A cluster of melioidosis infections in juvenile salt water crocodiles resolved using genome wide comparison of a common north Australian strain of Burkholderia pseudomallei

In May 2016 B. pseudomallei was isolated from two deceased juvenile crocodile hatchings at a Wildlife Park just outside of Darwin, Northern Territory. ST-109 was identified as the strain responsible for infection in both animals. Though case reports of melioidosis in crocodiles have been documented in Thailand, Malaysia and Australia, crocodiles are thought to be relatively resistant to infection. This prompted us to further investigate whether the two cases were caused by a single point source in the park environment. We undertook three rounds of environmental sampling at the park and performed WGS and MLST sequence typing on cultured isolates. Using WGS data already available through Menzies School of Health Research, we analysed the phylogenetic relatedness and population structure of clinical and environmental ST-109 isolates (n=140) collected and sequenced over the last 29 years from the Northern Territory. Used in conjunction with the crocodile pathology findings, these data allowed us to infer the most likely source of infection in the hatchlings.