Resistance of field isolates of Trichostrongylus colubriformis and Ostertagia circumcincta to ivermectin

Exploring the role of detoxification genes in the resistance of Bursaphelenchus xylophilus to different exogenous nematicidal substances using transcriptomic analyses

Bursaphelenchus xylophilus (Pine wood nematode, PWN) has become a worldwide forest disease due to its rapid infection ability, high lethality and difficulty in control. The main means of countering B. xylophilus is currently chemical control, but nematicides can present problems such as environmental pollution and drug resistance. The development of novel environmentally-friendly nematicides has thus become a focus of recent research. In this study, BxUGT3 and BxUGT34, which might be related to detoxification, were investigated by comparing transcriptomic and WGCNA approaches. Three other genes with a similar expression pattern, BxUGT13, BxUGT14, and BxUGT16, were found by gene family analysis. Further bioassays and qPCR assays confirmed that these five genes showed significant changes in transcript levels upon exposure to α-pinene and carvone, demonstrating that they respond to exogenous nematicidal substances. Finally, RNAi and bioassays showed that B. xylophilus with silenced BxUGT16 had increased mortality in the face of α-pinene and carvone stress, suggesting that BxUGT16 plays an important role in detoxification. Taken together, this study used novel molecular research methods, explored the detoxification mechanism of B. xylophilus at a transcriptomic level, and revealed a molecular target for the development of novel biopesticides.

In silico analysis of koranimine, a cyclic imine compound from Peribacillus frigoritolerans reveals potential nematicidal activity

Pine wilt disease (PWD) is a destructive vector-borne forest disease caused by the nematode Bursaphelenchus xylophilus. To date, several options are available for the management of pine wilt disease; however constant development and search for natural products with potential nematicidal activity are imperative to diversify management options and to cope with the possible future emergence of resistance in parasitic nematodes. Here, a combined metabolomics and genomics approach was employed to investigate the chemical repertoire and biosynthetic potential of the bacterial endophyte Peribacillus frigoritolerans BE93, previously characterized to exhibit nematicidal activity against B. xylophilus. Feature-based molecular networking revealed the presence of diverse secondary metabolites. A cyclic imine heptapeptide, koranimine, was found to be among the most abundant secondary metabolites produced. Genome mining displayed the presence of several putative biosynthetic gene clusters (BGCs), including a dedicated non-ribosomal peptide synthase (NRPS) BGC for koranimine. Given the non-ribosomal peptide nature of koranimine, in silico molecular docking analysis was conducted to investigate its potential nematicidal activity against the target receptor ivermectin-sensitive invertebrate α glutamate-gated chloride channel (GluCl). Results revealed the binding of koranimine at the allosteric site of the channel-the ivermectin binding site. Moreover, the ligand-receptor interactions observed were mostly shared between koranimine and ivermectin when bound to the α GluCl receptor thus, suggesting a possibly shared mechanism of potential nematicidal activity. This study highlights the efficiency of combined metabolomics and genomics approach in the identification of candidate compounds.

Enhanced production of pinosylvin stilbene with aging of Pinus strobus callus and nematicidal activity of callus extracts against pinewood nematodes

Pinosylvin stilbenes are phenolic compounds mainly occurring in the Pinaceae family. We previously reported that the accumulation of two pinosylvin stilbene compounds, dihydropinosylvin methyl ether (DPME) and pinosylvin monomethyl ether (PME), in Pinus strobus trees was highly enhanced by infection with pine wood nematodes (PWNs: Bursaphelenchus xylophilus), and these two compounds showed strong nematicidal activity against PWNs. In this work, we established a system of pinosylvin stilbene (DPME and PME) production via the in vitro culture of P. strobus calli, and we examined the nematicidal activity of callus extracts. Calli were induced from the culture of mature zygotic embryos of P. strobus. Optimized growth of calli was obtained in 1/2 Litvay medium with 1.0 mg/L 2,4-D and 0.5 mg/L BA. DPME and PME accumulation did not occur in nonaged (one-month-old) calli but increased greatly with prolonged callus culture. The concentrations of DPME and PME in three-month-old dark-brown calli were 6.4 mg/g DW and 0.28 mg/g DW, respectively. The effect of methyl jasmonate treatment on the accumulation of DPME and PME was evaluated in cell suspension culture of P. strobus. However, the treatment appeared to show slight increase of DPME accumulation compared to callus browning. A test solution prepared from crude ethanol extracts from aged calli (three months old) containing 120 µg/ml DPME and 5.16 µg/ml PME treated with PWNs resulted in 100% immobilization of the adult PWNs and 66.7% immobilization of the juvenile PWNs within 24 h. However, nonaged callus extracts did not show any nematicidal activity against juvenile PWNs and showed less than 20% nematicidal activity against adult PWNs. These results indicate that pinosylvin stilbenes can be effectively produced by prolonged culture of P. strobus calli, can be isolated using simple ethanolic extraction, and are applicable as beneficial eco-friendly compounds with nematicidal activity against PWNs.

Nematicidal Activity of Benzyloxyalkanols against Pine Wood Nematode

Pine wilt disease (PWD) is caused by the pine wood nematode (PWN; Bursaphelenchus xylophilus) and causes severe environmental damage to global pine forest ecosystems. The current strategies used to control PWN are mainly chemical treatments. However, the continuous use of these reagents could result in the development of pesticide-resistant nematodes. Therefore, the present study was undertaken to find potential alternatives to the currently used PWN control agents abamectin and emamectin. Benzyloxyalkanols (BzOROH; R = C2-C9) were synthesized and the nematicidal activity of the synthetic compounds was investigated. Enzymatic inhibitory assays (acetylcholinesterase (AChE) and glutathione S-transferase (GST)) were performed with BzOC8OH and BzOC9OH to understand their mode of action. The benzyloxyalkanols showed higher nematicidal activity than did benzyl alcohol. Among the tested BzOROHs, BzC8OH and BzC9OH showed the strongest nematicidal activity. The LD50 values of BzC8OH and BzC9OH were 246.1 and 158.0 ppm, respectively. No enzyme inhibitory activity was observed for BzC8OH and BzC9OH. The results suggested that benzyloxyalcohols could be an alternative nematicidal agent.

Screening, isolation and evaluation of a nematicidal compound from actinomycetes against the pine wood nematode, Bursaphelenchus xylophilus

BACKGROUND

Bursaphelenchus xylophilus is a migratory endoparasitic nematode known to cause severe environmental damage and economic losses in pine forest ecosystems. This present study investigated the nematicidal metabolites of actinomycetes in vitro and evaluated the disease control efficacy of the active compound and metabolites under greenhouse and field conditions.

RESULTS

Five thousand types of actinobacteria from Korean forest soil samples were screened to identify novel nematicidal agents against the pine wood nematode. Streptomyces sp. AN091965 showed the strongest nematicidal activity. One active compound, spectinabilin, was obtained by nematicidal asssy-directed fractionation, and it showed significant nematicidal activity against B. xylophilus, with an LC₅₀ value of 0.84 µg mL^-1 . Spectinabilin effectively suppressed the development of pine wilt disease in 5-year-old Pinus densiflora trees, even at 0.9 mg per tree under greenhouse conditions. Moreover, the acetone extract of the active strain's mycelia efficiently suppressed the development of pine wilt disease under field conditions.

