A four year longitudinal sero-epidemiological study of bovine herpesvirus type-1 (BHV-1) in adult cattle in 107 unvaccinated herds in south west England

Abstract

Background: Bovine herpesvirus type-1 (BHV-1) is an important pathogen of cattle that presents with a variety of clinical signs, including the upper respiratory tract infection infectious bovine rhinotracheitis (IBR). A seroepidemiological study of BHV-1 antibodies was conducted in England from 2002 - 2004: 29,782 blood samples were taken from 15,736 cattle from 114 herds which were visited on up to three occasions. Antibody concentration was measured using a commercial ELISA. Farm management information was collected using an interview questionnaire, and herd size and cattle movements were obtained from the cattle tuberculosis testing database and the British Cattle Movement Service. Hierarchical statistical models were used to investigate associations between cattle and herd variables and the continuous outcome percentage positive (PP) values from the ELISA test in unvaccinated herds.

Results: There were 7 vaccinated herds, all with at least one seropositive bovine. In unvaccinated herds 83.2% had at least one BHV-1 seropositive bovine, and the mean cattle and herd BHV-1 seroprevalence were 42.5% and 43.1% respectively. There were positive associations between PP value, age, herd size, presence of dairy cattle. Adult cattle in herds with grower cattle had lower PP values than those in herds without grower cattle. Purchased cattle had significantly lower PP values than homebred cattle, whereas cattle in herds that were totally restocked after the foot-and-mouth epidemic in 2001 had significantly higher PP values than those in continuously stocked herds. Samples taken in spring and summer had significantly lower PP values than those taken in winter, whereas those taken in autumn had significantly higher PP values than those taken in winter. The risks estimated from a logistic regression model with a binary outcome (seropositive yes/no) were similar.

Conclusion: The prevalence of BHV-1 seropositivity in cattle and herds has increased since the 1970s. Although the study population prevalence of BHV-1 was temporally stable during study period, the associations between serological status and cattle age, herd size, herd type, presence of young stock and restocked versus continuously stocked herds indicate that there is heterogeneity between herds and so potential for further spread of BHV-1 within and between herds.

Figure 1

Unadjusted age-specific profile of the mean BHV-1 antibody seroprevalence and 95% CI by quarter year (up to 2 years old) and year (after 2 years old) for seropositive herds. Herds are dichotomised by presence of seroconversion in the period of study. All cattle greater than 10 years of age were categorised in one age group. Data came from all visits to BHV-1 unvaccinated herds (Dataset A, see methods section).

Figure 2

Mean herd BHV-1 seroprevalence and 95% CI for seropositive herds by herd size (Numbers indicate the number of herds in each category). Herds are dichotomised by presence of seroconversion.

Figure 3

Mean and 95% CI BHV-1 PP value by the number of years after purchase for age group at purchase and herd purpose.

Figure 4

Normal probability plot of the standardised residuals for the herd level in Model 1 where all unvaccinated herd were included Table 3.