title: Modelling the consequences of targeted selective treatment strategies on performance and emergence of anthelmintic resistance amongst grazing calves.
abstract: The development of anthelmintic resistance by helminths can be slowed by maintaining refugia on pasture or in untreated hosts. Targeted selective treatments (TST) may achieve this through the treatment only of individuals that would benefit most from anthelmintic, according to certain criteria. However TST consequences on cattle are uncertain, mainly due to difficulties of comparison between alternative strategies. We developed a mathematical model to compare: 1) the most 'beneficial' indicator for treatment selection and 2) the method of selection of calves exposed to Ostertagia ostertagi, i.e. treating a fixed percentage of the population with the lowest (or highest) indicator values versus treating individuals who exceed (or are below) a given indicator threshold. The indicators evaluated were average daily gain (ADG), faecal egg counts (FEC), plasma pepsinogen, combined FEC and plasma pepsinogen, versus random selection of individuals. Treatment success was assessed in terms of benefit per R (BPR), the ratio of average benefit in weight gain to change in frequency of resistance alleles R (relative to an untreated population). The optimal indicator in terms of BPR for fixed percentages of calves treated was plasma pepsinogen and the worst ADG; in the latter case treatment was applied to some individuals who were not in need of treatment. The reverse was found when calves were treated according to threshold criteria, with ADG being the best target indicator for treatment. This was also the most beneficial strategy overall, with a significantly higher BPR value than any other strategy, but its degree of success depended on the chosen threshold of the indicator. The study shows strong support for TST, with all strategies showing improvements on calves treated selectively, compared with whole-herd treatment at 3, 8, 13 weeks post-turnout. The developed model appeared capable of assessing the consequences of other TST strategies on calf populations.
authors.affiliation: School of Agriculture Food and Rural Development, Newcastle University, Newcastle Upon Tyne, NE1 7RU, UK. Electronic address: z.berk@newcastle.ac.uk.
authors.forename: Zoe
authors.initials: Z
authors.lastname: Berk
authors.affiliation: Animal Science, School of Environmental and Rural Science, University of New England, Armidale, New South Wales, 2351, Australia.
authors.forename: Yan C S M
authors.initials: YCSM
authors.lastname: Laurenson
authors.affiliation: Scottish Centre for Production Animal Health and Food Safety, School of Veterinary Medicine, University of Glasgow, G61 1QH, Scotland, UK.
authors.forename: Andrew B
authors.initials: AB
authors.lastname: Forbes
authors.affiliation: School of Agriculture Food and Rural Development, Newcastle University, Newcastle Upon Tyne, NE1 7RU, UK.
authors.forename: Ilias
authors.initials: I
authors.lastname: Kyriazakis
mesh_terms: D000818:Animals; D000871:Anthelmintics / Q000008:administration & dosage* / Q000494:pharmacology*; D001835:Body Weight; D002417:Cattle; D002418:Cattle Diseases / Q000188:drug therapy* / Q000469:parasitology*; D004351:Drug Resistance; D008962:Models, Theoretical; D010028:Ostertagia / Q000187:drug effects*; D010029:Ostertagiasis / Q000188:drug therapy / Q000469:parasitology / Q000662:veterinary*; D010270:Parasite Egg Count; D016896:Treatment Outcome
chemical_list: D000871:Anthelmintics
keywords: Anthelmintic resistance; Cattle; Gastrointestinal parasitism; Mathematical modelling; Ostertagia ostertagi; Targeted selective treatment (TST)