CONCLUSION

To the best of our knowledge, this the first report to describe the nematicidal activity of spectinabilin against B. xylophilus. The cell extracts described herein merit further field studies as potential nematicides against the pine wood nematode. © 2018 Society of Chemical Industry.

Nematicidal and insecticidal activities of halogenated indoles

Parasite death via ion channel activations is the hallmark of anthelmintic and antiparasitic drugs. Glutamate gated chloride channel (GluCl) is a prominent targets for drug selection and design in parasitology. We report several iodine-fluorine based lead activators of GluCl by computational studies and structure-activity relationship analysis. 5-Fluoro-4-iodo-1H-pyrrolo [2, 3-b] pyridine and 5-iodoindole were bioactive hits that displayed in vitro anthelmintic and insecticidal activities against Bursaphelenchus xylophilus, Meloidogyne incognita, and Tenebrio molitor. Two important findings stood out: (i) 5F4IPP induced parasite death, and interacted proficiently with Gln²¹⁹ amino acid of pentameric GluCl in docking analysis, and (ii) 5-iodoindole appeared to act by forming giant vacuoles in nematodes, which led to a form of non-apoptotic death known as methuosis. The study suggests halogenated-indoles and 1H-pyrrolo [2, 3-b] pyridine derivatives be regarded potential biocides for plant-parasitic nematodes and insects, and warrants further research on the mode of actions, and field investigations.

Identification of Potential Nematicidal Compounds against the Pine Wood Nematode, Bursaphelenchus xylophilus through an In Silico Approach

Bursaphelenchus xylophilus is a destructive phytophagous nematode that mainly infects pine species and causes pine wilt disease (PWD). PWD is one of the most devastating diseases that has damaged the pine forests of eastern Asia and Portugal for the last four decades. B. xylophilus infects healthy pine trees through Monochamus beetles and its subsequent proliferation results in destruction of the infected pine trees. The poor water solubility and high cost of currently used trunk-injected chemicals such as avermectin and abamectin for the prevention of PWD are major concerns. Thus, for the identification of new compounds targeting the different targets, five proteins including cathepsin L-like cystein proteinase, peroxiredoxins, hsp90, venome allergen protein and tubulin that are known to be important for development and pathogenicity of B. xylophilus were selected. The compounds were virtually screened against five proposed targets through molecular docking into hypothetical binding sites located in a homology-built protein model. Of the fifteen nematicides screened, amocarzine, mebendazole and flubendazole were judged to bind best. For these best docked compounds, structural and electronic properties were calculated through density functional theory studies. The results emphasize that these compounds could be potential lead compounds that can be further developed into nematicidal chemical against B. xylophilus. However, further studies are required to ascertain the nematicidal activity of these compounds against phytophagous nematode.

Assessments of iodoindoles and abamectin as inducers of methuosis in pinewood nematode, Bursaphelenchus xylophilus

Bursaphelenchus xylophilus is a quarantined migratory endoparasite known to cause severe economic losses in pine forest ecosystems. The study presents the nematicidal effects of halogenated indoles on B. xylophilus and their action mechanisms. 5-Iodoindole and abamectin (positive control) at low concentration (10 µg/mL) presented similar and high nematicidal activities against B. xylophilus. 5-Iodoindole diminished fecundity, reproductive activities, embryonic and juvenile lethality and locomotor behaviors. Molecular interactions of ligands with invertebrate-specific glutamate gated chloride channel receptor reinforced the notion that 5-iodoindole, like abamectin, rigidly binds to the active sites of the receptor. 5-Iodoindole also induced diverse phenotypic deformities in nematodes including abnormal organ disruption/shrinkage and increased vacuolization. These findings suggest the prospective role of vacuoles in nematode death by methuosis. Importantly, 5-iodoindole was nontoxic to two plants, Brassica oleracea and Raphanus raphanistrum. Henceforth, the study warrants the application of iodoindoles in ecological environments to control the devastating pine destruction by B. xylophilus.

Genome-wide SNP analysis using 2b-RAD sequencing identifies the candidate genes putatively associated with resistance to ivermectin in Haemonchus contortus

Background

The excessive and uncontrolled use of anthelmintics, e.g. ivermectin (IVM) for the treatment of livestock parasites has led to widespread resistance in gastrointestinal nematodes, such as Haemonchus contortus. There is an urgent need for better management of drug-use in nematode control and development of novel anthelmintics. Discovery and identification of anthelmintic resistance-associate molecules/markers can provide a basis for rational anthelmintics-use and development of novel drugs. Recent studies have shown that ivermectin resistance in H. contortus is likely to be multi-genic in nature except for several genes coding for IVM target and efflux pump. However, no other IVM resistance-associated genes were characterized by conventional methods or strategies. In the present study we adopted a new strategy, i.e. using genome-wide single nucleotide polymorphism (SNP) analysis based on 2b-RAD sequencing, for discovering SNPs markers across the genomes in both IVM susceptible and resistant isolates of H. contortus and identifying potential IVM resistance-associated genes.

Results

We discovered 2962 and 2667 SNPs within both susceptible and resistant strains of H. contortus, respectively. A relative lower and similar genetic variations were observed within both resistant and susceptible strains (average π values were equal to 0.1883 and 0.1953, respectively); whereas a high genetic variation was found across both strains (average π value was equal to 0.3899). A significant differentiation across 2b-RAD tags nucleotide sites was also observed between the two strains (average F_ST value was equal to 0.3076); the larger differences in average F_ST were observed at SNPs loci between coding and noncoding (including intronic) regions. Comparison between resistant and susceptible strains revealed that 208 SNPs loci exhibited significantly elevated F_ST values, 24 SNPs of those loci were located in the CDS regions of the nine genes and were likely to have signature of IVM directional selection. Seven of the nine candidate genes were predicted to code for some functional proteins such as potential IVM target and/or efflux pump proteins, component proteins of receptor complex in membrane on neuromuscular cells, and transcriptional regulation proteins. Those genes might be involved in resistance to IVM.

Conclusions

Our data suggest that candidate genes putatively associated with resistance to IVM in H. contortus may be identified by genome-wide SNP analysis using 2b-RAD sequencing.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1959-6) contains supplementary material, which is available to authorized users.

