Targeted selective treatment of gastrointestinal nematodes in lambs: accuracy of the dynamic growth curves

With the objective of validating the use of dynamic growth curves as a targeted selective treatment (TST) criterion, 46 lambs (25 females and 21 males) from the Gralha Azul Experimental Farm of PUCPR were evaluated. The lambs, Texel and Ile de France crossbred, were evaluated from 45 to 150 days of age. Every two weeks, the lambs were weighed and evaluated for packed cell volume (PCV) and fecal egg count (FEC). The need for anthelmintic treatment was indicated by the dynamic growth curves whenever the observed weight was lower than the expected weight. The dynamic growth curves were weighted at each weighing in relation to the previous weight of the lamb. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy were calculated, considering 3 thresholds of FEC, based on the 80th, 85th, and 90th percentile of the cumulative distribution of the FEC as the gold standard. The mean PCV values (31.85% and 32.85%), FEC (1874.99 and 1037.35) and live body weight (27.23 kg and 33.12 kg), respectively for treated and untreated lambs, showed no difference (p > 0.05). For the criterion of FEC ≥ 3000, sensitivity was 55%, specificity 71%, PPV 17%, NPV 94% and accuracy 70%. It is concluded that the dynamic curves have a high capacity to identify lambs that do not need treatment (NPV 94%), and additionally that the association with another criterion, such as FEC, in cases where anthelmintic treatment is indicated by the curve, could increase the accuracy of the method.