Objective—To define the role of passively tranferred
immunity in protection against early infection with
ovine herpesvirus 2 (OvHV-2) in lambs.
Animals—15 adult sheep and 34 lambs.
Procedure—2 groups of animals were used, including
15 lambs born to OvHV-2-free ewes and 19 lambs
born to OvHV-2-positive ewes. After nursing
colostrum, all lambs and their dams were introduced
into a flock positive for OvHV-2. Blood was obtained
from the lambs every 2 weeks and examined by PCR
assay and competitive inhibition ELISA.
Results—None of the animals had positive results by
PCR analysis for samples obtained approximately 2
weeks after introduction into the flock. In the group of
lambs from OvHV-2-infected ewes, 5 of 19 had positive
results at 1 month of age and 17 of 19 by 5
months of age. In the group of offspring from OvHV-2-negative ewes, only 1 of 15 had positive results at 1
month of age, and the number reached 12 of 15 by 5
months of age. All lambs in both groups had positive
results by 6 months. An active antibody response to
the virus was detected in animals within 3 weeks
after viral DNA became detectable in the blood.
Conclusions and Clinical Relevance—Analysis suggests
that passively transferred immunity does not
play an important role in the delay of infection with
OvHV-2 in lambs. Age also does not seem to influence
susceptibility. The rate of infection in young
lambs may simply be a reflection of the intensity of
viral exposure in their environment. (Am J Vet Res
Case Description—Severe disease and death were identified in cattle exhibited at a state fair that were naturally infected with ovine herpesvirus type 2 (OvHV-2).
Clinical Findings—Most affected cattle had anorexia, signs of depression, diarrhea, fever, and respiratory distress ultimately leading to death. Mean duration of clinical signs prior to death was 6 days (range, 1 to 26 days). Mean number of days between apparent exposure and death was 71 days (range, 46 to 139 days).
Treatment and Outcomes—19 of 132 cattle cohoused in 1 barn died of malignant catarrhal fever (MCF). The diagnosis of sheep-associated MCF was confirmed on the basis of results of an OvHV-2–specific PCR assay performed on tissue samples obtained from affected cattle. The disease was associated but not significantly with distance from the center of the barn and was not associated with distance from the center of the sheep pens.
Clinical Relevance—Outbreaks of MCF in cattle are unusual, particularly in association with livestock exhibitions. Because the clinical signs may be similar to those of some transboundary diseases, cases of MCF should be reported and investigated. Findings for this outbreak provided evidence to suggest that fair boards and veterinarians should reexamine biosecurity recommendations for livestock exhibitions.