Risk factors associated with within-herd transmission of bovine leukemia virus on dairy farms in Japan

Abstract

BackgroundAlthough several attempts have been made to control enzootic bovine leukosis (EBL) at the local level, a nationwide control program has not been implemented in Japan, except for passive surveillance. Effective control of EBL requires that the transmission routes of bovine leukemia virus (BLV) infection should be identified and intercepted based on scientific evidence. In this cross-sectional study, we examined the risk factors associated with within-herd transmission of BLV on infected dairy farms in Japan. Blood samples taken from 30 randomly selected adult cows at each of 139 dairy farms were tested by enzyme-linked immunosorbent assay (ELISA). Information on herd management was collected using a structured questionnaire.ResultsInfected farms were defined as those with more than one ELISA-positive animal and accounted for 110 (79.1%) of the 139 farms in the study. Completed questionnaires obtained from 90 of these 110 farms were used for statistical analysis. Seroprevalence, which was defined as the proportions of animals that tested positive out of all animals tested on the farm, was 17.1%, 48.1%, and 68.5% for the 25th, 50th, and 75th percentiles, respectively. A mixed logistic regression analysis implicated a loose housing system, dehorning, and a large number of horseflies in summer as risk factors (coefficient = 0.71, 1.11, and 0.82; p = 0.03, < 0.01, and 0.01, respectively) and feeding of colostrum to newborn calves from their dams as a protective factor (coefficient = -1.11, p = 0.03) against within-farm transmission of BLV on infected farms.ConclusionControl of EBL in infected dairy farms in Japan will be improved by focusing particularly on these risk and protective factors.

Bovine leukaemia virus DNA in fresh milk and raw beef for human consumption

“…The pertinent literature has reported viral particles and viral DNA in cows' milk or colostrum that could be considered a risk factor for transmission to calves [33][34][35][36]. Viral DNA was also detected in fresh milk in the present study, thereby agreeing with previous reports, despite not having been described as a risk factor for humans.…”

Prevalence and Risk Factor Analysis of Bovine Hemoplasma Infection by Direct PCR in Eastern Hokkaido, Japan

“…In cats, both feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV) are regarded as important risk factors for feline hemoplasma infections [6,7,14,30]. Although the reasons why feline retroviral infections are related to hemoplasma infections remain unclear, the routes of infection are thought to be the same [6].…”

“…Vertical transmission of BLV in cattle across the placenta or through colostrums has been previously reported [16]. Horizontal transmission of BLV has also been reported, which can occur with hematophagous insect bites or inappropriate medical treatment [12,14]. One report described the use of molecular techniques to show how hemoplasma could be transmitted vertically and horizontally in cattle [11].…”

Nationwide Survey of Bovine Leukemia Virus Infection among Dairy and Beef Breeding Cattle in Japan from 2009–2011

“…These insects are known to be more abundant in southern regions of Japan with the horsefly a possible vector involved in the horizontal transmission of BLV. The presence of horseflies has been positively correlated to within-herd seroprevalence in Japan [11]. Furthermore, milk and colostrum from infected cattle can contain BLV-infected cells [4,6], although not all BLV-positive cattle produce milk containing BLV [23].…”

“…However, housing systems were changed to a loose housing system leading up to 1990 with a free-stall barn system introduced into 50% of dairy farms breeding more than 75 heads [9]. Our recent study showed that a loose housing system was utilized in 23% of dairy herds and was a risk factor for transmission of BLV [11,18]. We believe that the increase in within-herd prevalence for dairy herds can be partially attributed to the change of housing systems over the last two decades, as it has enabled dairy cows to have greater contact with each other.…”