Routine monitoring of anthelmintic efficacy against cattle gastrointestinal nematodes is necessary to conserve the efficacy of existing anthelmintics and limit resistance development; consequently, there is a need for robust field efficacy testing protocols. Efficacy is evaluated using slaughter studies with total parasite counts (to support product registration), and in the field using the faecal egg count reduction test (FECRT), and guidelines produced b the World Association for the Advancement of Veterinary Parasitology (WAAVP) are commonly followed for both approaches. WAAVP FECRT guidelines were originally developed for use in sheep and have been adapted, but not validated against the more rigorous slaughter studies, for use in cattle, and when following these guideline many authors report "lack of efficacy" as "evidence of resistance". The FECRT measures the egg production by gravid female worms. Experimental studies have shown that faecal egg counts (FEC) are not positively correlated with actual worm burden, and that egg suppression occurs in otherwise viable females following treatment with macrocyclic lactones, which may lead to overestimation of product efficacy when applying the FECRT. Interpreting cattle FECRTs can be challenging as the level of egg excretion is generally low and highly aggregated within groups of cattle, with the vast majority of animals shedding only low numbers of eggs. It is important to note that the FECRT does not measure 'resistance' per se, and a number of biological and statistical caveats apply to the FECRT in cattle (and other host species), which can lead to inaccurate test interpretation. Interpretation is further complicated by the presence of both adult and immature worms of different species, each with varying fecundity over time in relation to the level of acquired immunity. In addition, during autumn and winter months, worm egg output tends to decline due to hypobiosis and delayed development to sexual maturity. During late autumn and winter, up to 80% of ingested larvae may be come arrested (with development resuming in the sprint) so FECRT performed at this time of year can be misleading as immature worms may not become patent. Recent work shows that group size, the level and distribution of egg shedding, the egg detection limit of the FEC method, and the method of efficacy calculation all impact the precision of the FECRT, and without construction of 95% confidence intervals (CI) the results can easily be misinterpreted. Despite these caveats, the FECRT currently remains the most practical method for the field assessment of anthelmintic efficacy in cattle nematodes in the absence of validated in vitro and molecular tools for detecting anthelmintic resistance.
|Number of pages||2|
|Publication status||Published - Oct 2015|
- gastrointestinal nematodes
- faecal egg count
- faecal egg count egg count reduction