Bursaphelenchus xylophilus is killed by homologues of 2-(1-undecyloxy)-1-ethanol

2-(1-Undecyloxy)-1-ethanol, monochamol, is a male-produced aggregation pheromone of the Monochamus species, which are efficient vectors of the pine wood nematode (PWN), Bursaphelenchus xylophilus, which cause devastating damage to pines worldwide. The nematicidal activity of synthetic monochamol and its homologues (ROEtOH: R = C7-C13) were investigated to find potential alternatives to the currently used PWN control agents abamectin and emamectin. Compounds with C7-C13 chain length alkyl groups exhibited 100% nematicidal activity at a concentration of 1000 mg/L. At a concentration of 100 mg/L, 2-(1-nonyloxy)-1-ethanol (C9OEtOH), 2-(1-decyloxy)-1-ethanol (C10OEtOH), 2-(1-undecyloxy)-1-ethanol (C11OEtOH), and 2-(1-dodecyloxy)-1-ethanol (C12OEtOH) showed 100% nematicidal activity, but the others showed weaker activities. C11OEtOH showed similar nematicidal activity to abamectin in terms of LD90 values, which were 13.30 and 12.53 mg/L, respectively. However, C9OEtOH, C10OEtOH, and C12OEtOH (LC90 values: 53.63, 38.18, and 46.68 mg/L, respectively) were less effective than C11OEtOH and abamectin. These results indicate that monochamol could be an effective alternative agent against PWN. The relationship of insecticidal and nematicidal activity to different carbon chain lengths in compounds is discussed.

Field efficacy of four anthelmintics and confirmation of drug-resistant nematodes by controlled efficacy test and pyrosequencing on a sheep and goat farm in Denmark

We describe a case of anthelmintic resistance on one of the largest organic small ruminant farms in Denmark. The flock was established in 2007 by purchase of animals from other Danish farms and had history of clinical parasitism, high mortality of young stock and anthelmintic treatment failure. In October 2011, 40 lambs and 40 kids were selected for a faecal egg count reduction test (FECRT) with fenbendazole (FBZ), ivermectin (IVM), moxidectin (MOX) and levamisole (LEV). Lambs were treated with the recommended sheep dose of each product while kids received the sheep dose of IVM, 1.5× sheep dose of MOX and 2× sheep dose of FBZ and LEV. Untreated lambs and kids were also included and three methods for calculating faecal egg count (FEC) reduction were compared. In a subsequent investigation, a controlled efficacy test (CET) with FBZ and IVM was performed in lambs infected with Haemonchus contortus and Trichostrongylus colubriformis isolated from adult goats on the farm. Recovered specimens of H. contortus were subjected to pyrosequencing for detection of single nucleotide polymorphisms (SNPs) related to benzimidazole (BZ) resistance. During the FECRT, FECs in untreated lambs dropped significantly by 47%. No FEC reduction was detected in untreated kids. After FBZ treatments, FEC reductions in lambs and kids ranged from 15 to 54% and 49-56%, respectively, according to the different calculation methods. Post IVM treatments, FEC reductions in lambs and kids varied between 71-90% and 81-83%, correspondingly. LEV and MOX reduced FECs by 98-100% in both species. In the CET, FBZ reduced H. contortus worm counts by 52-56% and no reduction in T. colubriformis counts were detected after treatment. IVM eliminated 100% of H. contortus and reduced T. colubriformis counts by 84-92%, according to different calculation methods. Pyrosequencing of isolated H. contortus revealed increased frequencies of the BZ resistance-related SNP in codon 200 of the β-tubulin isotype 1 gene. Frequency of BZ resistance-related SNPs in codons 167 and 198 were very low and did not exceed levels as obtained in the susceptible reference isolate. Anthelmintic resistance was confirmed in this recently established organic farm and low field efficacy of FBZ was verified by CET and pyrosequencing. BZ-resistant populations of H. contortus and T. colubriformis were isolated for the first time in Denmark. Problems with correct dosing of goats, the observed FEC reduction in untreated lambs and the relevance of including a control group in the FECRT are discussed.

The effect of vitamin E supplementation on an experimental Haemonchus contortus infection in lambs

The objective of this study was to investigate the effect of vitamin E supplementation on an experimental Haemonchus contortus infection in lambs. Twenty lambs were stratified into two treatment groups based on fecal egg count. Worm-free lambs, 28-32 weeks of age, were supplemented with vitamin E (d-α-tocopherol) for 12 weeks following the recommendations of the National Research Council for the minimum daily requirement (control; 5.3 IU/kg body weight (BW)/day (d), n=10) or the requirement for optimal immune function (VE10; 10 IU/kg BW/d, n=10). Five weeks following initiation of vitamin E supplementation, lambs were infected with 10,000 H. contortus third stage larvae. Samples were taken weekly to quantify serum α-tocopherol, serum total non-specific immunoglobulin (Ig)G, whole worm antigen specific IgG, packed cell volume (PCV), and fecal egg count (FEC). Expression of cytokine genes IFN-λ and IL-4 were measured in peripheral blood collected prior to slaughter. Lambs were necropsied six weeks after infection and the α-tocopherol concentration of liver, muscle and lymph node were measured as well as abomasal worm burden and histologic evaluation of the abomasum for inflammation and enumeration of eosinophils and globule leukocytes. The livers of VE10 lambs contained slightly more α-tocopherol than control lambs. No differences were observed in serum, muscle or lymph node α-tocopherol concentration, serum IgG or peripheral mRNA expression of IL-4 or IFN-λ between control and VE10 lambs. However, lambs supplemented at 10IU/kg BW/d had a lower PCV reduction, FEC and worm burden 49% less than control lambs. Worm burden was negatively correlated with eosinophil (-0.720, P<0.05) and globule leukocyte count (-0.867, P<0.05). Strong positive correlations were observed within the inflammatory cell response in VE10 lambs that was absent in control lambs. These data indicate that additional vitamin E supplementation resulted in lower worm burden and greater recruitment of innate effector cells to the site of infection. Further studies are necessary to elucidate the mechanism by which vitamin E affects greater recruitment of innate effector cells to the abomasum during gastrointestinal nematode infection of lambs.

Identification of variant ABC-transporter genes amongOnchocerca volvuluscollected from ivermectin-treated and untreated patients in Ghana, West Africa

Treatment with ivermectin (IVM) is known to cause a loss of polymorphism at certain loci of the beta-tubulin, gamma-aminobutyric-acid-receptor, glutamate-gated-chloride-channel and ATP-binding-cassette (ABC) transporter genes of IVM-resistant Haemonchus contortus. The genetic variation of four ABC-transporter homologues from Onchocerca volvulus was therefore investigated, to determine if any change in genetic polymorphism occurs in these genes following repeated treatment with IVM. Samples were collected in the Northern, Brong-Ahafo and Volta regions of Ghana, in 1999 and 2002; nodules containing adult O. volvulus were removed from subjects who had either received multiple IVM treatments or never taken IVM. The ATP-binding domains of four ABC-transporter genes (OvMDR-1, OvMDR-3, OvABC-1 and OvABC-6) were amplified from individual O. volvulus and examined for polymorphism, using single-strand-conformation-polymorphism (SSCP) analysis. In the samples collected in 1999, OvMDR-1 and OvABC-1 showed significant reduction in polymorphism following IVM treatment whereas OvABC-6 and OvMDR-3 were not found to be polymorphic. The samples collected in 2002 also showed a reduction in polymorphism for both OvMDR-1 and OvABC-1. Several single-nucleotide polymorphisms, which resulted in either amino-acid-replacement substitutions or nonsense mutations, were identified in the alleles of OvMDR-1 and OvABC-1.

