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1.
Intestinal helminths are an important cause of equine disease. Of these parasites, the Cyathostominae are the commonest group that infect horses. These nematodes consist of a complex tribe of 51 species, although individual horses tend to harbour 10 or so common species, in addition to a few rarer species. The Cyathostominae can be extremely pathogenic, and high levels of infection result in clinical symptoms ranging from chronic weight loss to colic, diarrhoea and death. As part of their life cycle, immature cyathostomins penetrate the large intestinal wall, where they can enter a state of inhibited larval development. These larvae can exist in this state for months to years, after which they subsequently re-emerge. If larvae re-emerge in large numbers (i.e. several million), severe pathological consequences ensue. The inhibited larvae are also relatively refractory to several of the currently available anthelmintics, so that horses treated previously with anthelmintics can still carry life-threatening burdens of these parasitic stages. Little is known about the cyathostomin larvae during their mucosal phase, and current research efforts are focused on investigating the biology of these stages. Much of the research described here highlights this area of research and details studies aimed at investigating the host immune responses that the mucosal larvae invoke. As part of this research effort, molecular tools have been developed to facilitate the identification of larval and egg stages of cyathostomins. These molecular tools are now proving very useful in the investigation of the relative contributions that individual, common cyathostomin species make to the pathology and epidemiology of mixed helminth infections. At the more applied level, research is also in progress to develop an immunodiagnostic test that will allow numbers of mucosal larvae to be estimated. This test utilises antigen-specific IgG(T) serum antibody responses as markers of infection. As anthelmintic resistance will be the major constraint on the future control of the Cyathostominae, researchers are now actively investigating this area and studies aimed at elucidating the molecular mechanisms of drug resistance are described. Another parasite which has assumed a clinically important role in horses is the tapeworm, Anoplocephala perfoliata. This parasite is prevalent world-wide and has been shown to be a significant cause of equine colic. Because previous methods of estimating the infection intensity of tapeworm were inaccurate, recent research has been directed at developing an immunodiagnostic ELISA for these cestodes. Specific IgG(T) responses to antigens secreted by adult tapeworms have been shown to provide a reasonable indication of infection intensity. An ELISA based on these responses is now commercially available. The steps involved in the development of this ELISA are described here. In addition to these recent advances in research, this review also outlines the principle areas for future research into these important equine parasites.  相似文献   

2.
Avermectins and milbemycins (AM) are potent compounds against all major nematode parasites, but their continuous usage has led to the development of widespread resistance in many of the important species of ruminant and equine parasites. The exception to this has been the cyathostomins, where AM resistance was recently first reported only after decades of drug exposure. Data from a Brazilian study suggests that AM resistance has developed in cyathostomins and reports of shortened egg reappearance periods after ivermectin treatment have been published recently from USA and Germany. Thus, AM resistance in cyathostomins is an emerging worldwide concern, but there is only limited amount data on the extent of this problem. To limit the development and spread of AM-resistant cyathostomins the equine industry must implement new strategies for worm control, and the veterinary parasitology community must develop and validate improved protocols for detecting anthelmintic resistance in the field.  相似文献   

3.
Intestinal nematodes are an important cause of equine disease. Of these parasites, the Cyathostominae are the most important group, both in terms of their prevalence and their pathogenicity. Cyathostomin infections are complex and control is further complicated by ever‐increasing levels of resistance to some of the commonly used anthelmintics. There are no new equine anthelmintics under development, so it is imperative that the efficacy of any currently‐effective drug classes be maintained for as long as possible. It is believed that the proportion of refugia (i.e. the percentage of parasites not exposed to a drug at each treatment) is one of the most crucial factors in determining the rate at which anthelmintic resistance develops. It is important, therefore, that levels of refugia be taken into account when designing nematode control programmes for horses. This can be assisted by knowledge of the local epidemiology of the infection, supplemented by faecal egg count analysis to identify those animals that are making the major contribution to pasture contamination. This type of rational nematode control requires equine veterinary surgeons to get involved in designing and implementing deworming programmes. The advice given must be based on a combination of knowledge of cyathostomin biology and epidemiology as well as an awareness of the parasite population's current drug sensitivity and a sound history of husbandry at the establishment. As anthelmintic resistance will be the major constraint on the future control of cyathostomins, researchers are now actively investigating this area. Studies are underway to develop tests that will enable earlier detection of anthelmintic resistance and an assay that will help identify those horses that require anthelmintic treatments targeted at intestinal wall larvae.  相似文献   

