Efficacy of 3 anthelmintics in communally grazed sheep as reflected by faecal egg count reduction tests in a semi-arid area of South Africa

A survey was conducted on the occurrence of anthelmintic resistance of nematodes in communally grazed sheep in a semi-arid area near Mafikeng, South Africa, from January to March 2006. In the herds belonging to twelve smallholder sheep farmers, the efficacies of albendazole, levamisole and closantel were tested by faecal egg count reduction tests where 80 % efficacy was considered the cut off for anthelmintic resistance. The results of the faecal egg count reduction tests showed more than 80% efficacy with all the drugs used in most cases, but there were notable exceptions. In 1 case, closantel had an efficacy of 72%, albendazole had an efficacy of 68% and levamisole showed efficacies of 58%, 60% and 75% respectively on 4 farms. The occurrence of anthelmintic resistance in this farming sector is of concern and steps should be taken to prevent its further spread and development to avoid a situation developing as on numerous commercial sheep farms in South Africa where resistance is common.     

Sheep and goat nematode resistance to anthelmintics: pro and cons among breeding management factors

Although the molecular bases of resistance to anthelmintic families have been intensively studied, the contributing factors for the development of anthelmintic resistance are less well known. Clear recommendations must be given to farmers in order to delay the onset of anthelmintic resistance. Until now, the main advice has concerned the reduction of treatment frequency in order to slow down the spread of resistance. Anthelmintic resistance development depends mostly on an efficient selection pressure. This means that a high treatment frequency is neither necessary nor sufficient to select for resistance. The contribution of resistant worms, which have survived an anthelmintic treatment, to the subsequent generation is the key factor that controls resistance spread. This point is illustrated by five surveys conducted on sheep and goat farms from France and Morocco. In the 52 farms studied, less than three anthelmintic treatments were given each year. Three characteristics of breeding management can be identified in the build up of anthelmintic resistance: (1) the introduction of resistant worms through the purchase of sheep/goats or the use of common pastures, grazed by several herds/flocks, (2) under-dosing of hosts and the repeated use of one class of drugs, (3) the size of the population in refugia (infective larvae on pastures) at the time of the treatment. The role played by these breeding management factors in selecting for resistance is discussed. The most efficient way to limit the increase of anthelmintic resistance remains the reduction of the selection pressure by drugs, and optimal timing to maximise their efficacy.     

Anthelmintic resistance of nematodes in communally grazed goats in a semi-arid area of South Africa

A survey was conducted on the occurrence of anthelmintic resistance of nematodes in communally grazed goats in a semi-arid area in South Africa. In herds belonging to 10 small-holder goat farmers, the efficacies of fenbendazole, levamisole and rafoxanide were tested by faecal egg count reduction (FECR) tests. Efficacies of 80% were considered a threshold for anthelmintic resistance. The FECR tests showed that all drugs tested more than 80% effective in most instances, but there were notable exceptions. In 1 case, rafoxanide was only 31% effective and in another case fenbendazole was only 47% effective. The occurrence of anthelmintic resistance in this farming sector is of concern. Steps should be taken to prevent its further spread and to avoid the development of a situation as on numerous commercial sheep farms in South Africa where resistance is very common.     

Managing helminths of ruminants in organic farming

The use of anthelmintics is strongly limited in organic farming. This may induce a change in the intensity (no of worms) and diversity (proportions of species) of helminth infection. Helminths remain a major preoccupation in organic sheep farming: high levels of infection have been recorded on several farms and helminth diversity is always higher. The helminth infection in milk cattle of northern Europe is controlled and diversity is higher in organic farms, as recorded in sheep. The role of helminth diversity on intensity is still unclear. Grazing management is one of the means to controlling helminths. The use of safe pastures for calves and sheep after weaning is one of the major components of control. The use of alternate or mixed grazing is common for cattle in northern countries but is uncommon for sheep in France. Grazing management is not sufficient to controlling infection in sheep and conventional anthelmintic treatments are performed. Additionally, alternative treatments are used. The alternative therapies based on phytotherapy or homeopathy are largely recommended in organic farming but do not have any demonstrated efficacy. More research is needed to evaluate such therapies.     

