Chronic endometritis in subfertile mares with presence of chlamydial DNA

Chlamydia abortus is an obligate intracellular gram-negative
bacterium that infects a large number of mammalian species. It
is known to be the agent of the Enzootic Ovine Abortion, but an
important and subtle role is represented by its involvement in
genital tract infections of the bovine species, causing metritis
and infertility [1]. Currently, Sachse et al [2] adopt the classification
that groups the 11 Chlamydia species in a single genus, the
genus Chlamydia. Genital infection, occasional abortion, and
conjunctivitis have been reported in mares, but the relationship
between abortion and chlamydial infection is still under discussion
[3]. Microorganisms belonging to the genus Chlamydia
play a role in human infertility: Chlamydia trachomatis is one of
the main agents involved in pelvic inflammatory disease (PID)
and can determine chronic endometritis [4]. Chronic damages
due to the persistence of Chlamydia abortus infection appear to
be similar to the lesions found in chronic infection by
C. trachomatis [5] and similar, in histologic aspects, to ocular lesions
that are found in Trachoma [6].
Dealing with subfertility in mares, a particular attention should
be paid to chronic endometritis (CE). Chronic endometritis often
follows “postbreeding endometritis,” that is a common reaction in
response to semen introduction into the uterus or follows repeated
artificial inseminations or intrauterine treatments. Microorganisms
ascending from the lower genital tract can colonize the uterine
cavity; in normal conditions, mechanisms such as cervical mucus
plug, the endometrial epithelium and its immune cellular components
(neutrophils, macrophages, and natural killer cells), and elements
of the innate immune system, including natural
antimicrobial peptides seem to play an important role to restrict
bacterial proliferation and invasion [7,8]. When defense mechanisms
are ineffective or conformation anomalies impair uterine
clearance, we assist at the establishment of CE. Chronic endometritis
consists of the protraction of an inflammatory condition of
uterine endometrium characterized by an abnormal pattern of
lymphocyte subsets and, consequently, an aberrant endometrial
microenvironment. Although CE can be asymptomatic, recent
studies have shown that it is related with repeated implantation
failures after in vitro fertilization-embryo transfer, unexplained