A prospective investigation of bovine leukemia virus infection in young dairy cattle, using survival methods

Herd management practices associated with bovine leukemia virus incidence rate in Michigan dairy farms

The objective of this study was to identify associations between herd management practices and the incidence rate of bovine leukemia virus (BLV) infections in Michigan dairy herds. Previous management risk factor studies were of antibody prevalence rather than the rate of recent infections. Milk samples were collected from cohorts of cows on 112 Michigan dairy herds and tested for BLV using an antibody capture ELISA (n = 3849 cows). Cows were subsequently followed for an average of 21 months. Cows negative for anti-BLV antibodies and still present in their respective herds were retested by the same antibody capture ELISA to estimate within-herd incidence rates. The overall crude incidence rate was 1.46 infections per 100 cow-months at risk for the 1314 retested cows in 107 herds. The average within-herd incidence rate was 2.28 infections per 100 cow-months (range: 0 to 9.76 infections per 100 cow-months). A negative binomial regression model was used to identify herd management practices associated with the within-herd incidence rate. Results of the final multivariable model identified higher herd prevalence, milking frequency, needle reuse, as well as housing post-parturient cows separately, to be associated with increased incidence rate. Utilization of sand bedding for the lactating herd was found to be associated with decreased incidence rates. Results of this study suggest potential routes of BLV transmission which should be further investigated as disease control targets in ongoing control programs.

Using shared needles for subcutaneous inoculation can transmit bluetongue virus mechanically between ruminant hosts

Bluetongue virus (BTV) is an economically important arbovirus of ruminants that is transmitted by Culicoides spp. biting midges. BTV infection of ruminants results in a high viraemia, suggesting that repeated sharing of needles between animals could result in its iatrogenic transmission. Studies defining the risk of iatrogenic transmission of blood-borne pathogens by less invasive routes, such as subcutaneous or intradermal inoculations are rare, even though the sharing of needles is common practice for these inoculation routes in the veterinary sector. Here we demonstrate that BTV can be transmitted by needle sharing during subcutaneous inoculation, despite the absence of visible blood contamination of the needles. The incubation period, measured from sharing of needles, to detection of BTV in the recipient sheep or cattle, was substantially longer than has previously been reported after experimental infection of ruminants by either direct inoculation of virus, or through blood feeding by infected Culicoides. Although such mechanical transmission is most likely rare under field condition, these results are likely to influence future advice given in relation to sharing needles during veterinary vaccination campaigns and will also be of interest for the public health sector considering the risk of pathogen transmission during subcutaneous inoculations with re-used needles.

Herd-level determinants of bovine leukaemia virus prevalence in dairy farms

The prevalence of bovine leukaemia virus (BLV) was determined in 113 Michigan dairy herds by ELISA testing for anti-BLV antibodies in milk. Additionally, an interview regarding management practices with cooperating herd managers identified farm-level variables thought to be associated with prevalence of BLV. Twenty-three risk factors (P ⩽ 0·1) were identified on one-way ANOVA or simple linear regression. Multivariate analysis identified several management practices whose predictive value for increased prevalence of BLV may relate to transmission among herd mates, e.g. reuse of hypodermic needles, lack of fly control, gouge dehorning and increased use of injections in dry cows. Additionally, exclusive breeding of heifers with artificial insemination was associated with decreased BLV prevalence, as compared with at least some use of natural service by a bull. Although intervention studies are needed before causal relationships can be concluded, and unaccounted variables related to transmission exist among dairy herds, these findings suggest management practices that may help dairy producers reduce the transmission of BLV within their herds.

Using scenario tree modelling for targeted herd sampling to substantiate freedom from disease

Background

In order to optimise the cost-effectiveness of active surveillance to substantiate freedom from disease, a new approach using targeted sampling of farms was developed and applied on the example of infectious bovine rhinotracheitis (IBR) and enzootic bovine leucosis (EBL) in Switzerland. Relevant risk factors (RF) for the introduction of IBR and EBL into Swiss cattle farms were identified and their relative risks defined based on literature review and expert opinions. A quantitative model based on the scenario tree method was subsequently used to calculate the required sample size of a targeted sampling approach (TS) for a given sensitivity. We compared the sample size with that of a stratified random sample (sRS) with regard to efficiency.

Results

The required sample sizes to substantiate disease freedom were 1,241 farms for IBR and 1,750 farms for EBL to detect 0.2% herd prevalence with 99% sensitivity. Using conventional sRS, the required sample sizes were 2,259 farms for IBR and 2,243 for EBL. Considering the additional administrative expenses required for the planning of TS, the risk-based approach was still more cost-effective than a sRS (40% reduction on the full survey costs for IBR and 8% for EBL) due to the considerable reduction in sample size.