Anthelmintic resistance amongst sheep and goats in Kenya

Faecal egg count reduction tests (FECRT) and larval cultures were used to assess the prevalence of anthelmintic resistance amongst sheep and goats on farms served by Kenya's five regional Veterinary Investigation Laboratories. Twenty-four out of 42 farms tested (57%) showed resistance to at least one anthelmintic group. Resistance to levamisole was found in nine out of 35 sheep farms tested (26%) and 12 out of 24 goat farms (50%). Resistance to benzimidazole drugs was found in 10 out of 28 sheep farms (36%) and six out of 20 goat farms (30%). Larval cultures of post-treatment faecal samples showed resistance to be predominantly due to Haemonchus contortus. Twelve farms were routinely using substandard products which appeared to have no anthelmintic activity.     

A survey on anthelmintic resistance in nematode parasites of sheep in Lusaka,Zambia

While surveys in Southern Africa indicate anthelmintic resistance of gastrointestinal nematodes to be common in small ruminants in South Africa, Kenya and Zimbabwe there have been no reports of resistance in Zambia. The objective of this study was to determine whether anthelmintic resistance occurs in Zambia, and to obtain information on nematode control practices in the country. During the rainy season six commercial sheep farms were selected in and around Lusaka and Chisamba. Worm control practices were gauged by means of a questionnaire, and the Faecal Egg Count Reduction Test was performed for assessing anthelmintic efficacy of albendazole, levamisole and ivermectin. On all farms anthelmintic treatment was the only approach used to control nematode infections. Frequency of treatment varied from twice a year to every 6 weeks and drugs of different anthelmintic groups were alternated within the same year. There was a wide range in faecal egg counts of individual sheep before treatment, with some individual counts of up to 87000. Larval identification showed that Haemonchus was almost the only genus recovered from the faecal cultures before and after treatment. Albendazole resistance was found on five of the six farms. On each of the four farms where ivermectin gave less than 95% reduction in egg counts, there was resistance to albendazole as well. Levamisole showed an efficacy of 95% or higher on all six farms.     

Anthelmintic resistance and sheep management practices in south western Victoria

Twenty-eight farms in 7 shires in south western Victoria were selected and tested for presence of benzimidazole-resistant nematodes between November 1979 and June 1981. Mean faecal egg counts of sheep were less than 100 strongyloid eggs/g on 11 farms. Faecal egg count reduction tests were conducted on the remaining 17 farms and thiabendazole was less than 90% efficient in reducing egg counts in sheep from 5 (29%) of these farms. Thiabendazole-resistant Teladorsagia circumcincta were identified at necropsy of experimentally infected treated sheep. In further studies a survey of 104 farms was conducted in the Mount Rouse and Dundas shires of western Victoria in 1981 and 1982 respectively to determine the prevalence of anthelmintic resistance in these shires. Mean faecal egg counts among weaner sheep in the winter-spring of both years were less than 100 eggs/g which indicated low levels of parasitic nematode populations. A faecal egg count reduction test was conducted on 10 farms and thiabendazole was less than 90% efficient on 3; levamisole was greater than 90% efficient in all 10 tests. Most of the surveyed farms carried Merino or Merino crossbred sheep at 10 to 15 dry sheep equivalents per ha and weaners were treated with anthelmintics 3 to 6 times per year. Management procedures based mainly on anthelmintic therapy were effective in controlling nematode populations in weaner sheep, although many producers alternated between different groups of anthelmintics within the same year contrary to current recommendations for long-term preservation of anthelmintic efficacy. It was concluded that anthelmintic resistance was not of practical importance to the majority of sheep producers in the region.     

Anthelmintic resistance in New Zealand

Anthelmintic resistance was first confirmed in New Zealand in 1979 and since then has become common-place; more than 50 % of sheep farms now have detectable levels of resistance to one or more chemical classes of anthelmintic. Farmer drenching practices have changed little over the last 15–20 years and are clearly exerting a significant level of selection for resistance. In the absence of new chemical classes of anthelmintics, current parasite control practices will be unsustainable in the long-term. Once substantial resistance has developed, significant reversion to susceptibility is unlikely and re-introduction of failed drugs is likely to result in the rapid re-emergence of control problems. The number of anthelmintic treatments applied is not necessarily a reliable indicator of selection pressure and should not be the only factor considered in strategies for minimising the development of resistance. The relative potential of the different anthelmintics now available, particularly the long- acting products, to select for resistance varies with the way they are used and with other epidemiological and management factors; generalisations about their respective roles in the development of resistance are often unreliable. In many cases, literal extrapolation of recommendations for the management of resistance from Australia to New Zealand is unsupportable, given the differences in climate, parasite ecology and farming practices between the 2 countries. In the absence of a refuge for susceptible genotypes, as occurs when anthelmintic treatments are used as a means of generating low-contamination ‘safe’ pasture for young stock, the rapid development of resistance is likely. Anthelmintic treatments applied to animals with a high level of immunity, or which become immune while the anthelmintic is active, are likely to select for resistance faster than treatments applied to non-immune stock.     

