Emerging human and canine leptospirosis in New South Wales shows separate transmission pathways

As cases of leptospirosis continue to rise in both humans and dogs in New South Wales, a critical question has emerged: are our pets transmitting this dangerous bacterial disease to us?

A geospatial study recently published in Science in One Health suggests that there is limited evidence of dog-to-human transmission, human-to-dog transmission, or shared sources of infection at the spatial level examined (SA3). Instead, the findings indicate that humans and dogs may have largely independent transmission pathways.

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Researchers from the University of Sydney, who analyzed nearly 300 human cases and 79 canine cases, found that infections in the two species occurred in different hotspots and were driven by different bacterial strains. This discovery shifts the focus from pet contact to broader environmental sources—such as rodents and climate factors—and calls for a refined One Health strategy that addresses the distinct pathways of infection for each species.

A tale of two epidemics

This study challenges the conventional perception that simultaneously rising outbreaks may be interconnected. Canine cases were predominantly clustered in the Greater Sydney metropolitan area and the South Coast, while human cases were concentrated on the North Coast, with no statistical correlation between their geographical distributions. This clear spatial separation serves as a crucial clue, indicating that human and canine infections likely originate from distinct sources.

Dogs: rodent-contaminated urban environments 

Dominant serovars: In dogs, serovar Australis (29%) has recently overtaken Copenhageni (18%) as the most common cause of disease, representing a significant shift in the local leptospirosis landscape.

Reservoir Hosts: Both serovars are primarily associated with rats (Rattus rattus and Rattus norvegicus) as reservoir hosts. Serovar Australis may also be maintained by native animals, including small marsupials such as bandicoots, as well as native and introduced rodents.

Urban hotspots: The clustering in Sydney aligns with known rat populations. Construction activities and the presence of recreational parks are hypothesized to facilitate contact between dogs and environments contaminated with rat urine, the primary source of these serovars.

Humans: mouse-plague-driven transmission

Dominant serovar: In stark contrast, 58% of human cases were caused by a single serovar, Arborea. While both mice and rats can serve as reservoir hosts for Arborea, mice appear to be the primary driver in the Australian context.

Outbreak drivers: Major human outbreaks, including one among raspberry farm workers in 2018 and another linked to a widespread mouse plague in 2021, were overwhelmingly caused by serovar Arborea. Transmission is thought to occur through wounds or mucous membranes exposed to environments contaminated with infected rodent urine, or through direct contact in agricultural settings.

Distinct transmission pathways

The findings suggest limited evidence to support dog-to-human transmission or shared infection sources at the spatial scale examined in this study. At the SA3 level, no correlation was found between the number of human and canine cases (Spearman’s rank correlation coefficient r = −0.053, P = 0.641). In the nine geographical regions where both canine (n = 38) and human (n = 51) cases were reported, there was no overlap between specific serovars at the spatial level.

While five serovars (Australis, Copenhageni, Hardjo, Pomona, and Robinsoni) were detected in both species across NSW, the serovar mismatch at the spatial level, combined with distinct geographic clustering patterns, indicates that humans and dogs may have largely independent transmission pathways requiring separate intervention or prevention measures.

Separate threats, coordinated solutions

The study reveals a disparity in leptospirosis cases patterns between NSW and other developed regions: human cases significantly outnumber canine cases in NSW, while the reverse is observed in places like the United States and Europe. This contrast may be influenced by factors such as climate, occupational and recreational exposure risks, hygiene, public awareness, and disease surveillance practices. Therefore, control strategies should be tailored to address these species-specific transmission patterns.

For canine health: Stringent rodent pest control in urban areas and vaccination of dogs in endemic regions remain critical control measures.

For public health: Protecting outdoor and agricultural workers with personal protective equipment and targeted public health messaging about risks associated with exposure to rodent-contaminated environments, particularly during mouse plagues and after extreme weather events.

This study demonstrates that geospatial analysis, combined with serovar identification, is an important tool for deciphering distinct transmission pathways in a One Health context. Such information is essential for developing species-specific risk mitigation strategies and surveillance systems for both dogs and humans within a One Health framework.