Reaching a definitive diagnosis 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 is often a multi-step process with the common need for anesthetic procedures to procure 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, adverse 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 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 our diagnostic tests is that if the DNA of the pathogen in question is present, then that segment will be amplified, and once processed, the test will be positive. If the DNA is not present, the PCR test will be negative. Some PCR tests provide quantitative results as well.
As stated previously, primers are used to connect to a desired section of DNA in a sample. It 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 diseases we are trying to rule out.
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 Chalmyida 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.
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 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 would be more likely than a primary infection in this species.
What makes PCR testing so great is that we can take a single DNA molecule and turn it into millions of copies in a short time.
Veterinary PCR testing gives clinicians an opportunity 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. For more information on sample requirements please refer to our test menu.
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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.