The significance of venereally transmitted disease in horses is gaining importance since the number of mares being bred by
a single stallion has increased significantly. Thoroughbred stallions often breed 100 or more mares by natural cover while
stallions registered in an associations that allow artificial insemination can breed upwards of 200 mares in a single single.
Furthermore, with the acceptance of frozen semen virtually any country can spread disease in a country or continent far removed
from the stallion's direct area of influence.
The objective of this paper is to describe the most important potential diseases that can be transmitted by the stallion by
direct genital contact or through its semen. Management measures as well as some of the risk factors that would increase or
reduce the venereal transmission of disease are discussed.
The risk factors that may increase the chances of disease transmission through semen. 1) Natural Mating, 2) Poor Stallion
Management, 3) Inconsistent Breeding Method and 4) Artificial Insemination.
Direct sexual contact perhaps poses the biggest risk for venereally transmitted disease. All mares but particularly those
with poor fertility histories should be cultured prior to breeding. On the other hand stallions breeding by natural cover
should be monitored on a regular basis.
Unfortunately stallions that are not stood at a breeding farm tend to have lower numbers of mares per season and poorer reproductive
management. Hygienic procedures in these cases are often neglected and these stallions or mares can be carriers of different
microorganisms. In addition often these horses are poorly housed which can contribute to colonization of the penis by certain
In breeds that allow AI, it is not uncommon for stallion to breed several mares by natural cover at the farm, under no veterinary
supervision. However often times the owner will request for the stallion to be collected so semen can be shipped to other
mares. These inconsistent practices can increase the risk of a stallion getting contaminated or of spreading microorganisms
to several mares.
AI has been advocated as a technique that greatly reduces the risk of disease transmission. Stallions breeding artificially
could breed over 200 mares during the year. In general these horses have been carefully scrutinized for venereal diseases
and as housed with other animals of similar health status. However other factors such as the hygiene of the artificial vaginal,
lubricants, collection bottles, dummy mount, or teaser mare and semen packing material could serve as sources of bacterial
contamination of venereal disease transmission. However if some of these factors are overlooked, they can, instead serve as
a multiplier of disease.
Bacteria, Virus and Protozoa are the microbiological agents that can be transmitted through semen, cause diseases and affect
fertility. In addition genetic diseases are important to keep In mind when breeding horses.
The stallion can harbor bacteria in the external genitalia as well as in the internal genital organs. Except when only the
sperm rich fraction is collected with an open artificial vagina, virtually every stallion and all ejaculates have contaminants
that could be potential pathogens in the mare. The bacteria that constitute the normal flora of the stallion's penis rarely
produce genital infections in reproductively sound mares. It is not uncommon to culture an unwashed stallion penis or fossa
glandis and harvest a milieu of bacteria including; Escherichia coli, Sterptococcus zooepidemicus, Streptococcus equsimilis, Staphylococcus aureus, Bacillus spp, Klebsiella pneumoniae and Pseudomona aureginosa. However, when the normal bacterial flora is a disrupted, potentially pathogenic bacterium particularly, Pseudomonas aureginosa and Klebsiella pneumoniae can colonize the penis and prepuce. These organisms rarely produce clinical disease in stallions, but can be transmitted
to the mare's genital tract at the time of breeding, resulting in infectious endometritis and associated subfertility. The
factors that contribute to the colonization of the penis by these are not clearly determined. Some believe that the normal
bacterial microflora of the stallion's external genitalia possibly combats proliferation of pathogens. However the frequent
washing of the penis, with detergents or disinfectants could removes these non-pathogenic resident bacteria, increasing the
susceptibility of the penis and prepuce to colonization by pathogenic organisms. Others dispute this concept, asserting that
repeated washing of the external genitalia alone does not contribute to overgrowth of pathogenic microorganisms. The environment
in which a stallion is housed may influence the type of organisms harbored on the external genitalia. These organisms can
also be acquired at the time of breeding from a mare with a genital infection.
A sudden and unexplained drop in pregnancy rates should warn the stallion manager about a possible stallion genital infection.
Diagnosis of the colonization of the stallion's penis by a pathogen is done by careful evaluation of breeding records and
early pregnancy diagnosis. Definitive diagnosis is done by isolation of the microorganism in culture. In addition isolation
of the same microorganism with a similar sensitivity pattern from the non-pregnant mares will help confirm the diagnosis.
Treatment of penile colonization depends on the type of bacteria. And method of breeding. For stallions breeding by AI a thorough
penile wash prior to semen collection is recommended. The filtered semen is then diluted with extender containing antibiotic
for which the bacterial is sensitive. Incubation should be done for at least 30 min. prior to insemination. Stallions breeding
by natural cover should be washed and scrubbed thoroughly. After washing the penis is dried. The mare is covered and then
a uterine lavage is performed infusing the mare with appropriate antibiotic between 4 and 6 hrs post breeding.
Stallions with penile colonization by Klebsiella or Pseudomonas can be washed with a weak solution of HCl or sodium hypochlorite. Systemic antibiotic treatment should be avoided since it
has proved unrewarding in most cases.