The effect of oxfendazole terminated infections with Haemonchus contortus on the development of immunity in sheep

The relative contribution of third (L3), fourth (L4) or adult stages of Haemonchus contortus to the development of immunity was evaluated in three groups of sheep subjected to infections terminated by oxfendazole treatments at the L3, L4 or adult stage. A control group did not receive immunising infections. All the groups were challenged with 5000 L3, to evaluate the protection provided by the different protocols. All sheep were necropsied at the end of the experiment to count the abomasal worm burdens. A marked reduction in egg counts after challenge infection was only observed in sheep in which the infection was terminated in the adult stage (Group 4). A significant reduction in worm burden was also observed in Group 4. The immunising infections and/or the challenge infection resulted in moderately elevated IgG antibody levels against L3, L4 and adult somatic antigens in all the groups. In contrast, a strong IgG response against H. contortus excretory/secretory (ES) antigens was observed in the groups in which the immunising infection was terminated in the L4 and the adult stage. An elevated lymphocyte proliferation response against Haemonchus ES antigens was found only in the group that had their immunising infection terminated at the adult stage. The combined data suggest that exposure to and elicited immunological responses to ES antigens are important for the development of immunity against H. contortus.

Multiple lethal effects induced by a benzimidazole anthelmintic in the anterior intestine of the nematode Haemonchus contortus

A mechanism of benzimidazole efficacy against parasitic nematodes is postulated to involve inhibition of intestinal secretory vesicle transport via depolymerization of microtubules. We show that fenbendazole (FBZ) treatment of lambs causes pathology localized to the anterior intestine in the parasitic nematode Haemonchus contortus. The pathology included gross disintegration of the anterior intestine, DNA fragmentation in anterior intestinal nuclei with characteristics of an apoptosis-like process, and inhibition of host erythrocyte digestion. These lethal effects were associated with inhibited transport of apical secretory vesicles in the anterior intestine, and then generalized dispersal of these vesicles contents throughout the intestinal cytoplasm and worm body. Cytoplasmic accumulation of secretory vesicles and undigested erythrocytes preceded DNA fragmentation and vesicle content dispersal. Both DNA fragmentation and vesicle content dispersal were detected in disintegrated intestine and intestine that had not yet undergone disintegration. These pathologic effects in the anterior intestine appear sufficient to explain the efficacy of FBZ against adult H. contortus. The results implicate mechanisms in the anterior intestine that govern dispersal of apical secretory vesicle contents, DNA fragmentation and tissue disintegration as effectors of this pathology.

Multiple anthelmintic resistance in a goat herd

Anthelmintic resistance was monitored over a 30 month period within a goat herd in eastern Virginia, USA. Resistance to ivermectin, levamisole and benzimidazole drugs was detected in Haemonchus contortus using the fecal egg count reduction test (FECRT). When levamisole use was discontinued for 1 year, susceptibility to levamisole appeared to return. Although a single treatment with fenbendazole was able to reduce fecal egg counts by only 50%, two doses administered in a 12 h interval increased efficacy to 92%, however, confidence intervals indicated that resistance was still present. When fecal egg counts were determined the following year after several treatment using this protocol, the efficacy of fenbendazole had fallen again to 57% reduction in fecal egg counts. The predominant genus present in cultured composite fecal samples was Haemonchus. Trichostrongylus, Cooperia and Teladorsagia were also present in smaller numbers.

