RESUMEN
Haemonchus contortus is a nematode parasite that causes anaemia and affects the health of sheep. The mean corpuscular haemoglobin concentration (MCHC) is an excellent indicator to detect anaemia that could help to characterize resistant or susceptible lambs to gastrointestinal nematodes. The aim of this study was to evaluate the predictive value of MCHC in detecting changes in red blood cells and their relation to anaemia in lambs re-infected with H. contortus. An analysis of information was performed using 24 Pelibuey lambs previously infected in grazing, dewormed and experimentally re-infected with H. contortus. At the first haematological sampling (admission) the lambs were classified based on MCHC quartiles (Q). Subsequently, the lambs were housed for 56 days. Blood samples were taken every seven days to determine the haematological parameters using an impedance haematological instrument. Confidence limits were constructed with the records of the lambs that recovered their haematological parameters. Each quartile was analysed as a treatment in a repeated measures design over time. To know the optimal combination of sensitivity and specificity of MCHC to detect anaemia a curve of receiver operating characteristic (ROC) curve and the cut-off values were evaluated. In quartile 4 (Q4), lambs showed the highest faecal egg count (FEC, 764 eggs/g of faeces), mean corpuscular haemoglobin (17.0 pg) and MCHC (54.6 g/dL). This group also presented the lowest RBC values (5.8 × 106/mL), haematocrit (HCT, 18.3%), total plasma protein (5.7 g/dL), and HGB (9.7 g/dL). The optimal point of MCHC with ROC curve was 42.4 (sensitivity 88.2% and specificity 86.5%); the area under the curve was 0.91 (CI 95%, 0.86-0.96). These results are related to the haematological effects caused by H. contortus in susceptible lambs. In conclusion, the highest FEC and lower HCT in Q4 are important elements of the haematological damage caused by H. contortus and could identify susceptible lambs.