Studies on the barker (neonatal respiratory distress) syndrome in the pig.

A carrier boar was mated with 6 carrier sows to produce a total of 88 piglets (9 litters), 29.5% of which were 'barkers'. Analysis of these results implicated a recessive gene in the inheritance of the syndrome. Affected piglets were of normal size but developed respiratory distress with hypothermia soon after birth, and several died; survivors were killed within 8 hours of birth. A variety of lesions were seen in the lungs, e.g. immaturity and necrosis of alveolar epithelium, haemorrhages and hyaline membranes. In alveolar wash samples, phospholipids were markedly deficient, and surface tension was invariably higher than in unaffected piglets. All barkers had very small thyroid glands, and many showed signs of congenital hypothyroidism, including hairlessness, retarded ossification and reduced adrenocortical activity. Some of the abnormalities in barker piglets could be attributed to prenatal hypothyroidism, but pathogenesis of the lung abnormalities was less clear since normal maturation of fetal lung seems to depend on adrenocortical rather than thyroid hormones. However, hypothyroidism may have affected lung development indirectly via an effect on adrenal gland function. There are some similarities between this respiratory distress syndrome and parallel conditions in infants and foals.