Potential zoonotic transmission and environmental factors of Blastocystis sp. infection in wild roe deer (Capreolus capreolus) from Central Poland

In the hidden world beneath the surface of our digestive systems, a microscopic organism called Blastocystis lives alongside us. It is a single-celled parasite, a protist, that colonizes the intestines of humans and animals across the globe. While it is often found in people who feel perfectly healthy, its presence can sometimes be linked to stomach troubles, making its role in health a subject of ongoing debate. What makes this organism particularly interesting to scientists is its ability to jump between species. Just as a virus might move from a bat to a human, Blastocystis can travel between different animals and people, especially when they share the same water or land. Because these microscopic travelers do not look different under a microscope, researchers must examine their genetic code to tell them apart, sorting them into distinct groups called subtypes. Some of these subtypes are common in humans, while others are specific to certain animals, but the lines between them are often blurred, raising questions about how easily they might cross the boundary from the wild to our backyards.

A team of researchers in Poland set out to understand this boundary by looking at the roe deer, a small, agile forest animal that has become increasingly common in human-dominated landscapes. These deer are not just forest dwellers anymore; they frequently wander into farmlands and suburban edges, bringing them into close contact with livestock and people. The scientists wanted to know if these wild deer carried Blastocystis, which specific genetic types they carried, and whether the environment around them influenced the likelihood of infection. To find out, they collected 151 fecal samples from roe deer that had been hunted in three different regions of central Poland during the 2023 and 2024 hunting seasons. The samples were taken from the large intestine, preserved in alcohol, and frozen until they could be analyzed in a laboratory.

Inside the lab, the researchers extracted DNA from the samples and used a technique called polymerase chain reaction to amplify a specific segment of genetic material unique to Blastocystis. This process allowed them to detect the presence of the parasite even in tiny amounts. Once they confirmed the parasite was there, they sequenced the DNA to identify exactly which subtype was present. The results showed that nearly one-third of the deer, specifically 49 out of 151, were infected. The team identified three distinct subtypes: ST14, which was the most common, followed by ST13 and ST10. In a few cases, the specific subtype could not be determined, but the presence of the organism was clear. The genetic analysis revealed that these subtypes were not unique to the Polish deer; they matched closely with strains found in other animals around the world, including sheep, goats, cattle, and even other deer species in countries like China and the United Kingdom.

The study went beyond simply counting infections; it looked for patterns in the landscape that might explain why some deer were infected and others were not. The researchers mapped the areas where the deer lived, measuring how much of the land was covered by farms, water, human settlements, and wind farms. They found a clear connection between the environment and the infection. Deer living in areas with a high percentage of farmland and surface water were more likely to be infected. This makes sense, as agricultural fields often contain manure from livestock, which can carry the parasite, and water sources can become contaminated with fecal matter from various animals. Conversely, the researchers discovered that deer living near wind farms or in areas with a high density of human settlements were significantly less likely to carry the infection. This suggests that the presence of wind turbines or dense housing might alter the deer's behavior or movement patterns, perhaps causing them to avoid certain areas or reducing their contact with contaminated soil and water.

Despite finding these parasites, the researchers concluded that the risk of these specific deer subtypes infecting humans is likely low. While two of the subtypes found in the deer, ST10 and ST14, have been seen in people before, they are not the most common types found in human populations. The study highlights that while the deer act as a reservoir for these organisms, the specific genetic strains they carry are not the primary drivers of human infection. However, the presence of these subtypes in such a mobile and widespread animal serves as a reminder of the complex web of transmission that exists at the edge of the wild and the human world. The findings suggest that the way we shape our landscapes—through agriculture, urbanization, and infrastructure—directly influences the spread of microscopic life. By understanding these patterns, we gain a clearer picture of how diseases move through ecosystems, not as a sudden disaster, but as a steady flow shaped by the fields, forests, and farms we share with the wild.