Study of the impact of gastrointestinal nematodes infection on the performance of morada nova lambs for development of a targeted selective treatment

Gastrointestinal nematodes (GIN) are a major obstacle to sheep farming. Morada Nova lambs are resilient to GIN, and may present high infections without clinical signs, such as anemia, affecting the use of the FAMACHA method. In view of these factors, the objective of this study was to adapt a method to GIN control in Morada Nova animals through selective treatment (TST), using the average daily weight gain (DWG) of the lambs as a parameter. In addition, the applicability of near-infrared spectroscopy (NIRS) in the diagnosis of Haemonchus contortus infection was verified. Based on the data obtained, estimates of economic losses resulting from GIN infection were made in the different experimental groups. For this, 114 lambs in the rainy season and 102 in the dry season were randomized into three treatments: Control (CT – without treatment), Routine (RT - treated every 42 days), and Selective (TST - treated according to DWG). From the 63rd day of life (D63) to the D210, each 21 days, the animals were weighed and blood samples (packed cell volume (PCV) measurement and NIRS spectra evaluation), feces (egg count per gram of feces (EPG) and coprocultures) and FAMACHA grade reading were collected. To investigate the resistance of parasites to anthelmintic used in the experimental groups (levamisol 9.4 mg/kg), the EPG count reduction test (FECRT), the RESISTA-Test© for lethal concentration (LC) calculation and the molecular test (detection of indel 63 bp in the gene acr-8 in H. contortus) were performed. In the economic study, the parameters of operational cost of anthelmintic treatments and the economic result from them were analyzed and compared. In the CT, RT and TST groups, respectively, the mean EPG (p ≤ 0.05) were 4665.1, 3063.5 and 3462.1 in the rainy season and 4475.1, 1341.7 and 2863.4 in the dry season. The averages of the PCV were 32.1%, 33.4% and 32.3% in the rainy season and 33.9%, 36.0% and 35.1% in the dry season. Mean DWG was 0.087 Kg, 0.101 Kg and 0.094 Kg in the rainy season and 0.102 Kg, 0.113 Kg and 0.112 Kg in the dry season. The efficacy of levamisole in the FECRT was 66.4%, 24.1% and 76.4% in rainy and 90.7%, 12.4% and 64.8% in dry, respectively. In the RESISTA-Test© the LC50 were 0.482, 1.926 and 0.117 µg/mL in the rainy season and 12 0.437, 0.851 and 0.045 µg/mL in the dry season, respectively. In the molecular test, the frequency of homozygous resistance genotype was 57.1%, 71.4% and 40.0% in the rainy season and 47.8%, 55.9% and 41.9% in the dry season. No significant difference was observed between the TST and RT treatments in relation to the mean weight gain, final weight and DWG, especially in the dry season, in which the parasite load was lower. The efficacy of the anthelmintic treatments was more drastically reduced in the RT. Specificities of sheep breeds and environmental conditions need to be assessed prior to the application of TST by DWG for GIN control. The study of the NIRS spectra was performed based on the classification of Morada Nova lambs among sick and healthy for parasitic infection and presented a sensitivity of 93.33% and specificity of 73.21%. The technique was able to interact with blood components and make classifications, demonstrating that it can be more sensitive than FAMACHA, as well as a quick and versatile technique for field application. In relation to the economic survey, the animals of the RT arrived in the D210 heavier in relation to the CT, generating higher gross sales in both seasons. In relation to CT, one can verify an expressive economic result in favor of RT of R$ 33.00 per head in the rainy season and R$ 28.45 in the dry season. The RT presented economic results 14.4% and 10.9% higher than the CT and 7.2% and 1.9% higher than the TST, in the rainy and dry periods, respectively, but the fast development of resistance made its use unfeasible. Expenses with the use of anthelmintics are estimated at around 1.3% in the total economic result of the production system. Considering the reduction in weight gain triggered by GIN in national flocks, it is estimated a potential economic impact of R$ 472.6 million/year and that losses from deaths in the Brazilian sheep production are of R$ 65 million/year (AU)