Assessing the use of diagnostic laboratory accession data to support national bovine viral diarrhoea control in New Zealand

Non-Bovine Species and the Risk to Effective Control of Bovine Viral Diarrhoea (BVD) in Cattle

Bovine viral diarrhoea virus (BVDV) is an economically important and highly prevalent virus of domestic cattle. Infections with BVDV may lead to both, reproductive and immunological effects that can result in widespread calf losses and increased susceptibility to diseases, such as mastitis and respiratory disease. While BVDV is generally considered to be host specific, it and other Pestivirus species, such as Border disease virus (BDV) in sheep, have been shown to be infecting species other than those from which they were originally isolated from. Recently BVDV was placed on the OIE’s list of notifiable disease and control and eradication programmes for BVDV have been developed throughout much of Europe, the United States, and the United Kingdom. While some countries, including Sweden and Ireland have successfully implemented eradication programmes, other countries such as New Zealand and Australia are still in the early stages of BVDV control. Despite effective control methods, incursions of BVDV into previously cleared herds still occur. While the cause of these incursions is often due to lapses in control methods, the ability of ruminant pestiviruses to infect species other than cattle poses the question as to whether non-bovine species could be impeding the success of BVDV eradication and control. As such, the aim of this review is to make mention of what is known about the cross-species transmission of BVDV, BDV and other pestiviruses between cattle and non-bovine ungulate species and draw conclusions as to the risk non-bovine species pose to the successful control and eradication of BVDV from cattle.

Practices and opinions of New Zealand veterinarians regarding control of bovine viral diarrhoea

AIMS

To explore recommendations that New Zealand veterinarians make for diagnosing and managing bovine viral diarrhoea (BVD) in cattle herds under different clinical scenarios and their opinions towards potential barriers and opportunities for implementing BVD control programmes in New Zealand.

METHODS

A cross-sectional survey of registered veterinarians in New Zealand was conducted in 2019. Respondents were asked about the approaches they would use to manage BVD under different clinical scenarios as well as their opinions on national BVD control. A subset of veterinarians completed a more in-depth survey providing additional free-text responses on a range of different BVD topics. Descriptive statistics were provided for all quantitative study variables and the free-text responses were also analysed to generate further insights into veterinarians' perceptions towards BVD management.

RESULTS

The cross-sectional survey was completed by 101 of an estimated 870 (11.6%) cattle veterinarians. Thirty-five veterinarians completed the in-depth survey. There was wide variation in the BVD diagnostic testing and vaccination protocols that respondents recommended under different clinical scenarios. Annual bulk milk BVD testing was perceived as a valuable tool for initiating BVD discussions with dairy farmers. Respondents indicated that beef farmers were more difficult to engage in BVD control largely due to the logistical challenges of yarding cattle at the appropriate times to implement interventions, with many farmers only contacting veterinarians after experiencing a BVD outbreak Most respondents (91/101; 90%) believed it was possible to eradicate BVD from New Zealand, but cited lack of farmer awareness and poor compliance with management recommendations as significant barriers. The measure with the most support for inclusion in a compulsory national eradication programme was requiring farmers to declare the status of their animals prior to sale while the least supported measure was requiring farmers to double fence boundaries to prevent nose-to-nose contact with neighbouring stock. Although respondents highlighted the need for farmers and industry to support any national eradication programme in order for it to be successful, there was also recognition that veterinarians could be more pro-active in engaging with farmers particularly in discussions around the economics of BVD.

CONCLUSIONS AND CLINICAL RELEVANCE

While the survey respondents appeared to be highly supportive of BVD control, it was perceived that financial and logistical barriers existed that could impede farmer engagement. Further extension efforts may be needed to ensure that veterinarians are presenting clear and consistent recommendations about BVD management to farmers.Abbreviations: BVD: Bovine viral diarrhoea; NAIT: National Animal Identification and Tracing System; PI: Persistently infected.

