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.