Aspergillus spp.

Associated infections

• Invasive pulmonary aspergillosis in immunocompromised patients
• Disseminated aspergillosis with cerebral, cutaneous or sinus involvement
• Chronic pulmonary aspergillosis and aspergilloma
• Allergic bronchopulmonary aspergillosis
• Fungal sinusitis
• Aspergillus keratitis and endophthalmitis
• Cutaneous and surgical site infection following inoculation
• Otomycosis

Transmission route

The principal route of healthcare-associated exposure is inhalation of airborne conidia. Construction, demolition, renovation and excavation within or near a facility are the most frequently identified sources of elevated airborne spore counts and of case clusters in haematology and transplant units, and conidia can be carried substantial distances as airborne particles. Damp building materials, contaminated air handling and defective filtration are further recognised contributors. Person-to-person transmission does not occur.

The practical entry points in a healthcare building are worth listing, because they are the same ones that explain a mould in a water sample. Disturbed ceiling voids and above-ceiling work release accumulated dust. Damaged or water-stained plasterboard, ceiling tiles and insulation support fungal growth directly. Positive-to-negative pressure relationships that have drifted, doors propped open between construction zones and clinical areas, and air handling units with bypassed or loaded filters all allow spore-laden air into spaces that should be protected. Floods, roof leaks and condensation events are recognised triggers for subsequent case clusters.

A water-related route has also been proposed. Investigators have reported recovery of Aspergillus species from hospital water systems and from air adjacent to water sources such as showers, and have argued that aerosolisation of contaminated water may contribute to nosocomial exposure; this remains a secondary and less firmly established pathway than airborne dispersal from environmental dust. For endoscopy water monitoring, the practical interpretation is that Aspergillus is not a plumbing biofilm organism in the way that non-tuberculous mycobacteria or Pseudomonas aeruginosa are, and its recovery from a rinse water sample most often indicates ingress of environmental mould into the sampling or treatment pathway. Investigation should therefore focus on airborne and physical contamination routes: sampling technique, open or unsealed sample points, filter housings and storage vessels, damp or degraded fabric near the reprocessing area, ceiling and ventilation condition, and any recent construction or water damage. Where the facility serves immunocompromised patients, the finding warrants prompt escalation regardless of the presumed pathway.

Relevance in endoscopy and reprocessing

Aspergillus has limited direct relevance to flexible endoscope reprocessing compared with the bacterial and mycobacterial organisms that dominate rinse water monitoring, and it is more honest to say so than to construct a mechanism the evidence does not support. There is no substantial literature describing endoscope-transmitted invasive aspergillosis, and the organism does not colonise reprocessor plumbing or endoscope channels in the way that Pseudomonas or NTM do. Its conidia are hydrophobic and do not readily establish within a continuously wetted, low-nutrient system; they persist as inert propagules rather than as an actively growing plumbing population.

Susceptibility to disinfection also differs from the mycobacterial case. Aspergillus conidia are more resistant than vegetative bacteria to drying and to some low-level disinfectants, but they are considerably less resistant than bacterial spores and are inactivated by validated high-level disinfection with glutaraldehyde, ortho-phthalaldehyde or peracetic acid under the conditions used for semi-critical devices. Fungicidal activity is within the qualified spectrum of these agents. In practical terms, a correctly cleaned and correctly disinfected endoscope does not present an Aspergillus hazard, and the failure modes that produce mycobacterial persistence — channel biofilm, protected damage sites, matrix-associated tolerance — do not apply in the same way to a mould that is not building biofilm there.

Where the organism does matter operationally is in the environment around reprocessing and storage rather than in the water itself. Drying cabinets and storage cabinets draw air, and if that air is inadequately filtered, or if the cabinet is sited in a room with degraded ceiling fabric, water damage or nearby construction, conidia can be deposited on instrument exteriors during storage. The same applies to open sample points, to filter housings opened in a dusty environment, and to water storage vessels with poorly sealed vents. The relevant controls are therefore air-side and building-side: filtered air supply to drying and storage cabinets, intact ceilings and surfaces, controlled pressure relationships, prompt remediation of any water ingress or damp, and construction risk assessment with effective barriers whenever work is undertaken near reprocessing or storage areas. In CSD, the parallel concern is airborne contamination of the packing and sterile storage environment; in dental settings, damp fabric and inadequate ventilation in surgery and plant areas.

