Safety, Tolerability, and Immunogenicity of a Recombinant, Genetically Engineered, Live-Attenuated Vaccine against Canine Blastomycosis

Blastomycosis is a severe, commonly fatal infection caused by the dimorphic fungus Blastomyces dermatitidis in dogs that live in the United States, Canada, and parts of Africa. The cost of treating an infection can be expensive, and no vaccine against this infection is commercially available. A genetically engineered live-attenuated strain of B. dermatitidis lacking the major virulence factor BAD-1 successfully vaccinates against lethal experimental infection in mice. Here we studied the safety, toxicity, and immunogenicity of this strain as a vaccine in dogs, using 25 beagles at a teaching laboratory and 78 foxhounds in a field trial. In the beagles, escalating doses of live vaccine ranging from 2 10 4 to 2 10 7 yeast cells given subcutaneously were safe and did not disseminate to the lung or induce systemic illness, but a dose of <2 10 6 yeast cells induced less fever and local inflammation. A vaccine dose of 10 5 yeast cells was also well tolerated in vaccinated foxhounds who had never had blastomycosis; however, vaccinated dogs with prior infection had more local reactions at the vaccine site. The draining lymph node cells and peripheral blood lymphocytes from vaccinated dogs demonstrated gamma interferon (IFN-), tumor necrosis factor alpha (TNF-), and granulocyte-macrophage colony-stimulating factor (GM-CSF) specifically in response to stimulation with Blastomyces antigens. Thus, the live-attenuated vaccine against blastomycosis studied here proved safe, well tolerated, and immunogenic in dogs and merits further studies of vaccine efficacy. Blastomyces dermatitidis is the causative fungus of blastomycosis, which is a potentially life-threatening systemic infection in humans and other mammals, most often dogs. In the United States, blastomycosis is restricted to locations in the southeastern, south-central, and Upper Midwest states, where environmental conditions favor the growth of B. dermatitidis in the soil. In areas of high endemicity, annual incidence rates of canine blastomycosis of 1 to 2% have been reported (2). The fungus is presumably inhaled from infected soil into the lungs of a susceptible animal, initially causing a pneumonia. In dogs, the fungus often disseminates from the lungs to the skin, eyes, lymph nodes, and bones. The mortality rate exceeds 90% for dogs that do not receive prompt antifungal treatment (9). Blastomycosis is most commonly seen in large dogs, where the cost of treatment can exceed $1,000. Vaccine prevention would offer a cost-effective alternative for those dogs at risk in areas of high endemicity, but no commercial vaccine is available. Blastomyces dermatitidis is a dimorphic fungus that grows in the soil as a mold and, after the inhalation of infectious particles or spores, switches to the pathogenic yeast form in the lung. Yeast cells display a surface adhesin termed BAD-1 (Blastomyces adhesin 1). Gene targeting and deletion of BAD-1 attenuate pathogenicity of the yeast in a murine model of lethal pulmonary infection, defining BAD-1 as a virulence factor (3). This genetically modified, attenuated strain of B. dermatitidis given subcutaneously as a vaccine protects mice from lethal infection with wild-type virulent B. dermatitidis (for strains that are genetically related and unrelated to the vaccine strain) and produces sterilizing immunity (10). Thus, this recombinant attenuated strain holds promise as a potential vaccine against blastomycosis. In this report, we describe studies that were done to assess the safety and immunogenicity of this recombinant attenuated strain of B. dermatitidis yeast administered to dogs as a candidate vaccine against blastomycosis.