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Bovine Trichomoniasis

  Trichomoniasis is a venereal disease of cattle, characterized primarily by early pregnancy loss and, occasionally, by abortion and pyometra.  The causative agent,  Tritrichomonas foetus, is a flagellated protozoan parasite transmitted from infected, asymptomatic bulls to heifers or cows at the time of coitus.  Trichomoniasis has a worldwide distribution and is a major cause of infertility in naturally bred cattle in many countries.  The incidence in the United States is obscure because of the non-reportable status of the disease.

  Tritrichomonas foetus is confined to all regions of the reproductive tract where trophozoites multiply by binary fission to form two daughter trophozoites.  In cows, the trophozoites attach to the surfaces of epithelial cells lining the reproductive tract.  Examples of colonization in heifers and cows include the vagina, uterus, and oviduct.  T. foetus can be found in secretions from these sites, including the mild mucopurulent discharge associated with vaginitis and endometritis.  Bulls carry the protozoa only on the penis and preputial membranes, localizing  in the secretions (smegma) of the epithelial lining of the penis, prepuce, and distal portion of the urethra.  There are no lesions of diagnostic significance in bulls and the parasite does not affect either semen quality or sexual behavior.  A scant purulent preputial discharge may be noted within the first two weeks of infection, but generally, the infected bull serves as an asymptomatic carrier of the parasite.  Older bulls tend to become permanent carriers of T. foetus, perhaps as a result of the development of epithelial crypts in the preputial cavity of older bulls.  The parasite transmission rate from male to female at breeding may be as great as 42%.  T. foetus is rarely transmitted by artificial insemination of cattle if appropriate procedures for bull testing and hygiene are practiced.

  The pathogenesis of pregnancy loss is not yet well understood.  A likely cause of abortion is the direct cytotoxic insult of maternal endometrium and/or fibroblasts and the fetal chorionic trophectoderm.  Another potential virulence factor is the battery of extracellular cysteine proteinases that are elaborated by T. foetus.  At physiologic pH, these enzymes are very active against a wide variety of proteins including immunoglobulin, fibronectin, and lactoferrin.  Although T. foetus can bind immunoglobulin molecules in a nonspecific manner, whether this binding offers the parasite any protection from specific immune attack or whether it precedes proteinase degradation of immunoglobulin is not known.

  Overt clinical signs are rare as the apparent infertility due to embryonic death is the most common result.  Pyometra and abortion often are the first signs of trichomoniasis noticed in a herd, but they occur in relatively few animals.  When abortion occurs, it is usually within the first third to one-half of gestation.

  Grossly, the degree of autolysis in fetuses and placentas can vary from mild to marked.  Placentas are edematous, but otherwise unremarkable.  Fetuses may have no discernible lesions; however, enlarged livers and non-inflated, enlarged, firm lungs may be present on some fetuses.  Emphysematous bullae involving the splenic and hepatic capsules and the parietal peritoneum have been reported.

  Because infection is inapparent in bulls and mild vaginitis is found only occasionally in cows, a definitive diagnosis requires the identification of parasites in infected animals.  T. foetus is best located in preputial or vaginal secretions and, to a lesser extent, amniotic, allantoic, or abomasal fluids from the infrequently aborted fetuses.  The flagellated parasites can be identified by direct microscopic examination of these fluids.  More commonly, samples are inoculated into one of the several media, most notably Diamond’s or Claussen’s media, and allowed to grow in vitro until sufficient numbers of parasites are present to allow detection by light microscopy.  In bulls, the organisms are in the prepuce, frequently in small numbers.  Microscopic examination of preputial smegma for trichomonads is the most common method to confirm a herd diagnosis.  The characteristic aimless, jerky motion of the flagellate is diagnostic.  Culturing increases sensitivity markedly over direct exam.  Even with culturing, there is a 10-20% probability than an infected bull will be missed by a single culture; hence, multiple cultures are recommended.  In the field, the In-Pouch media system (Biomed Diagnostics, Santa Clara, CA) are convenient to use and have a long shelf-life.

  Reaching a diagnosis is only the beginning of the problem.  No legal treatment exists for bovine trichomoniasis.  Given the lifelong nature of most bull infections and lack of legal treatments, a veterinarian must recommend slaughter of infected bulls.  Artificial insemination has reduced the incidence of trichomoniasis over the past three decades and has proven to be the best control measure available.  Other control measures are possible if artificial insemination is not feasible.  The herd should be divided into exposed and unexposed groups.  The exposed group should be treated for recognizable uterine disease and allowed three months of sexual rest.  One may recommend eliminating all bulls greater than three years of age and using only younger bulls for mating.  This is based on the relative lack of susceptibility of young bulls to trichomonad infection.  Immunoprophylaxis for bovine trichomoniasis has been a priority due to the prevalence of the disease and its economic impact. A killed T. foetus vaccine is available which can be used in both cows and bulls.  Two infections are required, 2-4 weeks apart, prior to breeding season.  Extensive field testing reveals that when used properly, protection can be obtained in over 90% of the animals vaccinated.  Annual vaccinations are required.  Natural immunity in the cow will develop after 1-3 heat cycles, but is of short duration (6-12 months).  The available vaccine (Trich Guard-Fort Dodge) is partially efficacious in the cow, but has no known efficacy in the bull; hence, disease control involves both identification and culling of infected bulls along with vaccination (prebreeding) of cows to decrease the incidence of infertility until the disease is eradicated from the herd.  It is also important to remember that younger bulls are less susceptible to persistent infection than are older bulls.

 -by Trish LaSala, Class of 2000

 -edited by Arlen Mills, DVM, Purdue

  University Large AnimalHospital


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