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Anthelmintic

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D M Leathwick – One of the best experts on this subject based on the ideXlab platform.

  • the role of combination Anthelmintic formulations in the sustainable control of sheep nematodes
    Veterinary Parasitology, 2012
    Co-Authors: David Bartram, D M Leathwick, Ma Taylor, Thomas Geurden, Steven J Maeder

    Abstract:

    Combinations of Anthelmintics with a similar spectrum of activity and different mechanisms of action and resistance are widely available in several regions of the world for the control of sheep nematodes. There are two main justifications for the use of such combinations: (1) to enable the effective control of nematodes in the presence of single or multiple drug resistance, and (2) to slow the development of resistance to the component Anthelmintic classes. Computer model simulations of sheep nematode populations indicate that the ability of combinations to slow the development of resistance is maximised if certain prerequisite criteria are met, the most important of which appear to concern the opportunity for survival of susceptible nematodes in refugia and the pre-existing levels of resistance to each of the Anthelmintics in the combination. Combinations slow the development of a resistant parasite population by reducing the number of resistant genotypes which survive treatment, because multiple alleles conferring resistance to all the component Anthelmintic classes must be present in the same parasite for survival. Individuals carrying multiple resistance alleles are rarer than those carrying single resistance alleles. This enhanced efficacy leads to greater dilution of resistant genotypes by the unselected parasites in refugia, thus reducing the proportion of resistant parasites available to reproduce with other resistant adults that have survived treatment. Concerns over the use of Anthelmintic combinations include the potential to select for resistance to multiple Anthelmintic classes concurrently if there are insufficient parasites in refugia; the potential for shared mechanisms of resistance between chemical classes; and the pre-existing frequency of resistance alleles may be too high on some farms to warrant the introduction of certain combinations. In conclusion, Anthelmintic combinations can play an important role in resistance management. However, they are not a panacea and should always be used in accordance with contemporary principles for sustainable Anthelmintic use.

  • modelling the benefits of a new class of Anthelmintic in combination
    Veterinary Parasitology, 2012
    Co-Authors: D M Leathwick

    Abstract:

    Since 2009 two new classes of Anthelmintics have been registered for use in sheep in New Zealand. This raises challenging questions about how such new actives should be used, not only to minimise the development of resistance to them, thereby ensuring their availability as effective treatments for as long as possible, but also to minimise the further development of resistance to the other Anthelmintic classes. One strategy which appears to offer considerable potential for slowing the development of resistance is the use of combinations of different Anthelmintic classes, although this approach remains contentious in some countries. The potential benefit of using Anthelmintics in combination is particularly relevant to two recently released Anthelmintic compounds because one, monepantel, is presently only available as a single active product while the other, derquantel, is only available in combination with abamectin. A simulation modelling approach was used to investigate the potential benefits of using Anthelmintics in combination. The rate at which resistance develops to a new ‘active’ when used alone was compared to an equivalent compound used in combination with a second compound from an alternative class (in this case, abamectin), when various levels of resistance occur to the second active. In addition, the potential of a new active to reduce further development of resistance to the second compound in the combination was evaluated. Finally, the use of combinations as compared to sequential or rotational use patterns, in the presence of side resistance between two actives was investigated. The modelling simulations suggest a significant advantage to both compounds when they are used in combination, especially if both initially have high efficacy. The development of resistance to the new active was delayed, although to a lesser extent, even when the efficacy of the second active in the combination was only 50%. Under a ‘low-refugia’ management environment resistance to all actives developed more rapidly, and the advantage of using actives in combination was reduced. When used in conjunction with other resistance management strategies, a combination containing a new active prevented further development of resistance to the older class. Using actives in combination was superior to using them individually either sequentially or in rotation, even in the presence of side-resistance between the two Anthelmintic classes.

  • Anthelmintic resistance in nematode parasites of cattle a global issue
    Trends in Parasitology, 2011
    Co-Authors: I A Sutherland, D M Leathwick

    Abstract:

    Acceptable performance of grazing cattle frequently depends on the availability of effective broad-spectrum Anthelmintics to remove, or prevent infection with, gastrointestinal nematodes. This control is increasingly threatened by populations of nematodes resistant to the most commonly used Anthelmintics. Although this appears to have developed more slowly than in nematodes infecting small ruminants, the number of reports in the literature over the past five years suggests a rapidly escalating problem. This review discusses this literature, several issues unique to cattle parasitism and Anthelmintics, and how previous research in small ruminants can improve the management of Anthelmintic resistance in cattle.

Steven J Maeder – One of the best experts on this subject based on the ideXlab platform.

  • world association for the advancement of veterinary parasitology w a a v p guideline Anthelmintic combination products targeting nematode infections of ruminants and horses
    Veterinary Parasitology, 2012
    Co-Authors: Timothy G. Geary, Barry C. Hosking, P. J. Skuce, Georg Von Samsonhimmelstjerna, Steven J Maeder, Peter Holdsworth, W E Pomroy, Jozef Vercruysse

    Abstract:

    Increasing 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.

