Managing anthelmintic resistance – Use of a combination anthelmintic and leaving some lambs untreated to slow the development of resistance to ivermectin

ANTHELMINTIC RESISTANCE IN SMALL RUMINANTS FROM THE SEMIARID OF PARAÍBA STATE, BRAZIL/Resistência anti-helmíntica em pequenos ruminantes do semiárido da Paraíba, Brasil

“…Several studies around the world have confirmed the resistance of gastrointestinal helminthes of small ruminants to Ivermectine (LIFSCHITZ et al 2010;LIMA et al 2010a;LEATHWICK et al 2012).…”

“…Duarte et al (2012), testing this anthelmintic in Northern Minas Gerais State, Southeastern Brazil, achieved efficacy ranging from 90% to 100%. However, Leathwick et al (2012) observed resistance to this anthelmintic (efficacy<70%) in New Zealand sheep and George et al (2011) observed efficacies from 53% to 81% in sheep from Trinidad and Tobago.…”

World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) Guideline: Anthelmintic combination products targeting nematode infections of ruminants and horses

“…Importantly, experimental and modeling data suggest that the development of resistance to a new anthelmintic will not be accelerated (and in most circumstances it will be delayed) by its inclusion in a combination product (Dobson et al, 2011a,b;Leathwick, 2012;Leathwick et al, 2012). Indeed, this is a driving force for combination chemotherapy of HIV, tuberculosis and malaria in humans (see above) and may be considered one of the key advantages of anthelmintic combinations in geographic locations where AR in cattle parasites is not already intensely distributed (i.e., Europe, North America).…”

“…Similarly, adding a novel constituent active to an anthelmintic combination product that includes existing constituent actives, as opposed to using it alone or in rotation in areas where resistance already exists, should not predispose it to a more rapid selection for evolution of parasite resistance as demonstrated in recent modeling and empirical studies (Dobson et al, 2011a,b;Leathwick, 2012;Leathwick et al, 2012). Proof of efficacy of combinations against existing resistant parasite isolates in dose-determination experiments will alleviate this concern to some extent, although only field use will reliably reveal if selection of a resistance mechanism that crosses anthelmintic classes can occur in nematode populations following the use of a new drug.…”

Refugia and the evolutionary epidemiology of drug resistance

“…Consequently, the cost of maintaining effective interventions via the development and use of multiple drugs is considerable. These costs have driven the exploration of alternative strategies for integrated pathogen management, such as the use of refugia (untreated populations) for reducing levels of resistant pathogens in treated animals [5]. Importantly, refugia can serve to dilute drug-resistant pathogens through the force of infection from drug-susceptible pathogens.…”