The effect of combinations of anthelmintics on parasite populations in sheep

Oksfendazol, oksiklozanid ve oksfendazol-oksiklozanid kombinasyonunun koyun ve keçilerde biyokimyasal ve hematolojik parametreler üzerine etkileri

Mevcut araştırmada sağlıklı koyun ve keçilere oral yolla oksfendazol, oksiklozanid ve oksfendazol-oksiklozanid kombinasyon uygulamasının, farklı zamanlarda biyokimyasal ve hematolojik parametreler üzerine etkisinin belirlenmesi amaçlandı. Çalışma çapraz dizaynda 6 adet sağlıklı dişi, 1-3 yaştaki İvesi koyun ve Alpin keçi üzerinde gerçekleştirildi. Koyun ve keçilere oral yolla 7.5 mg/kg oksfendazol, 15 mg/kg oksiklozanid ve oksfendazol-oksiklozanid (7.5 mg/kg-15 mg/kg) kombinasyonu uygulandı. Kan örnekleri ilaç uygulamalarından önce (0.saat, kontrol) ve sonraki 8., 24., 72. saat ile 7. günde alındı. Biyokimyasal parametrelerden albümin, alkalen fosfataz, alanin aminotransferaz, aspartat aminotransferaz, kolesterol, trigliserid, total protein, kan üre nitrojen, kreatinin değerleri otoanalizatör cihazında belirlendi. Hematolojik parametrelerden alyuvar sayısı, hemoglobin, hematokrit, ortalama korpüsküler hacim, ortalama korpüsküler hemoglobin, ortalama korpüsküler hemoglobin konsantrasyonu, akyuvar sayısı, lenfosit, monosit, % granülosit, % lenfosit, % monosit ve % eritrosit dağılım genişliği değerleri kan hücresi sayım cihazında ölçüldü. İlaç grupları ve zamana bağlı olarak hematolojik ve serum biyokimya parametrelerinde anlamlı bir fark bulunamadı. Bu sonuçlar koyun ve keçilere oral yolla, 7.5 mg/kg oksfendazol ve 15 mg/kg oksiklozanid tek veya kombine olarak uygulandığında hematolojik ve biyokimyasal parametrelerin fizyolojik sınırlarda olduğu ve klinik olarak anlamlı bir etkisinin olmadığını göstermektedir.

An assessment of worm control practices used by alpaca farmers in Australia

This study aimed to assess current worm control practices used by Australian alpaca farmers with an online questionnaire survey. The questionnaire contained questions about farm demography and general husbandry practices, farmers' knowledge about gastrointestinal nematodes (GINs) and their importance, the use of worm control strategies and anthelmintics, and grazing management. A link for the questionnaire survey was sent to all (n = 954) registered members of the Australian Alpaca Association in July 2015. The response rate for the questionnaire was 25% (239/954). The majority of respondents were from small (≤ 50 alpacas; 64%, 153/239) followed by medium (50-100 alpacas; 24%, 57/239) and large (>100 alpacas; 12%, 29/239) farms. Findings revealed that the majority of respondents kept Huacaya alpacas to produce high-quality fibre and alpacas were usually kept with other domestic ruminants (e.g. cattle and sheep). Although half of alpaca farmers (114/220) perceived that GINs were an important health problem of alpacas, with Haemonchus spp. being the most common nematode, the majority of them (174/220) used anthelmintics for nematode control. Macrocyclic lactones, a commercial combination of four anthelmintics (abamectin, albendazole, closantel and levamisole) and monepantel were the three most commonly used dewormers by Australian alpaca farmers. Although a significant proportion (166/213) of respondents used a quarantine drench for alpacas, very few respondents were aware of strategic deworming and the issue of anthelmintic resistance. Alpaca farmers mostly used anthelmintics at the dose rate recommended for sheep (47%, 79/167) and cattle (9%, 15/167), though some used 1.5 (31%, 51/167) and 2 (13%, 22/167) times the dose rate recommended for sheep. The majority of small herds used anthelmintics at the dose rate recommended for sheep and cattle while medium and large herds used anthelmintics at 1.5 to 2 times the dose rate recommended for sheep. This study provides invaluable insights into the demography of alpaca farms in Australia, husbandry practices used by alpaca farmers and their knowledge about worms and their control, thereby paving the way for developing guidelines for the control of GINs of alpacas.

Sustainable use of anthelmintics in an Integrated Parasite Management Program for sheep nematodes

Anthelmintic resistance is a major problem affecting sheep nematode control; however, chemical treatments will always be a likely routine part of any parasite control program. The present paper, with the aim of minimising the selection pressure for worsening anthelmintic resistance, outlines important practical and strategic aspects of planning the anthelmintic component of an integrated approach to sheep worm management.

