GNSS-derived metrics as indicators of gastrointestinal parasitism in naturally infected grazing ewes and lambs

Gastrointestinal nematodes, particularly Haemonchus contortus , cause significant health and productivity losses in sheep. While Targeted Selective Treatment (TST) mitigates anthelmintic resistance by treating only individuals requiring clinical intervention, its large-scale implementation is limited by high labour demands. Precision Livestock Farming (PLF) offers continuous monitoring alternatives to overcome this barrier. Therefore, this study evaluated the use of Global Navigation Satellite System (GNSS) trackers to identify locomotion pattern alterations associated with gastrointestinal parasitism in grazing sheep, aiming to assess its potential as a screening tool for TST. Two experiments evaluated 15 lactating ewes and 15 weaned lambs under natural infection conditions. Custom-collared GNSS trackers recorded spatial parameters during grazing, while packed cell volume (PCV) and faecal egg counts (FEC) were evaluated weekly to classify individual health status. Animals were evaluated from a post-anthelmintic ‘healthy’ baseline until they became ‘unhealthy’ (PCV < 22%, or PCV < 27% + FEC > 2000). H. contortus was the prevalent nematode genus in both cohorts (ewes = 96%, lambs = 93.8%). Analysis of GNSS-derived metrics revealed that unhealthy lambs exhibited significantly higher peak speed, overall speed, peak acceleration, and total active time than their healthy counterparts ( p < 0.05). In ewes, poor health status also significantly increased active time and overall speed ( p < 0.05); however, peak speed and acceleration were influenced primarily by body weight rather than health status. GNSS metrics are promising non-invasive candidate indicators for TST in grazing sheep.