Overview of Trichomoniasis in Cattle
Trichomoniasis is a venereal disease of cattle characterized primarily by early fetal death and infertility, resulting in extended calving intervals. Distribution is probably worldwide.
The causative protozoan, Tritrichomonas foetus, is pyriform and ordinarily 10–15 × 5–10 μm, but there is considerable pleomorphism. It may become spherical when cultured in artificial media. At its anterior end, there are three flagella approximately the same length as the body of the parasite. An undulating membrane extends the length of the body and is bordered by a marginal filament that continues beyond the membrane as a posterior flagellum. Although T foetus can survive the process used for freezing semen, it is killed by drying or high temperatures.
T foetus is found in the genital tracts of cattle. When cows are bred naturally by an infected bull, 30%–90% become infected, suggesting that strain differences exist. Variation in breed susceptibility to trichomoniasis may also exist. Bulls of all ages can remain infected indefinitely, but this is less likely in younger males. By contrast, most cows are free of infection within 3 mo after breeding. However, immunity is not longlasting and reinfection does occur. Transmission can also occur when the semen from infected bulls is used for artificial insemination.
The most common sign is infertility caused by embryonic death. This results in repeat breeding, and attending stock persons often note cows in heat when they should be pregnant. This, along with poor pregnancy test results (eg, too many “nonpregnant normal” and late-bred cows) is usually the presenting complaint. In addition to a reduced number of cows estimated to calve during the regular calving season, an increased number of cows with a “nonpregnant abnormal” reproductive tract diagnosis is seen. These include cows with pyometra, endometritis, or a mummified fetus.
Fetal death and abortions can also occur but are not as common as losses earlier in gestation. T foetus has been found in vaginal cultures taken as late as 8 mo of gestation and, apparently, live calves can be born to infected dams. Pyometra occasionally develops after breeding.
History and clinical signs are useful but are essentially the same as those of bovine genital campylobacteriosis (see Bovine Genital Campylobacteriosis). Confirmation depends on isolation of T foetus, which may be difficult to differentiate from other trichomonads resident in the digestive tract. Diagnostic efforts are directed at bulls, because they are the most likely carriers. Suction is applied to a pipette while it is used to vigorously scrape the epithelium in the preputial fornix. Alternatively, douching with saline or lactated Ringer’s solution (without preservatives) can be used. Aspirates or douches, concentrated by centrifugation, are examined using darkfield contrast microscopy. This material is also transferred immediately to the surface of a liquid culture medium such as Diamond medium. Better success culturing the organism has been reported when using commercially available media-filled pouches. In addition, incubating the media beyond the standard 48 hr may also enhance the accuracy of the diagnosis. Sampling every 48 hr for 10 days from the bottom of the tube and examining at 100–400× may reveal the rolling jerky movements of T foetus. Studies have examined the possibility of using PCR assays to identify T foetus directly from the preputial samples without an intervening culture. These tests are being used by some laboratories to reduce the time required for a definitive diagnosis of trichomoniasis. Some diagnostic laboratories are suggesting that the scraping be placed in phosphate buffered saline before transport to the diagnostic laboratory. A sample prepared in this way can be effectively examined for both trichomoniasis and campylobacteriosis by using a specific PCR application.
Studies suggest that 90%–95% of infected bulls will be positive on culture, and that three successive cultures at weekly intervals will detect ~99.5% of infected bulls. A vaginal discharge (after treatment of pyometra) or vaginal mucus (obtained toward the end of a luteal phase) may also be of diagnostic value.
The number of times the bull battery should be sampled and cultured to ensure the bulls are negative depends on the prevalence of fetal wastage in the cow herd. With more open and late cows, the frequency of testing should increase to improve the probability that bulls are negative for the protozoan.
Various imidazoles have been used to treat bulls, but none is both safe and effective. Ipronidazole is probably most effective but, due to its low pH, frequently causes sterile abscesses at injection sites. In addition, bulls are probably susceptible to reinfection after successful treatment. Resistance to ipronidazole may also be a concern. The biggest problem, however, is that the success of treatment is measured by repeated sampling, which may mean the individual bull can never be definitively called negative. Therefore, an unqualified recommendation for the bull's use cannot be given.
Control consists of eliminating the infection from the bull battery by culling all bulls and replacing them with virgin bulls or by testing and culling positive bulls. Repeated testing in older bulls may be unsatisfactory, and it may be prudent to cull them all. Reinfection is prevented by exposing only the uninfected (clean) bulls to uninfected (clean) cows. Clean cows are assumed to be those with calves at foot (even though some infected cows may produce a live calf) and virgin heifers. In situations in which several herds are commingled on the same range, caution must be exercised to ensure that cows and heifers are not exposed to potentially infected bulls at the home ranch before they are turned out on the common grazing pasture.
T foetus can be safely eliminated from semen with dimetridazole.
Vaccines developed some time ago for use in cows and evaluated in the field were not highly effective, especially in the absence of other control measures. However, the efficacy of whole-cell T foetus vaccines has recently been critically reviewed. Although there is some evidence to suggest that timely vaccination will improve reproductive performance in heifers, there is a distinct lack of evidence that vaccination will reduce "bull-associated outcomes."