Potential contribution of P-glycoproteins to macrocyclic lactone resistance in the cattle parasitic nematode Cooperia oncophora

Resistance against macrocyclic lactones such as ivermectin is widespread among parasitic gastrointestinal nematodes of small ruminants and is rapidly increasing in cattle parasites. ABC transporters of the subfamily B, the so-called P-glycoproteins (Pgps) have been frequently implicated in ivermectin resistance and are a major cause of multi-drug resistance in protozoa and helminths. The Pgp inhibitor verapamil (VPL) dramatically enhanced susceptibility of the cattle parasitic nematode Cooperia oncophora to ivermectin in vitro as measured in a larval developmental assay and a larval migration inhibition assay using third stage larvae. Moreover, VPL completely restored susceptibility to ivermectin in a resistant isolate resulting in virtually identical dose-response curves of susceptible and resistant isolates in the presence of VPL. Further characterisation of the molecular mechanisms resulting in Pgp-mediated ivermectin resistance is still hampered by the lack of molecular and biochemical information for Pgps of parasitic nematodes. Using PCR with degenerate primers, fragments of four different C. oncophora Pgps could be amplified and the Conpgp-2, previously implicated in macrocyclic lactone resistance in Haemonchus contortus, and Conpgp-3 full-length cDNAs were obtained by RACE PCR. The pgp sequences presented here contribute important data required to systematically screen resistant C. oncophora isolates for up- or down-regulation of Pgps and for the detection of single nucleotide polymorphisms in Pgps to detect selection of specific Pgp alleles by anthelmintics as early as possible.

Resistance to macrocyclic lactone anthelmintics byHaemonchus contortusandOstertagia circumcinctain sheep in New Zealand

AIM

To establish the efficacy of oral formulations of ivermectin and moxidectin against naturally acquired abomasal nematode infections on a North Island sheep farm.

METHODS

Two controlled slaughter trials were undertaken. In the first, 30 sheep on pasture were randomly allocated on the basis of faecal egg count to 1 of 3 groups, comprising an untreated control group and 2 treatment groups. One treatment group was given a single oral dose of ivermectin and the other a single oral dose of moxidectin, both at the manufacturer's recommended dose rates of 0.2 mg/kg liveweight. Six days after treatment, all animals were slaughtered and their abomasa recovered for worm counting. The second trial, which involved 47 animals, was essentially the same as the first except that, as well as involving the slaughter of 30 sheep from all 3 groups, 6 days after treatment, it also included a further 8 untreated control animals and 9 moxidectin treated animals which were slaughtered 27 days after treatment.

RESULTS

At 6 days after treatment, moxidectin was highly effective against all 3 of the abomasal nematodes present. While ivermectin was similarly effective against Trichostrongylus axei 6 days after treatment, it was not effective against either Ostertagia circumcinta or Haemonchus contortus, against which average efficacies of only 63.6% and 61.6%, respectively, were recorded. At 27 days after treatment, moxidectin, was also highly effective against T. axei (97.3% reduction) but not against either H. contortus (71.4% reduction) or O. circumcinta (61.0% reduction).

CONCLUSIONS

These results provide the first record of macrocyclic lactone resistance in H. contortus in sheep or in any other host in New Zealand, and the first case where such resistance has been exhibited in more than one parasite species at a time. Although the therapeutic efficacy of moxidectin was high against these resistant H. contortus and O. circumcincta strains, resistance to moxidectin was indicated by its diminished prophylactic activity against them. It is suggested that this reduction in the prophylactic activity of moxidectin is also likely to reduce its apparent current high therapeutic efficacy.

CLINICAL RELEVANCE

As well as providing further evidence that it can no longer be automatically assumed that macrocylic lactone anthelmintics will be effective on sheep farms in this country, these findings also present a warning that increasingly complex parasite control options may have to be faced in the future.

Effect of vitamin E supplementation on naturally acquired parasitic infection in lambs

Gastrointestinal nematode infections cause substantial economic losses in pasture-based sheep farming worldwide. Host nutritional status has been identified as a key component of immune function. While vitamin E supplementation is known to have broad-spectrum effects on immune function in livestock, to our knowledge, there are no reports on the effect of vitamin E supplementation on trichostrongylid parasite infections in lambs. This study evaluated the effect of parenteral vitamin E supplementation on naturally acquired parasite infection in lambs. Twenty-seven spring lambs were sequentially assigned to receive injections of vitamin E (15 or 30 IU D-α-tocopherol/kg body weight (BW) or placebo, every two weeks, from birth to 28 weeks of age. Blood was collected at weeks 0, 2, 4, 8, 12, 16, 20, 24, and 28 to determine serum α-tocopherol concentration. Once the youngest animal reached 15 weeks of age all lambs were dewormed and grazed together on a pasture known to be contaminated with trichostrongylid larvae. Fecal egg count and blood packed cell volume (%) were determined on each lamb immediately prior to deworming and for the first seven weeks of pasture infection. Lambs were euthanized when they reached 28 weeks of age for determination of parasite worm burdens. Vitamin E supplementation at 30 IU/kg BW increased serum α-tocopherol over that of placebo (P<0.001) however, there was no effect of vitamin E supplementation on liver (P=0.804) or muscle (P=0.16) α-tocopherol content. There was no effect of vitamin E supplementation on fecal egg counts, packed cell volume, worm burden or nematode species distribution. Nematode genera identified were Haemonchus (30%), Trichostrongylus (42%), Nematodirus (27%), Strongyloides sp. (<1%), and Aonchotheca sp. (<1%). These results indicate that biweekly injections of vitamin E at 15 and 30 IU d-α-tocopherol/kg BW, had no effect on parasitological parameters used in the study to assess gastrointestinal nematode infection.

Determination of the effective dose rate for monepantel (AAD 1566) against adult gastro-intestinal nematodes in sheep

Monepantel (AAD 1566) is the first compound from the recently discovered amino-acetonitrile derivative (AAD) class of anthelmintics to be developed for use in sheep. Three dose determination studies were conducted in Australia and Europe to identify the therapeutic dose of monepantel, when formulated for the oral treatment of sheep, to control adult gastro-intestinal nematodes. In each study, sheep infected with various nematode species were treated with either 1.25, 2.5 or 5.0mg monepantel/kg bodyweight. Following euthanasia and worm counting, their worm burdens were compared with those from untreated control groups. At a dose rate of 1.25mg/kg, monepantel showed efficacy above 91.9% against all major nematode species, with the exception of Chabertia ovina and Oesophagostomum venulosum. Efficacy against these two species was 93.6% and 94.0%, respectively, at a dose of 2.5mg/kg. At this dose, efficacy was above 99.2% against nine other nematode species including Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus spp. and Nematodirus spp. It was concluded that 2.5mg/kg would be a suitable dose rate for a commercial product.

