Detection of antimicrobial substances in individual cow and quarter milk samples using delvotest microbial inhibitor tests

The use of antibiotic therapy to treat and prevent udder infections of cows during the dry period is a key component of mastitis control in many count ries.' At the same time, the general public is becoming increasingly aware of potential hazards from antibiotic residues in foods. Consequently, Delvo test Cow Test (Royal Gist-brocades NV, Delft, The Netherlands), an on-farm version of Delvotest P, a microbial inhibitor test for antimicrobials, is b eing increasingly used by farmers to assess that milk from individual cows is fit for consignment to the bulk tank. Occasional reports of unexplained positive test results have led to suggestions of possible false-positive re actions in milk from individual cows. To investigate the potential causes o f such positive test results, three separate investigations were undertaken . In a field survey of unexplained positive reports from farmers, 14 milk s amples from six farms that tested positive were all found to contain antibi otic residues. In more formal investigations of individual quarter milk sam ples from an experimental herd, none of 134 milk samples from midlactation cows yielded positive reactions; for cows that had just calved, 16 of 144 m ilk samples were positive, and, of those, 13 had somatic cell counts >4,000 ,000/ml. Natural inhibitors were responsible for 1 positive reaction, 8 pos itive reactions were related to incomplete milking, and 7 samples contained beta-lactam antibiotics. Positive reactions caused by antibiotic persisted in individual quarter samples for up to 7 d postcalving compared with 4 d for milk samples from the whole udder. Delvotest was sensitive to cephaloni um, the active ingredient of Cepravin Dry Cow (Mallinckrodt Veterinary Ltd. , Uxbridge, United Kingdom), which is the market-leading product in the Uni ted Kingdom. Test results yielded a partial purple color reaction in the pr esence of 8 mu g/kg of cephalonium and a completely purple reaction at 16 m u g/kg. These results confirm the validity of Delvotest when used to examin e composite milk samples from individual cows supplying the United Kingdom dairy industry and suggest that, with proper attention to milk withdrawal p eriods and complete milking, there is no obvious risk of antibiotic contami nation of milk.