Etiology

• Difficult, protracted or assisted foaling, especially where fetotomy has been performed.
• Occasionally occurs where there is no history of a difficult foaling or only fetal oversize.
• Occasionally seen post-breeding due to damage from stallion's penis when mare's cervix incompletely relaxed at mating or where stallion is excessively vigorous and/or oversized.
• Could occur as a result of poor technique during artificial insemination   Reproduction: artificial insemination  .
• Irritant chemicals used for intrauterine treatment.
• Chronic endometritis or long-standing pyometra   Uterus: pyometra  .

Specific

• Previous cervical damage.

Pathophysiology

• The cervix is made up of longitudinal muscle along its body and circular muscle at the internal and external os.
• The shape and function of the cervix is determined by hormonal control.
• At oestrus, the cervix is open and relaxed, allowing the expulsion of any contaminants accumulating after foaling, breeding or treatment.
• During diestrus, the cervix is tight and closed, preventing the loss of the early embryo and the ascent of external infections.
• Any interference with this dynamic function of the cervix will lead to conceptual failure by obstructing the lumen or preventing adequate closure (thus predisposing the mare to endometritis   Uterus: endometritis - bacterial  ).
• Trauma to the cervix may lead to mucosal defects, adhesions, lacerations and incompetence.
• Areas of roughened mucosa or exposed submucosa may epithelialize spontaneously or with treatment.
• If mucosal defects are not treated or are slow to heal, then submucosal tissue will be in opposition allowing fibrous adhesions to form.
• The scar tissue may organize and contract leding to cervical canal distortion or obliteration and cervical incompetence.
• Cervical incompetence can also result from tearing of the circular cervical muscularis without associated mucosal damage, and poor healing following surgical closure of a cervical laceration if the submucosal tissue becomes trapped between the edges of the muscularis.
• Excessive cervical stretching during parturition can cause total disruption of the muscularis and of the vaginal/cervical mucosal layers leading to a wedge-shape defect in the body of the cervix, with the base at the external os.