Effect of immunosuppressants on dogs exposed two and one-half years previously to Blastomyces dermatitidis

Observation of 33 dogs exposed to Blastomyces dermatitidis over a 2 1/2 year period is reported. During the first 1 1/2 years six dogs died and 23 (77%) developed skin lesions. Clinical evidence of disease in all the animals then subsided and remained so through another year of observation. Nine animals were sacrificed after 120 weeks and several tissues were cultured from each one. The remaining 18 were given immunosuppressants--azathioprine, prednisone, and cyclophosphamide--for eight weeks prior to autopsy. About one-half of the dogs were found to be infected with Histoplasma capsulatum and all were negative for B. dermatitidis by culture. The dogs were thought to have been inadvertently exposed to H. capsulatum during the first eight weeks of observation from a nearby soil plot before being moved to a more remote shelter. Skin testing with histoplasmin and blastomycin and complement-fixation (CF) tests, performed at various intervals after exposure to B. dermatitidis, appeared to reflect the course of disease with the number of reactors to the H. capsulatum antigens remaining relatively high while the number of reactors to B. dermatitidis antigens fell with the passage of time. The blastomycin skin test results in these dogs, however, persisted far longer than positive Blastomyces CF results. These data suggest that most dogs exposed to B. dermatitidis exhibit clinical symptoms of disease after about 12 weeks, persisting for about one year. The majority of animals recover spontaneously and completely. With H. capsulatum, however, the infection can be subclinical for at least a two-year period.