Research Updates: Antigen and antibody tests for diagnosing and monitoring blastomycosis in dogs

Although not particularly sensitive or specific, agar gel immunodiffusion is the most widely available commercial test for detecting Blastomyces dermatitidis antibodies. The enzyme immunoassay, however, is highly sensitive for detecting B. dermatitidis antigen (galactomannan) in people and can also be used to detect anti-B. dermatitidis antibodies.

Erika Meler, DVM, MS

In this multicenter study, sera from 46 dogs with confirmed blastomycosis were tested by enzyme immunoassay (MVista Blastomyces dermatitidis antigen EIA—MiraVista Diagnostics) for B. dermatitidis antigen and by agar gel immunodiffusion (Meridian Bioscience) and enzyme immunoassay (MVista Blastomyces dermatitidis antibody EIA—MiraVista Diagnostics) for anti-B. dermatitidis antibodies. Urine samples from these dogs were tested by enzyme immunoassay for B. dermatitidis antigen. Forty-four dogs without fungal infections were used as a control group. Serum and urine antigen concentrations were measured in sequential samples from 23 infected dogs at baseline and during treatment.

Barrak Pressler, DVM, PhD, DACVIM

The enzyme immunoassay and agar gel immunodiffusion detected anti-B. dermatitidis antibodies in the sera of 76.1% and 17.4% of infected dogs, respectively. Blastomyces dermatitidis antigen was detected by enzyme immunoassay in 93.5% of urine and 87% of serum samples. The results showed that enzyme-immunoassay-based B. dermatitidis antigen detection is more sensitive than either enzyme immunoassay or agar gel immunodiffusion antibody testing for diagnosing blastomycosis in dogs. Furthermore, the B. dermatitidis antigen concentration decreased over time in both the serum and urine of infected dogs during treatment.

COMMENTARY

Blastomycosis is a commonly encountered disease in small-animal practice. Diagnosis usually requires identifying organisms by cytologic or histologic examination of tissues or, more rarely, by culture of exudates or body fluids. In a recent study, transthoracic fine-needle aspiration or transtracheal wash successfully identified B. dermatitidis organisms in 80.7% and 69.2% of cases, respectively, whereas anti-B. dermatitidis antibodies were detected by agar gel immunodiffusion in only 50% of cases.1

The study discussed here demonstrates that enzyme immunoassay detection of B. dermatitidis antigen is a valuable addition to the arsenal of diagnostic tests. In fact, antigen testing may eventually prove to be useful in the early diagnosis of blastomycosis and the most accurate test for diagnosis and therapeutic monitoring of this disease, with fewer potential complications than direct sampling of the respiratory tree. The role of antigen enzyme immunoassay in monitoring response to treatment must be explored further, but pilot results reported here are promising.

Unfortunately, the enzyme immunoassay cannot distinguish between histoplasmosis and blastomycosis galactomannan, thus decreasing the specificity of this test. Cross-reactivity also occurs in people with other mycoses. Pending further studies to determine the specificity of the enzyme immunoassay, B. dermatitidis antigen testing should be considered as a reasonable ancillary diagnostic test in dogs with suspected infection.

Spector D, Legendre AM, Wheat J, et al. Antigen and antibody testing for the diagnosis of blastomycosis in dogs. J Vet Intern Med 2008;22(4):839-843.

The information in "Research Updates" was provided by Erika Meler, DVM, MS, and Barrak Pressler, DVM, PhD, DACVIM, Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University, West Lafayette, IN 47907.

REFERENCE

1. Crews LJ, Feeney DA, Jessen CR, et al. Utility of diagnostic tests for and medical treatment of pulmonary blastomycosis in dogs: 125 cases (1989-2006). J Am Vet Med Assoc 2008;232(2):222-227.

Spector D, Legendre AM, Wheat J, et al. Antigen and antibody testing for the diagnosis of blastomycosis in dogs. J Vet Intern Med 2008;22(4):839-843.