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Scottish BVDV spot tests: Why do we get single animals testing positive and what is their significance?
Carty, H.; Caldow, G.; O'Connor, R.
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Objectives: Young stock spot tests are used to determine if a herd has current evidence of exposure to bovine viral diarrhoea virus (BVDV). Serum samples from a representative proportion of young stock are tested for the presence of antibodies to BVDV. Within the Scottish eradication programme, typically five animals per management group are tested. If one animal in the group tests positive for antibodies to BVDV, the herd is deemed to be "not-negative" for BVDV. Movement restrictions are placed on herds that are "not-negative" for BVDV. This study aims to determine the reasons for and sig- nificance of single antibody positive results in Scotland over a one year testing period.
Material & Methods: 4728 young stock spot tests from Scottish herds were analysed for the presence of BVDV anti- bodies by Scotland's Rural Colleges (SRUC) Veterinary Services between 1st October 2017 and 30th September 2018.
The spot tests were divided into one of three categories: (1) all animals tested negative for antibodies to BVDV; (2) more than 20% of the animals tested positive for antibodies to BVDV or (3) 20% or less of the animals tested positive for antibodies to BVDV. The data and notes from the spot tests in the third category were analysed to determine if there was a clear reason for low numbers (usually a single animal in a group of five) testing positive for antibodies to BVDV. Retest results, herd status for that year's calf crop and the subsequent calf crop were also assessed to determine the significance of these results.
Results: 4290 of the spot tests were in category 1, 209 were in category 2 and 229 were in category 3 (20% or less of the animals tested positive for antibodies to BVDV). After further data cleansing to remove inappropriate tests (those that were originally booked in as spot tests but in fact were tested for another reason), 216 spot tests in category 3 from 192 Scottish herds were used for this study. 40% of these spot tests were identified as having avoidable reasons i.e. use of vaccination, maternally derived antibody or incorrect calf tested. Following review of herd information, and in some cases retesting, 56% of the herds were assigned a "negative" status for the year and 44% a "not-negative" status. The subsequent calf crops from these herds were also tested, where applicable, and following analysis of those results 80% had a "negative" status, 17% a "not-negative" status and 3% had been declared as non-breeding herds. Animals persistently infected with BVDV were identified in three herds, despite apparent limited evidence of exposure to BVDV in the initial spot test.
Conclusion: 5% of spot tests carried out in the study period identified 20% or less of the tested animals as positive for antibodies to BVDV. 40% of these however had a clear, avoidable explanation. The results of this study can therefore be used to give clearer, evidence based guidance to minimise mistakes and misleading results with the spot test. The significance of these results on the herd status in the current and subsequent calf crops can also inform future policy on whether the cut-point for determining herd status should be changed.
Keywords: BVDV, eradication, testing, antibody.
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About
Affiliation of the authors at the time of publication
SRUC Veterinary Services, Ayr, United Kingdom;
SRUC Veterinary Services, Edinburgh, United Kingdom;
SRUC, Ayr, United Kingdom.
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