Janelle S. Renschler, DVM, PhD, Dipl ACVP and Lawrence Joseph Wheat, MD
We receive many calls from veterinarians across the U.S. and Canada requesting assistance in diagnosing or monitoring fungal infections in dogs and cats. The systemic mycoses often occur in localized areas (“hyperendemic” regions); therefore, a few vets see many fungal cases, but most vets are somewhat unfamiliar with these diseases. Many practitioners have only heard of our testing through continuing education or consultation with a specialist. This leads to confusion in selecting tests, interpreting test results, and treating the diseases.
Often there is a lack of understanding of the cross reactivity between the various fungal antigen tests. The surface antigens of many fungi are similar, and it is difficult to develop antigen assays that are highly specific for only one fungus. This is particularly problematic for Blastomyces and Histoplasma; therefore, the antigen assays for these two fungi display almost complete cross-reactivity. In addition, blastomycosis and histoplasmosis have overlapping endemic ranges and the clinical diseases may be very similar in animals. We do maintain separate assays for these fungi, but we do not recommend that a specimen be screened in both assays. Factors such as geographic region, travel history, clinical signs, etc. can be used to decide which test to submit. Ultimately, the assays do not differentiate Blastomyces from Histoplasma, so either assay could be used if the clinical features or locale cannot distinguish the infections. Our canine Blastomyces and Histoplasma antibody EIAs may be useful to help make a specific diagnosis. The antibody EIAs display less cross reactivity, and true positive results are usually higher than cross reactive results.
We are often asked if we offer a panel of fungal tests. Usually this is for cases with vague signs (e.g., fever of unknown origin) or pyogranulomatous inflammation with no etiologic diagnosis. Perhaps a specialist had recommended “fungal testing” and the practitioner lives in an area with no common systemic mycoses. Testing for all our fungal antigens and antibodies might be an easier way to randomly screen, but it would also lead to much unnecessary testing--transferring a cost burden to the pet owner. Instead, we provide numerous educational resources and clinical consultation to help vets pinpoint the right tests for the right patient. Usually the test selection can be greatly narrowed based on clinical features, geography, and travel history.
For vets practicing in the southwestern US, or those in northern states who see dogs owned by travelers or “snowbirds”, our coccidioidomycosis (Valley Fever) testing is beneficial. We are excited to offer a canine Coccidioides IgG antibody EIA as it outperforms the standard immunodiffusion antibody test with faster turnaround time and potentially higher sensitivity. The diagnosis of Valley Fever is often challenging, and a combination of serology and search for the organism may be required. Our Coccidioides antigen EIA has rather low sensitivity overall; therefore, it should not be used as the sole diagnostic. However, we have seen that the test is more often positive in severe cases, sometimes when antibody tests are negative. Testing in both the antigen and antibody EIA for Valley Fever screening will yield the best sensitivity.
Table 1. Summary of diagnostic sensitivity and specificity for MVD veterinary fungal assays