Interpreting a detection

A mould detection in a reprocessing water sample should be interpreted differently from a bacterial or mycobacterial detection, and the first question is how the organism entered the sample rather than how it colonised the system. The initial checks are physical and environmental: sampling technique and container sterility, whether the sample point was open to room air or poorly disinfected before collection, whether filter housings or storage vessels have been opened recently and in what conditions, whether vents and breathers on tanks are fitted with appropriate filters, and whether the water treatment plant room or reprocessing area shows damp, water staining, degraded ceiling tiles or visible mould. Recent construction, demolition, excavation or a water ingress event anywhere nearby is a strong candidate explanation and should be established early. Air handling condition — filter loading, pressure differentials, and whether any air path exists between construction zones and the reprocessing area — should be reviewed alongside.

A single isolate at low count, with a plausible airborne or sampling explanation and no clinical linkage, generally warrants correction of the identified pathway and a repeat sample rather than a full system shutdown. A trend is a different matter. Repeated recoveries across successive rounds, high counts, recovery from more than one sample point, or recovery coinciding with a building event indicate an ongoing ingress route that has not been closed, and should prompt a wider environmental investigation including air sampling and building fabric inspection rather than further water sampling alone. Any clinical linkage — a case or cluster of invasive aspergillosis in patients associated with the area — changes the picture immediately and moves the response from water investigation to a full environmental and infection prevention investigation.

The patient population determines the escalation threshold, and this is the most important judgement in interpreting an Aspergillus result. In a facility serving predominantly immunocompetent patients, a single low-count mould detection with an obvious sampling explanation is a quality issue. In a facility with haematology, transplant, intensive care or other severely immunocompromised patients, the same result warrants prompt escalation to infection prevention regardless of the presumed pathway, because the consequence of a genuine airborne exposure in that population is high-mortality invasive disease. Escalation should include infection prevention, facility engineering and, where construction is implicated, the works contractor and project team, since the controls that matter are barriers, pressure relationships and air filtration. It is also worth noting that routine water monitoring is not designed to detect fungi reliably — bacterial media, short incubation and standard temperatures will recover some moulds incidentally but will not quantify them meaningfully, so a mould result should be understood as a signal to investigate rather than as a measurement of fungal burden.

Antimicrobial resistance

Aspergillus species are intrinsically resistant to several antifungal classes, including the echinocandins for some species and amphotericin B in the case of A. terreus, and treatment of invasive disease relies principally on mould-active triazoles such as voriconazole, isavuconazole and posaconazole. The fluconazole class has no useful activity against moulds, and species-level identification therefore has direct therapeutic consequences. Acquired azole resistance in A. fumigatus, mediated by cyp51A mutations including the TR34/L98H and TR46/Y121F/T289A alleles, has emerged internationally and is attributed in large part to selection by azole fungicides used in agriculture and horticulture rather than to clinical exposure alone. Azole-resistant invasive aspergillosis is associated with substantially worse outcomes, and susceptibility testing or resistance-marker detection is increasingly recommended where resistance is prevalent.

The environmental origin of that resistance is worth understanding, because it means resistant strains can be acquired by patients who have never received an azole. Fungicides structurally related to the medical triazoles are used widely in crop protection and in the treatment of timber, flower bulbs and compost, and resistant A. fumigatus selected in those settings enters the general environmental spore pool. A patient's first exposure to a resistant strain may therefore be an ordinary inhalation of environmental dust.

Conidia are also physically robust and resistant to desiccation, which supports persistence in dust and on building surfaces. They tolerate wide temperature ranges, ultraviolet exposure and nutrient deprivation, and can remain viable in accumulated dust for extended periods — which is why disturbing long-undisturbed ceiling voids or wall cavities releases a substantial viable spore load. In terms of chemical disinfection, conidia sit above vegetative bacteria and below bacterial spores in the conventional hierarchy: they resist low-level disinfectants and drying but are reliably inactivated by validated high-level disinfection and by sterilisation. For a reprocessing service this means that Aspergillus control is achieved through air quality, building fabric integrity and containment of construction dust rather than through water treatment chemistry.