The recent prevalence of bovine leukemia virus (BLV) infection among Japanese cattle

Abstract

A seroepidemiological survey of bovine leukemia virus (BLV) infection was conducted in Japan in 2007 using an enzyme-linked immunosorbent assay (ELISA) and an agar gel immunodiffusion (AGID) test. A total of 5420 cattle (dairy, 3966; breeding beef, 797; fattening beef, 657) from 209 farms in seven prefectures in Japan were tested. The overall prevalence of BLV infection was 28.6%. The prevalence of BLV infection in dairy cattle (34.7%) was higher than for both fattening beef cattle (7.9%) and breeding beef cattle (16.3%). Age-specific prevalence showed that BLV prevalence increased with age in all types of cattle and was notably different between dairy and beef cattle under 1 year of age. Among 207 farms, 141 herds (68.1%) had one or more positive animals. The proportion of these positive farms was significantly higher among dairy farms (79.1%) than among beef breeding farms (39.5%) and beef fattening farms (51.9%) (P < 0.001). Dairy farms (40.5%) also showed a significantly higher within-herd prevalence than beef breeding (27.4%) and fattening (14.9%) farms (P = 0.001). This study indicated that BLV is more widely spread in dairy cattle than in beef breeding cattle in Japan. Given the prevalence of BLV infection in dairy and beef cattle was 8- and 1.7-fold higher, respectively, than rates previously found in 1980–1982, BLV appears to be spreading particularly among the dairy cattle population during the last two decades. Further investigation is required to determine the risk factors necessary to control BLV infection that take into account the different farming practices that exist between dairy and beef sectors.

Introduction

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL), a neoplasm of lymphatic tissue in bovine species (Kettmann et al., 1994). BLV is classified into the genus Deltaretrovirus in the family Retroviridae (Hunter et al., 2000). The majority of infected animals remain healthy with no apparent negative economic effects, but some BLV carriers develop a form of the disease known as persistent lymphocytosis (PL) and a low percentage of BLV-infected animals develop lymphoid tumors (Kettmann et al., 1994). BLV infection has a worldwide distribution (Burny et al., 1980, Schwartz and Levy, 1994) and seroepidemiological studies have indicated higher prevalence in some countries (Bause et al., 1978, Cockerell et al., 1992). EBL was successfully eradicated in some countries through national control programs in Europe in recent years (Acaite et al., 2007, Nuotio et al., 2003).

EBL is specified as a notifiable disease and has been subject to passive surveillance in Japan since 1998. There were 838 outbreaks of EBL on 677 farms in 2007, whereas there were only 159 outbreaks on 157 farms in 2000 according to the animal hygiene statistics of Japan (MAFF, 2008). However, a nationwide serological survey of antibodies against BLV has not been performed since 1982. The objective of this study was to estimate the recent prevalence of BLV infection in Japan. Moreover, the herd prevalence and within-herd prevalence in each sector (dairy and beef) were also investigated.