Bovine herpes virus type-4 infection among postpartum dairy cows in California: Risk factors and phylogenetic analysis

Abstract

The aims of this study were to determine the prevalence and associated risk factors of bovine herpes virus type-4 (BoHV-4) infection and describe the genetic characteristics and predominant genotypes of the virus in Yolo and Tulare counties, California. A cross-sectional study involving multi-stage sampling technique was used. One hundred and forty-eight post-partum cows were enrolled from 11 dairy farms. Uterine/vaginal samples were collected and tested for BoHV-4 and other co-infecting viruses using real-time PCR. Data were analysed using multilevel logistic mixed-effect model. Phylogenetic analysis of 10 BoHV-4 isolates was conducted by targeting three open reading frames (ORF3, ORF8 and ORF 22) of the viral genome. The prevalence of BoHV-4 infection was 22.3% (33/148), while post-partum metritis was 33.8% (48/142). Strong association was found between BoHV-4 infection and lactation number, lactation stage and post-partum metritis. The odds of being positive for BoHV-4 infection were 6.47 times (95% CI 1.17-35.92; P < 0.05) and 6.79 times (95% CI 1.19-38.55; P < 0.05) higher for cows in the fourth and fifth lactation, respectively, compared with cows in the first lactation. BoHV-4 was 8.27 times more likely (95% CI 1.43-47.94; P < 0.05) among cows in the early stage of lactation (0-120 days) compared with those in late lactation (>240 days). Cows with post-partum metritis were 4.51 times (95% CI 1.27-16.02; P < 0.05) more likely to test positive for BoHV-4 infection compared with those without post-partum metritis. Phylogenetic analysis of BoHV-4 based on sequencing of glycoprotein and thymidine kinase (TK) genes revealed genetic variability of the virus with glycoprotein B genotype 1 and TK genotype 2 as being dominant genotypes. The reported high genetic variability of BoHV-4 indicates the possibility of co-infection with multiple genotypes.