Conclusions

As the model depends on RF selected through literature review and was parameterised with values estimated by experts, it is subject to some degree of uncertainty. Nevertheless, this approach provides the veterinary authorities with a promising tool for future cost-effective sampling designs.

Biosecurity and risk management for dairy replacements

Biosecurity, biocontainment, and disease risk management on dairy replacement operations are time- and labor-intensive, planned programs. Oftentimes the value of these programs is realized only after disease is introduced to a facility or a disease outbreak occurs. There is no “one-plan-fits-all;” each plan must be tailored to meet the needs of management's goals and expectations and problems specific to a production enterprise or geographic region. A standard framework applicable to biosecurity programs includes: (1) hazard identification, (2) exposure assessment, (3) risk characterization, and (4) risk management. The discussion presented here helps lay the framework for development and implementation of biosecurity and risk-management programs within dairy replacement facilities.

Associations between farm management practices, productivity, and bovine leukemia virus infection in Ontario dairy herds

The objective of this study was to investigate the associations between herd-level bovine leukemia virus (BLV) status and herd-level management and production variables. The study population consisted of 1330 cows sampled from 102 Ontario dairy herds. The individual-cow prevalence of BLV infection in the population (based on AGID testing) was 23%, with 69.6% of herds having one or more positive animals. The herd-level explanatory variables were divided into two datasets containing winter housing variables and all non-seasonal variables, and summer housing variables and all non-seasonal variables. In both datasets, multivariable analyses found a negative association between herd-level milk production and BLV status, and positive associations between weaning age and purchasing animals from outside sources, and BLV status. Housing pre-weaned calves in hutches or separate calf buildings in either season was associated with an increased risk of BLV. The model containing winter housing variables also included positive associations between contact with older animals and BLV status, and between BLV status and the facilities used to house dry cows in the winter.

Economics of bovine leukemia virus infection

A herd infected with bovine leukemia virus suffers a direct economic loss due to clinical lymphosarcoma. A major indirect cost associated with infection is restriction of the sale of animals and germplasma to foreign markets. Reports on the economic effects of infection on production have been variable and are reviewed in this article. In order to develop cost-effective bovine leukemia virus control programs, costs associated with the disease, the cost of prevention, and expected economic returns from a program need to be considered.

Natural transmission of bovine leukemia virus in dairy and beef cattle

Many potential routes of bovine leukemia virus (BLV) transmission are reviewed in this article. Vertical transmission, in utero, or through colostrum and milk, accounts for a relatively small proportion of infections. Iatrogenic horizontal transmission, through procedures permitting the transfer of blood between cattle, has been shown to be a major route of transmission in most settings. Contact transmission stems from a mixture of natural sources of blood, exudates, and tissues that enter the body through mucosal surfaces or broken skin. Careful analysis of management procedures and environmental conditions present in individual dairy and beef herds affords the greatest opportunity to develop effective BLV prevention programs.

Regression models for time to seroconversion following experimental bovine leukaemia virus infection

This paper develops a parametric model for time to seroconversion after experimental bovine leukaemia virus (BLV) infection, and examines the effects of inoculation route, volume of inoculum, type of inoculation material, and antigen status of donor on seroconversion time. We used parametric and nonparametric statistical methodology to analyse interval data on 150 animals from 13 published reports. The log-logistic model fitted the observed times to seroconversion better than the log-normal or Weibull models, which were the considered alternatives.

Production and related variables in bovine leukaemia virus-infected cows

A newly developed milk dot blot test was used to detect anti-bovine leukaemia virus (BLV) antibody in milk samples from 2079 lactating adult cows from among 61 herds. The milk dot blot test was highly repeatable; the concordance rate, compared with the agar gel immunodiffusion test performed on serum, was 83.5%. All herds contained BLV-positive cows; the prevalence rate was 36%. BLV-positive cows tended to come from larger herds and were older and more often later in lactation. Fourteen production and related variables (herd size, age, days open, days in milk, milk somatic cell count, milk, fat, and protein produced in the current lactation, projected production of milk, fat, and protein, and breed class average deviations for milk, fat, and protein) were compared between BLV-positive and BLV-negative cows. Although somatic cell count, milk produced, and projected production of milk and protein were related significantly to BLV status using simple tests of association, once the variables herd size, age and days in milk were controlled, these differences were removed. Further analyses using logistic (outcome: individual cow BLV status) and least-squares regression (outcome: herd proportion of BLV-positive cows) failed to show an association between any of the measured production or related variables and BLV-positivity. We concluded that the effect of BLV on production and related variables in dairy cows was below the sensitivity of our analytical techniques or was non-existent.