Temporal trends in bulk tank milk antibody ELISA and PCR test results for bovine viral diarrhoea in New Zealand pastoral dairy herds

Aims: To describe temporal trends in bulk milk antibody ELISA and PCR testing for bovine viral diarrhoea (BVD) in New Zealand pastoral dairy herds and to assess the use of historical accession data to predict herd-level BVD incursions. Methods: Data on all diagnostic testing of bulk milk for BVD performed by the Livestock Improvement Corporation (Hamilton, NZ) over eight lactation seasons from 1 June 2010 to 31 May 2018 were analysed. This included anonymised herd identification, geographic location, herd size, sample collection date, sample to positive (S/P) ratio for antibody ELISA results, and cycle threshold values for PCR detecting viral RNA. Multivariable logistic regression was used to explore the relationship between historical accession data and the risk of herds having at least one positive bulk milk PCR test result in the 2017 season. Results: There were 156,034 bulk milk BVD diagnostic testing accessions for 10,495 uniquely identified dairy herds over the 8-season period. The prevalence of tested herds with at least one positive bulk milk PCR test result decreased from 14.6% (407/2,786) in the 2010 season to 5.6% (355/6,309) in the 2017 season with similarly marked declines in S/P ratios. In the 2017 season, 2,961/6,309 (46.9%) herds had S/P ratios greater than the 0.75 cut-off value indicating recent or active BVD virus transmission within the herd while 1,422/6,309 (22.5%) herds were classified as having negative or low S/P ratios. Herds that cleared BVD from the milking herd experienced a mean decline in S/P ratio of 0.11 units per year (min 0.05; max 0.18). In the multivariable analysis, the overall incidence risk of herds experiencing a BVD incursion in the 2017 season was 3.8% (146/3,848) and there were three significant predictors in the final model: herd size, PCR status in the 2014 season, and change in S/P ratio between the 2014 and 2015 seasons. The area under the receiver operating curve for the final model was 0.695 indicating poor discrimination. Conclusions and clinical relevance: The prevalence of dairy herds in New Zealand with positive bulk milk PCR test results and high S/P ratios has decreased over time, suggesting fewer herds are actively infected with BVD and that herd immunity may also be declining. Although monitoring trends in bulk milk test results provides useful information on changes in individual herd status, it is difficult to accurately predict when new incursions will occur and farmers should continue to maintain good biosecurity.

Modelling the economics of bovine viral diarrhoea virus control in pastoral dairy and beef cattle herds

Bovine viral diarrhoea virus (BVDV) is a prevalent pathogen in the New Zealand cattle industries, yet few studies to date have evaluated the economics of BVDV in pastoral dairy and beef herds to help inform management decisions. To address this knowledge gap, we developed stochastic individual-based simulation models to represent the transmission dynamics of BVDV in typical spring-calving dairy and beef farms in New Zealand. The models conservatively estimated the direct losses due to a BVDV outbreak at NZ$ 22.22 and NZ$ 41.19 per mixed-age cow per year for a naïve dairy and beef farm, respectively, over a 5-year period. The greatest economic impacts for the dairy farm occurred when persistently infected replacement heifers joined the lactating cow group and caused transient infection of cows to drop in milk production, whereas the greatest impacts for the beef farm was through the loss of fattening stock for sale due to lowered pregnancy rates. Various combinations of diagnostic testing, vaccination, and biosecurity measures were then explored to evaluate the cost-efficiency of different strategies for controlling BVDV at the farm-level. Providing farmers with the estimates of economic impacts of BVDV in their herds may further encourage the uptake of control measures, but close collaboration with a veterinarian to determine the optimal strategy for their unique farm circumstances is still required.

