Understanding CAPC's Parasite Prevalence Maps
Amanda Carrozza is a freelance writer and editor in New Jersey.
Factors including motivation, population, and sample size have an impact on CAPC’s prevalence maps for parasitic diseases such as heartworm, Lyme disease, and ehrlichiosis.
The Companion Animal Parasite Council (CAPC) is a trusted and often used source for veterinary professionals, media outlets, and pet owners as a means to track the presence of vector-borne diseases confirmed in a specific area. For instance, a quick glance at the agency’s 2018 prevalence map for Lyme disease shows that the risk of infection in the northeastern United States is high. Similarly, the canine heartworm prevalence map reveals that there have been nearly 6500 positive cases of heartworm in North Carolina this year—more than 3 times the number of confirmed cases in California.
How exactly are these maps created and updated?
In a recent article on the CAPC website, Heather Walden, MS, PhD, an assistant professor at the University of Florida College of Veterinary Medicine, explained the key factors that contribute to CAPC’s prevalence maps.
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Antibody and Antigen Testing
The tests used to detect vector-borne diseases pick up antigen or antibody biomarkers in a blood sample. When translating the results for prevalence maps, it’s important to understand the difference between the 2 types of tests and which is used for a particular disease.
For instance, Dr. Walden wrote, “the canine heartworm test is antigen-based, so the prevalence map is indicative of actively infected animals.” Tests used to detect feline heartworm, on the other hand, may be based on antigen or antibody levels. This raises some concern about validity because, by nature, antibodies can remain present after infection has been cleared. A positive result from an antibody test indicates that the subject has been exposed to the pathogen at some point in time, but it does not necessarily represent a new infection.
When creating the prevalence maps, analysts must remain cognizant of factors in a pet’s history that make it more likely to produce a positive result. For instance, a dog that recently moved between rescue groups in different states prior to being adopted is more likely to harbor parasites due to lack of previous protection. That same dog may have acquired an infection before moving across state lines. If tested after arrival, that case would be considered a new positive result in an area of the country where infection historically may have been rare.
Because a complete patient history is not always available, Dr. Walden said positive results in areas where there is frequent turnover in residents—major cities, retirement facilities, and military bases—must be interpreted with caution to account for infections that present in translocated animals.
In epidemiologic studies like the ones used to create the maps, the study population is a smaller representation of the general population and mainly consists of test results that were presented to a veterinary hospital. Because these animals are client-owned, the results do not account for feral or stray animals, those not tested or seen by a veterinarian, and a majority of animals in rescue or shelter facilities. Because of the smaller size of the study population, and the likelihood that the owned animals are exposed to preventive medications more than the general population, the prevalence estimates displayed on the maps are conservative. It is believed that the incidence rates for each pathogen are much higher in the total population.
An insufficient sample size is the most likely limitation in accurately understanding the results from any given survey tool. Beyond having no pets tested in a given practice area, analysts also have to be leery of situations in which too few pets are tested because a small sample size will exaggerate the results. “Caution should be employed in interpreting any percentages generated by testing a small number of pets, particularly when the pathogen presence is entirely unexpected,” Dr. Walden wrote.
It’s unlikely that a veterinarian would test every patient for a vector-borne disease regardless of practice location or the presence of clinical signs. As such, the motivation for testing should be another consideration when evaluating the map’s data. “When pets that are more likely to be infected are disproportionately tested, the number of positive results increases as does the calculated percentage of positive results,” she wrote.
An increase in positive results due to motivation is most likely to be seen in areas where testing is used more for diagnostic verification that for routine screening. Currently, Dr. Walden explained, the data do not include the reason for testing.
Types of Tests Used
When interpreting map data, it is worth noting that individual tests, like those used to detect Lyme and heartworm disease, vary in both sensitivity and specificity. Tests that lack sensitivity may underestimate the true rate of infection. Conversely, tests with poor specificity can inflate the number of reported infections due to false positive results.