Sensibilidade dos nematóides gastrintestinais de caprinos a anti-helmínticos na mesorregião do Sertão Paraibano

As helmintoses gastrintestinais ocupam um lugar de destaque na estatística de problemas sanitários da caprino-cultura moderna, criando continuamente novas dificuldades para o seu controle, sendo a resistência a drogas anti-helmínticas fator estratégico limitante para o seu controle. O presente estudo teve o objetivo de verificar a sensibilidade de nematóides gastrintestinais de caprinos a ação de compostos anti-helmínticos, convencionais e alternativos. Foram utilizados 120 animais, de ambos os sexos, sendo distribuídos em grupos de 24 animais cada (12 animais machos e 12 animais fêmeas), totalizando 10 grupos, submetidos, cada grupo ao tratamento com um composto anti-helmíntico específico. As drogas utilizadas nos ensaios foram a moxidectina 0,2%, albendazole, cloridrato de levamisol, ivermectina (drogas convencionais) e extrato aquoso de batata de purga (Operculina hamiltonii). Para avaliar a resistência, aplicou-se o teste de redução na contagem de ovos por grama de fezes (RCOF) e a larvacultura. As amostras fecais foram coletadas no dia em que foi realizada a medicação (dia base), aos 7, 14 e 21 dias após tratamento. Foram obtidos os seguintes resultados para a redução de ovos para a família Trichostrongyloidea, no tratamento de fêmeas com a moxidectina reduziu 92,8%, 88,7% e 89,8%; nos machos: 92,6%, 96,2% e 98,1%; com o levamisol as fêmeas reduziram 96%, 97,1% e 91%; nos machos: 85,7%, 94,2% e 100%; com o albendazol as fêmeas reduziram 65%, 60,3% e 75,4%; nos machos 88,8%, 88,8% e 55,5%; com a ivermectina reduziram 92,2%, 68,6% e 70,6%; nos machos 41,7%, 73,6% e 59,7%; com a batata de purga as fêmeas reduziram 31,8%, 34,1% e 49,4%, nos machos 61,5%, 80,7% e 50%. Na cultura de larvas o gênero Haemonchus, seguido de Bunostomum., Trichostrongylus e Oesophagostomum, foram identificados mesmo após os tratamentos.

The resistance of nematode parasites in sheep against anthelmintic drugs widely used in Western Turkey

This study was carried out in seven sheep herds infected by gastro-intestinal nematodes in the Afyonkarahisar district (Western Turkey) from June to July 2005. Sixty sheep in each herd were distributed into 3 groups as anthelmintic treatment groups along with a control group containing 15 animals. The anthelmintic groups were treated with albendazole (7.5 mg/kg BW), oxfendazole (7.5 mg/kg BW)-oxyclosanide (15 mg/kg BW) combination and ivermectin (0.2 mg/kg BW-injectable). The faecal samples were collected before treatment and after 10 days of treatment. Nematode egg counts per gram of faeces (EPG) were examined in the faecal samples using the McMaster technique. To identify the genus, the faecal samples were cultured. The faecal egg count reduction test (FECR) was employed to determine the resistance. In ivermectin treated groups, the FECR% values and the lower confidence limits were estimated as A: 68.57(14.77), B: 46.42(22.47), C: 84.41(35.38), D: 95.23(61.64), E: 97.14(76.94), F: 65.21(80.46) and G: 91.66(31.69), respectively. In the A, B, C, F and G herds, statistical analysis showed that the FECR% values were less then 95% and the estimated lower confidence limits were less then 90%. In all herds, albendazole treated, oxfendazole-oxyclosanide treated and control group, there were no eggs in the feacal samples. In these groups, the FECR% values were 100. In ivermectin treated groups, the third stage larvae from the genera Haemonchus, Oesophagostomum, Trichostrongylus, Cooperia, Nematodirus and Ostertagia were detected in coprocultures before treatment and the larvae from the genera Haemonchus and Oesophagostomum were detected in post-treatment coprocultures in the resistant herds. In conclusion, there was no evidence of resistance in sheep gastro-intestinal nematodes against albendazole and oxfendazole-oxyclosanide; in contrast, the resistance against ivermectin was detected in five sheep herds in the Afyonkarahisar district.

Avermectin-resistance in gastrointestinal nematodes of Boer goats and Dorper sheep in Switzerland

Anthelmintic resistance in gastrointestinal nematodes among small ruminants is widespread in South Africa and Dorper sheep and Boer goats have been imported into Switzerland from this country on a number of occasions. Therefore, this study aimed to investigate the occurrence of avermectin (AVM) resistant gastrointestinal nematodes (GIN) in these breeds in Switzerland. A total of 24 Boer goat farms and 12 Dorper sheep farms participated in the study. According to the faecal egg count reduction test (FECRT) AVM-resistant GIN populations were confirmed in 7 of the 24 Boer goat farms and suspected in a further 8 farms. Likewise AVM-resistance was confirmed in 2 of 12 Dorper sheep farms and suspected in a further 6 farms. Haemonchus contortus and Trichostrongylus spp. were the dominant resistant species according to larval cultures. In the farms with detected AVM-resistance the animals were additionally treated with levamisole after natural reinfection. With the exception of one farm with a 'close-to cutoff-result' the FECRT gave no indication for resistance against levamisole. The results indicate that AVM-resistance is widespread in Swiss small ruminant farms keeping Boer goats and Dorper sheep. The common tradition of grazing animals from different farms on prealpine and alpine pastures could favour the spread of resistant populations within the country.

Anthelmintic action of plant cysteine proteinases against the rodent stomach nematode, Protospirura muricola, in vitro and in vivo

Cysteine proteinases from the fruit and latex of plants, including papaya, pineapple and fig, were previously shown to have a rapid detrimental effect, in vitro, against the rodent gastrointestinal nematodes, Heligmosomoides polygyrus (which is found in the anterior small intestine) and Trichuris muris (which resides in the caecum). Proteinases in the crude latex of papaya also showed anthelmintic efficacy against both nematodes in vivo. In this paper, we describe the in vitro and in vivo effects of these plant extracts against the rodent nematode, Protospirura muricola, which is found in the stomach. As in earlier work, all the plant cysteine proteinases examined, with the exception of actinidain from the juice of kiwi fruit, caused rapid loss of motility and digestion of the cuticle, leading to death of the nematode in vitro. In vivo, in contrast to the efficacy against H. polygyrus and T. muris, papaya latex only showed efficacy against P. muricola adult female worms when the stomach acidity had been neutralized prior to administration of papaya latex. Therefore, collectively, our studies have demonstrated that, with the appropriate formulation, plant cysteine proteinases have efficacy against nematodes residing throughout the rodent gastrointestinal tract.

Efficacy of short-term feeding of sulla (Hedysarum coronarium) to young goats against a mixed burden of gastrointestinal nematodes

The aim was to investigate the efficacy of feeding sulla to young goats (<6 months of age) against a naturally acquired infection of gastrointestinal nematodes. Eleven goats were removed from the field, housed and were randomly divided into two treatment groups. Group 1 (n = 5) was fed a diet comprising lucerne-based pellets and aged hay. Group 2 (n = 6) were fed a diet of only sulla offered ad lib. The sulla contained 2.6% free condensed tannins (CT), 1.8% protein-bound CT and 0.1% fibre-bound CT. Goats in Gp 2 ate the sulla very readily from the first day. The goats were fed these diets for 10 days, then killed and total worm counts obtained. The arithmetic mean worm counts (in parentheses) for Gp 1 and Gp 2, respectively, were Teladorsagia circumcincta (5792, 5690), Trichostrongylus colubriformis (272, 262), T. vitrinus (1060, 1804) and Trichuris spp. (32, 30). There were no significant differences (p > 0.05) between any of these treatments. Short-term feeding of sulla had no effect on mixed burdens of gastrointestinal nematodes.

