Tick panels, or vector-borne disease panels, help to identify patients with tick-borne illness. But they have also likely
led to overdiagnosis and overtreatment of disease. In her presentation "Use and misuse of diagnostic panels in canine vector-borne
disease testing," Linda Kidd, DVM, PhD, DACVIM, described the benefits of and drawbacks to polymerase chain reaction (PCR)
and serologic testing.
WHICH TO CHOOSE: PCR OR SEROLOGY?
Start with a reasonable suspicion and test accordingly
A clinician's best tool in determining not only which organisms to test for but which test methodology to choose is the knowledge
of diseases endemic to the area and their common clinical presentations. For example, tick-borne disease testing in a dog
with immune-mediated hemolytic anemia may include tests that can identify Ehrlichia canis and Babesia, Anaplasma, Bartonella, and hemotropic Mycoplasma species. Whether to choose PCR or serology to make a diagnosis is a common conundrum and, unfortunately, not one that comes
with a simple answer, explained Dr. Kidd.
Keep in mind that each test looks for different types of evidence of infection:
- Serology provides indirect evidence by documenting the presence of antibodies against an organism. So it documents exposure,
not necessarily active infection.
- A PCR test detects the organisms' DNA, as long as the organisms are present in the peripheral blood.
Think it through
For organisms that circulate in low numbers or intermittently during chronic infections, PCR testing may prove to be an insensitive
diagnostic tool. For example, infection with Anaplasma phagocytophilum typically results in an increased number of circulating organisms during the acute phase, so PCR would be an appropriate
and sensitive test when anaplasmosis is suspected, whereas Rickettsia rickettsii infections typically have low numbers of circulating organisms, making PCR insensitive in this case. Documenting a fourfold
serologic increase in acute and convalescent R. rickettsii antibody titers would be a more sensitive diagnostic tool in that scenario.
Previous antibiotic therapy, such as with doxycycline, will also decrease the number of circulating organisms and may result
in false negative PCR test results. Dr. Kidd said this is why storing samples obtained at the time of the initial presentation
can be helpful. Both serum and EDTA samples can be held while awaiting other test results. Or if one test type produces a
negative result but the disease is still suspected, storing ensures that untreated blood will be available for alternative
testing. The patient can be started empirically on therapy during this process, without affecting test results.
Patients in the acute phase of illness often may not have had time to mount an antibody response, or seroconvert, and would,
therefore, have negative serologic results early on. In these cases, PCR would be the better test to choose. But for chronic
infections, such as Lyme borreliosis in which the clinical signs manifest weeks after exposure, there will likely be decreased
numbers of organisms in the peripheral blood at the time of presentation. Thus, serologic testing would be preferred, as PCR
test results would likely be negative.
Take other factors into account
As mentioned previously, PCR testing detects the presence of the organisms' DNA, which can apply even to dead organisms and
result in a positive test result. In this case, treatment and response to therapy will help determine if the clinical signs
are due to the tick-borne disease in question.
As with any diagnostic test, it is important to interpret results in light of the clinical findings. For example, infection
with A. phagocytophilum or R. rickettsii typically causes acute signs of illness, so positive serologic results for these organisms in patients with chronic signs
may simply indicate a previous infection.
Assuming no laboratory error, a positive result for a disease not commonly seen in an area also warrants concern, as it may
represent disease emergence. According to Dr. Kidd, the geographic distribution of many tick-borne illnesses is expanding,
and many are no longer geographically isolated.
1. Kidd L. Use and misuse of diagnostic panels in canine vector-borne disease testing. Presented at the Annual Meeting of the
American College of Veterinary Internal Medicine; June 2012.
This "Lecture Link" summary from the 2012 American College of Veterinary Internal Medicine Forum was contributed by Jennifer
L. Garcia, DVM, DACVIM, a veterinary internal medicine specialist at Sugarland Veterinary Specialists in Houston, Texas.
Dr. Jennifer L. Garcia