Undetected serovars: leptospirosis cases in the Cairns region during the 2021 wet season.

Background: Leptospirosis infection can lead to serious renal and cardiopulmonary complications and can be fatal. Following heavy rainfall and localised flooding in early 2021, Tropical Public Health Services in Cairns were alerted to an increase in leptospirosis cases in the region, with notifications almost three times higher than usual by mid-February. An epidemiological investigation was undertaken.

Methods: Leptospirosis notification data were obtained from the Queensland Notifiable Conditions System. Confirmed and probable cases residing in the Cairns region, with an onset date between 1 January and 31 May 2021, were included in the investigation. Case demographics, pathology results, symptoms, hospital stay information and presumed exposure sources were obtained from Queensland Health records; local rainfall data was obtained from the Australian Bureau of Meteorology. Case characteristics and rainfall were compared to the prior ten-year period and the distribution of cases by week of onset, address, exposure source and infecting serovar analysed.

Results: A total of 43 leptospirosis cases were notified between January and May 2021, the highest number recorded for the region since 2011. Presumed exposure sources were available for 40 cases (93.0%), with 33 cases (82.5%) exposed occupationally, including 25 cases working on banana farms. Infecting Leptospira serovars were identified for five cases (11.6%), with four infected with serovar Australis and one with serovar Zanoni. Limited information about the specific exposure sites for each case and a low serovar detection rate hampered the ability to confirm the presence or absence of a leptospirosis outbreak. While heavy rainfall is likely to have contributed to the spike in cases, no factors were identified as clearly associated with the increase.

Conclusions: A number of pathways are proposed to improve the collection of exposure site data and the identification of infecting serovars, in order to strengthen local leptospirosis surveillance and the ability to detect outbreaks in the Cairns region.