The detection of anthelmintic resistance in nematodes of veterinary importance

Before revised World Association for the Advancement of Veterinary Parasitology (WAAVP) guidelines on the detection of anthelmintic resistance can be produced, validation of modified and new methods is required in laboratories in different parts of the world. There is a great need for improved methods of detection of anthelmintic resistance particularly for the detection of macrocyclic lactone resistance and for the detection of resistant nematodes in cattle. Therefore, revised and new methods are provided here for the detection of anthelmintic resistance in nematodes of ruminants, horses and pigs as a basis for discussion and with the purpose that they are evaluated internationally to establish whether they could in the future be recommended by the WAAVP. The interpretation of the faecal egg count reduction test has been modified and suggestions given on its use with persistent anthelmintics and continuous release devices. An egg hatch test for benzimidazole (BZ) resistance is described. A microagar larval development test for the detection of benzimidazole and levamisole resistance provides third stage larvae for the identification of resistant worms. The sensitivity of these two tests can be increased by using discriminating doses rather than LD(50) values. Details are given of a PCR based test for the analysis of benzimidazole resistance in strongyles of sheep and goats, horses and cattle. Although promising for ruminant trichostrongyles, quantitative determination of gene frequency using real time PCR requires further development before PCR tests will be used in the field. Apart from faecal egg count reduction tests there are currently no satisfactory tests for macrocylic lactone resistance despite the great importance of this subject. Except for treatment and slaughter trials there are no validated tests for fasciolicide resistance or for the detection of resistance in cestodes.

Confirmation of resistance to ivermectin by Cooperia curticei in sheep

AIM

To investigate the occurrence of resistance to a full dose of oral ivermectin by Cooperia curticei in sheep.

METHODS

Twelve lambs on a sheep and cattle property in the North Island of New Zealand were randomly allocated to one of two equal-sized groups. One group was treated orally with a single dose of ivermectin at the manufacturer's recommended dose rate of 0.2 mg/kg, while the other remained as an untreated control. Worm counts were carried out post mortem on the abomasa and small intestines of all animals in both groups 7 days after treatment.

RESULTS

While treatment with ivermectin reduced the numbers of all other worm genera to almost zero, those of Ostertagia(= Telodorsagia) circumcincta and C. curticei were reduced by only 37% and 19%, respectively.

CONCLUSIONS

These results provide clear evidence of resistance to ivermectin by O. circumcincta and C. curticei. They also appear to represent the first record of macrocylic lactone (ML) resistance in C. curticei in sheep in New Zealand or elsewhere.

Resistance to moxidectin and abamectin in naturally acquired Ostertagia circumcincta infections in sheep

AIM

To investigate the occurrence of resistance to macrocyclic lactone (ML) anthelmintics by Ostertagia circumcincta in lambs on a sheep and cattle property in the North Island of New Zealand.

METHODS

Thirty lambs were randomly allocated to one of five equal-sized groups, consisting of an untreated control and four treatment groups. The treatments, which were administered at the manufacturer's recommended dose rates, included oral moxidectin, oral abamectin (both at 0.2 mg/kg), an albendazole-levamisole combination, and an albendazole-levamisole-ivermectin combination. Post mortem worm counts were undertaken 7 days after treatment to determine the efficacy of each anthelmintic.

RESULTS

The albendazole-levamisole and albendazole-levamisole-ivermectin combinations both reduced O. circumcincta burdens to zero whereas for moxidectin and abamectin efficacies of only 72% and 29%, respectively, were recorded.

CONCLUSIONS

These results clearly demonstrated the occurrence of resistance to MLs by O. circumcincta. Although this is not the first occasion where resistance to this anthelmintic family has been detected in this parasite in sheep in New Zealand, it is the first instance that resistance to either moxidectin or abamectin has been reported.

Multiple anthelmintic resistance in Haemonchus contortus isolated from South African Boer goats in Switzerland

A suspected case of multiple anthelmintic resistance on a farm in the canton of Zurich, Switzerland, into which South African Boer goats had previously been imported, was confirmed in a controlled test. Twenty sheep were allocated into one control group and three treatment groups to determine the efficacy of mebendazole, ivermectin and moxidectin applying the faecal egg count reduction test (FECRT). The sheep were slaughtered 1 week later and post-mortem worm counts were performed. Benzimidazole and ivermectin resistance were found in Haemonchus contortus with an efficacy of 55 and 61%, respectively. Moxidectin appeared to be effective when the data was analysed according to the recommended analytical techniques with an efficacy of 96%. This is the first description of resistance of gastrointestinal nematodes to one of the macrocyclic lactones in small ruminants in Switzerland. The results are discussed in relation to the importance of controlling livestock before importation.

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.

Drug resistance in nematodes: a paper tiger or a real problem?

PURPOSE OF REVIEW

The purpose of this review is to illustrate where drug resistance in parasitic nematodes has become a major problem. The mechanisms underlying anthelmintic resistance, the possible reasons for the development of anthelmintic resistance, and recommendations to minimize the further development of anthelmintic resistance in humans will be addressed.

RECENT FINDINGS

Resistance has developed to all drugs of the few anthelmintic classes currently available. Drug resistance has become a major threat to sheep production in Australia, New Zealand, Great Britain and South Africa. In addition, the treatment of nematode infections in horses has become insufficient because of anthelmintic resistance, whereas resistance in cattle has recently been found in New Zealand and southern America. In contrast, anthelmintic resistance to human helminths has only been reported anecdotally. The rate of development of anthelmintic resistance is determined by the resistance gene frequency and by selection pressure, as illustrated by the concept of 'worms in refugia' in ovine nematode infections.

SUMMARY

Although anthelmintic resistance has become a major constraint in livestock production of small ruminants, drug resistance is currently not a major issue in the treatment of human nematode infections. However, if recent community treatment programmes are pursued injudiciously, the inevitable evolutionary consequence of anthelmintic resistance in humans might emerge sooner rather than later.

