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
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
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 Diamonds or Claussens
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