4.
Roundworms and flatworms that affect donkeys can cause disease. All common helminth parasites that affect horses also infect donkeys, so animals that co‐graze can act as a source of infection for either species. Of the gastrointestinal nematodes, those belonging to the cyathostomin (small strongyle) group are the most problematic in UK donkeys. Most grazing animals are exposed to these parasites and some animals will be infected all of their lives. Control is threatened by anthelmintic resistance: resistance to all 3 available anthelmintic classes has now been recorded in UK donkeys. The lungworm, Dictyocaulus arnfieldi, is also problematical, particularly when donkeys co‐graze with horses. Mature horses are not permissive hosts to the full life cycle of this parasite, but develop clinical signs on infection. In contrast, donkeys are permissive hosts without displaying overt clinical signs and act as a source of infection to co‐grazing horses. Donkeys are also susceptible to the fluke, Fasciola hepatica. This flatworm can be transmitted, via snails and the environment, from ruminants. As with cyathostomins, anthelmintic resistance is increasing in fluke populations in the UK. A number of the anthelmintic products available for horses do not have a licence for use in donkeys, and this complicates the design of parasite control programmes. As no new equine anthelmintic classes appear to be near market, it is important that the efficacy of currently effective drugs is maintained. It is important that strategies are used that attempt to preserve anthelmintic efficacy. These strategies should be based on the concept that the proportion of worms in a population not exposed to anthelmintic at each treatment act as a source of ‘refugia’. The latter is an important factor in the rate at which resistance develops. Thus, it is imperative that parasite control programmes take into account the need to balance therapy to control helminth‐associated disease with the requirement to preserve anthelmintic effectiveness.  相似文献   

5.
Most veterinarians continue to recommend anthelmintic treatment programmes for horses that derive from knowledge and concepts more than 40 years old. However, much has changed since these recommendations were first introduced and current approaches routinely fail to provide optimal or even adequate levels of parasite control. There are many reasons for this. Recent studies demonstrate that anthelmintic resistance in equine parasites is highly prevalent and multiple‐drug resistance is common in some countries, but few veterinarians take this into account when making treatment decisions or when recommending rotation of anthelmintics. Furthermore, the current approach of treating all horses at frequent intervals was designed specifically to control the highly pathogenic large strongyle, Strongylus vulgaris. But this parasite is now quite uncommon in managed horses in most of the world. Presently, the cyathostomins (small strongyles) are the principal parasitic pathogens of mature horses. The biology and pathogenesis of cyathostomins and S. vulgaris are very different and therefore require an entirely different approach. Furthermore, it is known that parasites are highly over‐dispersed in hosts, such that a small percentage of hosts harbour most of the parasites. The common practices of recommending the same treatment programme for all horses despite great differences in parasite burdens, recommending prophylactic treatment of all horses without indication of parasitic disease or knowing what species of parasites are infecting the horses, recommending use of drugs without knowledge of their efficacy and failing to perform diagnostic (faecal egg count) surveillance for estimating parasite burdens and determining treatment efficacy, are all incompatible with current standards of veterinary practice. Consequently, it is necessary that attitudes and approaches to parasite control in horses undergo a complete overhaul. This is best achieved by following an evidence‐based approach that takes into account all of these issues and is based on science, not tradition.  相似文献   

