Today’s veterinary medicine isn’t your parent’s veterinary medicine. As science marches forward with new discoveries and techniques, advances in veterinary medicine follow suit. One such advance in the way we diagnose disease is the use of the polymerase chain reaction, or PCR. The polymerase chain reaction amplifies DNA so that one copy becomes millions of copies in a matter of hours. This is useful to detect and identify DNA where small amounts are present.
PCR was developed in 1983 by a biochemist named Kary Mullis, who I was actually lucky enough to meet in my days as an undergraduate. He won the Nobel Prize for his work in 1993. If the term PCR still isn’t ringing any bells, you’re not alone, but I know you know at least one common application for the technique—its use in forensics and court cases. Small amounts of DNA (say, those found in tiny drops of blood at a crime scene) are turned into large amounts of blood, and from there, a DNA match can be made. PCR has also helped those who have been imprisoned for crimes that they did not commit.
But back to the world of veterinary medicine, shall we? PCR has made the diagnosis of disease easier for veterinarians because while we may associate DNA with killers or paternity tests, the truth is that every living thing possesses unique genetic material. This means that finding a viral, bacterial, or protozoal cause for your pet’s symptoms can be as easy as submitting fluid or tissue samples to a laboratory.
PCR is different than other serology tests we run because it tests for the actual presence of the offending organism instead of the presence of antibodies against the organism. This important difference means that we can tell that the disease causing organism is there in your pet instead of just knowing that your pet has been exposed to and made antibodies against it.
A positive PCR test indicates that the bacteria, virus, or fungus is present in your pet. However, a negative test might not give your pet the all-clear. While negative tests can mean that your pet is free from the organism, it may also mean that the organism just wasn’t present in that particular sample, or that it was present in an amount that was too low for even PCR to detect it.
As stated previously, PCR can be used anywhere we want to amplify DNA. This means that we can use PCR to diagnose a plethora of infectious veterinary diseases. Whether we’re looking for tick-borne diseases like Lyme and Ehrlichia, other bacterial diseases like Leptospirosis in dogs and Mycoplasma (Hemobartonella) in cats, fungal diseases like Cryptococcus and Coccidiodes, or viruses like feline leukemia or canine distemper, PCR can aid us in our diagnoses. PCR can also detect the presence of protozoal parasites like Neospora and Toxoplasma.
While PCR seems like a magic bullet, it does have some limitations. If antibiotics or other treatment have already been started, a positive result may not occur. This can confound the diagnosis of a disease for which treatment has started. It’s always best to perform this test before treatment begins. Additionally, since the test has to be sent to an outside laboratory, time is a factor. It may take one to three days for the laboratory to send results. Add on the time it takes the sample to travel, and the results may be too late for a sick patient.
All in all, though, veterinarians and their patients are happy to have PCR as another tool in their diagnostic arsenal!