SHIV susceptibility changes during the menstrual cycle of pigtail macaques.

Background Hormonal changes during menstrual cycling may affect susceptibility to HIV. Methods We determined the simian human immunodeficiency virus (SHIV) acquisition time point in 43 cycling pigtail macaques infected by repeated vaginal virus exposures initiated randomly in the cycle. Results SHIV infection was first detected in the follicular phase in 38 macaques (88%), and in the luteal phase in five macaques (12%), indicating a statistically significant timing difference. Assuming a 7-day eclipse phase, most infections occurred during or following a high-progesterone period associated with menstruation, vaginal epithelium thinning, and suppressed mucosal immunity. Conclusions This raises questions whether other high-progesterone conditions (pregnancy, hormonal contraception) similarly affect HIV risk.