Selection for anthelmintic resistant Teladorsagia circumcincta in pre-weaned lambs by treating their dams with long-acting moxidectin injection

Abstract

Administration of long-acting anthelmintics to pregnant ewes prior to lambing is a common practice in New Zealand. Today, most of these products contain macrocyclic lactone (ML) actives, which because of their lipophilic nature, are detectable in the milk of treated animals and in the plasma of their suckling offspring. This study was conducted to confirm the transfer of ML actives to lambs in the ewe's milk, and to assess whether this could result in selection for ML resistant nematodes in the lamb. Ninety, twin bearing Romney ewes were treated before lambing with a long-acting injectable formulation of moxidectin, a 100-day controlled release capsule (CRC) containing abamectin and albendazole, or remained untreated. After lambing, seven ewes from each treatment group were selected for uniformity of lambing date and, along with their twin lambs, relocated indoors. At intervals, all ewes and lambs were bled, and samples of ewe's milk were collected, for determination of drug concentrations. Commencing 4 weeks after birth all lambs were dosed weekly with 250 infective larvae (L3) of either an ML-susceptible or –resistant isolate of Teladorsagia circumcinta. At 12 weeks of age all lambs were slaughtered and their abomasa recovered for worm counts. Moxidectin was detected in the plasma of moxidectin-treated ewes until about 50 days after treatment and in their lambs until about day 60. Abamectin was detected in the plasma of CRC-treated ewes until the last sample on day 80 and in the plasma of their lambs until about day 60. Both actives were detectable in milk of treated ewes until day 80 after treatment. Establishment of resistant L3 was not different between the treatment groups but treatment of ewes with moxidectin reduced establishment of susceptible L3 by 70%, confirming the potential of drug transfer in milk to screen for ML-resistance in the suckling lamb.

Climate change is likely to increase the development rate of anthelmintic resistance in equine cyathostomins in New Zealand

“…maintaining an adequate parasite refugia ( Barnes et al, 1995 ; Kaplan and Nielsen, 2010 ; Miller et al, 2012 ; Leathwick et al, 2012 ) and utilize selective treatment ( Gomez and Georgi, 1991 ; Nielsen et al, 2019 ), ensuring the current resistance status is identified and effective treatments are used, including combination products ( Leathwick et al, 2009 ; Dobson et al, 2011 ; Bartram et al, 2012 ; Leathwick, 2012 ; Miller et al, 2012 ), avoiding unnecessary use of drugs, e.g. long acting anthelmintics or prophylactic applications without evidence of necessity ( Sutherland et al, 1997 ; Leathwick et al, 2009 , Leathwick et al, 2015a , Leathwick et al, 2015b ).…”

“…The development of the model and incorporation of genetic mechanisms for anthelmintic resistance have been described in detail elsewhere ( Leathwick et al, 2019 ; Leathwick et al, 2015 ; Sauermann et al, 2019 ). Briefly, the model replicates the dynamics of the cyathostomin free-living and parasitic stages, the administration of anthelmintics and the subsequent build-up of resistant genotypes.…”

“…The recent development of a whole lifecycle simulation model for the cyathostomin species complex infecting horses Leathwick et al, 2019 ; Leathwick et al, 2015a , Leathwick et al, 2015b ; Sauermann et al, 2019 ) enabled the investigation of the development of anthelmintic resistance under different treatment regimens and for different climatic locations ( Leathwick et al, 2019 ; Nielsen et al, 2019 ). In these modelling studies the size of the parasite population increased, and anthelmintic resistance developed more rapidly in warmer climates when compared to colder climates.…”

Short communication: A practical farm-based trial to compare ewe nematode control strategies in peri-parturient ewes

“…As such neither product used in this study is licensed for use in dairy ewes. [24] have shown that a macrocyclic lactone can be detected in the plasma of suckling lambs up to at least 20 days after it was administered to the dam and thus may select for resistant parasites within the lamb [25]. The transfer of anthelmintic via the milk to the lambs may also contribute to the reduction in FEC of lambs suckling dams receiving short-acting and long-acting macrocyclic-lactones [26].…”

Peripartum Phenomenon in Crioula Lanada Sheep Susceptible and Resistant to Gastrointestinal Nematodes

“…However, its efficacy has been diminished due to the occurrence of parasitic resistance in herds (5). Therefore, alternatives to chemical control have really been sought because of anthelmintic resistance and especially the potential for transfer of sub-effective doses to lambs in milk during lactation (6), among them, the ability of animals to generate an effective immune response. The ability for sheep to acquire and to express immunity against gastrointestinal nematodes is genetically controlled, and this varies substantially among different breeds (7).…”