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

Abstract

BackgroundBuruli 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.MethodRetrospective 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.ResultsAmong 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.ConclusionsOur 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.

High antibody titres induced by protein subunit vaccines against Buruli ulcer usingMycobacterium ulceransantigens Hsp18 and MUL_3720

“…In this study, all mice succumbed to infection in a relatively short period (40 days) compared to previous mouse tail infection models [70] and human BU, where the incubation period is estimated at 4.8 months before the onset of ulceration [71]. All BALB/c and C57BL/6 mice succumbed to infection by 40 days after MU infection, even mice that were vaccinated by M. bovis BCG.…”

Buruli Ulcer: History and Disease Burden

“…This approach was applied in south-eastern Australia by systematic identification of BU patients with a single visit exposure to one of the well-known focal BU endemic areas. The mean incubation period determined for patients infected in Victoria was 4.5 months with a wide variation from 32 to 264 days [91,92]. In the Daintree region of FNQ, a large spike of BU cases was observed in September and October 2011, 7-8 month after an exceptionally long and very wet rainy season with peak rainfall in February 2011 that was suspected to be connected with the occurrence of the infections [44].…”

Individual and clinical variables associated with the risk of Buruli ulcer acquisition: A systematic review and meta-analysis

“…Unclear effects of selection and detection biases should be considered when analyzing all studies, mainly due to the lack of incorporation of the social history and BCG vaccination and HIV infection statuses of BU patients, or the use of the unreliable BCG scar as a source of vaccination status check [47]. Considering a mean incubation period of M. ulcerans of 4.8 months, it is also impossible to warrant that the studies reviewed only included individuals free-of-outcome ad initium [48]. This timeframe was also considered in the evaluation of exposure periods, which was here established as minimum for the correct sorting of controls.…”