  • the role of combination Anthelmintic formulations in the sustainable control of sheep nematodes
    Veterinary Parasitology, 2012
    Co-Authors: David Bartram, D M Leathwick, Ma Taylor, Thomas Geurden, Steven J Maeder

    Abstract:

    Combinations of Anthelmintics with a similar spectrum of activity and different mechanisms of action and resistance are widely available in several regions of the world for the control of sheep nematodes. There are two main justifications for the use of such combinations: (1) to enable the effective control of nematodes in the presence of single or multiple drug resistance, and (2) to slow the development of resistance to the component Anthelmintic classes. Computer model simulations of sheep nematode populations indicate that the ability of combinations to slow the development of resistance is maximised if certain prerequisite criteria are met, the most important of which appear to concern the opportunity for survival of susceptible nematodes in refugia and the pre-existing levels of resistance to each of the Anthelmintics in the combination. Combinations slow the development of a resistant parasite population by reducing the number of resistant genotypes which survive treatment, because multiple alleles conferring resistance to all the component Anthelmintic classes must be present in the same parasite for survival. Individuals carrying multiple resistance alleles are rarer than those carrying single resistance alleles. This enhanced efficacy leads to greater dilution of resistant genotypes by the unselected parasites in refugia, thus reducing the proportion of resistant parasites available to reproduce with other resistant adults that have survived treatment. Concerns over the use of Anthelmintic combinations include the potential to select for resistance to multiple Anthelmintic classes concurrently if there are insufficient parasites in refugia; the potential for shared mechanisms of resistance between chemical classes; and the pre-existing frequency of resistance alleles may be too high on some farms to warrant the introduction of certain combinations. In conclusion, Anthelmintic combinations can play an important role in resistance management. However, they are not a panacea and should always be used in accordance with contemporary principles for sustainable Anthelmintic use.

J A Van Wyk – One of the best experts on this subject based on the ideXlab platform.

  • The role of targeted selective treatments in the development of refugia-based approaches to the control of gastrointestinal nematodes of small ruminants
    Veterinary Parasitology, 2009
    Co-Authors: F. Kenyon, Jacques Cabaret, J A Van Wyk, M. Várady, A.w. Greer, G C Coles, G. Cringoli, E. Papadopoulos, B. Berrag, E. Thomas

    Abstract:

    Anthelmintic resistance is recognised as a major problem affecting small ruminant production worldwide and now threatens the sustainability of many of these systems. One method that has been proposed to prolong the efficacy of our current Anthelmintics is the maintenance of a parasite population in refugia (unexposed to a drug) which will maintain the genes for susceptibility within the parasite population. Management strategies that employ refugia-based methods include targeted or strategically timed whole flock treatments, targeted selective treatments (TST), whereby only a proportion of the flock is treated at any one time, and the dilution of resistant with susceptible parasites. The ability to effectively target Anthelmintic use relies on the identification of those animals that will most benefit from treatment. This review explains the concept of refugia, describes the role of refugia-based approaches to the management of Anthelmintic resistance and reviews the markers that have been studied as indicators for TSTs as well as the implementation of refugia-based strategies. Recent results suggest that targeting Anthelmintic treatment on the basis of anaemia, milk production and liveweight gain may offer a means of reducing Anthelmintic usage whilst still maintaining animal performance.

  • Anthelmintic resistance in south africa surveys indicate an extremely serious situation in sheep and goat farming
    Onderstepoort Journal of Veterinary Research, 1999
    Co-Authors: M O Stenson, J S Van Der Merwe, R J Vorster, P G Viljoen, J A Van Wyk

    Abstract:

    VANWYK, J.A., STENSON, M.O , VAN DER MERWE J.S. , VORSTER, R.J. & VILJOEN, P.G. 1999. Anthelmintic resistance in South Africa: Surveys indicate an extremely serious situation in sheep farm­ ing. Onderstepoort Journal of Veterinary Research, 66:273-284 Surveys to determine the prevalence and degree of resistance of Haemonchus spp. of sheep and goats to the available Anthelmintics in South Africa indicate that small ruminant production is enter­ ing a crisis situation . Three surveys employing the faecal egg count reduction (FECR) test to determine resistance were conducted in some of the main sheep-producing areas in the summer rainfall region of South Africa , where H. contortus is the principal worm species in sheep. After analyzing the data recorded in the surveys by six different methods, including the RESO test at two different levels of confidence, the results obtained in the least stringent one (geometric mean reduction of the worm egg counts of drenched, vs untreated group of sheep) are reported in this paper, so that if any bias was obtained it would be in the favour of the Anthelmintic. In Mpumalanga and KwaZulu-Natal there was Anthelmintic resistance in Haemonchus spp. on all the 52 farms surveyed. Sixteen percent of the strains of H. contortus were < 60 % susceptible to three of the four Anthelmintics tested, and 8 % of the strains were < 40 % susceptible to all four of the Anthelmintics. FECR tests of sheep in six localities in the Lebowa district of Northern Province indi­ cated that even in previously disadvantaged communities where Anthelmintic treatment is less inten­ sive, Anthelmintic resistance is developing, and is possibly at the level at which the situation on com­ mercial sheep and goat farms in South Africa was 25 years ago. From the data it appears that the level of Anthelmintic resistance of H. contortus in South Africa is possibly the highest that has so far been recorded in the world and that strains of it are emerging that may soon not be controllable by treatment with any of the existing Anthelmintics. Farmers in the sum­ mer rainfall region , if not the whole country, must be alerted to the immediate need for testing the parasite burdens of their sheep for susceptibility to preparations in all four groups of Anthelmintic compounds currently available. Alternative methods of integrated worm control , including biological , must be sought and implemented with urgency, to reduce further selection for resistance and to in­ duce reversion of the re sistance that has already developed.