Transmission of ovine herpesvirus 2 in lambs

Abstract

The pattern of acquisition of ovine herpesvirus 2 (OHV-2) infection in lambs was examined by a competitive-inhibition enzyme-linked immunosorbent assay and PCR. Newborn lambs (n = 118) did not exhibit antibody at birth. Viral DNA in peripheral blood leukocytes was detected in only 3% (n = 77) of newborn lambs before suckling. After nursing, viral DNA was sporadically present in about 10 to 20% of lambs until about 3 months of age. Thereafter, strong DNA signals began to appear in increasing numbers of lambs, reaching 100% by 5.5 months of age. Viral DNA in nasal secretions began to be detectable in about 30% of lambs at 5.5 months of age, achieved significant levels in 88% of lambs by 7.5 months of age, and then declined. The kinetics of the humoral immune response in lambs paralleled those of viral DNA in nasal secretions but did not parallel its presence in blood leukocytes. In the experiment to define the time of infection of OHV-2 in lambs, all five lambs separated from the flock at 2.5 months of age remained uninfected until the termination of the experiment at 1 year of age. In contrast, lambs weaned at 2.5 months of age and returned to the flock had become infected at 3.5 months of age. Weaning and separation from the flock at 3.5 months of age did not prevent infection. The study showed that OHV-2 infection does not commonly occur in perinatal lambs and that OHV-2-free sheep can be established by separation of lambs at the proper time, which has important implications for potential control measures.

FIG. 1

OHV-2 DNA in PBLs (open squares) and in nasal secretions (open triangles) and serum antibody (open circles) in lambs from OHV-2-infected ewes under normal husbandry conditions over a period of a year (n = 43 for first 3 months; n = 26 thereafter).

FIG. 2

OHV-2 antibody in lambs separated from the flock at weaning (open squares) (n = 12) and under normal husbandry conditions (open circles) (n = 15).

FIG. 3

Agarose gel electrophoresis of PCR products amplified from PBL DNA of lambs at 2.5 (A) and 3.5 (B) months of age. Lanes 1, 100-bp DNA ladder; lanes 2 to 6, samples from lambs isolated from infected sheep at 2.5 (A) or 3.5 (B) months of age; lanes 7 to 11, samples from lambs weaned at 2.5 (A) or 3.5 (B) months of age and returned to the sheep flock immediately after the weaning period. Lambs whose samples are in lanes 2 to 6 are twins of the lambs whose samples are in lanes 7 to 11. Lanes 12, no-DNA control; lanes 13, OHV-2-positive DNA control.