Blastomyces dermatitidis

Associated infections

• Acute and chronic pulmonary blastomycosis
• Acute respiratory distress syndrome in severe pulmonary disease
• Cutaneous blastomycosis, including verrucous and ulcerative lesions
• Osteoarticular blastomycosis
• Genitourinary blastomycosis, notably prostatic involvement
• Central nervous system blastomycosis
• Disseminated disease in immunocompromised patients

Transmission route

Infection is acquired almost exclusively by inhalation of conidia aerosolised when contaminated soil or decaying organic matter is disturbed. Reported exposures typically involve outdoor activity near waterways, excavation, forestry, or disturbance of rotting wood and leaf litter. The incubation period is long and variable, commonly ranging from around three weeks to several months. Direct cutaneous inoculation is described but rare, and person-to-person transmission does not occur under ordinary circumstances. There is no recognised reservoir in treated municipal water, building plumbing or plumbing biofilm.

The long and variable incubation period has a practical consequence for investigation. A patient presenting with blastomycosis may have been exposed weeks or months earlier and in an entirely different location, and travel history to an endemic region is more informative than any local environmental sampling. In a non-endemic setting such as Australia, a confirmed case is far more likely to represent acquisition abroad than local exposure, and this should be established before any environmental investigation is contemplated.

B. dermatitidis has no established relevance to endoscope reprocessing, final rinse water or biofilm control. It does not colonise water distribution systems, does not persist in reprocessor plumbing, and is not transmitted by contaminated instruments. Consequently, recovery of B. dermatitidis from a reprocessing water sample is not an expected finding and should first raise the possibility of misidentification, since several environmental moulds and dimorphic fungi can be confused morphologically. The appropriate response is to have the identification confirmed by a reference mycology laboratory using molecular methods before any conclusion is drawn. If identification is confirmed, investigation should address environmental ingress of mould into the sampling or water treatment pathway rather than biofilm control, alongside review of sampling technique and laboratory handling to exclude cross-contamination.

It is also worth noting that culturing this organism carries laboratory biosafety implications. The mould form is highly infectious by inhalation and is handled under appropriate containment; a laboratory that suspects a dimorphic fungus should be alerted so that plates are not opened on an open bench.

Relevance in endoscopy and reprocessing

B. dermatitidis has no established role in flexible endoscope reprocessing, and the useful contribution of a reference page on this organism is to say that clearly rather than to imply relevance it does not have. There are no documented cases of blastomycosis transmitted by a contaminated endoscope, no evidence that the organism establishes in reprocessor plumbing or endoscope channels, and no mechanism by which a rinse water supply would sustain it. It is not a biofilm organism, and comparisons with Pseudomonas aeruginosa or the non-tuberculous mycobacteria do not hold: those organisms are adapted to persist and replicate within engineered water systems, whereas this fungus requires a specific soil environment that a plumbing system does not resemble in any respect.

Behaviour against high-level disinfection is not a limiting factor. Fungal conidia are more resistant than vegetative bacteria to drying and to low-level disinfectants, but they are markedly less resistant than bacterial spores and are inactivated by validated high-level disinfection with glutaraldehyde, ortho-phthalaldehyde or peracetic acid at the conditions used for semi-critical devices, as well as by steam sterilisation. The organism has no recognised tolerance mechanism that would allow it to survive a correctly executed reprocessing cycle. Drying and storage considerations, which are central for mycobacteria because residual channel moisture permits regrowth, are not relevant here — B. dermatitidis will not multiply in an endoscope channel.

The honest framing for a reprocessing service is that if this organism appears in a water result, the finding is about the sample, the sampling environment or the laboratory, not about the reprocessing system. Where a genuine mould ingress route into the water pathway is identified, the controls are the same environmental ones that apply to any airborne fungus — sealed sample points, filtered vents on storage vessels, intact building fabric, control of construction dust, and clean technique during filter changes — and the response is not an escalation of water treatment chemistry or of disinfection parameters.

Interpreting a detection

A report of B. dermatitidis from a reprocessing water sample should not be actioned as a water-system finding until the identification has been confirmed. This is the single most important operational point about this organism. Several environmental moulds and dimorphic fungi resemble it morphologically, identification on macroscopic and microscopic appearance alone is unreliable, and the base rate of genuine Blastomyces in an Australian treated water sample is effectively zero. The isolate should be referred to a reference mycology laboratory for molecular identification, and the laboratory should be asked explicitly whether the identification was made on morphology or by sequencing. In parallel, the laboratory should be alerted to handle the isolate under appropriate containment, since the mould form is hazardous by inhalation.

While confirmation is pending, the reasonable operational posture is to investigate the sampling and environmental pathway rather than to shut down reprocessing. That means reviewing sampling technique and container sterility, whether the sample point was open to room air, whether filter housings or storage vessels have recently been opened, the condition of vents and breathers on tanks, the state of building fabric near the reprocessing and water treatment areas, and any recent construction, excavation or water ingress. Laboratory cross-contamination should also be considered, particularly if the same laboratory has recently handled a genuine dimorphic fungus or if other unusual environmental isolates have been reported in the same period. The absence of any plausible pathway, combined with an identification made on morphology alone, points strongly toward a reporting error.

If molecular identification confirms the organism, the response is an environmental and epidemiological investigation rather than a plumbing one. Relevant questions are whether any associated patient has travelled to an endemic region, whether soil or organic material has been introduced into the building through construction fill or landscaping, and whether the sampling pathway has any exposure to outdoor air or soil-derived dust. Infection prevention should be involved, and given the unusual nature of the finding in a non-endemic country, notification to the public health unit is appropriate for advice even where formal notification is not mandated. What should not happen is a programme of water system remediation, filter replacement and repeat mycological water sampling, because that addresses a transmission route that does not exist for this organism and delays identification of the real explanation.

Antimicrobial resistance

Acquired antifungal resistance is not a recognised clinical problem in Blastomyces species, and routine susceptibility testing is not standard practice. Itraconazole is the usual agent for mild to moderate disease, with lipid formulations of amphotericin B used for severe pulmonary, disseminated or central nervous system disease, typically followed by prolonged azole step-down therapy. Central nervous system disease is managed with agents achieving adequate penetration, and therapy is generally prolonged over many months with monitoring of azole levels. Treatment failure is more commonly attributable to inadequate duration, poor absorption or sub-therapeutic azole levels, or to delayed diagnosis, than to intrinsic or acquired resistance.

Delayed diagnosis is the recurring theme in poor outcomes, particularly outside endemic regions. Pulmonary blastomycosis is radiologically and clinically non-specific and is commonly mistaken for bacterial pneumonia, tuberculosis or malignancy, and in a country where the disease is not endemic it is unlikely to be considered at all unless a travel history prompts it. The consequence is that patients may receive several courses of ineffective antibacterial therapy before the diagnosis is reached.

From an environmental control perspective, the organism's relevance lies in avoiding disturbance of contaminated soil in endemic regions rather than in water disinfection strategy. There is no meaningful disinfectant-tolerance discussion to be had in a reprocessing context: the conidia are inactivated by validated high-level disinfection and by sterilisation, and the organism has no capacity to persist in a treated water system where disinfectant tolerance would matter. Facilities should not adjust water treatment or disinfection parameters in response to a Blastomyces report; the appropriate action is to confirm the identification and then to investigate the environmental or laboratory pathway that produced it.