A multi‐species model to assess the effect of refugia on worm control and anthelmintic resistance in sheep grazing systems

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. The model was run with variable proportions of the flock untreated (0, 10, 20, 30, 40 and 50%), with ewes selected at random so that reductions in the mean worm burden or egg count were proportional to the treated section of the flock. Treatments to ewes were given either in summer (December; low refugia potential, hence highly selective) or autumn (March; less selective due to a greater refugia potential), and the use of different anthelmintics was simulated to indicate the difference between active ingredients of different efficacy. Each model scenario was run for two environments, specifically a lower rainfall area (more selective) and a higher rainfall area (less selective) within a Mediterranean climatic zone, characterised by hot, dry summers and cool, wet winters. Univariate general linear models with least square difference post-hoc tests were used to examine differences between means of factors. The results confirmed that leaving a proportion of sheep in a flock untreated was effective in delaying the development of anthelmintic resistance, with as low as 10% of a flock untreated sufficient to significantly delay resistance, although this strategy was associated with a small reduction in worm control. Administering anthelmintics in autumn rather than summer was also effective in delaying the development of anthelmintic resistance in the lower rainfall environment where all sheep were treated, although the effect of treatment timing on worm control effectiveness varied between the environments and the proportion of ewes left untreated. The use of anthelmintics with higher efficacy delayed the development of resistance, but the initial worm egg count or number of annual treatments had no effect on either the time to resistance development or worm control effectiveness. In conclusion, the modelling study suggests that leaving a small proportion of ewes untreated, or changing the time of treatment, can delay the onset of anthelmintic resistance in a highly selective environment.

Section: Discussionmentioning

confidence: 99%

Computer modelling of anthelmintic resistance and worm control outcomes for refugia-based nematode control strategies in Merino ewes in Western Australia

Cornelius

Jacobson

Dobson

et al. 2016

“…Also, earlier computer simulation modelling suggests that for a single treatment given in summer in this environment, leaving even a small percentage of adult sheep untreated can provide a significant amount of refugia for non-selected worms, hence delaying anthelmintic resistance (Dobson et al, 2011). In conjunction with those studies, results from this investigation suggest that summer treatments based on a random selection index (with a minimum BCS limit to identify animals at risk of disease or compromised welfare), with up to 50% of adult sheep untreated would be expected to significantly delay the development of anthelmintic resistance, with minimal adverse effect on sheep health or production.…”

Section: The Use Of Bcs As a Tst Selection Index Has Been The Subjectmentioning

confidence: 99%

Application of a body condition score index for targeted selective treatment in adult Merino sheep—A modelling study

Cornelius

Jacobson

Besier³

2015

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractThis study aimed to establish whether sheep flock production losses due to nematode (worm)infections are typically greater in mature sheep selected for anthelmintic treatment at random compared to sheep selected for treatment based on low (poorer) body condition score (BCS).The study also examined the proportion of sheep in flocks that could be left untreated before production losses became evident, and projected worm egg pasture contamination. Sheep were monitored at two experimental sites in Western Australia (Mediterranean climate).Sheep were stratified for BCS, liveweight and faecal worm egg count (WEC) and allocated into treatment groups (treated or untreated), with equal numbers for each. Liveweight, BCSand WEC measurements were taken on 6 occasions at Farm A and 10 occasions at Farm B.Comparisons of sheep production (liveweight and BCS change) and pasture contamination potential (WEC) were conducted by generating "virtual flocks" of varying proportions sheep untreated (10%, 20%, 30%, 40%, and 50% untreated). For the comparison of the selection mode of sheep for treatment, the untreated sheep were either selected at random, or as the highest BCS animals at the commencement of observations. Univariate general linear models with least square difference post-hoc tests were used to examine differences between flocks for liveweight, BCS and WEC, and regression analysis was used to examine relationships between BCS and WEC, and liveweight and WEC. No difference in body weights was observed between flocks with varying proportions of ewes notionally left untreated at Farm B, and until more than 30% were left untreated at Farm A. There was no difference in BCS between flocks with varying proportions of ewes left untreated at either site. At no point were there differences in cumulative liveweight change or BCS between selection methods (BCS versus random) where the same proportion of sheep in virtual flocks were left untreated, suggesting that effort committed to individual BCS assessment would be of no benefit under these circumstances except for identifying low BCS sheep at risk of falling below critical 3 limits associated with health or welfare risks. No consistent relationship between WEC and BCS or bodyweight was observed, indicating that BCS selection would have no lesser or greater impact on worm pasture contamination compared to random selection. Summer treatments based on a random selection index (with a minimum BCS limit), with up to 30% of adult sheep untreated can be expected to delay the development of anthelmintic resistance, with mini...

“…Examples of generally accepted strategies to establish adequate refugia include: grazing untreated adults with younger animals that are treated; ensuring that the interval between treatments allows some contamination of pasture with unselected parasites; treating animals several days after moving to relatively worm-free pasture to contaminate the area with unselected nematodes; or leaving a proportion of animals with a group untreated. This mix of factors creates an extremely complex environment in which simulation models can be of more benefit than field experiments (Hosking, 2010;Dobson et al, 2011b). Thus, the propensity for selection of resistant nematode populations through an inadequate population of parasites in refugia is a matter of concern for all anthelmintic products and hence is another technology transfer problem for any new product, whether composed of single or multiple constituent actives.…”

Section: Best-practice Management Of Combination Anthelminticsmentioning

confidence: 99%

“…The principles of continuing education are the same whether producers use singleconstituent active or combination anthelmintic products. This includes testing for AR to identify suitable constituent actives, estimating (however inadequately) nematode burdens and species by fecal egg counts (FEC) and preferably larval culture (or PCR) to determine appropriate treatment regimens, and the management of pasture exposures to reduce the overall parasite challenge in balance with the maintenance of drug-susceptible populations in refugia, which can help slow the development of AR in nematodes (Barger, 1999;Dobson et al, 2001Dobson et al, , 2011bvan Wyk, 2001;Baker et al, 2012;Bartram et al, 2012).…”

Section: Best-practice Management Of Combination Anthelminticsmentioning

confidence: 99%

“…These infections are addressed as individual cases, unlike the population treatment paradigms typically employed in anthelmintic treatment strategies for ruminant livestock (and sometimes horses). Indeed, a key principle for slowing the emergence of AR in parasitic nematodes of ruminant livestock or horses is to obtain the highest possible removal of the parasites in significantly infected animals while ensuring that surviving parasites are diluted into a pool of susceptible, untreated parasites in the local environment as a refugial population (Dobson et al, 2001(Dobson et al, , 2011bBartram et al, 2012). This principle applies to singleconstituent active and combination products containing two or more distinct constituent actives (e.g., anthelmintics from different chemical classes with distinct mechanisms of resistance, or at least demonstrably different mechanisms of action).…”

Section: Rationales For the Use Of Anthelmintic Combination Productsmentioning

confidence: 99%

See 1 more Smart Citation

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

Geary

Hosking

Skuce

et al. 2012

a b s t r a c tIncreasing threats from anthelmintic resistant nematode populations warrant and motivate a reappraisal of chemotherapeutic strategies for nematode control in ruminant livestock and horses. The objective of this paper is to present a guideline for the evaluation of products containing two or more constituent anthelmintic actives in a single dosage form for the treatment of nematode infections in these animals. At present, regulatory policies on the approval of such products vary across jurisdictions, and this World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) guideline should enable the harmonization of the requirements. This guideline makes clear recommendations on the minimal standards needed, but stresses that registration dossiers for combination anthelmintic products submitted for approval must conform to the standards and practices already established in existing guidelines for anthelmintics.