Prevalence of Mycoplasma spp., Chlamydia spp., and Moraxella species in ocular samples from European bison (Bison bonasus) and their association with infectious keratoconjunctivitis

In the world of wild animals, eyes are windows that can easily become clouded by infection. A condition known as infectious keratoconjunctivitis is a common ailment for many hoofed mammals, from domestic cattle to wild herds roaming the mountains. It is a frustratingly complex problem, often caused not by a single villain, but by a mix of bacteria, parasites, viruses, and environmental irritants like dust or bright sunlight. When this disease strikes, it can cause painful swelling, excessive tearing, and corneal damage that ranges from mild irritation to total blindness. For conservationists and wildlife managers, understanding exactly what triggers these outbreaks is vital, especially when the animals involved are rare and ecologically significant.

In Poland, a specific population of European bison has been grappling with a wave of these eye infections since 2019. While researchers had previously identified parasitic worms as a frequent guest in these sick eyes, they knew that the parasites alone could not explain the full severity of the disease. To solve this puzzle, a team of scientists turned their attention to the bacterial suspects most commonly blamed for eye trouble in other ruminants: groups of bacteria called Mycoplasma, Chlamydia, and Moraxella. These microscopic organisms are known to cause similar issues in sheep, goats, and cattle, but their role in the lives of European bison had remained a mystery. The researchers set out to see if these bacteria were the primary culprits behind the bison's suffering or merely innocent bystanders living in the eye.

The investigation took place between 2023 and 2025, involving a careful examination of 55 European bison from various parts of Poland. These animals were part of a managed population, and the samples were collected during the official culling process, which is a standard method for managing herd sizes and health in the wild. The scientists did not just look at the eyes; they also recorded the age, sex, weight, and genetic background of each animal to see if certain types of bison were more likely to carry these bacteria. Using advanced molecular tools, they swabbed the conjunctival sacs—the moist inner lining of the eyelids—to search for the genetic fingerprints of the target bacteria.

The results painted a picture that was both revealing and surprisingly inconclusive. The researchers found that nearly half of the bison, specifically 26 out of the 55 animals, carried at least one of the bacteria they were hunting. The most common finds were Mycoplasma species and Moraxella bovoculi, each appearing in about a quarter of the herd. A less common guest, Chlamydia, was found in only a few individuals, while the specific bacterium Moraxella bovis, which is notorious for causing severe eye disease in cattle, was completely absent. Interestingly, the presence of these bacteria was not random; they were found significantly more often in younger bison, particularly those under the age of 13, and in those belonging to a specific genetic line known as the Lowland-Caucasian line.

However, the most critical discovery was what the bacteria did not do. Despite finding these organisms in so many animals, the researchers could not find a link between their presence and the actual eye disease. The severity of the eye lesions, which ranged from mild redness to severe ulceration, did not correlate with whether a bison was carrying the bacteria. A bison with a heavily infected eye was just as likely to test negative for these specific bacteria as one with a healthy eye. Furthermore, the bacteria were often found in animals that showed no signs of eye trouble at all. This suggests that while these microbes are part of the normal community living on the surface of a bison's eye, they are not the sole cause of the painful outbreaks seen in the population.

The physical examination of the eyes provided further clues. When the researchers looked at the tissue under a microscope, they saw that the inflamed areas were dominated by a specific type of immune response involving single-celled defenders, rather than the massive, pus-filled infections often seen with aggressive bacterial attacks. The corneas, the clear front windows of the eye, showed mostly mild swelling and cellular changes rather than deep, destructive damage. These findings reinforce the idea that the disease is a complex event, likely triggered by a combination of factors such as the environment, the animal's immune system, and perhaps other pathogens, rather than a simple infection by one of these bacteria.

Ultimately, this study clarifies the role of these specific bacteria in the lives of European bison. They are present, they are common in younger animals, and they are part of the eye's natural microbiome, but they do not appear to be the primary drivers of the infectious keratoconjunctivitis outbreaks. The disease remains a multifaceted challenge, where no single pathogen tells the whole story. For the future, this means that managing the health of these majestic animals will require looking beyond a single suspect and understanding the intricate web of interactions between the host, the environment, and the diverse community of microbes that call the bison's eye home.