Effects of pregnancy on the febrile responses in sheep.

Pyrogens were injected into unshorn pregnant and non-pregnant Soay and Clun Forest sheep and the effects on rectal temperature and thermoregulatory mechanisms monitored. Plasma concentrations of unconjugated oestrogens, progesterone and cortisol were measured in the sheep tested in late pregnancy. The effect of pyrogens was also examined in ovariectomized ewes treated with oestrogen and with oestrogen plus progesterone. In contrast to reports of work on shorn sheep all sheep developed a fever even when close to term but the increase in peripheral vasomotor tone normally induced by pyrogens was attenuated in the pregnant Clun Forest ewes. The hormone-treated ovariectomized animals developed normal fevers and provided little indication of a possible relation between peripheral levels of ovarian steroids and the degree of fever. The question of whether an endogenous antipyretic substance is present in pregnant sheep is discussed.