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School of Molecular & Biomedical Science
The University of Adelaide
AUSTRALIA 5005

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Dr David Ellis
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Blastomycosis

Description:

Blastomycosis is a chronic granulomatous and suppurative disease having a primary pulmonary stage that is frequently followed by dissemination to other body sites, chiefly the skin and bone. Although the disease was long thought to be restricted to the North American continent, in recent years autochthonous cases have been diagnosed in Africa, Asia and Europe. All available clinical and epidemiological evidence indicates that humans and lower animals contract blastomycosis from some source in nature. However, the natural habitat of B. dermatitidis has yet to be clearly delineated, despite some reports of its isolation from soil.

Clinical manifestations:

Pulmonary blastomycosis: In most individuals pulmonary lesions are asymptomatic and are not detected until the infection has spread to other organs. Others develop symptoms after an incubation period of 3-15 weeks. In most cases blastomycosis is indolent in onset and patients present with chronic symptoms such as cough, fever, malaise and weight loss. The lesions become more extensive, with continued suppuration and eventual necrosis and cavitation. Occasional patients present with an acute onset of infection, with development of high fever, chills, productive cough, myalgia, arthralgia and pleuritic chest pain. Often these patient appear to recover after 2-12 weeks of symptoms, but some will return months later with lesions at other sites. Other patients with acute onset will fail to recover and will develop a chronic chest infection or disseminated infection. Chest radiographic findings are variable and are not diagnostic.

Cutaneous blastomycosis: Haematogenous spread gives rise to cutaneous lesions in over 70% of patients. These tend to be painless and present either as raised verrucous lesions with irregular borders, or as ulcers. The face, upper limbs, neck and scalp are the most frequent sites involved.

Ulcerated granuloma on the nose
Ulcerated granuloma due to B. dermatitidis. (Courtesy of John Rippon, USA).

Cutaneous blastomycosis on the hand
Cutaneous blastomycosis. (Courtesy of John Rippon, USA).

Osteoarticular blastomycosis: Occurs in about 30% of patients with the spine, pelvis, cranial bones, ribs and long bones most commonly involved. Patients often remain asymptomatic until the infection spreads into contiguous joints, or into adjacent soft tissue causing subcutaneous abscesses. Radiological findings are often non-specific and arthritis occurs in up to 10% of patients.

Other forms include genitourinary blastomycosis involving the prostrate, epididymis or testis; haematogenous spread to the brain causing meningitis, and spinal or brain abscess. Other organs may also be involved and choroiditis and endophthalmitis have been reported.

AIDS patient have developed fulminant blastomycosis with widespread dissemination following endogenous reactivation of previous infection.

Laboratory diagnosis:

1. Clinical material: Skin scrapings, sputum and bronchial washings, cerebrospinal fluid, pleural fluid and blood, bone marrow, urine and tissue biopsies from various visceral organs.

2. Direct Microscopy: (a) Skin scrapings should be examined using 10% KOH and Parker ink or calcofluor white mounts; (b) Exudates and body fluids should be centrifuged and the sediment examined using either 10% KOH and Parker ink or calcofluor white mounts, (c) Tissue sections should be stained using PAS digest, Grocott's methenamine silver (GMS) or Gram stain.

Histopathology is especially useful and is one of the most important ways of alerting the laboratory that they may be dealing with a potential pathogen.

Tissue sections Tissue sections
Tissue sections showing large, broad-base, unipolar budding yeast-like cells, 8-15 um in diameter.

Note: tissue sections need to be stained by Grocott's methenamine silver method to clearly see the yeast-like cells, which are often difficult to observe in H&E preparations.

Interpretation: As a rule, a positive direct microscopy demonstrating characteristic yeast-like cells from any specimen should be considered significant.

3. Culture: Clinical specimens should be inoculated onto primary isolation media, like Sabouraud's dextrose agar and Brain heart infusion agar supplemented with 5% sheep blood.

Interpretation: A positive culture from any of the above specimens should be considered significant.

WARNING: Cultures of Blastomyces dermatitidis represent a severe biohazard to laboratory personnel and must be handled with extreme caution in an appropriate pathogen handling cabinet.

4. Serology: Serological tests are of limited value in the diagnosis of Blastomycosis.

5. Identification: In the past microscopic morphology, conversion from the mould form to the yeast form, and animal pathogenicity have all been used; however exoantigen tests are now the method of choice for identifying Blastomyces dermatitidis.

Causative agents:

Blastomyces dermatitidis

Treatment Guidelines

 

Further reading:

Ajello L and R.J. Hay. 1997. Medical Mycology Vol 4 Topley & Wilson's Microbiology and Infectious Infections. 9th Edition, Arnold London.Howard, DH (ed) 1983-1985.

Kwon-Chung KJ and JE Bennett 1992. Medical Mycology Lea & Febiger.

Richardson MD and DW Warnock. 1993. Fungal Infection: Diagnosis and Management. Blackwell Scientific Publications, London.

Rippon JW. 1988. Medical Mycology WB Saunders Co.