NATURAL TRANSMISSION OF ORF VIRUS FROM CLINICALLY NORMAL EWES TO ORF-NAIVE SHEEP

The apparent natural transmission of orf virus from clinically normal ewes to susceptible sheep was observed during a border disease vaccine experiment. The 14 susceptible sheep were persistently infected with border disease virus and had been reared indoors in isolation from oth er sheep since birth. Their ages ranged from two to four years and the y were housed in two groups: group 1 consisted of four sheep persisten tly infected with the Moredun strain of border disease virus and group 2 consisted of 10 sheep persistently infected with the Oban strain of the virus. On day 0, six sheep were removed from group 2 and rehoused . To the remaining four sheep in each group were added eight four- to six-year-old pregnant conventionally reared ewes at 48 days gestation. Fourteen days later the four sheep in group 1 were moved to another p en housing eight similar five-year-old pregnant en es at 48 days' gest ation, and the four sheep from group 2 were rehoused with their origin al stallmates. Twenty-one days later lip lesions typical of orf were f irst observed on the sheep from both groups and the disease spread to all the sheep persistently infected with border disease virus over the next four weeks. Virological and serological evidence demonstrated th at the source of infection for the sheep was almost certainly the conv entionally reared ewes, on which no lesions resembling orf were observ ed at any time during the study.