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Blastomycosis
in dogs and cats |
Among domestic animals, blastomycosis is most common and of
most importance clinically in dogs.
Blastomycosis in dogs:
Blastomycosis occurs most commonly in the late spring to fall
in sporting breeds and hounds (1-5 years of age). This probably
only reflects the likelihood of environmental contact by such
dogs, not any specific genetic breed predilection to infection.
However, some reports suggest Beagles, Dobermans, German Shepherds
and Dalmatians may be specifically over-represented among cases
of blastomycosis.
Clinical disease in dogs:
The incubation period for blastomycosis is relatively long
(5-12 weeks).
Blastomycosis in dogs occurs in 3 major forms:
1. pulmonary disease
- Conidia are inhaled from the soil, undergo reversion to a
yeast form, reproduce within macrophages and are released into
the alveoli, leading to inflammation.
- Clinical signs may include anorexia, weight loss, weakness,
a dry cough and dyspnea.
- Fever occurs in only about 20-40% of cases.
- Pulmonary lesions may be diffusely distributed throughout
the lungs ("cotton ball" appearance radiographically)
or they may occur as solitary mass lesions or consolidated lung
lobes. (See radiography below)
2. extrapulmonary (systemic) disease
- The yeast disseminate by lymphatic and hematogenous spread
from the lungs to other organs if the immune system cannot contain
the infection.
- The organs most commonly affected are:
- lymph nodes (lymphadenopathy)
- eyes (uveitis, glaucoma, retinal granulomas and/or detachment,
conjunctivitis, corneal edema)
- In a recent survey (Bloom et al., 1996) of ocular
blastomycosis in dogs, 6/108 eyes had anterior segment disease,
24/108 eyes had posterior segment disease and 78/108 had endophthalmitis.
- Ocular disease occurs in ~40% of cases.
- bones (osteomyelitis of the long bones, primarily below the
stifle/elbow)
- Bone involvement occurs in about 30% of cases.
- skin (dermatitis, cellulitis, fistulous tracts)
- Skin lesions occur in ~20-40% of cases.
3. localized cutaneous disease
- Cutaneous
disease can follow direct inoculation of the organism from
the soil into the skin, but it is far more commonly a manifestation
of systemic disease.
Blastomycosis in cats:
Blastomycosis is much less common in cats (~100-fold) than in
either dogs or people. When they are infected, cats most commonly
manifest systemic disease.
Diagnosis of blastomycosis in dogs and cats:
- cytologic evaluation on wet mounts of exudates or fine-needle
aspirates for the presence of BROAD-BASED BUDDING YEASTS
- Organisms may not be detectable in transtrracheal washes
or bronchoalveolar lavages because the organism exists in the
lung primarily in interstitial rather than bronchial locations.
- radiography
- lungs - variable appearances:
- non-specific interstitial/alveolar pattern
- diffuse, miliary, nodular to "cotton ball" pattern
with hilar lymphadenopathy (This is often considered the "classic"
presentation of pulmonary blastomycosis.)
- solitary mass-like lesions or lung lobe consolidation
- bone:
- osteomyelitis
(generally single lesions, often in the epiphyseal regions-
can be either lytic or proliferative in appearance)
- histopathology:
- granulomatous/suppurative inflammation, with PAS or methanamine
silver staining of fungal elements
- serology:
- An agar-gel immunodiffusion (AGID) assay is commercially-available,
using the A-antigen of B. dermatitidis. While highly specific,
the sensitivity of this assay has been reported to be 50-90%.
A radioimmunoassay (RIA) using the WI-1 antigen has been developed
and is reported to be both highly specific (100%) AND sensitive
(92%). However, dogs may be seronegative early in the course
of infection and, conversely, diagnosis based on serology alone
can be problematic since dogs may recover spontaneously from
subclinical infections and remain seropositive.
- ELISA assays are under development.
- PCR assays are available - the assay targeting the WI1 gene
appears to be more sensitive and specific than that targeting
the 18s ribosomal RNA gene.
- culture (This is rarely done because of the potential danger
to laboratory technicians from inhalation of hyphae and conidia.)