Teladorsagia circumcincta resistance to moxidectin and multiple anthelmintic groups in ewes following use of the persistent drug before lambing

This paper describes the nematode control strategy adopted by a Scottish hill sheep farmer. It provides an example of the limited use of anthelmintic drugs, targeted towards control of the periparturient rise in faecal nematode egg output in ewes and exploitation of grazing management, resulting in limited exposure of naive sheep to infective larvae on pasture. Resistance to benzimidazole, imidazothiazole and macrocyclic lactone anthelmintics, and to a combination of a macrocyclic lactone and imidazothiazole anthelmintic drugs was diagnosed. The targeted use of a persistent anthelmintic drug to control a periparturient rise in faecal nematode egg output in ewes may be unnecessary and selects strongly for resistance when the reservoir of anthelmintic-susceptible nematodes in refugia is small at the time of treatment. However, the use of a persistent anthelmintic drug in a selective proportion of ewes can be important and probably does not select strongly for resistance when the reservoir of anthelmintic-susceptible nematodes in refugia is large at the time of treatment. The former circumstances might arise on many Scottish hill sheep farms, whereas the latter may occur on upland and lowground farms, depending on previous grazing management, anthelmintic use and winter weather conditions. These factors must be taken into account when preparing sustainable health plans for nematode parasite control in individual sheep flocks.     

Anthelmintic resistance in New Zealand

Anthelmintic resistance was first confirmed in New Zealand in 1979 and since then has become common-place; more than 50% of sheep farms now have detectable levels of resistance to one or more chemical classes of anthelmintic. Farmer drenching practices have changed little over the last 15-20 years and are clearly exerting a significant level of selection for resistance. In the absence of new chemical classes of anthelmintics, current parasite control practices will be unsustainable in the long-term. Once substantial resistance has developed, significant reversion to susceptibility is unlikely and re-introduction of failed drugs is likely to result in the re-emergence of control problems. The number of anthelmintic treatments applied is not necessarily a reliable indicator of selection pressure and should not be the only factor considered in strategies for minimising the development of resistance. The relative potential of the different anthelmintics now available, particularly the long-acting products, to select for resistance varies with the way they are used and with other epidemiological and management factors; generalisations about their respective roles in the development of resistance are often unreliable. In many cases, literal extrapolation of recommendations for the management of resistance from Australia to New Zealand is unsupportable, given the differences in climate, parasite ecology and farming practices between the 2 countries. In the absence of a refuge for susceptible genotypes, as occurs when anthelmintic treatments are used as a means of generating low-contamination 'safe' pasture for young stock, the rapid development of resistance is likely. Anthelmintic treatments applied to animals with a high level of immunity, or which become immune while the anthelmintic is active, are likely to select for resistance faster than treatments applied to non-immune stock.     

Minimising the development of anthelmintic resistance, and optimising the use of the novel anthelmintic monepantel, for the sustainable control of nematode parasites in Australian sheep grazing systems

Objective To compare the risk of different treatment scenarios on selecting for anthelmintic resistance on Australian sheep farms. Design A computer simulation model predicted populations of Trichostrongylus colubriformis, Haemonchus contortus or Teladorsagia (Ostertagia) circumcincta, and the frequency of anthelmintic resistance genes. Method Nematode populations and the progression of drug resistance for a variety of treatment options and management practices in sheep‐rearing areas of Western Australia (WA), Victoria (VIC) and New South Wales (NSW) were simulated. A scoring system was devised to measure the success of each option in delaying resistance to each anthelmintic and in controlling nematode populations. Results The best option at all sites was combining the new anthelmintic (monepantel) with a triple mixture of benzimidazole, levamisole and abamectin (COM). The next best option was: in NSW, rotation at each treatment between monepantel, moxidectin and COM; in VIC, rotation at each treatment between monepantel and COM; and in WA, rotation at each treatment between monepantel (used in winter) and COM or moxidectin (used in summer–autumn). In WA, rapid selection for resistance occurred as a consequence of summer–autumn treatments; however, if a small percentage of adult stock were left untreated then this selection could be greatly reduced. Despite purposely assuming relatively high resistance to benzimidazole and levamisole, COM was still effective in controlling worms and delaying resistance. Conclusions Because of cost constraints, it may not be feasible or profitable for producers to always use the combination of all drugs. However, the second‐ and third‐best options still considerably slowed the development of anthelmintic resistance.     

