Substitution of benzimidazole-resistant nematodes for susceptible nematodes in grazing lambs

Abstract

Multi-drug-resistant gastrointestinal nematode parasite populations are becoming more and more prevalent. Since anthelmintic treatments are of limited effectiveness, one solution could be to replace the anthelmintic-resistant population by a susceptible population, in order to re-establish the possibility of drug-based anthelmintic control. We investigated this substitution strategy in 4 paddocks of 0.7 ha, each of which was seeded with a benzimidazole-resistant Teladorsagia circumcincta population. The proportion of benzimidazole-resistant worms in these paddocks ranged from 20% to 89%. A 2-step replacement was performed: first, the paddocks were not grazed for 6 months (from December to July), and then the grass was cut to eliminate any residual infective larvae, before contaminating each of the paddocks with 10 seeder lambs experimentally infected with a benzimidazole-susceptible strain of T. circumcincta (from July to November). At the end of the experiment, all the populations on the 4 paddocks were phenotypically benzimidazole-susceptible, but genotyping indicated that 2 populations harboured 1% and 3% resistant worms respectively. This study demonstrates that nematode replacement is feasible in temperate areas, using semi-intensive stock management, even when the initial levels of benzimidazole-resistance are very high. Further research should next assess replacing the whole community to cope with the species diversity observed under field conditions.

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

“…To date, several studies have been conducted to examine the feasibility of replacing BZ resistant H. contortus (Van Wyk and Van Schalkwyk, 1990) or T. circumcincta (MoussavouBoussougou et al, 2007), or multi-drug resistant nematodes (Bird et al, 2001;Van Wyk et al, 2001;Sissay et al, 2006) with susceptible nematodes on pasture. These studies have shown that it is possible to reintroduce susceptible nematodes either by reducing the resistant parasites to negligible levels on pasture and infecting either entire flocks or seeder lambs with susceptible parasites to re-contaminate pastures (Van Wyk and Van Schalkwyk, 1990;Bird et al, 2001;Van Wyk et al, 2001;Moussavou-Boussougou et al, 2007) or by reducing resistant nematodes to very low levels in the hosts and transferring the hosts to pastures with high levels of nematodes in natural refugia (Sissay et al, 2006). Whilst most of these studies were conducted in subtropical and tropical areas where high temperatures during the summer months (Van Wyk and Van Schalkwyk, 1990;Bird et al, 2001;Sissay et al, 2006) or cold, dry winters ) render the pasture safe or even clean of infective larvae in only a few months, the experiment by Moussavou-Boussougou et al (2007) was performed in a temperate region where infective larvae can survive for many months on pasture.…”

Reduction of benzimidazole resistance in established Haemonchus contortus populations in goats using a single infection with a benzimidazole-susceptible isolate

“…The protocol for restoring BZ susceptibility investigated in this paper does not depend on the elimination of pre-existing resistant worm populations, a task difficult to achieve on most farms. It should therefore be easier to implement than previously attempted worm substitution schemes (Van Wyk & Van Schalkwyk, 1990; Bird et al , 2001; Sissay et al, 2006; Moussavou-Boussougou et al , 2007). Further studies are required to explore the potential of this approach for application in practical worm-control strategies.…”

“…Phenotypic resistance has been reduced experimentally on farms by deliberately introducing susceptible strains of Haemonchus contortus (Van Wyk & Van Schalkwyk, 1990) and Teladorsagia circumcincta (Moussavou-Boussougou et al , 2007). Most protocols for achieving this aim require the use of suppressive AH treatments to eliminate established worm populations from livestock, which are then kept indoors until external life-cycle stages disappear from pasture through natural environmental processes.…”

“…The hosts are subsequently re-infected with BZ-susceptible worms, either artificially (Bird et al , 2001) or by grazing a naturally infected pasture (Sissay et al, 2006). This process is termed ‘parasite substitution’ (Moussavou-Boussougou et al , 2007).…”

Αξιολόγηση Της Εφαρμογής Επιλεγμένων Ατομικών Ανθελμινθικών Αγωγών Για Την Καταπολέμηση Των Γαστρεντερικών Νηματωδών Παρασίτων Των Μικρών Μηρυκαστικών

“…υλοποίηση της αραίωσης μπορεί να γίνει με την είσοδο στην εκτροφή νεαρών ζώων που έχουν μολυνθεί τεχνητά με ευαίσθητα στελέχη παρασίτων, τα οποία και διασπείρουν μέσα στο λειμώνα(Moussavou-Boussougou et al, 2007).…”