Submitted to: Veterinary Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/4/2002
Publication Date: N/A
Citation: Interpretive Summary: The purpose of this study was to further define the factors that affect the time at which young lambs become infected under natural flock conditions with sheep-associated malignant catarrhal fever virus. Two groups of animals were used in the study, including 15 lambs born to the virus-free free ewes and 19 lambs born to the infected ewes. The infection rate in each group was monitored. The data suggest that passively transferred maternal immunity does not play any role in the protection of lambs from infection early in life. Neither does age seem to affect susceptibility. The rate of infection in young lambs may be dependent simply upon intensity of viral exposure in their environment. Based on current study, it should be noted that earlier separation of lambs than 2.5 months of age originally recommended is advisable in order to maximize the chances of achieving virus-free status in animals.
Technical Abstract: This study was designed to further define the factors that affect the time at which young lambs become infected with ovine herpesvirus 2 (OvHV-2) under natural flock conditions. Two groups of animals were used in the study, including 15 lambs born to OvHV-2 specific- pathogen free (SPF) ewes and 19 lambs born to OvHV-2 positive ewes. At about 1 day of age, after the lambs had acquired colostrum, all lambs and their mothers were introduced into an OvHV-2 positive sheep flock and raised under normal husbandry conditions. None of the animals (0%) were PCR-positive at about 2 weeks after mixing. By one month, 7% of the offspring from the SPF ewes were PCR-positive. The percentage of positive lambs in this group increased slowly, reaching 80% by five months post-exposure. In the group of lambs from infected ewes, 26% were PCR-positive at one month and the number increased to 92% by 5 months. All animals (100%) in both groups were PCR-positive 6 months after introduction to the flock. An active antibody response to the virus was detected in animals within 3 weeks after viral DNA become detectable in the blood. The data suggest that passively transferred immunity does not play a significant role in the protection of lambs from infection by OvHV-2 early in life. Neither does age seem to influence susceptibility. The rate of infection in young lambs may be dependent simply upon intensity of viral exposure in their environment.