Production of malignant catarrhal fever virus-free sheep.

The study was designed to better define the variables affecting the success of the establishment of ovine herpesvirus 2 (OHV-2)-free sheep flocks. A total of 38 lambs born to OHV-2-positive ewes was selected and divided into four groups. Three groups of 10 lambs each were separated from the positive ewes at 2, 2.5 and 3 months of age, respectively, and maintained in isolation facilities. One group of eight remained in the positive flock as controls. Peripheral blood samples from each lamb were examined regularly by PCR for OHV-2 DNA. All lambs (100%) that were weaned and maintained in isolation from the ages of 2, 2.5 and 3 months remained negative until the termination of the experiment at 1 year of age. One lamb was discovered to be PCR-positive on the day of isolation at 2.5 months of age, and was promptly removed from the isolation group. In contrast, all lambs (100%) that remained with the flock became PCR-positive by 6 months of age. The data confirmed that, with rare exceptions, separation of lambs from OHV-2 infected animals at around 2 months of age reliably yields OHV-2-free sheep. Appropriate PCR monitoring will enable the rare exceptions to be removed from the group, and is recommended as a safety measure.