Serological evidence for exposure to bovine viral diarrhoea virus in sheep co-grazed with beef cattle in New Zealand

Aims: To determine whether sheep that co-grazed with cattle that were suspected to be positive for bovine viral diarrhoea (BVD) virus had serological evidence of exposure to the virus.Methods: Eighteen commercial farms that routinely co-grazed cattle and sheep in the same paddocks were recruited through purposive sampling. The recruiting veterinarians identified nine farms with cattle herds that were known or highly suspected to be positive for BVD and nine farms that were considered to be free of BVD. Blood samples were taken from 15 ewes aged 1 year on each farm and samples were submitted to a commercial diagnostic laboratory to test for antibodies against pestiviruses using an ELISA. All samples that were positive were then tested using a virus neutralisation test (VNT)for antibodies against BVD virus.Results: Of the 270 blood samples, 17 were positive for pestivirus antibodies by ELISA and these originated from two farms that were known or suspected to have BVD virus-positive cattle. None of the samples from the nine flocks co-grazed with cattle herds that were known or suspected to be BVD virus-negative were positive for pestivirus antibodies. Within the two positive farms, 2/15 samples from the first farm and 15/15 samples from the second farm were antibody-positive. When the 17 positive blood samples were submitted for VNT, all 15 samples from the second farm tested positive for BVD virus antibodies with the highest titre being 1:512.Conclusions and clinical relevance: In this small sample of New Zealand sheep and beef farms with suspected BVD infection in cattle, there was evidence of pestivirus exposure in co-grazed sheep. Although we were unable to confirm the origin of the exposure in these sheep, these findings highlight that farmers who are trying to eradicate BVD from their cattle should be mindful that the infection may also be circulating in sheep, and both populations should be considered a possible risk to each other for generating transient and persistent infections. Further work is needed to estimate the true prevalence of New Zealand sheep flocks that are affected by BVD and the associated economic impacts.

Practices and opinions of New Zealand beef cattle farmers towards bovine viral diarrhoea control in relation to real and perceived herd serological status

Aims: To investigate the seroprevalence of infection with bovine viral diarrhoea (BVD) virus among 75 beef herds and seroconversion in cattle during early pregnancy, and to determine the practices and opinions of farmers towards BVD control and their association with real and perceived herd serological status.Methods: Blood samples were collected before mating in 75 beef herds across New Zealand from 15 unvaccinated heifers that had delivered their first calf that season. Serum samples were tested for BVD antibodies using ELISA individually, and after pooling samples for each farm. Animals that were antibody-negative were retested at either pregnancy diagnosis or weaning. Farmers were asked to complete a detailed survey about herd demographics, BVD testing and vaccination practices, and opinions towards national BVD control.Results: Based on the pooled serum antibody ELISA results, there were 28/75 (37%) negative herds, 15/75 (20%) suspect herds, and 32/75 (43%) positive herds. Of 1,117 animals sampled 729 (65.3%) tested negative for BVD virus antibodies; when retested, 47/589 (8.0%) animals from 13/55 (24%) herds had seroconverted. Among 71 famers providing survey responses 11 (15%) believed their herd was infected with BVD, 24 (34%) were unsure and 36 (51%) did not think their herd was infected. Only 19/71 (18%) farmers had performed any BVD testing within the past 5 years and 50/70 (71%) had not vaccinated any cattle for BVD. Support for national BVD eradication programme was strong in 51/71 (56%) respondents, but the biggest challenge to BVD control was considered to be famer compliance. Compared to farmers who did not think their herd was infected, more farmers who thought BVD was present in their herds had previously tested for BVD, would consider testing all replacement calves, and would support establishing a national BVD database; fewer would consider purchasing BVD tested or vaccinated cattle only.Conclusions and clinical relevance: Only 15% of the beef farmers in this study believed their herds were infected with BVD virus and few of them had undertaken BVD screening. Nevertheless many were supportive of implementing a national BVD control programme. It is likely that the lack of farmer awareness around BVD and the failure of farmers to recognise the potential impacts in their herds are hindering progress in controlling the disease in New Zealand. There are opportunities for New Zealand veterinarians to be more proactive in helping beef farmers explore BVD management options.