6.
Equine gastrointestinal nematodes are ubiquitous; in horses that graze contaminated pasture and that are not treated appropriately, large numbers of worms can accumulate, which can lead to serious clinical disease. Nematode control has traditionally followed interval treatment regimens, which involve regular anthelmintic administration to all horses based on the strongyle egg reappearance periods of each drug, usually defined around the time of licensing. Interval treatment programmes have resulted in substantial reductions in large strongyle disease, but have made major contributions to the development of anthelmintic resistance, particularly in cyathostomins. Cyathostomin resistance to 2 of the 3 available anthelmintic classes is widespread, and resistance to both classes in single populations is not uncommon. Reduced efficacy of the most commonly used macrocyclic lactone anthelmintics, as measured by shortened egg reappearance periods after treatment, is emerging in cyathostomins. Macrocyclic lactone resistance is also now commonly reported in Parascaris equorum on stud farms. Faecal worm egg counts (FWEC) are increasingly being used as part of targeted approaches to parasite control, whereby only those horses with moderate to high FWEC within a group are treated with an anthelmintic. The objective of this approach is to reduce environmental contamination, while leaving a proportion of the worm population in some horses unexposed to selection pressure for anthelmintic resistance. This article reviews recent findings in equine parasitology research that will underpin guidelines for control, with a particular focus on how to optimise the value of FWEC methodologies and anthelmintic efficacy analyses.  相似文献   

7.
During the past two decades anthelmintic resistance in equine parasites has been found in the group of small strongyle species (cyathostomins) and in the ascarid species Parascaris equorum. The ubiquitous nature and possible severe consequences of disease with these nematodes make them the prime targets of current worm control programmes. Traditional control strategies mainly rely on the strategic application of anthelmintics, currently represented by three major drug classes: benzimidazoles (BZ), the tetrahydropyrimidine pyrantel (PYR) and macrocyclic lactones (ML). Following decades of routine and frequent anthelmintic applications, many cyathostomin populations on horse farms in industrialised countries must be considered as resistant to BZ anthelmintics. However, to date no published cases of cyathostomin disease specifically associated with anthelmintic resistance were reported. Possibly this is due to the generally subclinical and unspecific symptoms associated with cyathostomin infections. Nevertheless, exclusive reliance on the ML drug class may increase the threat of clinical disease due to drug-resistant cyathostomins. More recently, P. equorum has been reported as having developed resistance against ivermectin and moxidectin, two representatives of the ML-class. These anthelmintics are currently the most frequently used drug class in horses. This nematode species is mainly found in foals and in younger horses due to the development of immunity following exposure to infection. Infection with P. equorum can result in clinically drastic consequences such as obstruction and/or penetration of the small intestine, the latter usually leading to death. In conclusion, on horse farms the efficacy of anthelmintic treatments should be examined routinely for each drug class. Several factors can influence the rate at which anthelmintic resistance develops; high frequency of treatment being one of the most important. Modern control strategies should therefore attempt to significantly reduce anthelmintic treatments. Several pasture and farm management practices found to be negatively associated with nematode and anthelmintic resistance prevalence will be discussed in the review presented here.  相似文献   

8.
The future implementation of improved and sustainable control strategies for the major equine parasites will be dependent on a greater insight into their basic biology, pathogenicity and epidemiology together with an enhanced ability for accurate diagnosis. This paper will provide a review of the current molecular methods under development for the detection of equine parasites and their application to current scientific questions. In particular, the strongyles are recognised as important pathogens of horses and recent advances made in the study of this parasitic group at the single species level will be addressed. The ribosomal (r)DNA region of the parasite genome has been employed to distinguish between closely related species. Molecular probes designed to this target region were used in combination with PCR technology to allow the identification of individual species within mixed infections. They have been applied to all parasite stages to look at the role of individual species in natural infection, disease and drug resistance. Similar techniques have been developed to detect other equine parasites and these will also be discussed. Further opportunities for employing existing techniques and the need for new diagnostic tools will be highlighted.  相似文献   

