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The prefemoral efferent lymphatics of four sheep persistently infected with a non cytopathic (NCP) isolate of border disease virus (BDV) were cannulated. Recovered lymphocytes were examined for the presence of virus by an immunocytochemical technique employing a pool of monoclonal antibodies which recognise the 120K non-structural polypeptide of NCP BDV. The results revealed that 9.5% of the lymphocytes carried virus antigen. Lymphocytes from two of the sheep were studied by in situ hybridisation using a viral antisense RNA probe complementary to the region of the BDV genome coding for the 120K polypeptide. This showed that 70-80% of the cells were infected, confirming the greater sensitivity of the in situ hybridisation technique.

Most lambs affected with border disease die early in life but those which survive gradually loose their body tremors and their fleece abnormalities become less clear. Seven female lambs persistently infected with border disease virus were reared to maturity and bred from when they were 2 to 3 years old. Two failed to conceive but five gave birth to 6 live lambs with clinical signs of border disease characterized by hairy and pigmented fleece with or without body tremors. The epidemiological significance of persistently infected sheep is discussed.