Poverty and Weakened Immunity, Not Climate Alone, Drive Leishmaniasis in Southern Europe

Leishmaniasis, a parasitic disease transmitted by sand flies, has long been considered a problem shaped by warming temperatures and expanding insect ranges. But a new study published in PLOS Neglected Tropical Diseases suggests that the story in southern Europe is far more human than that. Analyzing data from five countries on the northern rim of the Mediterranean, researchers found that socioeconomic conditions—demography, occupation, and housing quality—along with human immunosuppressive conditions were among the strongest predictors of where people actually fall ill. Climate mattered, but mainly in drawing the boundary between endemic and non-endemic areas, not in determining disease intensity within them.

The research team, led by Danyang Wang of Wageningen University together with collaborators in Spain, Portugal, Italy, France, and the Netherlands, set out to fill a persistent gap. Although leishmaniasis is endemic in southern Europe and public health authorities worry about outbreaks and geographic spread, the risk factors driving human disease in this region have been understudied. Most previous work focused narrowly on the parasite, its sand fly vectors, or climatic suitability models. The new analysis instead took a deliberately broad view, testing associations between leishmaniasis incidence and an array of variables spanning socioeconomics, immunocompetence, climate, land use, biodiversity, and the ecology of the pathogen, its vectors, and its animal reservoirs.

The findings carry a pointed message for policymakers. Socioeconomic factors were strongly associated with leishmaniasis incidence in endemic regions, although the specific factors and the magnitude of their effects varied among the five countries studied. In practice, this means that the profile of vulnerability looks different in, say, rural Portugal than in suburban Italy: occupational exposure, crowded or poorly constructed housing, and demographic composition each weighed differently depending on local context. The authors argue that this heterogeneity is precisely why one-size-fits-all control programs have struggled, and why national surveillance systems need to incorporate social and economic data alongside entomological monitoring.

Perhaps the most striking single result concerned human immunosuppression. The prevalence of immunosuppressive conditions in the human population was highly correlated with leishmaniasis risk in Spain and Italy, the two countries where relevant data were available. This aligns with the clinical understanding that Leishmania parasites, which persist inside host macrophages, are normally held in check by cell-mediated immunity. People receiving immunosuppressive therapy, living with HIV, or otherwise experiencing compromised immune function can experience reactivation of dormant infections or severe primary disease. In epidemiological terms, a growing population of immunocompromised individuals acts as an amplifier of apparent disease incidence, even where transmission intensity has not changed.

Climate, by contrast, played a more limited and geographically specific role. In the analysis, climatic variables likely delineated leishmaniasis-free regions from endemic regions in France, marking the northern edge of suitable conditions for the parasite and its vectors. But within endemic areas, climate did not emerge as a dominant driver of incidence. The authors draw a consequential inference from this pattern: climate change alone may not drive the spread of leishmaniasis within this century. That conclusion challenges a common narrative in which warming temperatures are expected to march the disease steadily northward across the continent, and it suggests that factors governing human exposure and susceptibility deserve at least equal attention in forecasting models.

The study also weighed the relative importance of the pathogen’s ecological partners. Where data permitted, the hazard posed by the parasite itself and the abundance of animal reservoirs—typically dogs and other canids that maintain the infection cycle—affected leishmaniasis risk more strongly than the hazard posed by sand fly vectors. This is a subtle but important distinction for control strategy. Interventions that target sand flies, such as insecticide-treated nets and residual spraying, address only part of the transmission equation. Reducing reservoir abundance, improving veterinary screening and treatment of infected dogs, and monitoring the circulation of Leishmania in animal populations may yield greater epidemiological returns, particularly in foci where reservoirs are numerous and closely associated with human dwellings.

Biodiversity emerged as a quieter influence. The researchers found that biodiversity was weakly but negatively related to leishmaniasis incidence, a pattern consistent with the dilution effect hypothesis, in which more diverse ecological communities reduce the probability that a vector encounters an competent reservoir host. The relationship was modest, and the authors are careful not to overstate it, but it adds leishmaniasis to the growing list of infectious diseases whose dynamics may be partly regulated by the integrity of surrounding ecosystems. Land use change, urbanization, and habitat simplification could therefore carry hidden costs for public health, even when their direct effects on sand fly habitat appear neutral or favorable.

Methodologically, the work illustrates the value of integrative risk modeling. Rather than testing one category of variables at a time, the team assembled a multi-country dataset and evaluated competing explanations—social, immunological, climatic, and ecological—within a common statistical framework. This design allowed them to rank the relative importance of drivers, something single-factor studies cannot do. It also exposed the data gaps that hamper such analyses: reservoir abundance estimates, standardized surveillance of vectors, and accessible human incidence records were unavailable or uneven across the region, constraining the precision of the models and complicating cross-country comparison.

The practical implications are broad. The authors highlight the need for surveillance of Leishmania species in humans, vectors, and reservoirs; systematic assessment of reservoir abundance; and improved data accessibility as crucial elements of disease prevention and preparedness. Because socioeconomic vulnerability and immunosuppression emerged as major drivers, they argue, control and prevention policies should extend well beyond entomology—into housing improvement, occupational protection, and clinical vigilance for at-risk patients. Public health systems in Spain, Italy, France, Portugal, and neighboring countries could integrate these findings into screening protocols, targeting immunocompromised individuals in endemic foci for earlier detection and treatment.

Finally, the study’s authors advocate a One Health approach to understanding and controlling leishmaniasis, one that brings together epidemiologists, social scientists, ecologists, veterinarians, and clinicians. The disease, they note, sits at the intersection of human poverty, animal reservoirs, vector ecology, and environmental change, and no single discipline can map that intersection alone. As southern Europe’s climate warms and its rural landscapes transform, the new evidence suggests that the leishmaniasis map of the coming decades will be drawn less by thermometers than by the living conditions, immune status, and surrounding ecosystems of the people at risk—a reframing that could reshape surveillance priorities across the Mediterranean basin.

Subject of Research: Socioeconomic and immunological risk factors for human leishmaniasis in southern Europe

Article Title: Socioeconomic risk factors and human immunosuppressive conditions are major drivers of human leishmaniasis in southern Europe

Article References: Wang, D., Matson, K. D., Hof, A. R., Berriatua, E., Maia, C., Bruno, F., Castelli, G., Pérez Cutillas, P., Risueño Iranzo, J., Verdú Serrano, E., Vitale, F., Xufre, Â., & van Langevelde, F. (2026). Socioeconomic risk factors and human immunosuppressive conditions are major drivers of human leishmaniasis in southern Europe. PLOS Neglected Tropical Diseases, 20(10), e0014787. https://doi.org/10.1371/journal.pntd.0014787

Image Credits: AI Generated

DOI: 10.1371/journal.pntd.0014787

Keywords: leishmaniasis, Leishmania, sand flies, socioeconomic risk factors, immunosuppression, southern Europe, climate change, One Health, disease reservoirs, biodiversity, PLOS Neglected Tropical Diseases, epidemiology