Aaron D. Blackwell

Department of Anthropology, Tsimane Health and Life History Project, San Borja, Bolivia

 

Helminth infection causes TH2 biasing of immune responses, with effects on autoimmunity, microbiota, and health such that some refer to helminths as “old friends”. These changes are similar to those caused by infection with another parasite: the human fetus. Maternal immune function is TH2 biased during pregnancy in order to tolerate a genetically distinct fetus. Given these similarities, we hypothesized that infection with helminths might increase human fecundity. We investigate with seven years of longitudinal data from the Tsimane, Bolivian forger-horticulturalists, experiencing both natural fertility and a 70% helminth prevalence. We observed 184 nulliparous women, 45 of whom became pregnant during the study period, and 511 intervals following births for 432 women. Controlling for BMI, sea.son, and acculturation, roundworm (Ascaris lumbricoides) was associated with earlier first pregnancy (HR=2.24, p=0.002) and shortened IBIs at younger ages (at age 20: HR=2.33, p<0.001). In contrast, hookworm was associated with both delayed first pregnancy (HR=0.38; p=0.003) and extended interbirth intervals (HR=0.77, p=0.042). A potential explanation is that hookworm and roundworm induce different immune responses, with a mixed TH1/TH2 response to hookworm and a TH2 polarized response to A. lumbricoides. Alternatively, these two species may entail different costs and thus differentially affect life history schedules.