Weather Patterns Shape Which Leptospira Strains Infect Humans in Ukraine

Leptospirosis is one of the most widespread zoonotic diseases on the planet, a bacterial infection passed from animals to humans through water and soil contaminated with the urine of infected hosts. Yet despite its global reach, the disease remains notoriously difficult to track, because its transmission is tightly woven into local environmental conditions: rainfall, flooding, temperature, and the ecology of the animals that carry the spirochete. A new study of Ukraine, published in BMC Infectious Diseases, has now mapped how different serogroups of Leptospira, the genus of bacteria responsible for the disease, are distributed across the country’s regions and years, and how those patterns track with climate variables. The findings offer a rare, nationally comprehensive picture of a neglected disease in a country undergoing profound socio-environmental upheaval.

The research team, led by Pavlo Petakh of Uzhhorod National University and the European Molecular Biology Laboratory in Heidelberg, together with colleagues from Ternopil, Kyiv, and Uzhhorod, analyzed national surveillance records from Ukraine covering the years 2018 through 2023. The dataset was substantial: 1,332 human Leptospira serogroup identifications and 2,440 animal identifications, aggregated at the level of Ukraine’s administrative regions, known as oblasts. To these epidemiological records the researchers joined regional meteorological data, including average annual temperature, total annual precipitation, and the annual number of rainy days. The goal was to determine whether different serogroups, which represent immunologically distinct variants of the bacterium, respond differently to climatic conditions, and whether those differences could be quantified in a way useful for public health planning.

Serogroups matter because they are the primary lens through which leptospirosis surveillance operates worldwide. The microscopic agglutination test, the reference standard for diagnosis, classifies infections according to the antibodies a patient produces against a panel of Leptospira reference strains. Different serogroups are associated with different reservoir animals and different ecological niches. Icterohaemorrhagiae, for example, is classically linked to rats, while Pomona is often carried by livestock such as pigs and cattle, and Hebdomadis by a range of small mammals. Knowing which serogroups circulate where, and under what weather conditions, therefore tells epidemiologists something about which animal reservoirs are driving transmission and which human activities are most likely to bring people into contact with contaminated environments.

The surveillance data revealed a striking asymmetry between human and animal findings. In humans, two categories dominated: Icterohaemorrhagiae and a residual grouping labeled Others each accounted for 30.2 percent of all detections. That concentration suggests that rodent-associated transmission remains the leading recognized route of human infection in Ukraine, with a long tail of other serogroups making up the remainder. Animal data, by contrast, showed considerably greater serogroup diversity, reflecting the heterogeneous ecology of Leptospira across livestock, wildlife, and commensal species. This divergence between the human and animal pictures is itself informative: it implies that human case detection captures only a filtered subset of the bacterial diversity circulating in the environment, shaped by which serogroups cause severe, recognizable disease and which host species live closest to people.

Spatially and temporally, the distribution of serogroups was far from uniform. Different oblasts reported different serogroup profiles, and the composition shifted across the six-year study window, a period that included the COVID-19 pandemic and the full-scale war that began in 2022. Both events have disrupted agriculture, sanitation, and healthcare delivery in ways that could plausibly alter exposure patterns, for example by changing rodent populations, displacing people and animals, or reducing diagnostic capacity in affected regions. The study’s descriptive analyses documented this variation but stopped short of attributing specific shifts to specific disruptions, an appropriate caution given the observational nature of surveillance data.

The most technically interesting results came from the quantitative modeling. In simple correlation analyses, two human serogroups stood out for their association with rainfall: Hebdomadis and Pomona counts were positively correlated with average annual precipitation, with correlation coefficients of 0.36 and 0.32 respectively, both statistically significant at p less than 0.001. These are moderate correlations by epidemiological standards, but they are notable because they were serogroup-specific. Icterohaemorrhagiae, the dominant serogroup, did not show the same rainfall dependence, hinting that rodent-associated transmission operates on a different environmental logic than transmission involving livestock-associated serogroups.

To probe this further, the team built a multinomial logistic regression model, a statistical framework suited to outcomes with more than two categories. Here the outcome was the serogroup identified in each human case, and the predictors were the regional meteorological variables, with Icterohaemorrhagiae serving as the reference category. The model confirmed that both average annual precipitation and average annual temperature were significantly associated with serogroup composition, each at p less than 0.001. The effect sizes were expressed as relative risk ratios. Higher precipitation increased the relative risk of Hebdomadis by a factor of 1.64 and of Pomona by a factor of 1.66, compared with Icterohaemorrhagiae, with both estimates significant at p less than 0.001. Higher temperature, meanwhile, most strongly increased the relative risk of the Others category and of Pomona.

One of the study’s more subtle findings concerns the difference between the amount of rain and the frequency of rain. The authors report that temperature and the number of rainy days appear to have stronger associations with leptospirosis incidence than cumulative rainfall totals. This distinction has a plausible mechanistic basis. Leptospira bacteria survive in water and moist soil, and individual rain events create the temporary wet environments, puddles, flooded fields, saturated ground, through which the bacteria move from animal reservoirs into human contact. Frequent rainfall keeps these transmission bridges open more often than a large total volume of rain delivered in few events. For a pathogen whose survival outside a host depends so delicately on moisture, the rhythm of wet weather may matter as much as its quantity.

The implications extend beyond Ukraine. Climate change is altering precipitation patterns and temperature regimes across Europe and globally, and zoonotic diseases with environmental transmission stages are expected to shift in response. A framework that links specific pathogen variants to specific climatic conditions, as this study does for Leptospirosis serogroups, is exactly the kind of tool needed to build climate-informed surveillance and early warning systems. If regional weather forecasts can predict when conditions favor particular serogroups, health authorities could target warnings to the people most at risk, farmers working with livestock during wet periods, for instance, or residents of rodent-dense urban areas after flooding. The authors explicitly frame their findings as a step toward such strategies.

The study also carries a cautionary note about the limits of surveillance data. Serogroup identifications depend on testing capacity, healthcare-seeking behavior, and reporting infrastructure, all of which varied across Ukraine’s regions and years, particularly during wartime. Correlation at the oblast level cannot establish causation, and unmeasured factors, from land use to animal husbandry practices, surely shape the observed patterns. Still, by combining a large national dataset with rigorous statistical modeling, the researchers have produced one of the most detailed pictures to date of how a neglected zoonotic pathogen distributes itself across a real landscape and a real climate. As extreme weather intensifies across the region, that picture may prove essential for anticipating where leptospirosis strikes next, and which variant of the bacterium will be waiting in the water.

Subject of Research: Spatial and meteorological determinants of Leptospira serogroup distribution in humans and animals in Ukraine

Article Title: Spatial, temporal, and meteorological distribution of Leptospira serogroups in humans and animals in Ukraine (2018–2023)

Article References: Petakh, P., Huber, W., Halabitska, I., Myronyuk, I., Lushchak, O., & Kamyshnyi, O. (2026). Spatial, temporal, and meteorological distribution of Leptospira serogroups in humans and animals in Ukraine (2018–2023). BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14517-7

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14517-7

Keywords: Leptospira, leptospirosis, Ukraine, zoonoses, epidemiology, climate, precipitation, temperature, serogroups, surveillance, One Health, BMC Infectious Diseases