Anthelmintic resistance and alternative control methods

Anthelmintic resistance has been a problem almost from the first use of the drugs introduced during the past 50 years. Evaluation of anthelmintics on a specific farm is essential to determine which drugs may be used in controlling parasite numbers. Treatment of livestock during the season in which parasite transmission is unfavorable lessens pasture contamination. Targeting treatment places less pressure on susceptible worms, diluting the resistant parasite population. Pasture management and using safe pastures for animals at highest risk lower pasture exposure. Selection for individual animals resistant to the effects of parasites lessens the need for use of anthelmintics. Alternatives to anthelmintics include cryptic antigen vaccines, copper wires, and biologic compounds.

Anthelmintic resistance: The state of play revisited

Helminthosis is one of the major constraints in the successful wool and mutton industry throughout the world. Anthelmintic Resistance (AR) is said to have been established when previously effective drug ceases to kill exposed parasitic population at the therapeutically recommended dosages. Anthelmintic resistance is almost cosmopolitan in distribution and it has been reported in almost all species of domestic animals and even in some parasites of human beings. Some of the most important species of parasites of small ruminants in which AR has been reported include: Haemonchus spp., Trichostrongylus spp. Teladorsagia spp., Cooperia spp. Nematodirus spp., and Oesophagostomum spp. All the major groups of anthelmintics have been reported for development of variable degrees of resistance in different species of gastrointestinal nematodes. This paper describes the global scenario of prevalence and methods used for detection of AR in small ruminants. Different mechanisms and contributory factors for the development of AR are discussed. Various options and alternate strategies for the control and/or delay in the onset of AR are suggested in the light of available information.

Characterisation of two triple resistant field isolates of Teladorsagia from Scottish lowland sheep farms

The anthelmintic resistance status of two field isolates derived from farms (farm A and B) located near Edinburgh were examined using both controlled efficacy tests (CET) and faecal egg count reduction tests (FECRT). Efficacies against fenbendazole (FBZ), levamisole (LEV) and ivermectin (IVM) and, for one isolate, against combinations of these anthelmintics and moxidectin were determined in naïve lambs, artificially infected with the isolates and treated with the compounds at the manufacturers recommended dose rates. (FBZ, 5mg/kg bodyweight (BW); LEV, 7.5mg/kg BW; IVM, 0.2mg/kg BW; Moxidectin (MOX) 0.2mg/kg BW). In both field isolates, the predominant species found pre-treatment and the only species found post-treatment was Teladorsagia circumcincta. Resistance to FBZ, LEV and IVM was confirmed in CET and FECRT on farm A and to the latter two compounds on farm B, which had a history of benzimidazole resistance and where TBZ resistance was also demonstrated using an egg hatch assay (EHA). For the farm A isolate CET efficacies against FBZ; IVM; LEV; FBZ + IVM; FBZ + LEV; FBZ, LEV + IVM and MOX were 59, 60, 88, 94,93, 92 and 98%, respectively. The CET efficacies for the farm B isolate were 51% and 72% for LEV and IVM, respectively.

The efficacy of levamisole, and a mixture of oxfendazole and levamisole, against the arrested stages of benzimidazole-resistant Haemonchus contortus and Ostertagia circumcincta in sheep

Sheep were allowed to graze pasture that had been seeded with benzimidazole-resistant Haemonchus contortus and Ostertagia circumcincta in order to acquire a burden of arrested larvae. Following housing, sheep were dosed orally with either oxfendazole at a dose rate of 4.7 mg/kg (to confirm the benzimidazole-resistant status of the species of nematode), levamisole at a dose rate of 7.5 mg/kg, or an oxfendazole/levamisole mixture at a dose rate of 4.6 mg/kg oxfendazole and 8.1 mg/kg levamisole. The efficacies of the treatments were assessed by estimation of the arrested larval burden in the abomasum of each sheep, either at 10 or 11 days (oxfendazole and oxfendazole/levamisole mixture), or 12 or 13 days (levamisole), after treatment. Compared to the untreated controls, the protection afforded by a single dose of either levamisole or the oxfendazole/levamisole mixture was >99% against the arrested stages of both Haemonchus contortus and Ostertagia circumcincta. Treatment with oxfendazole confirmed the benzimidazole-resistance status of the two species.

Economic evaluation of three anthelmintic strategies for lamb flocks affected by benzimidazole-resistant nematodes

OBJECTIVE

To compare the profitability of three anthelmintic strategies in growing lambs in flocks with nematodes resistant to benzimidazole anthelmintics.