Pharmaceutical treatments of gastrointestinal nematode infections of sheep-Future of anthelmintic drugs

Various interacting factors have been identified to explain why health plans for nematode parasite control, based on conventional epidemiological knowledge and involving pharmaceutical treatments of their sheep hosts have become unsustainable. Of these, the emergence of anthelmintic resistance has had a major impact on the economics of sheep farming, necessitating fundamental managemental changes. This review focusses on the use of anthelmintic drugs for the control of gastrointestinal nematode infections in sheep, emphasising the need to develop sustainable strategies in the face of inevitable parasite evolution in response to exposure to anthelmintic drugs and other noxious stimuli, or favourable opportunities resulting from changing animal management and climatic factors.     

The effective life of ivermectin on Western Australian sheep farms--a survival analysis

A mail survey of 235 Western Australian sheep farmers who had performed faecal egg count reduction tests for anthelmintic resistance in 1999 or 2000 was conducted, with some telephone follow-up. A response of 56% was achieved. Resistance to ivermectin, a member of the macrocyclic lactone class of anthelmintics, had developed on 44% of the farms surveyed. We used time to occurrence of resistance to ascertain factors that contributed to extending the time ivermectin remained an effective drench on these farms (median time = 10.5 years). This time was significantly longer when farmers implemented more worm control practices on their farms (P = 0.003). We developed a multivariable survival model that contained the following main effects: reduced winter drenching frequency, 0–2 flock treatments in 5 years (hazard ratio (HR) 0.52); availability of alternative effective anthelmintic classes on the farm (HR 0.30); always using safe pastures (HR 0.23); and veterinarians as the primary source of worm control advice (HR 0.58). The relationship of these findings to the understanding of anthelmintic resistance is discussed.     

Anthelmintic resistance in South Africa: surveys indicate an extremely serious situation in sheep and goat farming

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 entering 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 indicated that even in previously disadvantaged communities where anthelmintic treatment is less intensive, anthelmintic resistance is developing, and is possibly at the level at which the situation on commercial 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 summer 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 induce reversion of the resistance that has already developed.     

Managing anthelmintic resistance in small ruminant livestock of resource-poor farmers in South Africa

Gastrointestinal parasitism is one of the most important disease complexes of sheep and goats impacting on the resource-poor livestock farmer. Of the responsible nematodes, Haemonchus contortus, a blood-sucking worm of the abomasum, poses possibly the greatest threat. Over the past several decades, the worm has been controlled through the use of anthelmintics, but the emergence of anthelmintic resistance has threatened this chemotherapeutic approach. In Africa, the overall prevalence of anthelmintic resistance has not been extensively investigated, particularly within the resource-poor farming sector, but resistance has been reported from at least 14 countries with most of the reports emanating from Kenya and South Africa and the majority concerning H. contortus. While levels of resistance under commercial sheep farming systems in South Africa is considered to be amongst the worst in the world, resistance has also been reported from the resource-poor farming sector. Increases in productivity and reproduction of livestock and the development of markets for sale of animals are seen by international funding bodies as a way out of poverty for communities that keep livestock. This must lead to the greater need for parasite control. At such times, the risk of levels of anthelmintic resistance escalating is much greater and there is therefore a need to look at alternatives to their use. Proposed strategies include the appropriate, but judicious use of anthelmintics by application of the FAMACHA system and the use of alternatives to anthelmintics such as strategic nutrient supplementation. It is also very clear that there is a strong demand for knowledge about animal diseases, including helminthosis, and their effective management in the resource-poor livestock farming communities. This is an important challenge to meet.     

Anthelmintic drug resistance in the UK

The term 'anthelmintic drug resistance' describes the heritable ability of some nematode parasites to survive treatment with anthelmintic drugs at the recommended therapeutic dose levels. Genes for resistance appear to be present in many of the important pathogenic nematodes of ruminants and horses. Under intensive management systems, where heavy reliance is placed on anthelmintic drugs for worm control, the selection of resistant genotypes may result in increased reports of the drugs failing to control the nematode populations against which they are aimed. Anthelmintic resistance has been reported from many parts of the world, and in some countries multiple drug-resistant strains have emerged. In the UK, recent investigations indicate an increasing level of resistance to benzimidazoles among nematodes of sheep and horses. The incidence, diagnosis, epidemiology and implications of anthelmintic resistance are discussed.     

