LONGITUDINAL ASSESSMENT OF ANTHELMINTIC RESISTANCE IN GASTROINTESTINAL NEMATODES AMONG GRAZING LIVESTOCK IN MIXED FARMING SYSTEMS

Abstract

Anthelmintic resistance among gastrointestinal nematodes presents a growing threat to the sustainability of livestock production, particularly in mixed farming systems where sheep, goats, and cattle graze collectively. A three-year longitudinal study with integrated diagnostic and epidemiological approach was conducted to examine prevalence, genetic basis and risk factors of anthelmintic resistance on ten farms in geographically diverse locations.  Fecal Egg Count Reduction Tests (FECRT) demonstrated a concerning decline of imidazothiazoles and macrocyclic lactones in several farms, and also a significantly reduced effect of benzimidazoles (mean reduction: 59%).  Molecular diagnostics has identified high rates of resistance-associated mutations, most notably the F200Y variant of the beta-tubulin gene, to be highly predictive of phenotypic resistance outcomes.  Coproculture studies revealed that Haemonchus contortus was the most predominant species comprising over 60 per cent of nematode populations on most of the farms contributing to the burden of resistance.  Although the spatial GIS analysis identified specific resistance hotspots where intervention measures should be carried out considering region-specific factors, the logistic regression analysis indicated that a high frequency of anthelmintic treatment, lack of grazing rotation, and prior use of drugs were significant predictors of resistance (p < 0.01).  The time frame in which chemotherapeutic options may be effective is becoming increasingly narrow with the dissecting fact that 40 percent of the farms possessed dual or triple-class resistance.  These findings highlight the importance of having combined parasite management (IPM) approaches which marry non-chemical methods such as pasture management, strategic selective treatment and genetic selectivity of hardy breeds with careful drug use.  This study, besides providing valuable information in the development of sustainable control measures needed to protect the future of mixed livestock farming systems, also provides vital, field-confirmed indications of the passionate and complex nature of anthelmintic resistance.