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

Surveys to determine the prevalence and degree of resistance of Haemonchus spp. of sheep and goats to the available anthelmintics in South Africa indicate that small ruminant production is entering a crisis situation. Three surveys employing the faecal egg count reduction (FECR) test to determine resistance were conducted in some of the main sheep-producing areas in the summer rainfall region of South Africa, where H. contortus is the principal worm species in sheep. After analyzing the data recorded in the surveys by six different methods, including the RESO test at two different levels of confidence, the results obtained in the least stringent one (geometric mean reduction of the worm egg counts of drenched, vs untreated group of sheep) are reported in this paper, so that if any bias was obtained it would be in the favour of the anthelmintic. In Mpumalanga and KwaZulu-Natal there was anthelmintic resistance in Haemonchus spp. on all the 52 farms surveyed. Sixteen percent of the strains of H. contortus were < 60% susceptible to three of the four anthelmintics tested, and 8% of the strains were < 40% susceptible to all four of the anthelmintics. FECR tests of sheep in six localities in the Lebowa district of Northern Province indicated that even in previously disadvantaged communities where anthelmintic treatment is less intensive, anthelmintic resistance is developing, and is possibly at the level at which the situation on commercial sheep and goat farms in South Africa was 25 years ago. From the data it appears that the level of anthelmintic resistance of H. contortus in South Africa is possibly the highest that has so far been recorded in the world and that strains of it are emerging that may soon not be controllable by treatment with any of the existing anthelmintics. Farmers in the summer rainfall region, if not the whole country, must be alerted to the immediate need for testing the parasite burdens of their sheep for susceptibility to preparations in all four groups of anthelmintic compounds currently available. Alternative methods of integrated worm control, including biological, must be sought and implemented with urgency, to reduce further selection for resistance and to induce reversion of the resistance that has already developed.

Effects of the multidrug-resistance-reversing agents verapamil and CL 347,099 on the efficacy of ivermectin or moxidectin against unselected and drug-selected strains of Haemonchus contortus in jirds (Meriones unguiculatus)

The development of anthelmintic resistance is making parasite control in small ruminants problematic. Following the discovery that the drug transporter P-glycoprotein may be involved in macrocyclic lactone resistance in Haemonchus contortus, we determined the effect of two multidrug-resistance modulators, verapamil and CL347,099, on the efficacy of ivermectin and moxidectin against unselected and drug-selected strains of H. contortus. CL347,099 is an analog of verapamil that has multidrug-resistance properties but weaker calcium-channel-blocking activity than the parent drug. The combinations of verapamil with either ivermectin or moxidectin significantly reduced worm counts of the selected strains as compared with the untreated controls, whereas ivermectin or moxidectin alone did not significantly reduce worm counts as compared with the untreated controls. The CL347,099 plus moxidectin combination was significantly more efficacious than moxidectin alone against the ivermectin-selected strain. The drug-combination regimes were without adverse effect on the jirds. However, higher levels of verapamil (> or =40 mg/kg) produced some toxicity.

Decreased ivermectin and moxidectin sensitivity in Haemonchus contortus selected with moxidectin over 14 generations

Ivermectin resistance in the nematode Haemonchus contortus has been reported in many parts of the world and many ivermectin resistant isolates have been found to have reduced sensitivity to moxidectin. However, it is unclear whether parasites that are selected with moxidectin would demonstrate reduced sensitivity to ivermectin. In this study, the effects of moxidectin and ivermectin on an unselected strain and a strain of H. contortus derived from the unselected strain but selected over 14 generations with moxidectin, were compared in jirds. The recovery of adult worms and fourth stage (L4) larvae following treatment were compared between strains and anthelmintics. Moxidectin-selected H. contortus showed reduced sensitivity to ivermectin as well as to moxidectin. Doses of 0.1 mg/kg of moxidectin and 0.4 mg/kg of ivermectin were necessary to obtain an efficacy of 95% or above against the moxidectin-selected strain of H. contortus compared with 0.025 mg/kg for moxidectin and 0.1 mg/kg for ivermectin required for a similar efficacy in the unselected strain.

Selection for anthelmintic resistance by macrocyclic lactones in Haemonchus contortus

