Seasonal trends of sand fly abundance and Leishmania infection: The case of Bologna province, Italy (2016–2023)

Leishmaniasis is a vector-borne disease, caused by <i>Leishmania</i> parasites, transmitted by phlebotomine sand flies. Recently, shifts in its geographic distribution have been observed across Europe, including Italy, with climate changes believed to influence sand fly proliferation, altering disease transmission. Using meteorological, entomological, and visceral leishmaniasis (VL) data (2016-2023) from the province of Bologna, Italy, this study aimed at examining the relationships between meteorological factors and sand fly abundance, <i>Leishmania</i> infection in sand flies, and human VL cases. Entomological data showed the peak of sand fly abundance in July-August, followed by a peak of the infection rate one month later. Sand fly abundance resulted negatively correlated with spring cumulative precipitation (<i>r</i> = -0.93, 95 % CI: -1.00, -0.62) and mean relative humidity (<i>r</i> = -0.81, 95 % CI: -1.00, -0.29) while a positive correlation was observed between yearly sand fly abundance and number of VL cases in the following year (<i>r</i> = 0.82, 95 % CI: 0.34, 1.00). A negative correlation was also found between cumulative precipitation from March to June and number of VL cases in the following year (<i>r</i> = -0.71, 95 % CI: -1.00, -0.07). These findings suggest that reduced precipitation may contribute to increased sand fly abundance within the same season and potentially lead to higher number of notified human VL cases in the following year. Our study highlights the importance of meteorological factors as potential predictors of leishmaniasis. Considering these findings, we propose that public health measures, such as information campaigns and the use of repellents, could be strengthened during drier years, provided that our results can be replicated in other regions with different environmental contexts to ensure broader applicability.