Reproductive problems in the cow herd: trichomoniasis, BVD, and others (Proceedings)

Key points

     • Outline basic methods for maintaining reproductive success, including: designing appropriate immunization programs and implementing relevant biosecurity practices.

     • Provide an overview and update on identification and control of important diseases including: Bovine viral diarrhea and Tritrichomonas fetus in beef cow-calf herds.

Preventing reproductive loss

Optimizing reproductive success in cow-calf herds relies on combining appropriate immunization and biosecurity practices with the current production system management techniques in the herd. The goal of the immunization program is to match herd immunity to the risks faced. Modifications to the immunization program include not only selection of appropriate antigens, but also matching the timing of immunizations to the time of greatest disease challenge in the environment.

Importing cattle into a cow-calf operation represents a potential source of disease exposure to the resident herd. This risk can be limited through diagnostic testing and an effective quarantine program. The diagnostic tests selected for the operation depend on goals of management, current on-farm disease status, and other state/federal regulations. Prior to implementing any test as a part of import procedures, the veterinarian and owner should decide how test results will impact future decisions (if tests are positive, what will be done with the animals?) Using diagnostic tests as a screening tool to reduce risk of disease introduction from new imports is most effective in diseases with a carrier state.

Risk of disease transfer from imported cattle to the resident herd can also be reduced by implementing a quarantine period at arrival. This technique is most helpful for pathogens that result in a transient infectious state. The length of the quarantine period is based on the estimated length of the infection, and as a rule of thumb, new arrivals are typically housed separately from the resident herd for 21-35 days.

In addition to general control measures, veterinarians and producers should be aware of preventative and control measures for specific diseases, including trichomoniasis.

Tritrichomonas infections in beef herds

Tritrichomonas foetus causes early embryonic death, abortions, and infertility in beef herds. Definitive diagnosis requires culture and identification of the organism from an animal in the herd. In infected herds, ranchers may not notice any indications of a problem until the time of pregnancy examination when an increased number of open cows are detected. If the breeding season is long (more than 90 days) the astute rancher may notice an increased number of cows cycling at the end of the breeding season. The number of cows that calve can be reduced by 20% to 40% and the mean calving date will be later and the calving season will be spread out longer than in non-infected herds. Proper herd management techniques are necessary to limit losses due to the disease in future years.

Bulls colonized with Tritrichomonas foetus are the primary reservoir for infection in the herd, yet are typically asymptomatic. The infection is localized in the prepuce and penile crypts and there is no known treatment to clear infection. Bulls less than 3 years of age may clear the infection, while those 3 years of age and older are usually infected for life. All non-virgin bulls should be considered potential sources of infection for a herd. Although young bulls may not become permanent carriers, they can still spread the infection to susceptible females during the period that they are infected. Culturing bulls (preputial samples) for the organism is an important component of control programs, but the test produces many false-negative results. In order for a bull to be considered negative for trichomoniasis, he must have three samples taken at weekly intervals be culture-negative. All positive bulls should be sent to slaughter. Bulls that were not previously tested should also be tested 3 times. All bulls including new imports should be tested in cases where Tritrichomonas is suspected.

Infection in the cow occurs primarily by exposure to an infected bull at breeding. Initial infection of the female does not cause rapid conception failure, rather the pregnancy progresses to about 60-120 days with a peak loss at 70-90 days, at which time the embryo/fetus dies and is resorbed or aborted. The first sign of Trichomoniasis in a herd is that infected cows and heifers return to estrus one to three months after breeding. A period of infertility may last for another two to six months as a result of the infection. Cows that are infected with Tritrichomonas foetus typically clear the infection within a few months. Immunity, however, is not permanent and the cow is subject to re-infection and abortion in subsequent breeding periods. Occasionally an open cow will fail to clear the infection or a pregnant cow will remain infected through pregnancy and be a source of infection for the herd at the next breeding season. Ideally open cows should be culled at the end of each breeding season. Infected cows will also occasionally have an abortion due to Tritrichomonas. All abortions should be examined to be sure they are not caused by Tritrichomonas. Cervical mucus from the cow can be inoculated into the In Pouch and cultured for identification. Cows that are open or abort due to Tritrichomonas should be culled prior to the breeding season.

A vaccine is available and may help infected animals clear the infection more rapidly resulting in improved calving rates compared to non-vaccinates. Cows may be vaccinated before breeding in accordance with label instructions. Vaccination may be a helpful adjunct to other Tritrichomonas control measures, but vaccination without institution of other controls has limited value.

Bovine viral diarrhea in cow-calf herds

Herd history can be used to categorize breeding herds as high or low risk for potential presence of a PI animal. Wittum et al., surveyed randomly selected herds and BVDV-suspect herds and found one or more BVDV positive calves in 4% of the randomly selected herds and 19% of herds with a history of BVD compatible syndromes.(Wittum, Grotelueschen et al. 2001) Accurate classification is an approximation of true prevalence of herds with at least one PI animal and this can be used to evaluate the potential economic consequences of identifying and removing a persistently infected animal from the herd. If true prevalence of herds with at least one PI animal is 1%, the average annual dollars available for screening is only $0.15 vs. $4.60 if true prevalence in herds is 30%.(Larson, Pierce et al. 2002) Therefore, screening testing may not be economically justifiable for all clients, yet in selected herds identification and removal of BVD PI animals can be financially beneficial.

High risk herds are animal groups that have historical information increasing the index of suspicion of BVD problems. Examples include: abortions, decreased fertility, increased calf death loss and low level illness in the herd. If a herd has several of the risk factors, an aggressive diagnostic plan should be identified to find a potential PI. The timing and number of calves tested is important for optimal outcome. Ideally, calves will be tested and PIs removed prior to the breeding season to prevent further exposure of pregnant dams to viral shedding from PI calves. All calves on the farm should be tested for PI status and dams of confirmed PI calves should be tested as follow up.

Low risk herds are herds without disease signalment leading to suspicion of BVD. In many of these herds, identifying that they are free from BVD and maintaining BVD free status is an important goal for marketing rather than production purposes. To meet this target in an economically feasible manner, a diagnostic surveillance program should be pursued with follow up testing as appropriate.

The diagnostic plan should reveal the herd status regarding BVD infection. This information should be used to guide the herd BVD control strategy. If the herd is positive for the virus, efforts should be placed on finding all reservoirs and eliminating them from the herd. Removal of PI calves prior to exposure to pregnant dams during the susceptible period is critical to prevent formation of future PI animals.

If the herd is BVD virus negative; the herd should allocate resources toward providing a strong biosecurity program. Again, protecting the pregnant cow prior to four months of gestation is important to eliminate the formation of PI calves. New animals exposed to the cow herd (bulls, replacement females) should be screened prior to introduction.

Immunization against BVD virus is an important component of prevention and control programs. Modified-live viral vaccines have been shown to provide some protection against fetal infection with BVD. The BVD virus is genetically diverse and concern has been expressed over the ability of a single genotype in a vaccine to provide cross protection against other field strains. Numerous vaccines are present that offer both genotypes (type 1 and type 2) and as we do not know which strain will be present in the field, utilization of a bivalent vaccine may be advantageous.

Summary

Reproductive success is critical to the beef cow-calf herd and the veterinarian plays an important role in identifying problems in this area. Optimizing reproductive success in cow-calf herds relies on combining appropriate immunization and biosecurity practices with the current production system management techniques in the herd. Trichomoniasis is an emerging disease in beef herds and appropriate testing techniques are important for prevention and control of this disease.