Abstract

DIEZ, Violeta et al. Evaluation of screening tests for antimicrobial residues in milk from individual cows treated with a combination of penicillins G and streptomycin. Ces. Med. Vet. Zootec. [online]. 2013, vol.8, n.2, pp.52-60. ISSN 1900-9607.

Abstract Background: in Colombia, the law (Resolution 1382, 2013) prohibits the sale of milk that contains any antimicrobial drug residues, although no specific official screening tests and detection limits have been specified. At present, milk with positive results to both the Delvotest^® and Snap^® assays is simply rejected. To avoid contaminating bulk tanks with milk from individually treated cows, producers would benefit from having on-farm screening tests to conduct their own quality controls. In addition, on-site testing would allow farmers to check if the withdrawal times of commercially-available generic products are in accordance with labeled recommendations. Material and Methods: In this study, two commonly used rapid detection tests (Delvotest^® and SNAP^® specific for beta-lactams) were used on milk from 39 subclinical mastitic Holstein cows that were prescribed with daily intramuscular injections of a commercial suspension containing 8.000.000 IU of penicilin G (75% procaine penicilin G, 25% potasium penicillin) and 8 g of streptomycin sulfate, for a total of 4 days. Cows were individually milked and samples collected every 12 hours the day before, and for 3 days after the recommended withdrawal time of three days post-treatment. To inactivate the potential action of natural inhibitors of microbial growth that may be present in milk (ie., lysozyme and lactoferrin), the results of the Delvotest^® were compared before and after milk samples were subjected to heat treatment (82°C for 5 minutes). Results: When the Delvotest^® was used as per manufacturer's instructions (i.e., without heating), 7 of 39 cows were positive for one more day past the recommended withdrawal period. However, the results of the Snap^® specific for beta-lactams and Delvotest^® post-heating showed that only 2 of those 7 cows were positive, suggesting that 5 animals gave false positive results. For the 312 milk samples analyzed, a high degree of concordance was observed (Kappa coefficient=0.74±0.1) between the Snap^® and Delvotest^® post-heating. Conclusions: Considering that the streptomycin in this product is known to be eliminated faster than penicillin-G, the results suggest that the efficacy of the Delvotest^® (after heat treatment) is similar to that of the Snap^® beta-lactams for the detection of penicillin residues. However, when the Delvotest^® was not preceded by heat treatment to inactivate potential natural inhibitors, it yielded a high number of false-positive results. The results also showed that in 95% (37/39) of the cows treated with this commercial product, the labeled instructions of a 3 day withdrawal period were adequate for compliance within the law.

Keywords : delvotest; milk; Penicillin G; residues; SNAP; Streptomycin.

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