9.
Over the past 10-15 years, we have witnessed a rapid increase in both the prevalence and magnitude of anthelmintic resistance, and this increase appears to be a worldwide phenomenon. Reports of anthelmintic resistance to multiple drugs in individual parasite species, and in multiple parasite species across virtually all livestock hosts, are increasingly common. In addition, since the introduction of ivermectin in 1981, no novel anthelmintic classes were developed and introduced for use in livestock until recently with the launch of monepantel in New Zealand. Thus, livestock producers are often left with few options for effective treatment against many important parasite species. While new anthelmintic classes with novel mechanisms of action could potentially solve this problem, new drugs are extremely expensive to develop, and can be expected to be more expensive than older drugs. Thus, it seems clear that the "Global Worming" approach that has taken hold over the past 40-50 years must change, and livestock producers must develop a new vision for parasite control and sustainability of production. Furthermore, parasitologists must improve methods for study design and data analysis that are used for diagnosing anthelmintic resistance, especially for the fecal egg count reduction test (FECRT). Currently, standards for diagnosis of anthelmintic resistance using FECRT exist only for sheep. Lack of standards in horses and cattle and arbitrarily defined cutoffs for defining resistance, combined with inadequate analysis of the data, mean that errors in assigning resistance status are common. Similarly, the lack of standards makes it difficult to compare data among different studies. This problem needs to be addressed, because as new drugs are introduced now and in the future, the lack of alternative treatments will make early and accurate diagnosis of anthelmintic resistance increasingly important.  相似文献   

10.
牛羊线虫病是严重影响牛羊生产性能和经济效益的主要寄生虫病，化学药物是目前防治本病的主要手段，近年来线虫抗药性。已经成为世界各地牛羊线虫控制中的首要问题。建立线虫抗药早期检测技术可以时了解抗药性及其产生的条件等，便于采取有效对策，本文简要阐述牛羊线虫抗药性的分布与检测，现有抗药性检测方法的缺点以及PCR检测技术的最新进展。  相似文献   

11.
Moxidectin has broad‐spectrum anti‐nematodal and anti‐arthropodal activities in the horse but is not effective against tapeworms or flukes. Moxidectin and ivermectin have the same efficacy against internal, adult parasites of horses. Moxidectin, however, is highly effective in eliminating encysted and hypobiotic larval stages of cyathostomins, whereas ivermectin is not. Treatment of horses with moxidectin results in an egg‐reappearance period (ERP) of 15–24 weeks. Because of its long ERP, moxidectin is labelled to be used at 12 week intervals. Moxidectin may provide protection against infection by ingested cyathostomin larvae for 2–3 weeks after it is administered. The larvicidal activity of moxidectin has often been compared to that of fenbendazole administered at either 7.5 or 10 mg/kg bwt for 5 consecutive days. The efficacy of fenbendazole, when administered daily for 5 consecutive days at 7.5 or 10 mg/kg bwt, against all stages of cyathostomins is often less than that of moxidectin because resistance of cyathostomins to benzimidazoles is prevalent worldwide, and the 5 day course of fenbendazole does not overcome this resistance. There are now reports of resistance of ascarids to moxidectin. Overt resistance of cyathostomins and a shortened egg re‐emergence period after treatment with moxidectin have been reported. Rapid removal of manure by natural fauna can significantly reduce larval nematode concentrations and thereby reduce intervals of anthelmintic treatment. Of the macrocyclic lactones, moxidectin has the least deleterious effect on faecal fauna.  相似文献   

