The incubation period of Buruli ulcer (Mycobacterium ulcerans infection) in Victoria, Australia – Remains similar despite changing geographic distribution of disease

Background: Buruli ulcer (BU) is a geographically-restricted infection caused by Mycobacterium ulcerans; contact with an endemic region is the primary risk factor for disease acquisition. Globally, efforts to estimate the incubation period of BU are often hindered as most patients reside permanently in endemic areas. However, in the south-eastern Australian state of Victoria, a significant proportion of people who acquire BU are visitors to endemic regions. During a sustained outbreak of BU on the Bellarine peninsula we estimated a mean incubation period of 4.5 months. Since then cases on the Bellarine peninsula have declined but a new endemic area has developed centred on the Mornington peninsula. Method: Retrospective review of 443 cases of BU notified in Victoria between 2013 and 2016. Telephone interviews were performed to identify all cases with a single visit to an endemic region, or multiple visits within a one month period. The incubation period was defined as the time between exposure to an endemic region and symptom onset. Data were subsequently combined with those from our earlier study incorporating cases from 2002 to 2012. Results: Among the 20 new cases identified in short-term visitors, the mean incubation period was 143 days (4.8 months), very similar to the previous estimate of 135 days (4.5 months). This was despite the predominant exposure location shifting from the Bellarine peninsula to the Mornington peninsula. We found no association between incubation period and age, sex, location of exposure, duration of exposure to an endemic region or location of BU lesion. Conclusions: Our study confirms the mean incubation period of BU in Victoria to be between 4 and 5 months. This knowledge can guide clinicians and suggests that the mode of transmission of BU is similar in different geographic regions in Victoria. Author summary: Buruli ulcer (BU) is a destructive skin and soft tissue infection caused by Mycobacterium ulcerans. Contact with an endemic area is the key risk factor for disease acquisition. Incubation period cannot be estimated in long-term residents of endemic areas but in Victoria, Australia a proportion of our cases are short term visitors. We previously estimated a mean incubation period of 4.5 months during a sustained outbreak on the Bellarine peninsula. Since then there has been a significant increase in Victorian cases, a decline in cases on the Bellarine peninsula and the rapid expansion of a new endemic area on the Mornington peninsula. We performed a new retrospective review of 443 BU notifications between 2013 and 2016, identifying a further 20 cases with single or tightly clustered (within 1 month) exposures to BU-endemic areas. We estimated a mean incubation period in the new group of 4.8 months, very similar to our previous estimate. This knowledge assists clinicians assessing patients with compatible symptoms and a distant exposure to an endemic area, and public health practitioners seeking to prevent transmission. Our finding also suggests that the mode of transmission of BU may be similar in different geographic regions in Victoria.