Exaggerated and potentially false claims of resistance to triclabendazole may be hampering control of fasciolosis by encouraging farmers to use less effective products, says Ian Fairweather , who argues for more accurate diagnosis before such claims are made THERE is much evidence to indicate that the incidence of fasciolosis has increased in recent years and that this trend is likely to continue well into the future (Kenyon and others 2009, Fox and others 2011). The other major threat to control of the disease is the emergence of resistance to triclabendazole (TCBZ), which remains the most important flukicide because of its wide spectrum of activity, particularly against the migrating juvenile and most pathogenic stages of infection in sheep. Internationally, a number of cases of resistance have been confirmed and they indicate that resistance does exist. In the UK and Ireland, several reports of possible TCBZ resistance have appeared in the literature in recent years (Fairweather 2011a). Various phrases are used to describe reports of resistance: ‘lack of efficacy’; ‘failure of efficacy’; ‘flukicide inefficacy’; ‘possibility of inefficacy’; ‘evidence of inefficacy associated with use of TCBZ’; ‘drench … appeared to be unsuccessful’; ‘a suspicion of lack of efficacy of fluke (TCBZ) treatment’; ‘treatment failure’; ‘flukicide failure’; ‘resistance suspected following identification of fluke eggs in faecal samples taken three weeks post-treatment’; ‘flock deaths … despite treatment with a flukicide. Parasite resistance to treatment could not be ruled out’. Reports often talk about ‘flukicide’, without specifying which one. However, to my …