Pathologic copulatory lock in a genetically engineered laboratory mouse breeding pair

A breeding pair of genetically engineered laboratory mice (Mus musculus) presented in apparent copulatory lock (coital tie). After anesthetizing the animals, gentle traction was used to separate the pair at which point a vaginal prolapse was detected and the penis was covered with black, firm, dry crusts and noted to have a solid pale, tan, firm cylindrical mass adhering to its glans. The vaginal prolapse was reduced and the female was returned to its cage. The male mouse had a severely distended bladder which could not be expressed and was euthanized. Histopathologic examination of the distal two-thirds of the penis revealed diffuse, acute coagulative necrosis. The mass adhered to the distal penis was a homogenous granular eosinophilic material consistent with a copulatory plug. While copulatory plugs and locks have been described in some rodent species, they have not been reported in laboratory mice. While the cause of the adherence of the plug to the penis could not be determined, we hypothesize that its adherence to both the penis and the vagina led to the lock and subsequently to ischemic necrosis of the distal penis.