Clinical performance of a point-of-care Coccidioides antibody test in dogs | VETgirl Veterinary Continuing Education Podcasts
In this VETgirl online veterinary continuing education podcast, we discuss a point-of-care (POC) antibody assay for Coccidioides in dogs. As this VETgirl rarely treats this disease – living in the frozen tundra of Minnesota – let’s do a quick refresher on coccidioidomycosis. Recall that this is a systemic fungal infection also known as Valley Fever, common in the southwest United States and parts of northern Mexico (Brown). The fungal organisms live in the soil, but unfortunately can become aerosolized and inhaled, leading to disease. Affected dogs often suffer from respiratory disease, but a quarter of cases can result in more serious, disseminated infections (Davidson).
In a study by Reagan et al entitled Clinical performance of a point-of-care Coccidioides antibody test in dogs, the authors dive into the diagnostic options available for detecting Coccidioides, and they offer a quick and helpful review of the available options. Briefly, several diagnostic options are available, though cytology, histopathology, or fungal culture represent the gold standards. An agar gel immunodiffusion (AGID) assay is used commonly in dogs to detect IgM or IgG against fungal antigens, but this test is expensive, has certain technical challenges, and varies in performance. Additionally, an enzyme immunoassay can also be performed for rapid antibody detection, but this assay still requires sending samples to a reference laboratory. Lastly, an immunochromatographic lateral flow assay (LFA) is another type of rapid detection assay that detects antibodies against two Coccidiodies antigens. This is a POC test that can be performed in approximately 30 minutes! Therefore in the current study, the authors sought to determine the agreement between this LFA and the standard AGID assay in dogs with suspected coccidioidomycosis.
The authors collected serum from client-owned dogs with a clinical suspicion of coccidioidomycosis. Previously stored samples at the University of California Davis Coccidioidomycosis Laboratory for Coccidioides antibody testing were used, and an additional set of prospectively collectively samples were also included. Unfortunately, complete medical records were not available for all of the retrospectively included samples, though all included samples had an AGID assay performed at this UC Davis laboratory. Control dogs were dogs that had coccidioidomycosis as an initial differential, but ultimately were diagnosed with a different condition. A cohort was selected using convenience sampling for LFA analysis, which was performed by a single individual for consistency. Both a positive and negative control were used as recommended. This LFA shows a red control line that helps you determine it is working appropriately, and an additional test line appears if the sample is positive. Therefore, the authors documented the results as positive (both control and test lines present), negative (only control line present), or invalid (control line absent).
In total, 48 serum samples were included in this study, which again were collected both prospectively and retrospectively. The majority of samples were from California and Texas, with several other states having a smaller number of submissions. Of these 48 samples, 36 were positive for Coccidioides antibodies using the AGID assay, and quantitative IgG titers ranged from 1:1 to 1:128 in the 35 dogs that had them performed. Recall that the control dogs were those that had an initial clinical suspicion of coccidioidomycosis, but ultimately were diagnosed with a different condition. These dogs’ final diagnoses included neoplasia (n = 3), multisystemic autoimmune disease (n = 2), respiratory disease (n = 2), other infectious disease (n = 2), granulomatous meningoencephalomyelitis (n = 1), panuveitis (n = 1), and a dog with a polyostotic lesion thought secondary to previous septic arthritis (n = 1).
All 48 dogs had the LFA assay performed. Of the 36 samples that were AGID positive, 32 were positive using the LFA, and 4 were negative. Of the 12 samples that were AGID negative, all 12 were negative using the LFA. In other words, of these 48 samples, the AGID and LFA assays agreed in 91.7%, and the 4 discordant samples were all AGID positive and LFA negative. The Cohen’s kappa statistic was 0.80. Recall that if a control line was absent on the LFA assay, the test was recorded as invalid, but there were no tests that had invalid results in this study. If we look more closely at the 4 samples with discordant results, all 4 had AGID IgG titers performed. Three had titers of 1:4, and 1 had a titer of 1:16. When comparing the LFA to AGID, the positive predictive value in this population was 100%, and the negative predictive value was 75%.
The authors highlight a few limitations for the readers to consider. First, the study did not use identification of actual fungal organisms, such as cytologic visualization or microbiologic methods. Secondly, the dogs included in this study were at various stages of disease progression, and the authors note that evaluating the LFA at the time of diagnosis and during the course of treatment would also be valuable. Lastly, only 1 control dog was diagnosed with a different fungal infection, which was Cryptococcus, but more animals with various types of fungal infections would help further interrogate the LFA assay’s specificity. So with that all said, what can we take away from this VETgirl podcast? Well the LFA appears to have good agreement with the AGID, demonstrating 89% positive percentage agreement and 100% negative percentage agreement. That said, if this VETgirl had a high clinical suspicion of coccidioidomycosis and had a negative LFA result, I may perform additional testing for verification. Overall though, this test appears to have great diagnostic potential and to also be quick and technically feasible. It will be exciting to see how it performs in additional studies!
1. Reagan K, McHardy I, Thompson III G, et al. Clinical performance of a point-of-care Coccidioides antibody test in dogs. J Vet Intern Med 2021; 35: 965-969.
2. Brown J, Benedict K, Park BJ, et al. Coccidioidomycosis: epidemiology. Clin Epidemiol 2013;5:185.
3. Davidson AP, Shubitz LF, Alcott CJ, Sykes JE. Selected clinical features of Coccidioidomycosis in dogs. Med Mycol 2019;57:S67-S75
AGID – Agar gel immunodiffusion
LFA – Lateral flow assay
POC – Point-of-care
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