12.
AIMS: To test the hypotheses that when untreated adult ewes are rotationally grazed (follow behind) on pastures after lambs receiving routine anthelmintic treatments, the ewes can function as a source of unselected parasites in refugia, capable of slowing the development of anthelmintic resistance, and suppress the build-up of parasites resulting from the development of anthelmintic resistance. METHODS: Firstly, the potential of untreated adult ewes to slow the development of anthelmintic resistance, and to suppress parasite populations under differing levels of anthelmintic efficacy, was investigated using a simulation model. Secondly, a field trial with three replicates of each treatment compared two grazing systems (lambs only vs lambs followed by ewes) and two types of anthelmintic, viz albendazole (ALB), to which resistance was present (faecal nematode egg count reduction (FECR)=57-59%) and ivermectin plus levamisole (IL), to which resistance was absent (FECR=97-99%), in a factorial treatment structure. Parasite populations were monitored using faecal nematode egg counts (FEC), faecal larval cultures, pasture larval sampling, and slaughter of tracer lambs. Animal performance was measured using liveweight, dag score, body condition score, and fleece weights. RESULTS: Model simulations indicated that parasites cycling in the untreated ewes could slow the development of resistance being selected for by the anthelmintic treatments given to lambs and this could occur without a nett increase in larval numbers on pasture. Further, as worm control in the lambs declined with increasing levels of anthelmintic resistance the ewes increasingly functioned as nett removers of parasite larvae, effectively reducing parasite population size. In the field trial, untreated adult ewes contributed to pasture infestations of most parasite species, but not Nematodirus spp. Parasite species on pasture and infecting lambs changed when ewes were present, but larval populations on pasture in the autumn were no greater than when lambs grazed alone. In the presence of anthelmintic resistance, parasite populations were reduced when ewes grazed in rotation with lambs, implicating the ewes as nett removers of parasite challenge. CONCLUSIONS: Untreated adult ewes were a source of unselected genotypes, capable of slowing the development of anthelmintic resistance in most, but not all, parasite species. Further, the potential of adult ewes to remove from pasture more parasite larvae than they contribute through faecal contamination indicates a potentially useful role in suppressing parasite populations, particularly when worm control in lambs is less effective as a result of anthelmintic resistance.  相似文献   

13.
Mode of action of common anthelmintics   总被引：1，自引：0，他引：1  
The primary physiological mode of action of nearly all drugs used to control helminth parasites is not well understood. Examination of the general modes of action in terms of parasite requirements for survival may, however, provide guidelines to continue our present investigations and direct us for future research.
Two general areas of drug activity are (1) interference with energy-generating metabolism, and (2) interference with proper neuromuscular coordination. Few exceptions are found to these areas because the target organisms require little else for survival.
The target of most previously effective and many presently effective anthelmintics is an adult parasite. Adult helminth parasites must maintain an advantageous feeding site and must transport and metabolize substrates (primarily glucose) to generate life-maintaining energy. Adult worms have almost no other short-term requirements for survival because they have little or no metabolic activity in either lipid synthesis or oxidation, nucleic acid synthesis, or protein synthesis (except egg-laying).
Examination of specifics of energy-generating metabolism and putative neurochemical transmitters allows us certain predictions in relating a given drug activity to an individual species. These examinations indicate that such diversity exists among helminth metabolic or nervous systems that a broad-spectrum anthelmintic would most likely inhibit at several sensitive points.
Published information on drug action and differences in data interpretation are discussed. Two novel areas for future anthelmintic investigations may be (1) juvenile hormone or ecdysone analogs, and (2) pheromones. A combined empirical and rational approach to the development of new drugs may be the most efficient avenue of future research.  相似文献   

14.
Effective parasite control is essential for the maintenance of optimal health and performance in the horse. The worldwide escalation of parasite resistance is a major cause of concern for the horse industry. Parasite resistance to every main class of equine anthelmintic has been documented. Furthermore, dual and cross resistance also have been widely reported, despite different climatic and management practices seen throughout the world. Studies documenting parasite resistance to major classes of equine anthelmintics are discussed. Disagreement among researchers exists regarding how to effectively control equine internal parasites. Current theories of factors leading to resistance and control programs are discussed. It is clear that parasite resistance and control in the horse is an area requiring continued intensive study.  相似文献   

15.
Cyathostome resistance to the benzimidazole fenbendazole (FBZ) and other anthelmintic medication has been documented worldwide. Parasite resistance to anthelmintic medication is of great concern to the anthelmintic industry and to horse owners. The present study examines the efficacy of FBZ anthelmintic treatments in horse herds from ranches in four different geographical locations within Texas. In addition, the load reduction method was compared with the traditional fecal egg count reduction test to determine the incidence of parasite resistance to FBZ. Four ranches in different areas of Texas were surveyed to determine cyathostome resistance to FBZ. Two of the four ranches had young (aged ≤2 years) and older animals (aged >2 years). The number of animals with parasites varied widely between the study's locations. Differences were observed in both the fecal egg count reduction test and load reduction method across the four study sites (P < .001). Cyathostome resistance to FBZ seemed to be prevalent in three of the four ranches, whereas FBZ was highly efficacious against cyathostomes on one ranche. There was a trend toward increased parasite resistance in the younger animals (P = .081). These results show the importance of testing anthelmintic medication effectiveness.  相似文献   

16.
The recent registration in New Zealand of the first new class of broad-spectrum anthelmintic, for use against nematode parasites of ruminants, in nearly three decades has raised the possibility that parasite management practices could be improved to minimise the emergence of resistance to the new drug. A review of knowledge pertaining to the selection of anthelmintic resistance in nematode parasites of sheep highlights a number of management practices which could be altered to achieve this.