Evaluation of a larval development assay (DrenchRite®) for the detection of anthelmintic resistance in cyathostomin nematodes of horses

A larval development assay (LDA, DrenchRite) was evaluated to determine the effectiveness of this method in detecting anthelmintic resistance in cyathostomin nematodes of horses. A total of 15 horse farms from Georgia and South Carolina (USA) and Population S ponies from the University of Kentucky (USA) were included in this study. Nematode eggs were extracted from pooled fecal samples and placed into the wells of a DrenchRite plate for testing against thiabendazole (TBZ), levamisole (LEV) and 2 ivermectin (IVM) analogs (IVM-1, IVM-2). After a 7-day incubation larvae in each well were counted and data were analyzed by logistic regression. Resistance status of each farm for different drugs was determined in a separate study using a fecal egg count reduction test. LDA were performed on the 15 farms once, however, the Population S cyathostomins were assayed on 3 separate occasions to estimate the consistency of results between assays. Mean TBZ LC50 for oxibendazole resistant, suspected resistant and sensitive farms were 0.2015, 0.1625, and 0.1355 microM, respectively. For LEV, mean LC50 for PYR resistant, suspected resistant and sensitive farms were 1.590, 1.8018 and 1.4219 microM, respectively. All 15 farms had worms susceptible to IVM; mean LC50 for IVM-1 and for IVM-2 were 7.5727 and 87.9718 nM, respectively. A linear mixed model was fitted to the data to determine the relationship between LC50 and LC95 and resistance status for each farm. No meaningful relations were found. Consistency of assays varied between drugs, being best for TBZ and worst for LEV and IVM-1. All farms in this study had benzimidazole-resistant nematodes; therefore usefulness of DrenchRite for discriminating susceptibility versus resistance to this drug class could not be accurately assessed. Moreover, since all farms tested were sensitive to IVM and resistance to this drug class has not yet been reported in cyathostomins, it is not possible to assess accurately the usefulness of DrenchRite LDA for detecting IVM resistance at this time. Assay results for LEV suggest that LEV in a LDA does not yield data that is useful in estimating PYR efficacy in vivo. Based on results for PYR/LEV, the current high prevalence of benzimidazole resistance, no known cases of IVM resistance, and the sometimes extreme variation in results seen in many of the assays, DrenchRite LDA cannot be considered a useful tool for the diagnosis of resistance in cyathostomins of horses at present.

Efficacy of abamectin against ivermectin-resistant strain of Trichostrongylus colubriformis in sheep

The efficacy of two formulations of abamectin, i.e. oral and injectable was determined against ivermectin-resistant strain of T. colubriformis in sheep. Twenty-four lambs were infected with 10,000 third stage larvae of ivermectin-resistant strain of T. colubriformis. Twenty-four days post-infection, the lambs were divided randomly into four groups of six animals each according to egg counts. The first group was left untreated and kept as a control. The second group was treated with ivermectin (oral) at 0.2mg kg(-1) body weight. The third group was treated with oral formulation of abamectin at 0.2mg kg(-1) body weight. The fourth group was treated with injectable formulation of abamectin at 0.2mg kg(-1) body weight. Fecal egg count and controlled slaughter tests were employed to determine the efficacy of abamectin (oral and injection) against ivermectin-resistant strain of T. colubriformis in sheep. Reduction in arithmetic mean fecal egg counts achieved by ivermectin (oral), abamectin (oral) and abamectin (injection) was 66, 98 and 76%, respectively 10 days after treatment. Ivermectin (oral), abamectin (oral) and abamectin (injection) reduced arithmetic mean worm burden by 63, 97 and 74%, respectively. The findings demonstrated that abamectin oral formulation was more effective than abamectin injection against ivermectin-resistant strain of T. colubriformis in sheep.

Performance of hair sheep composite breeds: Resistance of lambs to Haemonchus contortus1

This study was designed to evaluate breed differences in resistance to Haemonchus contortus in lambs. A total of 181 ewe lambs representing crossbred Dorsets (DO) and Dorpers (DP; out of 1/2-Dorset, 1/4-Rambouillet, 1/4-Finnsheep ewes) and straight-bred Katahdins (KT) were evaluated over 3 yr. An additional 144 DO, DP, KT, and Barbados Blackbelly x St. Croix (HH) wethers were evaluated over 2 yr. Lambs were weaned at 60 to 90 d of age. After deworming at about 4 mo of age, ewe lambs received approximately 10,000 infective larvae and were evaluated for parasite resistance in drylot, whereas wethers were evaluated on pasture under conditions of natural infection. Each sex was analyzed separately. Egg counts per gram of feces (FEC), log-transformed FEC (LFEC), packed cell volumes (%), and body weights (kg) measured at 3, 4, 5, and 6 wk after deworming and reinfection were analyzed. Breed influenced all traits (P < 0.05) except BW in ewe lambs and PCV in wethers. Year and week influenced (P < 0.05) all traits. At most times, DP had the highest FEC, DO had the lowest PCV, and KT and HH had lower FEC and higher PCV than either DO or DP. Clearly, Dorper sheep were not more resistant to parasites than DO, but they were able to cope with infection better by maintaining similar or higher (P < 0.05) PCV and similar BW. Katahdin and HH were more resistant, with lower FEC (P < 0.05) than DO or DP. Breed differences were more apparent when infection levels were higher; DO and DP were less affected when infection levels were low and when animals were on a better plane of nutrition. Caribbean hair sheep originated in hot, humid regions of West Africa. They and the derivative Katahdin breed seem relatively resistant to parasitism. The Dorper, in contrast, was developed in more arid regions and exhibited little parasite resistance.

The comparative serum disposition kinetics of subcutaneous administration of doramectin, ivermectin and moxidectin in the Australian Merino sheep

This study evaluates the comparative serum disposition kinetics of injectable formulations of doramectin (DRM), ivermectin (IVM) and moxidectin (MXD) in Australian Merino sheep. Thirty-six, 2-year-old sheep were allocated by weight into six groups of six animals. Animals in each group received 200 microg/kg of DRM, MXD, IVM or a combination of two of these drugs by subcutaneous (s.c.) injection. Blood was collected at designated intervals (between 1 h and 40 days after treatment) and the serum analysed by high performance liquid chromatography (HPLC) using fluorescence detection. The results indicated that MXD administration produced a significantly higher maximum serum concentration and a more rapid absorption as compared with DRM and IVM. MXD and DRM had a significantly larger area under the concentration vs. time curve (AUC) than IVM, suggesting a more persistent effect for the former two products in sheep. The AUC for DRM was significantly higher when administered alone as compared with that observed when given in combination with MXD or IVM, suggesting preferential elimination of DRM compared with IVM and MXD from concurrent s.c. administration.

Resistance to prophylactic treatment with macrocyclic lactone anthelmintics in Teladorsagia circumcincta

Sixty-four Romney ewe lambs were allocated to 12 groups on the basis of liveweight. Four groups (n=5) were administered oral ivermectin (IVM), 4 (n=6) oral moxidectin (MOX) and the remaining 4 (n=5) controlled-release capsules containing IVM (IVM-CRCs). Nine and 10 days later, the groups within each treatment type were challenged with infective-stage larvae (L3) of 1 of 4 different isolates of Teladorsagia circumcincta (two doses each of 5000 L3). The first of these (S) was known to be anthelmintic-susceptible; the second (OR) was a multiple anthelmintic-resistant strain recovered from the field following therapeutic failure of both ivermectin and moxidectin and subsequently maintained in the laboratory without further anthelmintic selection; the third (R) was derived from OR but had been passaged for five generations indoors with each generation being screened with all three broad-spectrum anthelmintic classes; and the fourth (RxS) was an F1 cross between the R and S isolates. As anticipated, because of its limited residual activity, IVM had no significant effect on the establishment, 9 and 10 days post-treatment, of any of the parasite isolates. In contrast MOX, which has greater residual activity, was highly effective at preventing the establishment of the S isolate but showed no significant effect against the OR, R or RxS isolates. The IVM-CRC was also highly effective at preventing the establishment of the S isolate and furthermore it significantly reduced establishment of both the OR and RxS isolates, although it had no significant effect against the R isolate. The results suggest that with respect to the establishment of T. circumcincta L3s following anthelmintic treatment, macrocyclic lactone (ML) resistance is effectively a dominant trait in the presence of MOX, while it behaves as a partially dominant/recessive trait under treatment with IVM-CRCs. The potential implications of this finding in relation to selection for ML resistance in T. circumcincta are discussed.

