Diagnostic concordance and animal-level epidemiological patterns of bovine tuberculosis, paratuberculosis, and enzootic bovine leukemia in Colombian dairy cattle

In the high-altitude valleys of Colombia, dairy farming is a vital engine of the local economy, providing milk for millions and supporting countless families. Yet, like many agricultural systems worldwide, these herds face invisible threats from chronic infections that can linger in animals for years without showing obvious symptoms. Three such infections are particularly concerning: bovine tuberculosis, a bacterial disease that can spread to humans; paratuberculosis, a bacterial infection that causes severe weight loss and diarrhea in cattle; and enzootic bovine leukemia, a viral infection that can lead to tumors. Detecting these diseases is notoriously difficult because the animals often look healthy while carrying the pathogens. Furthermore, the tools scientists use to find these infections—some looking for antibodies the immune system has made, others looking for the genetic material of the germ itself—do not always agree with one another. When different tests tell different stories about the same animal, it becomes hard for farmers and veterinarians to know how widespread the problem really is or how best to stop it.

A team of researchers set out to untangle this complexity in the Ubaté Valley, a region known for its specialized dairy production. They visited thirty different farms and examined nearly four hundred cattle, ranging from young calves to older cows. For each animal, the team performed a battery of tests. They checked for tuberculosis by injecting a small amount of protein under the skin and measuring the reaction after three days. They drew blood to look for antibodies against the leukemia virus and the paratuberculosis bacteria, and they also analyzed blood and fecal samples using molecular techniques to search for the genetic fingerprints of the viruses and bacteria themselves. The goal was not just to count how many animals were sick, but to see if the different methods of testing were telling the same story.

The results painted a picture of a landscape where the choice of test dramatically changes the view of the herd's health. When the researchers looked for bovine tuberculosis, the news was reassuring: not a single animal tested positive, and only four showed a very weak, uncertain reaction. Similarly, when they searched for the genetic material of the paratuberculosis bacteria in the animals' feces, they found nothing. However, the story was very different for enzootic bovine leukemia. When they tested the blood for antibodies, a sign that the immune system had fought the virus, they found that 58 percent of the animals were positive. This suggested that more than half the cattle had been exposed to the virus at some point. Yet, when they used the molecular test to look for the virus's genetic material inside the blood cells, the number of positive animals dropped sharply to just 14 percent.

This huge gap between the two tests for leukemia reveals a crucial biological reality. The antibody test detects a history of infection, showing that the animal's immune system has encountered the virus, even if the virus is currently dormant or present in very low numbers. The molecular test, by contrast, only finds the virus when it is actively replicating inside the blood. The fact that the two methods agreed so poorly—matching in only a small fraction of cases—means that relying on just one test gives an incomplete picture. An animal might test positive for antibodies but negative for the virus, suggesting it carries a latent infection that is not currently active. Conversely, the molecular test might miss animals that have been infected but whose immune systems have kept the viral load too low to detect.

The researchers also looked at how factors like age, breed, and the size of the farm influenced these results. They found that older animals were more likely to test positive for the virus's genetic material, which makes sense as they have had more time to accumulate exposure. Interestingly, the type of test mattered for the breed as well. Holstein cattle, a common dairy breed, were more likely to test positive for the virus's genetic material than other breeds, but they were not necessarily more likely to test positive for antibodies. The size and productivity of the farm also played a role. The four largest and highest-producing herds had a lower rate of antibody-positive animals but a higher rate of animals with detectable viral genetic material compared to the smaller farms. This suggests that the larger herds might be in a different stage of the infection cycle, perhaps dealing with more recent, active transmission, while the smaller herds might have older, more established infections that are less active.

Ultimately, this study highlights that there is no single "truth" when it comes to diagnosing these chronic diseases in a field setting. The high number of animals with antibodies but no detectable virus suggests a large reservoir of cattle that have been infected but are not currently shedding the virus in a way that the molecular test can catch. This has important implications for how farmers manage their herds. Simply removing every animal that tests positive for antibodies might not be the most efficient strategy, as many of those animals may not be spreading the disease. Instead, the findings suggest that a more nuanced approach is needed, one that considers the specific type of test used and the stage of infection. By understanding that different tests reveal different parts of the infection's life cycle, veterinarians and farmers can make better decisions to protect their herds and ensure the safety of the milk supply, moving beyond simple counts to a deeper understanding of the invisible battles happening within their cattle.