Two morphologically marked strains of Haemonchus contortus, CAVRS (smooth-macrocyclic lactone resistant) and McMaster (linguiform-macrocyclic lactone susceptible), were used to investigate the selection for anthelmintic resistance following exposure to ivermectin (IVM), a non-persistent anthelmintic. and a more persistent anthelmintic, oral moxidectin (MOX). Three types of selection were investigated: (1) selection of resident worms at the time of treatment (Head selection); (2) selection of incoming-larvae post-treatment (Tail selection); and (3) selection of both resident population and incoming larvae (Head + Tail selection). The experimental animals were adult sheep and lambs. In the controls where there was no anthelmintic selection, the proportion of CAVRS in the adult worm population was the same as the proportion in larvae given to both adults and lambs indicating that CAVRS and McMaster H. contortus were equally infective. There was a significant effect of anthelmintic on total worm numbers in adult sheep with MOX treated adults having less worms, but selection type was non-significant. Anthelmintic type had a significant effect on numbers of resistant worms in adult sheep with less resistant worms in the MOX treated groups, but selection type had no effect. Analysis of variance of arcsine-transformed proportions of resistant worms found that the type of anthelmintic had a highly significant effect, with MOX treated adults having a higher proportion of resistant worms, while type of selection was not significant. In the lambs, nil treated controls and IVM Head + Tail and Tail selected groups had similar geometric mean total worm burdens while Head selected had less total worms. In the MOX treated lamb groups the worm burdens were similar within selection type but less than the IVM treated groups. In the lambs, the types of selection that resulted in more resistant worms were IVM Tail, MOX Head + Tail and MOX Tail. Resistant worm numbers were similar in both adult and lamb groups with Head selection by either MOX or IVM. Moxidectin selected out higher proportions of resistant worms than did IVM in the lambs, with Tail and Head + Tail being stronger selectors than Head. Computer simulations were used to estimate the rate at which resistance developed in the field using the information generated in the present study. The anthelmintic treatments used in the simulation followed a strategic parasite control program for H. contortus in which all sheep receive three Closantel (CLS) treatments in summer. all sheep receive a broad-spectrum (BS) drench or capsule at weaning and lambs receive an additional two BS drenches insummer or no further treatment in the case of the capsule. Moxidectin, IVM-capsule and IVM were the broad spectrum anthelmintics simulated. All simulations were run four times assuming high or low efficacy against resident resistant worms and in the presence or absence of CLS resistance. The simulations indicated that the presence of CLS resistance hastened selection for macrocyclic lactone (ML) resistance. While the IVM-capsule will select most rapidly for ML resistance, IVM oral is expected to be least selective. Moxidectin treatment is intermediate, except in simulations with no CLS resistance and when MOX is assumed to be highly effective against resident ML-resistant worms, in which case MOX can be expected to select more slowly than IVM oral treatments.

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.

Pharmacology of anthelmintic resistance

Anthelmintic resistance has grown from a curiosity to an important economic problem in several animal industries and is now set to threaten the control of human parasites. The pharmacology of anthelmintics and anthelmintic resistance has been studied most extensively in the nematode parasites of sheep. Here, Nick Sangster and Jenny Gill discuss this veterinary experience, summarizing the progress made in understanding anthelmintic resistance and highlighting the tools available for research.

Further studies on anthelmintic resistance in sheep at an organised farm in arid region of Rajasthan

Faecal egg count reduction test (FECRT) and in vitro egg hatch assay (EHA) was performed on Indian Karakul (3/4 crosses of Karakul with Malpura, Marwari and Sonadi) transported from arid region campus, Bikaner, where resistance was reported in 1996. FECRT revealed that fenbendazole and levamisole were 100% effective in reducing the egg counts. LC50 value on EHA was 0.074 +/- 0.015 microg thiabendazole ml(-1). The faecal culture examination revealed the presence of Haemonchus contortus only. It was concluded that H. contortus was fully susceptible to both benzimidazole and levamisole.

Influence of ruminal bypass on the pharmacokinetics and efficacy of benzimidazole anthelmintics in sheep

Oxfendazole, fenbendazole and albendazole were each administered at 5mgkg(-1) to sheep fitted with abomasal cannulae as a single bolus intra-ruminally or infused intra-abomasally at a declining exponential rate, with half-life equivalent to the rate of rumen fluid outflow. The pharmacokinetic disposition of parent compound and metabolites in plasma and abomasal fluid was determined by high performance liquid chromatography. Compared with intra-ruminal administration, intra-abomasal infusion of fenbendazole lowered the area under the concentration-time curve of drug in both plasma and abomasal fluid; intra-abomasal infusion of albendazole substantially increased maximum drug concentration and the concentration-time curve in abomasal fluid and lowered the plasma concentration time curve of the sulphoxide metabolite; intra-abomasal infusion of oxfendazole increased maximum concentration and the concentration-time curve of drug in plasma and abomasal fluid. The greater availability in abomasal fluid of oxfendazole and albendazole when given at commercial dose rates of 5 mg kg(-1) and 3.9 mg kg(-1), respectively, by intra-abomasal infusion correlated with increased efficacy of both drugs against benzimidazole-resistant Trichostrongylus colubriformis and of albendazole against benzimidazole-resistant Haemonchus contortus over that achieved by intra-ruminal administration as a single bolus.