A number of previously common practices such as whole-flock treatment of adult ewes around lambing, and treatment of lambs as they are moved onto pastures with low parasite contamination have been clearly identified as high risk for selecting resistant parasites. Once high-risk practices have been identified steps can be taken to either eliminate their use or mitigate the associated risk. Much of the focus on the management of resistance around the world is on the retention of susceptible genotypes in refugia. While approaches to retaining unselected parasites are likely to vary around the world, empirical studies indicate that the practice is likely to be effective at slowing the development of resistance. The challenge for farmers and advisors will be to strike a balance between retaining sufficient susceptible para-sites to usefully delay the development of resistance while not unduly compromising animal performance and farm profitability. The merits of combining different classes of anthelmintic in order to slow the development of resistance remains somewhat contentious in some countries. However, the attributes of oral anthelmintics are such that they seem likely to meet most, if not all, of the criteria for combinations to be highly effective at slowing the build-up of resistance in nematode parasites.

It is evident that considerable progress has been made in understanding the factors involved in selecting anthelmintic-resistant nematodes since the last broad-spectrum anthelmintic class was released in the early 1980s. Therefore, it should be possible to manage a new class of anthelmintic in such a way as to significantly extend its effective life. The challenge is likely to be in convincing farmers of the merits of adopting such pro-active strategies.  相似文献   

17.
Sustainable equine parasite control: perspectives and research needs   总被引：1，自引：0，他引：1  
Clinically important equine parasites are ubiquitous in managed horse populations. The traditional approach to parasite control is frequent administration of anthelmintics to all horses on a farm. However, increasing levels of anthelmintic resistance is forcing horse owners and veterinarians to shift this control paradigm. Treatment regimens involving routine deworming of all horses throughout the year are now being replaced by more sustainable approaches, which take in to account the importance of maintaining adequate parasite refugia. The selective therapy principle has been recommended for more than 15 years, but there is limited experience with this approach. The relative magnitude of the faecal egg count for an individual horse is a consistent trait, and this provides a reliable basis for selective therapy. But no studies have evaluated the consequences of selective therapy in the long-term, and such studies are strongly needed to validate this approach. Importantly, it remains unclear how selective therapy may affect the prevalence and intensity of other parasites of significant pathogenic potential (e.g. Strongylus vulgaris), which have become uncommon due to years of intensive chemotherapy. Consequently, a selective approach requires vigilant surveillance of the parasite fauna and intensity. This places a demands for reliable diagnostic tools. Also noteworthy is the fact that the majority of equine nematode parasites are more pathogenic during their larval stages, when they cannot be detected by traditional egg counting techniques. Consequently, parasite-specific diagnostic tools capable of assessing prepatent parasite burdens, and able to differentiate between strongyle species of different pathogenic potentials, would be of great value to the equine clinician. Tools for detecting infections with the tapeworm Anoplocephala perfoliata are laborious, difficult to interpret, and at present there is no established method to evaluate treatment efficacy. Thus, better diagnostic tools are needed for tapeworms as well. Biological control, especially the predacious fungi have demonstrated good potential as an adjunct for strongyle control and such a product could easily have a market in equine establishments. In summary, there is general agreement that the traditional treat-all at frequent interval approach should be abandoned, and that optimal parasite control can be maintained with far fewer anthelmintic treatments. But better diagnostic techniques and more evidence documenting the long-term consequences of selective therapy programs are needed to develop and validate systems for sustainable equine parasite control.  相似文献   

