Strongyloides stercoralis

Associated infections

• Chronic intestinal strongyloidiasis
• Larva currens, a migratory serpiginous rash
• Loeffler-like pulmonary syndrome with eosinophilia
• Hyperinfection syndrome
• Disseminated strongyloidiasis
• Secondary Gram-negative bacteraemia, sepsis and meningitis
• Asymptomatic chronic infection with eosinophilia as the sole finding

Transmission route

Infection is acquired almost exclusively by percutaneous penetration of infective filariform larvae from faecally contaminated soil, typically through bare skin contact with damp ground in areas with inadequate sanitation. Autoinfection sustains the infection thereafter. Rare additional routes include transmission via solid organ transplantation from an infected donor and, uncommonly, person-to-person spread through contact with faecally contaminated material. Environmental control depends on sanitation infrastructure; in the Australian setting, community-wide ivermectin mass drug administration has been shown to produce sustained reductions in seroprevalence.

The transplantation route deserves note because it is the one circumstance in which the parasite moves between people in a healthcare setting, and it does so through transplanted tissue rather than through any environmental vehicle. Donor-derived strongyloidiasis in recipients receiving immunosuppression has resulted in fatal hyperinfection, which is why donor screening in endemic-exposure histories is now standard practice in transplant programmes. This is a tissue and organ safety issue, not an environmental or reprocessing one, and it has no bearing on water systems or device decontamination.

Strongyloides stercoralis has no relevance to endoscope reprocessing, final rinse water quality or water-system biofilm, and this should be stated without qualification. The parasite is not a waterborne pathogen in the sense that Cryptosporidium and Giardia are: it has no environmentally robust cyst or oocyst stage, its infective larvae require moist soil rather than treated water, and infection is established through the skin rather than by ingestion of contaminated water. Larvae do not survive or replicate in a treated water distribution system, do not form or colonise biofilm, and are not recognised contaminants of purified water, washer-disinfector rinse water or endoscope channels.

A report of Strongyloides from a healthcare water or reprocessing sample should therefore be treated as almost certainly anomalous. The appropriate response is to contact the reporting laboratory to confirm sample identity and chain of custody, establish which method generated the result and whether it was validated for the matrix tested, and exclude transcription error, specimen mix-up with a clinical faecal sample, or cross-contamination during processing. Escalation should be to laboratory quality and clinical microbiology rather than to the water safety group unless a confirmed result is obtained, in which case gross faecal contamination of the sample or the supply must be excluded.

Relevance in endoscopy and reprocessing

Strongyloides stercoralis has no relevance to flexible endoscope reprocessing, CSD final rinse water, or dental unit waterlines, and there is no reason to qualify that statement. There are no documented endoscope-associated transmission incidents, no pseudo-outbreaks, and no plausible mechanism for either. The parasite does not colonise channels, does not form or participate in biofilm, and has no stage capable of persisting on reprocessed equipment or in a water system.

The intersection with endoscopy is diagnostic only. Duodenal aspirate or small bowel biopsy obtained at upper gastrointestinal endoscopy is one route to diagnosing strongyloidiasis, and larvae are occasionally identified incidentally in duodenal histology taken for another indication. This is a matter of specimen handling and pathology reporting, not of contamination risk. Larvae present in such a specimen have no capacity to survive a reprocessing cycle: they are fragile, motile organisms with no protective wall, readily destroyed by detergent, mechanical cleaning and any high-level disinfectant, and they cannot survive drying or the absence of a host. Behaviour against high-level disinfection is not a meaningful question here because there is nothing durable to inactivate.

Drying and storage implications are likewise absent. The controls that matter for water-associated organisms, verified channel drying, storage cabinet performance, elimination of stagnation, have no relevance to this parasite because it neither multiplies nor persists in those environments. A facility should not modify its water treatment, disinfection chemistry, filtration or storage practices in response to a Strongyloides report. Where the organism is genuinely relevant to a health service is on the clinical side: screening patients with endemic-area exposure history before commencing corticosteroids or other immunosuppression, which is a prescribing and clinical governance matter entirely separate from anything managed through a water quality programme.

Interpreting a detection

A report of Strongyloides stercoralis from a water or final-rinse sample is almost certainly an analytical or reporting artefact, and the facility should be told so clearly and early. There is no water reservoir for this parasite, no mechanism by which larvae could establish or persist in a distribution system, and no basis for treating the result as a water-quality event. Framing this correctly at the outset prevents a facility from suspending equipment, mounting a plant investigation or escalating to public health on the strength of a result that will not survive verification.

The checks belong to the laboratory. Contact the reporting laboratory to confirm sample identity and chain of custody, because specimen mix-up with a clinical faecal sample is the single most likely explanation, especially in laboratories that process clinical parasitology and environmental samples in the same facility. Establish which method generated the result: microscopy of a concentrated environmental sample is prone to misidentification of free-living nematodes and other larval forms, several of which are common in environmental matrices and morphologically similar to an untrained or unwary observer, while a PCR assay validated for faecal specimens has no established performance characteristics in water. Ask whether the method was validated for the matrix tested, whether controls in the run were clean, and whether other samples in the same batch were positive. Exclude transcription and reporting error, including selection of the wrong organism from a laboratory information system, which is a mundane but genuine source of implausible results.

Escalation should be directed to laboratory quality and clinical microbiology, not to the water safety group. A single anomalous result that does not reproduce on a correctly identified sample requires no water-system action, and the corrective action sits within the laboratory's quality system. If a result is genuinely reproducible on a correctly identified environmental sample using a properly controlled method, the conclusion is still not that the water system harbours the parasite but that the sample has been contaminated with faecal material, and the investigation should follow that pathway, in which case the accompanying findings for faecal indicator organisms would be expected to be grossly abnormal and would be the more informative result. Any genuine clinical concern about strongyloidiasis in a patient belongs with the treating team and infectious diseases service, and is addressed through serological screening and pre-emptive ivermectin treatment in at-risk patients rather than through any water-side intervention.

Antimicrobial resistance

Ivermectin is the treatment of choice for both chronic strongyloidiasis and hyperinfection, with albendazole as an alternative of lower efficacy. Clinically significant anthelmintic resistance in S. stercoralis is not an established problem in human infection; apparent treatment failure more commonly reflects inadequate dosing or impaired absorption in critically ill patients, incomplete clearance in the immunosuppressed, or reinfection in an endemic setting. In hyperinfection, oral absorption may be unreliable because of ileus or mucosal disease, and prolonged or parenteral administration has been used in severe cases.

Because of the risk of hyperinfection, serological screening and pre-emptive treatment before initiating corticosteroids or other immunosuppression is recommended for people from endemic areas. This is the single most important preventive measure associated with the parasite in an Australian health service context, given the hyperendemic prevalence in parts of northern Australia and the frequency with which corticosteroids are prescribed. Serology has good sensitivity in chronic infection but may be falsely negative in the immunosuppressed and in early infection, and a negative result in a high-risk patient does not entirely exclude infection.

There is no disinfectant resistance to consider, because the parasite has no environmentally durable stage that would require inactivation in a healthcare water or reprocessing context. Filariform larvae are fragile, short-lived outside soil, and destroyed by detergents, mechanical cleaning, heat and any high-level disinfectant. No aspect of water treatment specification, disinfection chemistry, filtration rating, ultraviolet dosing or drying practice needs to account for this organism, and a facility should not adjust any of these in response to a Strongyloides report. Its resistance profile is a clinical pharmacology matter only.