Hypothermia in neonatal piglets: interactions and causes of individual differences.

Hypothermia is a major cause of mortality in neonatal piglets. However, there are considerable individual differences in the successful recovery from postnatal hypothermia in the common farrowing environment, and so far the causes and interactions of causes have not been studied in detail. Using 635 crossbred neonatal piglets, the aim of this study was to identify the links among different physiological and behavioral measures and their connections to the ability of piglets to overcome initial postnatal hypothermia, with rectal temperature at 2 h as the response variable. The data included birth weight, hypoxia at birth (viability score and lactate in umbilical cord blood), latency to first udder contact and first suckle, scans of individual piglet position during the first 2 h after birth, and rectal temperature at birth and 2 h postpartum. A graphical chain model was used to analyze data. The statistical variables were divided into blocks according to level (design and litter) and chronological order (prenatal, birth, perinatal, and thermoregulatory success at 2 h) before applying the graphical model to the data. Bayesian information criteria (BIC) was used for model selection. The BIC relates to maximum likelihood, but introduces an additional penalty term for the number of variables. The strength of an association between 2 variables is reported as the increase in BIC (BICinc) due to removing the link. Results indicate that at 2 h, 22.1% of the piglets had a rectal temperature below 37 °C. Out of the 16 variables included in the model, only 3 had direct links to the response variable of rectal temperature at 2 h. There was a positive relationship between rectal temperature at 2 h and birth weight (BICinc=26), and between being observed more often by the udder as opposed to alone during both the first (BICinc=8) and second hours (BICinc=19) after birth. Lighter piglets and piglets that had experienced hypoxia took longer to achieve first suckle, which in turn affected where the piglet positioned itself during the first and second hours after birth. Variables related to the birth process had no direct connection to thermoregulatory success, but were additive in the explanation of piglet behavior. The rectal temperature of individual piglets at 2 h depends largely on piglet birth weight and on piglet position in relation to sow and littermates. Birth weight is the most important single factor in successful recovery from postnatal hypothermia.