Using laboratory-bred natural rodent hosts that had been castrated and then
implanted with either testosterone or inert oil, we have shown that testos
terone causes prolonged and more intense infections of a tick-borne piropla
sm, Babesia microti. This will result in more ticks becoming infected while
feeding. Sexually active male rodents with high testosterone levels are al
so known to show increased locomotory activity and reduced innate and acqui
red resistance to tick feeding, so that more ticks are likely to be picked
up and then fed successfully by these hosts. As a result, the transmission
potential of B. microti is significantly increased via hosts with high rath
er than low testosterone levels. It is argued that testosterone helps to ge
nerate the observed aggregated distributions of parasites amongst their hos
ts, which also enhances parasite persistence.