Molecular characterization of Babesia ovis in an enzootic focus in eastern Bosnia and Herzegovina

In the rolling hills of eastern Europe, a microscopic parasite known as Babesia ovis waits for its moment to strike. This single-celled organism is the cause of a severe disease in sheep, one that brings high fever, deep exhaustion, and a dangerous drop in red blood cells. The parasite does not travel on its own; it relies entirely on a specific carrier, the brown tick Rhipicephalus bursa, to move from one animal to another. These ticks are active during the warm summer months, feeding on sheep and passing the infection along. In certain areas, the disease becomes a permanent fixture, appearing every year in a predictable pattern. Scientists call these areas enzootic foci, or simply, hotspots where the disease lives and circulates continuously. For decades, veterinarians have known that these hotspots exist, but they have struggled to understand the invisible world inside them. They knew the ticks were there and the sheep were getting sick, but they did not know if the parasite itself was changing as it moved through the population. Did the same version of the bug infect every sheep in a valley, or were there many different versions circulating, perhaps even changing from one summer to the next? Understanding this genetic variety is crucial because it tells us how the disease spreads, how it might evolve to resist treatment, and whether a single vaccine or test could work for all the different versions of the parasite.

A team of researchers set out to answer these questions in a specific hotspot located in the municipality of Rudo, in eastern Bosnia and Herzegovina. This area is a compact valley surrounded by hills, covering roughly 36 square kilometers. It is a place where sheep farming is a traditional way of life, with small flocks grazing on pastures that edge the pine forests. The scientists focused on this specific valley because it had a history of regular outbreaks, yet the sheep in these flocks had never been brought in from outside the area during the study period. This made the valley a perfect, isolated laboratory to watch the parasite in action. Over the course of four different transmission seasons—2020, 2021, 2025, and 2026—the team collected blood samples from 41 sheep that were either showing signs of the disease or carrying the infection without obvious symptoms. These animals came from 15 different farms spread across seven villages within the valley. The researchers then used a standard genetic tool to read a specific section of the parasite's DNA, a segment that acts like a unique barcode for the organism. By comparing these barcodes, they could see if the parasites found in one village were identical to those in another, or if they had changed over time.

The results revealed that the parasite population was not as uniform as one might expect. Although the scientists found one main version of the parasite that dominated the area, appearing in every village and every season they studied, it was not the only one. They discovered five other, much rarer versions of the parasite, which they identified as distinct genetic types. These rare versions were not spread evenly across the valley. Instead, they appeared in very specific, localized pockets. For instance, one rare type was found only in the village of Prebidoli, while two others were found only in the village of Ljutava, which lies just across the river. Furthermore, these rare types did not persist year after year. A rare type found in one summer might be completely absent the next, replaced by the common version or another rare type. In fact, none of these rare versions were found in more than one season, suggesting they might be fleeting visitors in the local population rather than permanent residents.

The researchers were careful to note that while these patterns were clear, they could not prove with absolute certainty that the location or the season was the sole reason for these differences. The number of samples from some villages was small, and the rare types appeared so infrequently that statistical tests could not confirm a strong link between a specific village and a specific genetic type. However, the data strongly suggests that the parasite population is structured on a very small scale. It appears that the disease does not mix freely across the entire valley like water in a bowl. Instead, the different versions of the parasite seem to stay within their own small neighborhoods, perhaps because the sheep do not mix much between farms, or because the ticks themselves have limited movement. The most common version of the parasite, which the team found everywhere, matched genetic sequences found in sheep across Turkey, Iraq, Spain, and other countries, indicating it is a widespread type that has traveled far and wide. The rare types, however, were unique to this local area, with some never having been seen in scientific records before.

This study changes how we view these disease hotspots. It shows that even in a small, contained area where the same disease appears every year, the microscopic world is dynamic and diverse. The fact that rare genetic types appear sporadically and stay in specific villages means that a single snapshot of the area might miss the full picture. If a researcher had visited only once, they might have concluded that the parasite was identical everywhere. By returning over several years, the team uncovered a hidden layer of complexity. They found that the genetic makeup of the parasite can shift from season to season and vary from one village to the next, even when the animals and the ticks are moving within a very small distance. This finding highlights the need for repeated sampling to truly understand the disease. It also suggests that to fully map the genetic structure of this parasite, scientists will need to use more sensitive genetic tools than the ones used in this study, which were designed to be broad and general. For now, the work in Rudo has established a baseline, showing that the enemy is not a single, static force, but a collection of variations that shift and hide in the details of the local landscape.