneumovirus, Beta-Coronavirus, Bordetella bronchiseptica, Mycoplasma cynos, Streptococcus zooepidemicus, Influenza A Virus
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2. Palmieri C, Roy P, Dhillon AS, Shivaprasad HL. Avian mycobacteriosis in psittacines: a retrospective study of 123 cases. J Comp Pathol. 2013;148(2-3):126-138.
3. Baron HR, Stevenson BC, Phalen DN. Comparison of In-Clinic Diagnostic Testing Methods for Macrorhabdus ornithogaster. J Avian Med Surg. 2021;35(1):37-44.