Laburn, Helen P.

The fetus is thermally clamped to its mother, and susceptible to infectious agents arising in maternal or amniotic tissue. We review the consequences of maternal fever and infection for the fetus. Possible consequences include noxious fetal hyperthermia, cytokine-induced damage to tissues and premature labour or abortion. [ABSTRACT FROM AUTHOR]

We measured core body temperatures of three pregnant ewes and their fetuses by means of chronically implanted temperature-sensitive data loggers. In standard laboratory conditions (dry-bulb temperature 20–22°C) core body temperatures of pregnant sheep and their fetuses during the last trimester of pregnancy demonstrated a constant feto-maternal temperature gradient, and a daily thermal circadian rhythm for the mother and fetus with a small amplitude (<1°C). When the same pregnant ewes were transferred to a natural field habitat where dry-bulb temperature varied between 7°C and 27°C in a 24 h period, there was a marked increase in the amplitude of ewe nychthemeral temperature rhythms (∼2°C). There was similarly an increase in fetal temperature rhythms, but the variations were smaller. Immediately after birth in the field, the core temperature of newborn lambs fell by up to 4.5°C. Thereafter, newborn lambs increased their core body temperatures and appeared to regulate core temperature and daily temperature rhythms at the same levels that they were as fetuses. Only in the second month post-birth do lamb core temperatures and temperature rhythms begin to resemble those of adult sheep. We have shown that fetal thermal protection, which is evident in ewes housed in in-door conditions, persists in natural field conditions also, where fetuses appear to be more protected from night-time falls- than daytime rises in maternal core temperature.

Infection causes fever and suppression of appetite, a combination of effects which threatens normal growth in infected children. We have used an animal model to study the effects on growth of recurrent simulated Gram-positive bacterial infection. After weaning, 10 guinea pig pups underwent surgery under general anaesthesia for the implantation of temperature-sensitive radiotelemeters and thereafter were assigned to receive intramuscular injections of either 50 μg/kg muramyl dipeptide (MDP), or sterile saline. During a 30-day period corresponding to their rapid growth phase, the pups were given eight injections. MDP resulted in fevers of about 1.5 °C on each occasion, but no significant change in body temperature occurred after saline injections. Food intake was suppressed during each febrile episode such that 24-h intake was significantly lower on days of injections of MDP, compared to days between MDP injections in the same animals, and compared to that of animals injected with saline. The rate of weight gain of the MDP-injected guinea pigs was significantly lower than that of the control group and failed even to achieve a rate similar to the saline-injected group in their more adult-like growth phase. Plasma zinc concentration was significantly lower in MDP-compared to saline-injected animals sampled 8 days after the last injection. Our results show that recurrent fever during the growth phase of young guinea pigs results in irreversible growth failure, and that reduced food intake on days when the animals were febrile was at least partly responsible for this reduced rate of growth.

We exposed Dorper-cross ewes at 􏰀120–135 days of gestation to a hot (40°C, 60% relative humidity) and a cold (4°C, 90% relative humidity) environ- ment and to treadmill exercise (2.1 km/h, 5° gradient) and measured fetal lamb and ewe body temperatures using pre- viously implanted abdominal radiotelemeters. When ewes were exposed to 2 h of heat or 30 min of exercise, body temperature rose less in the fetus than in the mother, such that the difference between fetal and maternal body temper- ature, on average 0.6°C before the thermal stress, fell signif- icantly by 0.54 􏰁 0.06°C (SE, n 􏰂 8) during heat exposure and by 0.21 􏰁 0.08°C (n 􏰂 7) during exercise. During 6 h of maternal exposure to cold, temperature fell significantly less in the fetus than in the ewe, and the difference between fetal and maternal body temperature rose to 1.16 􏰁 0.26°C (n 􏰂 9). Thermoregulatory strategies used by the pregnant ewe for thermoregulation during heat or cold exposure appear to protect the fetus from changes in its thermal environment.