Efficacy of eprinomectin pour-on against gastrointestinal nematode infections in sheep

Two field trials were conducted in two farms (farms A and B) in southern Italy, to assess the efficacy of eprinomectin applied topically at the dose rate of 500 micro g/kg to sheep with naturally occurring infections of gastrointestinal nematodes (GIN). The nematode population determined by necropsy consisted of Teladorsagia circumcincta, Haemonchus contortus, Trichostrongylus vitrinus, T. capricola, Nematodirus sp., and Chabertia ovina in sheep from farm A, and of T. circumcincta, T. vitrinus, T. capricola, T. colubriformis, and C. ovina in sheep from farm B. In each farm, 42 female sheep were assigned to a eprinomectin treated group (E-group) and a control untreated group (C-group) of 21 animals each. On farm A, the percentage reductions in strongyle faecal egg counts from E-group compared to C-group were 99.1% on day 10; 97.4% on day 30; and 67.0% on day 60. On farm B, on the same days, they were 95.4, 84.9, and 69.4%, respectively. In the course of the two trials, eprinomectin was well tolerated by all the animals with no adverse reactions following the topical treatment.

Nematodes and liver fluke in New Zealand

Abstract A general review of the epidemiology, significance and control of nematode parasitism of sheep, goats, cattle and deer in New Zealand, the emergence of anthelmintic resistance and its effects, and the search for parasite control strategies that reduce reliance on anthelmintic use, is provided. The research that has formed the basis for present levels of understanding of this complex and important topic is summarised and sources of further information are indicated. Aspects of nematode infections of horses, pigs, dogs and cats, and the history of the liver fluke, Fasciola hepatica, in New Zealand are also reviewed briefly.

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.

Macrofilaricides and onchocerciasis control, mathematical modelling of the prospects for elimination

BACKGROUND

In most endemic parts of the world, onchocerciasis (river blindness) control relies, or will soon rely, exclusively on mass treatment with the microfilaricide ivermectin. Worldwide eradication of the parasite by means of this drug is unlikely. Macrofilaricidal drugs are currently being developed for human use.

METHODS

We used ONCHOSIM, a microsimulation mathematical model of the dynamics of onchocerciasis transmission, to explore the potentials of a hypothetical macrofilaricidal drug for the elimination of onchocerciasis under different epidemiological conditions, as characterized by previous intervention strategies, vectorial capacity and levels of coverage.

RESULTS

With a high vector biting rate and poor coverage, a very effective macrofilaricide would appear to have a substantially higher potential for achieving elimination of the parasite than does ivermectin.

CONCLUSIONS

Macrofilaricides have a substantially higher potential for achieving onchocerciasis elimination than ivermectin, but high coverage levels are still key. When these drugs become available, onchocerciasis elimination strategies should be reconsidered. In view of the impact of control efforts preceding the introduction of macrofilaricides on the success of elimination, it is important to sustain current control efforts.

The difference in efficacy of ivermectin oral, moxidectin oral and moxidectin injectable formulations against an ivermectin-resistant strain ofTrichostrongylus colubriformisin sheep

AIM

To evaluate the efficacy of ivermectin oral, moxidectin oral and moxidectin injectable formulations against an ivermectin-resistant strain of Trichostrongylus colubriformis in sheep.

METHODS

Twenty-four mixed breed lambs were infected with 15,000 infective third-stage larvae of an ivermectin-resistant strain of T. colubriformis which had originally been isolated from a goat farm in Northland in 1997. Twenty-six days post infection, the lambs were divided into 3 treatment groups and a control group (n=6 lambs/group). Treatment consisted of either ivermectin oral formulation (0.2 mg/kg), moxidectin oral formulation (0.2 mg/kg), or moxidectin injectable formulation (0.2 mg/kg). Faecal egg counts (FECs) were determined at 0, 3, 5, 7 and 10 days after treatment. All animals were necropsied 12 days after treatment and worm counts were performed. Larval development assays were conducted 24 days post infection. A further 3 lambs were infected with 15,000 infective third-stage larvae of a fully susceptible strain of T. colubriformis for comparative purposes in the larval development assay. The efficacy of the moxidectin injectable formulation was also confirmed in these 3 lambs.

RESULTS

The FEC reduction test at day 10 after treatment revealed 62%, 100% and 0% reductions in arithmetic-mean FECs for ivermectin oral, moxidectin oral and moxidectin injectable groups, respectively. The ivermectin oral, moxidectin oral and moxidectin injectable formulations achieved 62%, 98% and 4% reductions in arithmetic-mean worm burdens, respectively. Larval development assays showed resistance ratios for ivermectin of 4:1, avermectin B2 of 2.7:1, ivermectin aglycone of 37:1, moxidectin of 1.4:1, thiabendazole of 14.6:1 and levamisole of 1.8:1.

CONCLUSIONS

The moxidectin oral formulation provided a high degree of control against ivermectin-resistant T. colubriformis whereas the moxidectin injectable formulation had very low efficacy. Ivermectin aglycone was the analogue of choice for diagnosis of ivermectin resistance in T. colubriformis in the larval development assay.

Ivermectin-resistantOstertagia circumcinctafrom sheep in the lower North Island and their susceptibility to other macrocyclic lactone anthelmintics

AIM

To confirm the ivermectin resistance status of a strain of Ostertagia circumcincta which was isolated from a sheep farm in the lower North Island of New Zealand and to assess the susceptibility of this strain to other macrocycliclactone anthelmintics.

METHODS

Twenty-five lambs housed indoors were each infected with 12,000 L3 larvae of the above parasite strain. Approximately 3 weeks after infection the lambs were allocated to 1 of 4 treatment groups (3 groups of 6, and 1 group of 7 lambs), one of which remained untreated while the others were drenched orally with ivermectin, moxidectin or abamectin at 0.2 mg/kg liveweight. Faecal egg counts (FECs) before and after treatment, and post-mortem worm burdens 10 days after treatment were examined to assess efficacies of each anthelmintic.

RESULTS

Treatment with ivermectin reduced the mean FEC by only 18% and the mean worm burden by only 42%, whereas moxidectin and abamectin reduced FECs by 92% and worm burdens by 95%.

CONCLUSION

These results, together with a similar case described recently from the South Island , confirm the emergence of ivermectin resistance in nematode parasites of sheep in New Zealand. The superior efficacy of moxidectin and abamectin in this case indicates that, following the emergence of resistance to ivermectin, some short-term practical use may still be made of these other anthelmintics. However, their continued use will undoubtedly result in increased levels of resistance and eventual therapeutic failure of these products also.