Experiments in Border disease: IX. The pathogenesis of the skin lesion

Abstract

The histology of the skin has been studied in control and Border disease-affected sheep foetuses and lambs. The foetuses, from dams injected at 50 days gestation, ranged in gestational age from 55 days to 145 days at sampling. The lambs, from dams injected at 50 to 130 days gestation, were sampled at birth and 3 weeks of age. In Border disease, the abnormally coarse birthcoat of lambs was found to be a result of aberrant differentiation of primary follicles, which was only initiated by infections occurring earlier than about 80 days gestation. Following maternal inoculation at 54 days gestation, foetal skin follicle development was temporarily retarded at about 95 days. There followed an extended phase of very rapid growth of primary follicles, so that they became enlarged compared with those of control foetuses, and produced larger and therefore more heavily medullated fibres. This was the main feature of foetal skin in Border disease, evident as early as 115 days gestation, and maximal at 125 to 135 days gestation. The normal physiological check to follicle growth which occurred perinatally reduced follicle size and lessened the difference between the follicle population of control and diseased animals at this time. Fewer secondary follicles developed in Border disease, and their development was retarded. This may have been an indirect effect of the disease process. Compared with control foetuses, foetuses affected with Border disease tended to have a larger number of lymphocytes in the dermis in late gestation. This might have been an indication of an immunological response to Border disease in the skin.