METHOD

A partial-budgeting analysis was carried out by means of a stochastic simulation model, which allows inputs to be described as distributions rather than as fixed values, and hence permits variation between farms to be considered in the analysis.

RESULTS

The results show that control of nematode parasites by use of an effective anthelmintic provides the highest net returns, yielding a margin over ineffectively treated lambs of A$114 per 100 lambs on average. Suppressive treatment based on the administration of two controlled-release capsules and monthly with moxidectin resulted in an average loss of A$131 per 100 lambs in comparison with animals treated with an ineffective anthelmintic. Analysis of the results from capsule-treated lambs did not take into account the unmeasured benefits associated with less contamination of pastures. Sensitivity analysis using a stochastic model indicates that apart from the effect of treatment on weight gain variation in carcase price greatly influences the profitability of all the parasite control programs examined.

CONCLUSION

The results suggest that it is economically important for farmers to adjust their strategy in the presence of anthelmintic resistance. But as a result of uncertainty in the factors influencing economic return, the expected economic benefit is likely to vary substantially.

Anthelmintic resistance in sheep and goat farms on Peninsular Malaysia

The faecal egg count reduction test (FECRT) was conducted on 39 sheep farms and 9 goat farms located in Peninsular Malaysia. The anthelmintic groups used in these tests were the benzimidazoles, levamisole, the benzimidazole/levamisole combination, macrocyclic lactones and closantel. Results indicated that the prevalence of resistance to the benzimidazole group was high, with approximately 50% of the sheep farms and 75% of the goat farms having resistant nematode parasite populations present. Resistance to levamisole, closantel and ivermectin was also detected. Differentiation of the infective larvae derived from faecal cultures indicated that by far the most predominant parasite species was Haemonchus contortus.

Anthelmintic resistance

Since the first reports of resistance to the broad spectrum anthelmintics were made some three decades ago, this phenomenon has changed from being considered merely as a parasitological curiosity to a state of industry crisis in certain livestock sectors. This extreme situation exists with the small ruminant industry of the tropical/sub-tropical region of southern Latin America where resistance to the entire broad spectrum anthelmintic arsenal now occurs. In contrast, the cattle industry does not appear to be threatened--or so it seems. Although field reports of resistance have been made to the range of broad spectrum anthelmintics in nematode parasites of cattle, it appears that the evolution of resistance in cattle parasites is not as dramatic as for sheep worms. However, one cannot remain confident that this state of affairs will remain static. Concern is shared amongst parasitologists that we have not looked closely enough. In regions of the world where internal parasites are considered a problem in cattle and drenching occurs frequently, no widespread surveys have been carried out. It appears that because of the very high costs and risks associated with taking a new active drug down the development track to marketing, that the pharmaceutical industry has, in general, turned away from this activity. By implication, the international small ruminant industry is too small for these companies to make the necessary investment. This begs two questions: What is the fate of the sheep (and goat) industries in those parts of the world where resistance is rampant and immediate ameliorative parasite control options are required? What will be the response if significant resistance is found in cattle parasites? There is a body of opinion which suggests that if resistance becomes an issue in the control of cattle parasites then the pharmaceutical industry will find it commercially attractive to re-enter the anthelmintic discovery and development business. This is based on the simple commercial fact that by far the greatest anthelmintic sales are associated with the cattle industry. However, this market is specific and sectoral, with by far the greatest sales in North America and Western Europe, where the prevalence of resistance is likely to be low and remain so, more-or-less indefinitely. So the chances of the above scenario occurring must be considered low. Remarkable developments have recently occurred in non-chemotherapeutic parasite control options, for example worm vaccines, host selection and biological control. Also, there seems to be greater acceptance of various grazing management practices designed to reduce the frequency of anthelmintic treatment. However, they collectively cannot be expected to offer immediate salvation to farmers now faced with chemotherapeutic failure to control nematode parasites in their flocks. The future for these farmers must be considered bleak, because compounded with these problems are the poor commodity prices for sheep and goat meat and fibre, resulting in relentless reductions in funding for research to support these industries. Perhaps the major social issues associated with re-structuring and possibly abandonment of sheep and goat farming in affected areas may precipitate action? As veterinary parasitologists, who in general have an interest and expertise in parasite control, we must promote the importance of the problem of anthelmintic resistance and ways to tackle it.

