Selective and on-demand drenching of lambs: Impact on parasite populations and performance of lambs

Abstract

Drenching strategies for lambs designed to slow the development of anthelmintic resistance, by increasing the pool of susceptible worms available to dilute resistant survivors after treatment, resulted in increased numbers of H. contortus and T. colubriformis but not other species of parasite on pasture. The increased parasite challenge to lambs in the autumn was associated with small production losses, which may be acceptable to farmers wishing to implement such strategies. It is clear that further work is required on the interaction between management practices and the population dynamics of parasites, especially with regard to creating pools of susceptible genotypes to slow the development of drench resistance.

Which is the best phenotypic trait for use in a targeted selective treatment strategy for growing lambs in temperate climates?

“…As such, a number of experimental studies have focussed on the impact of TST on performance traits. In comparison to a suppressive full flock treatment regime, the implementation of TST strategies for T. circumcincta infections have previously been shown to have non-significant impacts upon weight gain (Leathwick et al, 2006a;Greer et al, 2009;Stafford et al, 2009;Besier et al, 2010;Busin et al, 2013;Valcárcel et al, 2015). Predicted average weight gain benefits within this simulation study were very similar to the full flock treatment group for a single anthelmintic drenching occasion occurring within the later stages of the grazing season.…”

The role of targeted selective treatments in the development of refugia-based approaches to the control of gastrointestinal nematodes of small ruminants

“…in the targeted group. Another study where the heaviest 10% of the flock remained untreated found no significant differences in liveweight gain, compared to a flock where all animals were suppressively treated (Leathwick et al, 2006a). Increased numbers of H. contortus and T. colubriformis were found on pasture, although the long term implications of this with regard to increase pasture contamination leading to higher parasite challenge in subsequent years was not examined.…”

“…Leathwick et al (2006a) compared the effect of a suppressive drenching regime to that of a targeted regime where animals were treated on the basis of mean FEC (group mean exceeded 500 eggs per gram) on lamb performance and the parasite populations present. There was little difference between the numbers of anthelmintic treatments given to the two groups.…”

Validation of the FAMACHA© eye colour chart using sensitivity/specificity analysis on two South African sheep farms

“…The proportion of the parasite population that escapes drug selection is at present thought to be the most important factor in influencing the rate of development of resistance (Van Wyk, 2001;Leathwick et al, 2006). It has been largely in response to this that systems of sTST, notably FAMACHA ß , were developed.…”