Investigation and response to Australia’s largest outbreak of leptospirosis – New South Wales, Australia, 2018

  • Water or foodborne
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Background:
In June 2018 an outbreak of leptospirosis was recognised among raspberry workers from a mixed-berry farm. Initial detection was via polymerase chain reaction (PCR). We conducted an investigation to identify risk factors for infection, to inform control measures.

Methods:
We detected cases through mandatory laboratory notifications, awareness-raising among farm-employees, clinician alerts and syndromic surveillance of hospital presentations. Confirmed cases had a four-fold rise or single titre ≥400 on microscopic agglutination test (MAT) and positive IgM. Probable cases were IgM or PCR positive. In a case-control study among raspberry workers on the farm, we compared questionnaire responses on potential risk-factors between cases and seronegative controls (opportunistically recruited on-site). Adjusted odds ratios (aOR) were calculated from logistic regression using backwards elimination.
We assessed environmental risks on-site and tested trapped rodents for leptospirosis.

Results:
We identified 69 cases with onsets between April and August 2018 (50 confirmed, 19 probable). All were raspberry workers from the single farm. All confirmed cases were MAT-positive for Leptospira borgpetersenii serovar Arborea. Compared with controls, cases were less likely to wear gloves (39/67 [58%] versus 53/69 [77%], aOR 0.30, 95% CI 0.20-0.88) and more recently employed (median 0.54 versus 1 year employed; each additional year aOR 0.76, 95% CI 0.61-0.95). On unadjusted analysis, cases more commonly reported always having scratched hands (28/67 [42%] versus 18/69 [26%], OR 2.70, 95% CI 1.13-6.43). Rodent activity was evident around raspberry plants. Three of twelve mice tested PCR and MAT positive (for Leptospira Arborea).

Conclusion:
Workers were likely exposed through scratches inflicted during harvesting, which potentially came into contact with environmental leptospires from mice. Control measures included enhanced glove-use, short-term doxycycline prophylaxis and rodent control. The outbreak subsequently subsided. We think hand protection should be used to mitigate risk among raspberry workers. Chemoprophylaxis may assist in controlling outbreaks. Including PCR in surveillance definitions enhances case detection.

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