Lack of Cyathostomin sp. reduction after anthelmintic treatment in horses in Brazil

The increase of anthelmintic resistance in the last years in the nematode population of veterinary importance has become a major concern. The objective of the present study was to evaluate the efficacy of the main anthelmintic drugs available in the market against small strongyles of horses in Brazil. A total of 498 horses from 11 horse farms, located in the states of Paraná, São Paulo, Rio de Janeiro and Minas Gerais, in Brazil, were treated with ivermectin, moxidectin, pyrantel and fenbendazole, orally at their recommended doses. The fecal egg count reduction test (FECRT) was used to determine the product's efficacy and fecal culture was used to determine the parasite genus. Reduction on anthelmintic efficacy was found for fenbendazole in all horse farms (11/11), pyrantel in five yards (5/11) and ivermectin had low efficacy in one of the yards studied (1/11). Multidrug resistance of up to 3 drugs classes was found in one of the tested farms (1/11). Cyathostomin were the most prevalent parasite. The results showed that resistance to fenbendazole is widespread; the efficacy of pyrantel is in a critical situation. Although the macrocyclic lactones compounds still showed high efficacy on most farms, suspected resistance to macrocyclic lactones is of great concern.     

A survey of the trichostrongylid nematode species present on UK sheep farms and associated anthelmintic control practices

A survey of sheep farms from across the UK was conducted to establish information on farming practices, the trichostrongylid nematode species present and anthelmintic usage. Questionnaires and faecal samples were returned from 118 farms. First stage larvae (L(1)) were cultured from faecal samples and used for PCR analysis to determine the presence/absence of selected trichostrongylid species. Teladorsagia circumcincta was the only species present on 100% of farms. Haemonchus contortus was found on ～50% of farms and was widespread throughout the UK. The most common Trichostrongylus spp. was T. vitrinus, found on 95% of farms. Determining the anthelmintic dose rate based on the weight of the heaviest animal in the flock to avoid under dosing was carried out on 58% of farms and was associated with a significantly lower mean epg (p<0.001) in lambs. However, the weight of animals was only estimated (as opposed to animals weighed) on 32% of farms. Macrocyclic lactones (ML) were the most commonly used anthelmintic class for ewes, whilst benzimidazoles (BZ) were the most widely used in lambs. Twenty-two of the surveyed farms had confirmed anthelmintic resistance, of these, 18 had BZ resistance, one had levamisole (LEV) resistance and 3 had resistance to both BZ and LEV. No farms in this survey reported resistance to ML. Location had a significant effect on the incidence of anthelmintic resistance on the farms in this survey (p=0.002). There was evidence of a lower risk of anthelmintic resistance occurring on farms from Scotland compared to those in England (p(f)=0.047) and Wales (p(f)=0.012). Farm type, flock type and open or closed status did not have any significant effect on the incidence of anthelmintic resistance when all other factors were taken into consideration.     

Management of gastrointestinal nematode parasites on sheep farms in New Zealand

AIM: To report current farmer opinions and farming practices relating to control of gastrointestinal nematodes and anthelmintic resistance on sheep farms in New Zealand. METHODS: An interview-based cross-sectional study of grazing management and anthelmintic usage was conducted by veterinarians on 80 randomly selected sheep farms in New Zealand. RESULTS: Useable data were returned by 74/80 (92%) farmers who participated in the study. However, despite contacting 400 farmers the target sample size of 100 farms was not reached. The results indicated that only 31% of farms had previously tested for drench resistance, that effective quarantine-drenching of imported stock was not always carried out, and that farmers were more likely to integrate cattle than ewes into their grazing management of lambs. Furthermore, the number of drenches given to lambs had changed little in 25 years. The use of faecal egg counting by farmers has increased. CONCLUSIONS: Dependence on anthelmintics continues to be high on sheep farms in New Zealand. Whilst the number of drench treatments has changed little, there is more widespread use of persistent or long-acting treatments. Farmers need to be encouraged to monitor the resistance status of nematode populations on their farms and use this information to develop strategies aimed at maintaining susceptible alleles within the parasite populations and conserving the efficacy of existing drug families.     

A survey by questionnaire of parasitic worm control in cattle and sheep at the Glasgow University Lanark practice

The results of a questionnaire circulated in 1984 concerning parasitic worm control on cattle and sheep farms in a veterinary practice in the west of Scotland are reported. Control by grazing management or anthelmintic treatment was used in 92 per cent of the cattle herds and in all the sheep flocks. The use of anthelmintic drugs was greatest on farms where grazing control was also practised. Benzimidazoles were the most frequently used anthelmintic drugs.