A breeding season can appear routine until the pregnancy check tells a different story. Conception rates fall short of expectations, cows return to heat off schedule and open females begin to stack up. What looks like a management issue is often something far more specific and far more costly.
Across the U.S. beef industry, bovine trichomoniasis continues to drive significant reproductive loss. Economic modeling published in the Journal of Agricultural and Applied Economics shows even modest reductions in disease prevalence can produce meaningful gains, with losses largely tied to fewer calves born and extended calving intervals. In affected herds, pregnancy rates commonly fall by 20% to 40%, and the financial impact compounds quickly over time.
At the center of it all is a consistent and often underestimated risk: the bull.
The Biology That Drives the Problem
Trichomoniasis, caused by the protozoan Tritrichomonas foetus, is a venereal disease transmitted during natural breeding. While both sexes are involved in transmission, the disease behaves very differently in cows versus bulls.
Most cows clear infection within two to four months. However, that clearance comes after early embryonic loss, often delaying conception by 30 to 90 days and stretching the calving season.
“The cow can mount a short-term immune response and clear the infection. She can eventually rebreed and carry a calf to term, but she’s going to lose that initial pregnancy that she had,” said Jennifer Koziol, associate professor of food animal medicine and surgery at the School of Veterinary Medicine at Texas Tech University on a recent episode of DocTalk.
Bulls, by contrast, create the long-term problem.
“The bull is a silent carrier because he doesn’t have any symptoms. He’s just going to spread it from female to female during breeding,” Koziol says.
A recent review published in the International Journal of Molecular Sciences explains why. The organism colonizes the preputial crypts, where immune clearance is limited. As bulls age, these crypts deepen, increasing the likelihood of persistent infection. Once infected, bulls typically remain carriers for life.
This biological mismatch is what makes control so difficult. The cow eventually clears the infection, but only after reproductive loss. The bull never clears it and continues to transmit it.
Recognize the Pattern in the Herd
Trichomoniasis rarely presents as a single obvious sign. Instead, it emerges as a pattern of reproductive inefficiency that can be easy to misinterpret early on.
“We see low pregnancy rates ... cows returning to estrus at intervals they shouldn’t, we can see abortions, ” Koziol says, discussing the big indicators that something is wrong in the herd.
These clinical signs reflect disruption during early gestation. Most losses occur within the first 60 days of pregnancy, often before confirmation, which is why the problem may go unnoticed until later in the season.
The full impact often becomes clear at pregnancy check.
“We go out and do pregnancy checks, and we’re only getting 50% to 60% conception rates. That’s a pretty terrible day when you’re just saying open, open, open,” Koziol says.
In heavily affected herds, calf crops can drop into the 50% to 70% range, well below the 85% to 95% typically expected in well-managed operations. At that point, the biological effects have already translated into economic loss.
“This is not an individual animal problem — it’s a herd-level problem. Once we find a positive, we have to start thinking about the entire bull battery and the whole herd,” Koziol says.
Epidemiologic studies, including one published in Theriogenology, show herd structure and management decisions drive disease persistence. The use of older bulls, multi-sire breeding systems and the introduction of untested animals all increase risk. Even a single infected bull can maintain transmission within a herd, particularly when multiple bulls are breeding simultaneously.
How Trich Continues to Spread
Despite long-standing awareness, trichomoniasis persists because of how easily it moves between herds and how difficult it can be to detect with absolute certainty.
“A lot of times the way a herd gets infected is if a neighbor bull breeds cows, then your bull breeds behind him and becomes positive. That’s why testing before and after the breeding season is so important,” Koziol says.
Fence-line contact, shared grazing and commingling all create opportunities for exposure. Diagnostic research, including recent work on evolving testing approaches, highlights another challenge: no single test guarantees detection. While PCR has improved sensitivity compared to traditional culture, false negatives can still occur due to sampling technique, organism load or intermittent shedding.
For that reason, Koziol suggests repeated testing should be used to improve confidence in bull status, specifically at the start and end of the breeding season. Even virgin bulls should be screened.
“We don’t trust a virgin-status bull,” Koziol warns. “When we buy a bull, we want to test him and know that he’s negative.”
No Treatment, Only Prevention
A defining limitation of trichomoniasis control is the absence of an effective treatment for bulls. Prevention depends on verification, not assumption. Bulls must be tested prior to introduction, regardless of perceived risk.
Once a bull tests positive, removal from the breeding population is the only effective option. There is no reliable method to eliminate infection.
Vaccination plays a supportive role, primarily in cows, where it can reduce the severity and duration of infection. While experimental vaccine trials in bulls have been performed, they have not yet produced a practical solution for eliminating the carrier state. As a result, vaccination should be viewed as an adjunct, not a replacement for testing and culling.
The Economics of Getting it Wrong
For producers weighing the cost of testing, the comparison is straightforward.
“The test costs about $45 to $60 depending on the state. That’s pretty economical compared to losing multiple $2,000 cows,” Koziol says.
Economic analyses reinforce this at scale. Losses are driven not only by fewer calves, but also by extended calving seasons, reduced uniformity and increased replacement pressure. Even relatively small drops in pregnancy rate can have a measurable impact on profitability, particularly in larger herds.
Where Control Succeeds or Fails
Operations that successfully control trichomoniasis tend to follow a consistent set of practices:
- Test all bulls before and after each breeding season
- Remove positive bulls immediately
- Minimize commingling and fence-line exposure
- Verify the status of all incoming breeding animals
When these steps are applied consistently, trichomoniasis becomes a manageable risk. When they are skipped, even once, the disease can establish and persist.
Trichomoniasis is not a new disease, and it is not a mysterious one. Its persistence is tied to a single, well-defined weakness in herd management.
Bulls remain the weak link because they are both the reservoir and the vector, carrying infection silently and indefinitely. The visible losses show up in the cow herd, but the source remains easy to overlook. Control depends on consistent use of the tools already available.
Trichomoniasis erodes reproductive performance quietly, one breeding at a time. In most cases, the problem begins — and continues — with the bull.
