Border disease of sheep--aspects for diagnostic and epidemiologic consideration

Border Disease (BD) is a condition of newborn sheep that results from congenital infection by a non-cytopathic pestivirus, occurring during the first half of gestation. The variations in expression of the virus directly relate to the age of the fetus at the time of infection. There are four distinct disease syndromes: (1) early embryonic death, (2) abortion and stillbirth, (3) birth of lambs with malformations, and (4) birth of small, weak lambs, lacking characteristic clinical signs, but bearing features of immunosuppression. In the newborn, the BD virus may be recovered from all tissues and teratogenic lesions are found in the endocrine, nervous, skeletal, integumentary and immune systems. These effects of virus infection are manifest in the clinical signs characteristic of the disease, such as tremors, ataxia, hairy birthcoat, low birth weight, facial bone malformations, short-boxy stature, and eye abnormalities. The consequences of the BD compromised immune system is an increased susceptibility to infection, a failure to produce specific antibody to BD virus, and an inability to clear the virus; features characteristic of the immuno-tolerant state. The lifelong shedding and persistence of virus is of epidemiologic importance. The persistently infected BD ewe remains a source of infection for the flock both through horizontal transmission (virus shedding) and congenital transmission (a persistently infected ewe will always bear a BD lamb). Detection of persistently infected individuals within a flock is difficult: clinical signs abate with time and most frequently no antibody to BD is produced.