Heat exposure of pregnant sows modulates behaviour and corticotrope axis responsiveness of their offspring after weaning
Résumé
The aim of this study was to determine, in swine, what are the consequences of heat exposure during most of gestation on the development of stress response systems in the offspring. For this purpose, pregnant sows were housed at thermoneutrality (TN, 18-24 °C, n=12) or in heated rooms (HS, 28-34 °C, n=12) from the 7th (G7) to the 106th day of gestation (G106), the gestation length being of 115 days. Half of the groups were primiparous and the other half consisted of multiparous sows. From G107 to the end of the lactation period, all sows and their progeny were housed at thermoneutrality in the same maternity rooms. After 28 days of lactation, 5 males and 5 females per litter were weaned and housed in pens of 10 littermates in thermoneutral rooms. At 65 days of age, they were mixed within treatments to form groups of 12 piglets from at least 4 different litters. Two pigs per gender and per litter were randomly selected for blood sampling during lactation (d26) and post-weaning periods (d37). One pig per gender and per litter was randomly selected for saliva sampling before (d64 at 09.00, 13.00
and 17.00) and after mixing (d65 at 13.00, 17.00 and d66 at 09.00). Six piglets per litter were chosen by chance and their individual behaviour (postures, social and explorative activity) was observed by scan sampling on the 7-8th and 15-16th days of lactation. Behaviour was then observed at pen level on the day of weaning and on the 3 following days. Behaviour of sows was not investigated. After a transient increase in rectal temperature during the first week of exposure to elevated temperature, HS sows acclimatized well. Heat did not affect their body
weight gain during gestation nor the subsequent growth of their litter during lactation. In the progeny, total cortisol blood concentrations were similar in the two treatment groups at 26 and 37 days of age. Salivary cortisol concentrations were similar between treatments before the mixing (TN: 0.79 vs HS: 0.92±0.07 ng/ml, P>0.05), but higher in HS pigs after the mixing stress (TN: 1.6 vs HS: 2.2±0.1 ng/ml, P<0.001). Behaviour traits were not affected by treatment during lactation but there was an effect on the first two days after weaning. HS piglets spent less time involved in an activity (29 and 20% for HS vs 40 and 27% for TN on the 1st and 2nd day after
weaning, P<0.05), and this was especially noticeable for the time spent walking and exploring the pen on the 1st day (18% for HS vs 24% for TN, P<0.05). Accordingly, HS pigs spent more time lying on those 2 days (P<0.05). During the 3 days of observation, HS pigs spent more time sleeping in groups of 3 or more pigs (56 vs 50%, P<0.01). To conclude, long lasting exposure of pregnant sows to high ambient temperature increased the response of the corticotrope axis after a social stress, and altered the behaviour of piglets during both resting and active phases after weaning. These preliminary results suggest that prenatal heat exposure could alter both
endocrine and behavioural stress systems in pigs, reminding what can be observed after other kinds of prenatal stressors (social stress, restraint stress…).