Bat ecological traits behind Leptospira distribution and genetics in the north-eastern Palaearctic

Leptospirosis is a widespread infection caused by spiral-shaped bacteria that can live in the soil and water, waiting to enter the bodies of mammals. While humans and farm animals often catch this disease from contaminated environments, a vast array of wild animals also carry the bacteria, sometimes without showing any signs of illness. These wild carriers act as reservoirs, holding the bacteria and releasing them back into the environment through their urine. Among these wild hosts, bats have long been suspected of playing a role, yet scientists have struggled to understand exactly which types of bats are most likely to carry the infection and why. The question is not just about the bats themselves, but about the specific habits and living conditions that allow these microscopic organisms to thrive within them and spread to the wider world.

To answer this, researchers in the north-eastern part of the Eurasian landmass set out to examine thousands of bats, looking for the genetic signature of the bacteria in their urine. They did not simply count how many bats were infected; instead, they grouped the animals by how they live. They looked at where the bats slept, how they hunted for food, whether they traveled long distances, and how they organized their social groups. By comparing these daily habits with the presence of the bacteria, the team uncovered a clear pattern: the way a bat lives its life is deeply connected to whether it carries the infection.

The study examined 2,455 individual bats representing 37 different species across a vast region stretching from the Baltic Sea to the Russian Far East. The researchers found that nearly one in four bats tested positive for the bacteria. However, this infection was not spread evenly. Some species were rarely infected, while others carried the bacteria at very high rates. The most significant factor turning out to be the age of the bat. Adult bats were far more likely to be infected than younger ones, suggesting that the bacteria accumulates over a lifetime of exposure. In fact, the odds of an adult female being infected were significantly higher than for a young female, while young males and adult males of the same species showed similar, lower rates of infection.

Beyond age and species, the researchers discovered that specific ecological behaviors strongly predicted infection. Bats that formed large maternity colonies, where hundreds of females and their young gather in a single place, were much more likely to carry the bacteria than those living in small, scattered groups. The type of shelter mattered immensely as well. Bats that roosted in hollows, such as tree cavities or man-made voids, were far more likely to be infected than those that squeezed into narrow cracks or crevices. This preference for hollow spaces likely creates a humid, enclosed environment where the bacteria can survive longer in the air and on surfaces, increasing the chance of transmission between individuals.

The way a bat hunts also played a crucial role. Species that hunt by flying low over lakes and rivers, skimming the surface of the water, showed a much higher rate of infection compared to bats that hunt in dense forests or high in the open sky. Since the bacteria thrive in moist soil and water, bats that spend their time hovering over wet surfaces may be picking up the infection more frequently or living in areas where the bacteria persist longer. Interestingly, the distance a bat travels did not seem to be a major factor. Whether a bat stayed close to home or migrated hundreds of kilometers, the likelihood of infection remained similar, provided the other habits like roosting and hunting were the same.

The team also looked at the genetic makeup of the bacteria found in the bats. They identified several known types of the pathogen, including strains related to those that cause disease in humans and livestock. However, they also found unique genetic lineages that appeared to be specific to bats, suggesting that these animals may host their own distinct versions of the bacteria. In some cases, a single bat was found to carry more than one type of the bacteria at the same time. The genetic analysis revealed that while some bat species shared the same bacterial strains, others carried entirely different ones, reinforcing the idea that the relationship between the bat and the bacteria is complex and specific to each species.

One of the most striking findings was the role of male bats. In many bat species, males live alone or in small, loose groups, while females gather in large nurseries. The study found that when males formed their own colonies, the infection rates in those species rose significantly. In these male colonies, the difference in infection rates between males and females disappeared, suggesting that the social structure of the colony itself drives the spread of the bacteria. When males and females roost together or when males form their own large groups, the bacteria spreads more easily, likely because the conditions in these crowded shelters are perfect for the pathogen to survive and move from one animal to another.

The researchers also tracked individual bats over time, catching and testing the same animals months apart. They found that once a bat became infected, it tended to remain infected for a long time, sometimes shedding the bacteria for over a year. However, the shedding was not always constant; a bat could test positive one month and negative the next, perhaps because the bacteria temporarily hide in the kidneys or because the immune system briefly clears them from the urine. This fluctuation makes it difficult to predict infection based on a single test, but the overall trend showed that once a bat acquires the bacteria, it often carries it for a significant portion of its life.

This work provides a clear picture of the conditions that allow leptospirosis to persist in bat populations. It is not just about the species of the bat, but about the environment it creates for itself. Large, crowded hollows, hunting over water, and the formation of male colonies all create a perfect storm for the bacteria to thrive. By understanding these specific ecological drivers, scientists can better predict where these infections might be found and how they might move from wild animals to humans. The study confirms that the habits of the host are just as important as the biology of the bacteria in determining the spread of disease.