18.
It is increasingly being recognized that non-chemical parasite control strategies may need to be combined to control more effectively gastrointestinal parasitism, result in resilient production systems and reduce reliance on anthelmintics. Here, we consider if and how metabolizable protein (MP) supplementation and anti-parasitic plant secondary metabolites (PSM) may modulate parasite epidemiology through intervention in pasture contamination, development of infection on pasture and larval challenge as target processes. We then propose that combining two or more non-chemical parasite control strategies may have additive effects on host resistance, especially if the individual strategies target different drivers of parasite epidemiology, different processes in the parasite life cycle or different phases of acquired immunity to parasites. This epidemiological framework is used to review recent findings on combining maternal MP supplementation and grazing the PSM-rich bioactive forage chicory as an example of combining nutritional treatments to manipulate parasite epidemiology in a temperate production system. In the absence of available data for combined nutritional strategies in tropical production systems, we make predictions on the consequences of combining such strategies in these systems. We conclude that currently published studies on combining nutritional strategies under temperate conditions show potential to improve additively host resilience and reduce reliance on anthelmintics; however, effects on epidemiology have to date not shown the additive results hypothesized. The framework developed may assist further in evaluating combined (nutritional) strategies to manipulate parasite epidemiology.  相似文献   

19.
Three anthelmintic classes with distinct mechanisms of action are commercially available. Selection of nematode populations resistant to all these drugs has occurred, particularly in trichostrongyloid parasites of sheep. Anthelmintic resistance in cattle parasites has only recently been recognized and appears to be less pronounced, even though very similar species infect both hosts. To understand the bases for differences in the rate of resistance development in sheep versus cattle parasites, it is important to first demonstrate that the same kinds of resistance alleles exist in both. The benzimidazoles (BZ), which have been used for more than 40 years, were chosen as an example. BZ-sensitive (BZ(S)) and BZ-resistant (BZ(R)) nematodes that parasitize sheep have been distinguished at the molecular level by a single nucleotide change in the codon for amino acid 200 of a beta-tubulin gene, a switch from TTC (phenylalanine) to TAC (tyrosine). PCR primers were designed to completely conserved regions of trichostrongyloid beta-tubulin genes and were used to amplify DNA fragments from Haemonchus contortus (cDNA from a BZ(S) and a BZ(R) library) as positive controls. The technique was then extended to the cattle parasites, Cooperia oncophora and Ostertagia ostertagi (from genomic DNA). Sequence analysis proved the presence of amplified BZ(S) alleles in all three species and BZ(R) alleles in the BZ(R) population of H. contortus. Based on these data, nested PCR primers using the diagnostic T or A as the most 3' nucleotide were designed for each species. Conditions for selective PCR were determined. To demonstrate feasibility, genomic DNA was recovered from individual H. contortus L(3) larvae from both BZ(S) and BZ(R) populations. Genomic DNA was also isolated from >70 individual adult male C. oncophora collected from a cattle farm in New Zealand with reported BZ resistance. Allele-specific PCR discriminated among heterozygotes and homozygotes in both species. This method could find utility in studying the molecular epidemiology of BZ resistance in cattle parasites and for defining the variables that limit the development and spread of anthelmintic resistance in this host.  相似文献   

20.
Effective, sustainable control of nematode parasites of grazing livestock is becoming evermore challenging and difficult. This is largely due to two contrasting issues. One is the rapid escalation of resistance to anthelmintic drugs, which is arguably the greatest problem now facing the small ruminant industries worldwide. Secondly, there is the increasing trend towards organic farming, in which there is prohibition of the prophylactic use of all chemical compounds. Livestock producers urgently need non-chemotherapeutic alternatives in parasite control. Researchers have responded to this challenge and a variety of quite different approaches have been the subject of intense investigation in many countries for several decades now. These vary in relation to their stage of development for on-farm use, their utility, and their applicability across the spectrum of grazing livestock enterprises throughout the world. One relatively recent innovation is the biological control approach to nematode parasites. This has now reached the stage of commercialization. This review focuses on these issues and provides an overview of the possible ways in which the biological control of nematode parasites could be employed in grazing ruminant livestock systems worldwide.  相似文献   

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