Blastomycosis
What characteristic morphologic forms help identify this
pathogen?
- Size and broad based budding, refractile thick cell wall.
What are risk factors for acquisition of this organism, and what is
the portal of entry?
- Exposure to soil and warm moist wooded areas rich in organic
debris increase the risk of exposure to Blastomyces.
- Inhalation of
conidia and conversion to yeast phase in the lung provide the most
common portal of entry, with skin lesions being a manifestation of
dissemination; however primary skin disease after dog bites has been
reported.
What are the major host defenses against this pathogen?
- Inhaled conidia are phagocytized and killed by neutrophils ,
monocytes and macrophages .
- The yeast forms are more resistant to
phagocytic killing ; hence, conversion to yeast forms represents a
virulence characteristic of B dermatitidis. T cell mediated responses
are important .
- Humoral immunity has no demonstrable protective effect.
What clinical syndromes are caused by this pathogen?
- Pneumonia and empyema, ulcerated or verrucous lesions of the
skin which may mimic carcinoma, subcutaneous nodules, bone and joint
involvement, prostatitis and brain abscess are seen .
- Disseminated
infection is more frequent in immunocompromised hosts, in particular in
HIV disease.
What diagnostic procedures are helpful?
- Demonstration of characteristic yeast forms in clinical
specimens and silver stains are helpful .
- Presence of pyogranulomas or
pseudoepitheliomatous lesion of the skin or upper respiratory tract
should prompt a search for B. dermatitidis. Culture on Saborauds or
enriched media may take upto 4 weeks to grow .
- Conversion of the
mycelial form into the yeast phase is required for confirmation of the
diagnosis.
- Serologic tests lack sufficient positive or negative
predictive value to be of use in routine clinical practice.
What is the treatment of choice?
- In severely ill patients or immunocompromised patients,
especially in the presence of CNS involvement, treatment with
amphotericin B is preferred.
- In immunocompetent hosts with mild to
moderate disease itraconazole or ketoconazole may be used.