A survey of dairy cattle worm control practices in southeast Brazil

A survey on the nematode control strategies utilized by dairy farmers in Southeastern Brazil, a region accounting for 46.4% of the national milk production, was conducted through interviews with farmers. To select the producers to be interviewed the region was split into 16 non-contiguous clusters, according to the level of milk production. A systematic sample was then selected in each of the clusters. The interview questionnaire consisted of one-way, multiple-choice and open-ended questions. Data collected were represented by numbers and digitized on a data base (Epi Info, version 5.01b) and analyzed. Out of the 89 farmers interviewed, 37.5% deworm their herd after clinical signs and 62.5% preventively. Generally, anthelmintics are applied from one to 12 times a year (average of 3.79 times a year) in all age categories of animals. Of the anthelmintics used in the last deworming, imidazothiazole was used exclusively by 17.1% of the farmers, benzimidazole by 9.8% and avermectin by 18.3%, while 55.8% used more than one anthelmintic class to deworm their animals. To choose the dosage, most farmers consult the product label (94.8%) and determine the volume to be applied, based on an estimate of the average body weight of each animal (62.9%). Improvements in the general appearance of the herd and weight gains of growing animals were observed by most farmers after deworming (87.3%). However, most of them (66.2%) recalled interrupting the use of some compound in the last few years, due to the detection of no improvement following treatment (32.7%), rising costs of the medication (28.6%), adverse reaction (8.2%), product not available at the time of purchase (4.1%) and decision to change the compound in use (10.2%). Most farmers (95.3%) intend to continue using the same control measures in the following year. Veterinarians play an important role in the farmer's choice to deworm their animals, as many seek advice from them. Therefore, programs aimed at technology transfer should include continuous updates on the subject, especially for veterinarians.

The effectiveness of benzimidazole-levamisole combination drenches in the presence of resistance to both benzimidazole and levamisole anthelmintics in New Zealand sheep

Examination of 25 cases of multiple benzimidazole and levamisole resistance, identified in sheep by faecal egg count reduction testing at the Batchelar, Lincoln and Invermay Animal Health Laboratories, showed that benzimidazole-levamisole combinations provided effective control in eight (47%) of 17 cases in which they were tested. Overall, the use of combination drenches resulted in average improvements in faecal egg count reductions of 25.6% (p<0.01) and 23.2% (p<0.05), respectively, over those achieved by the use of benzimidazole or levamisole drenches alone. The results suggest that instances of multiple resistance in which combination drenches might be effective are unlikely to be predictable either by identification of the parasites involved, or from the levels of benzimidazole or levamisole resistance present.

Interaction between Ostertagia circumcincta and Haemonchus contortus infection in young lambs

Twenty-one-week-old, worm-free, pen-reared lambs were infected with either 6000 O. circumcincta L3 per week, or 3000 H. contortus L3 per week, or both (9000 L3 per week). Egg counts were monitored throughout the experiment, and worm burdens and larval establishment rates of both worm species were estimated after 4, 7, 10 and 13 weeks of infection. After 10-13 weeks of infection with H. contortus only, establishment of O. circumcincta was lower than in previously uninfected controls, demonstrating that a high level of immunity to H. contortus affords some cross-protection against O. circumcincta. Total H. contortus worm burdens and egg counts (about 2000 worms and 3000 e.p.g., respectively) in sheep infected with both worm species were less than half those observed in sheep infected with H. contortus alone (about 5000 worms and 10,000 e.p.g., respectively). Cross-protection between the two species was observed, but was probably less important than the reduction in H. contortus establishment that was caused by O. circumcincta disrupting abomasal physiology.

Bafilolides, potent inhibitors of the motility and development of the free-living stages of parasitic nematodes

Three Streptomyces isolates were identified as producing macrolide antibiotics of the bafilomycin or leucanicidin types during an evaluation of Australian actinomyces for the production of inhibitors of larval development in the parasitic nematode, Haemonchus contortus. Bafilomycins A1, B1, C1, and D were obtained from culture A239 and the 2-O-methyl-L-rhamnosyl derivative of bafilomycin A1, leucanicidin, from cultures A223 and A240. All these 'bafilolides' gave similar patterns of inhibition typified by an initial paralysis of newly hatched L1 larvae and a lethal toxicity within 24 h. LD50 values for inhibition of larval development of McMaster H. contortus ranged from 0.23 micrograms ml-1 for leucanicidin to 2.5 micrograms ml-1 for bafilomycin D. The bafilolides had broad spectrum nematocidal activity, being equi-potent as inhibitors of H. contortus, Trichostrongylus colubriformis and Ostertagia circumcincta larval development. Further, all bafilolides caused some inhibition of H. contortus L3 motility, with the semi-synthetic analogue, bafilomycin B2, the most potent inhibitor (LP50 against McMaster H. contortus 1.9 microgram ml-1). Nematode strains resistant to the known benzimidazole, levamisole and avermectin anthelmintics showed no cross resistance to the bafilolides, supporting the hypothesis that the bafilolides act by an independent mechanism.

