Shedding of ovine herpesvirus 2 in sheep nasal secretions: the predominant mode for transmission

Abstract

Ovine herpesvirus 2 (OvHV-2), the major causative agent of malignant catarrhal fever in ruminant species worldwide, has never been propagated in vitro. Using real-time PCR, a striking, short-lived, peak of viral DNA, ranging from 10(5) to over 10(8) copies/2 microg of DNA, was detected in nasal secretions from over 60.7% of adolescent sheep (n = 56) at some point during the period from 6 to 9 months of age. In contrast, only about 18% of adult sheep (n = 33) experienced a shedding episode during the study period. The general pattern of the appearance of viral DNA in nasal secretions was a dramatic rise and subsequent fall within 24 to 36 h, implying a single cycle of viral replication. These episodes occurred sporadically and infrequently, but over the 3-month period most of the 56 lambs (33, or 60.7%) experienced at least one episode. No corresponding fluctuations in DNA levels were found in either peripheral blood leukocytes or plasma. In a DNase protection assay, complete, enveloped OvHV-2 virions were demonstrated in the nasal secretions of all sheep examined during the time when they were experiencing an intense shedding episode. OvHV-2 infectivity in nasal secretions was also demonstrated by aerosolization of the secretions into OvHV-2-negative sheep. The data herein show that nasal shedding is the major mode of OvHV-2 transmission among domestic sheep and that adolescents represent the highest risk group for transmission.

FIG. 1.

Highest levels of OvHV-2 DNA recorded in nasal secretions of adolescent and of adult sheep. (A) Results for 56 adolescent sheep (groups 1, 2, 5, and 7) beginning at about 6 months and ending at about 9 months of age. (B) Results for 33 adult sheep samples taken over a 3-month period. Among these were 10 lambing ewes (group 2) and 23 nonpregnant ewes (group 4, nos. 11 to 20; group 5, nos. 21 to 26; group 8, nos. 27 to 33). The horizontal lines at 10⁵ viral DNA copies arbitrarily define a minimum shedding peak day for individual animals.

FIG. 2.

Real-time PCR depicting OvHV-2 shedding episodes from three individual adolescent sheep (6 and 9 months of age). (A) Single peak lasting less than 24 h (K2069); (B) two distinct peaks, both lasting less than 24 h (K2403); (C) multiple, almost overlapping peaks, with a total duration of about 96 h (K2357).

FIG. 3.

Aerosolization experiment. Two OvHV-2-negative sheep received aerosols, each with 2 ml of nasal secretion samples obtained from sheep experiencing an intense shedding episode. Both sheep that received aerosols seroconverted at 2 or 3 weeks after aerosolization. The horizontal line represents the cutoff value at 25% inhibition.