Possum bites man: case of Buruli ulcer following possum bite

Abstract

In June 2021, a previously well 81-year-old man, resident on the Mornington Peninsula, Victoria, presented to his general practitioner with a shallow, red ulcer on the dorsum of his left index finger over the distal interphalangeal joint, progressive over the previous 3 weeks. He recalled acquiring a small single-tooth puncture wound at this exact site from a ringtail possum (Pseudocheirus peregrinus) about 6 months previously at his home. No other finger injuries were recalled during those months. The bite occurred after the animal appeared to be ailing and the patient attempted to catch it for wildlife carers. While no lesions were observed on this possum, he had noticed other possums around his home with skin lesions. After immediately washing with soap and eucalyptus oil, the wound fully healed in 2 weeks without apparent complication. The ulcer with which the patient presented to his GP gradually started 6 months later, with induration, erythema and minor pain (Box, A). No fever, night sweats, lymphadenopathy or myalgia were reported. After failure to respond to one week of oral cephalexin, a referral was made to an infectious diseases physician (DPO) who suspected Buruli ulcer and organised a swab. Mycobacterium ulcerans infection was confirmed immediately by polymerase chain reaction (PCR) and subsequently by culture. The patient began an 8-week course of rifampicin and clarithromycin, complicated midway by a paradoxical flare of pain and swelling (Box, B), which settled with a course of oral prednisolone (0.4 mg/kg weaned over 11 weeks). Healing has progressed well since antibiotics were completed. Faecal pellets from common ringtail and common brushtail possums (Trichosurus vulpecula) were collected from two sites in the patient’s garden in July 2021. Eight pellets from each site were screened for the presence of M. ulcerans using IS2404 qPCR.1 All 16 samples were qPCR-negative. It is unknown whether any of the collected samples were from the implicated possum. The patient’s only travel outside his town of residence in the 6 months before ulcer development was to north-east Melbourne, not known to be endemic. He has not left Australia for over 20 years. His only regular outdoor activity is cycling. Buruli ulcer is a destructive skin and soft tissue infection caused by M. ulcerans. It is endemic in tropical sub-Saharan Africa, coastal areas of temperate south-east Victoria, and tropical Far North Queensland.2 The mode of transmission to humans remains controversial; however, compelling evidence exists for mosquitoes as the major vectors in temperate Australia, likely acting mechanically by carrying the pathogen on external body surfaces rather than biologically where the pathogen reproduces inside the host. Small marsupials are a likely environmental reservoir and amplifier in southern Australia, but no equivalent reservoir–vector pair has yet been identified overseas.1, 2 M. ulcerans has been identified in skin lesions and faeces of both ringtail and brushtail possums in south-east Victoria. The proportion of M. ulcerans-positive possum faeces in environmental surveys correlates closely with risk of Buruli ulcer in humans, suggesting a significant role for possums in the transmission cycle.1, 2 We present a case of an apparent M. ulcerans infection directly from a possum rather than indirectly from a mechanically contaminated mosquito.2 Notably, the possum’s observed “sickly” demeanour suggests it was unwell, but we have not been able to retrieve the affected individual. Possible transmission modes include contamination of saliva with M. ulcerans from the environment or from licking M. ulcerans-positive wounds. A paradoxical reaction, as featured in this case, is an inflammatory flare-up after commencing antibiotics, presumably from liberated antigenic material from the infection site stimulating the local immune system, which was initially suppressed by M. ulcerans’ immunosuppressive toxin, mycolactone. The proposed transmission event fits within the known incubation period range for Buruli ulcer of 2–10 months. Buruli ulcer lesions are more common on surfaces that are indirectly, not necessarily directly, exposed to the environment, such as forearms and legs rather than fingers,2 supporting transmission via an insect vector. Unusually, the lesion in this case occurred at a site where a mosquito is less likely to successfully bite, providing further circumstantial evidence supporting the possum bite itself as the transmission event. Lesion-free possums may still harbour M. ulcerans, as lesion-free possums with M. ulcerans-positive gut and faecal samples have been noted.1 The negative PCR results from the faecal samples may be explained by the collection occurring over 6 months after the biting incident, after which the animal may have died. We thank Dr Peter Meggyesy, who diagnosed the case, and Dr Adrian Murrie, who performed the debridement of the finger and clinical supervision of the patient, from the Sorrento Medical Centre. No relevant disclosures. Not commissioned; externally peer reviewed.

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