Anthelmintic resistance in Australian sheep nematode populations

The resistance status of gastro-intestinal nematodes to anthelmintics was evaluated on 881 sheep farms throughout Australia during 1991-92. Resistance was shown to be widespread. Overall, 85% of farms had sheep infected with nematodes resistant to benzimidazole, 65% to levamisole and 34% to combination (benzimidazole+levamisole) products. Resistance to ivermectin was not detected. On only 9% of farms did all anthelmintic groups reduce egg counts by greater than or equal to 95%. The culture of faeces from untreated sheep showed Telodorsagia circumcincta, Trichostrongylus spp, Chabertia ovina and Haemonchus contortus to be the principal species. The nature and prevalence of resistance was not significantly correlated with stocking rate. However, resistance to combination products was almost twice as prevalent on farms in areas with an average annual rainfall of greater than 500 mm.

Control strategies to prevent resistance

Anthelmintics will continue to play a central role in nematode control programs of livestock for the foreseeable future. However, to prevent the emergence of resistance, anthelmintics need to be either so effective that there are no survivors following treatment or treatment should be so infrequent that those that survive make little or no contribution to subsequent generations of the parasite populations. In this regard, the record for the sheep, horse and particularly the goat industry is not good, whereas the second factor is almost certainly responsible for the lack of a significant resistance problem in the cattle industry. In Australia, where high levels of resistance prevail, control programs designed to limit the increase and spread of resistance have been promoted for some time. However, their development, and certainly their implementation, has tended to be reactive rather than proactive--attempting to deal with the problem when it threatens future productivity, rather than preventing it in the first instance. Regions and/or livestock industries with relatively less of a resistance problem should heed these developments and endeavour to establish a responsible ethos amongst users of anthelmintics. In concert with the general philosophy of Integrated Pest Management (IPM), parasite control schemes should focus on the planned integration of a range of techniques, rather than relying almost entirely on anthelmintics. By these means, sustainable control strategies to prevent resistance could be achieved.

Anthelmintic resistance

In populations of livestock in which anthelmintics have become the only means of control, individual worms in some species of helminths evade the effects of specific anthelmintics. If these resistant individuals are selected (by removing the susceptible individuals in the population) then the resistant worm population on individual farms becomes dominant in both numbers and effect. This selection of anthelmintic resistant populations is most often reported in equids and small ruminants (the hosts in which anthelmintics are used most frequently). There is little evidence of anthelmintic resistance by Ostertagia in cattle but an understanding of the problem in other grazing species may aid in delaying its onset. Benzimidazole/phenothiazine resistance by small strongyles in horses in widespread. Resistance by various geographic strains of Haemonchus, Teladorsagia and Trichostrongylus against all of the commonly used classes of anthelmintics in small ruminants has been verified. The failure of levamisole to remove adult Ostertagia ostertagi from cattle has been reported in several geographical areas. Resistance is an individual farm problem and each premise has to be evaluated as to which anthelmintics can be used. Drug rotation, especially rapid rotation (less than 1 year) selects for multiple drug resistance. Epidemiologic treatment selects for resistance, but if properly used, requires considerably fewer treatments to prevent disease and the development of resistant populations will be slowed. A combination of strategic and tactical treatments along with evaluation of anthelmintics will be required to enable producers to raise livestock in maximum production units.

Population dynamics of Trichostrongylus colubriformis and Ostertagia circumcincta in single and concurrent infections

Twenty-one-week-old worm-free pen-reared lambs were infected weekly with either 10,000 T. colubriformis larvae, 5000 O. circumcincta larvae, or with both species (15,000 larvae per week). Larval establishment and total worm burdens were estimated after 4, 7, 10 and 13 weeks of infection. Faecal egg counts and lamb bodyweights were measured weekly, and numbers of eosinophils in blood were estimated before infection and at weeks 5, 8 and 14. For both species of worms, the dynamics of infection (establishment, worm burdens, egg counts) were not affected by concurrent or pre-existing infection with the other species. Infection with T. colubriformis alone did not protect against O. circumcincta, but infection with O. circumcincta alone provided slight protection against the T. colubriformis larvae. Blood eosinophils increased between 5 and 8 weeks of infection and were similar for the three infections. This corresponded to the reduction in establishment for both species.