Comparison of six in vitro tests in determining benzimidazole and levamisole resistance in Haemonchus contortus and Ostertagia circumcincta of sheep

Six in vitro methods for the detection of anthelmintic resistance were compared using benzimidazole/levamisole-resistant Haemonchus contortus and benzimidazole/levamisole/ivermectin-resistant Ostertagia circumcincta as well as susceptible strains of both parasite species. The degree of resistance to thiabendazole and levamisole was compared by (1) an egg hatch assay, (2) an egg hatch paralysis assay, (3) a larval development assay, (4) a larval paralysis assay (5) a larval paralysis assay with physostigmine and (6) larval micromotility assay. The degree of resistance for each assay was expressed as resistance factor--RF. For the detection of thiabendazole and levamisole resistance, the larval development test was observed as the most sensitive to measure quantitatively a degree of resistance between susceptible and resistant strains. For this test the RF for thiabendazole and levamisole was 14.3 and >32.5, respectively in H. contortus strains and 21.1 and 3.5 in strains of O. circumcincta. Egg hatch assay was also found to be sensitive and accurate in determining of resistance to benzimidazole. For measurement of levamisole resistance the egg hatch paralysis assay and larval paralysis assay were found to be able to distinguish between strains, but some disadvantages of these techniques make it unsuitable for field detection of levamisole resistance. Other in vitro assays as larval paralysis assay with physostigmine and larval micromotility assay were also investigated. Significant differences in paralysis of the larvae were observed using larval paralysis assay.

PF1022A--a novel anthelmintic cyclooctadepsipeptide. Modification and exchange of the N-methyl leucine residues

The first structure-activity relationships of the anthelmintic cyclooctadepsipeptide PF1022A have been established via a systematic exchange of the leucine residues by a series of related N-alkylated amino acids. The data presented strongly suggest that (L)-N-methyl-leucine is crucial for high in vivo activity.

Synthesis and endectocidal activity of novel 1-(arylsulfonyl)-1-[(trifluoromethyl)sulfonyl]methane derivatives

We have recently synthesized a series of novel disulfonylmethane compounds that have shown anthelmintic and insecticidal (endectocidal) activity. Several analogues have shown activity against the internal nematode Haemonchus contortus. In sheep studies, these analogues have shown 100% control of this internal parasite at a 10 mg/kg rate. In vitro activity against the biting flies, Stomoxys calcitrans and Haematobia irritans, has been observed at rates as low as 25 and 2.3 ppm, respectively. Only marginal activity against the liver fluke Fasciola hepatica and Trichostrongylus colubriformis was seen. Respiratory control index values on rat liver mitochondria for this series suggested uncoupling of oxidative phosphorylation as a mechanism of action. Compound 1 is considered to be a promising agent for treatment of parasitized sheep.

Ivermectin resistance in nematodes may be caused by alteration of P-glycoprotein homolog

Resistance to ivermectin and related drugs is an increasing problem for parasite control. The mechanism of ivermectin resistance in nematode parasites is currently unknown. Some P-glycoproteins and multidrug resistance proteins have been found to act as membrane transporters which pump drugs from the cell. A disruption of the mdrla gene, which encodes a P-glycoprotein in mice, results in hypersensitivity to ivermectin. Genes encoding members of the P-glycoprotein family are known to exist in nematodes but the involvement of P-glycoprotein in nematode ivermectin-resistance has not been described. Our data suggest that a P-glycoprotein may play a role in ivermectin resistance in the sheep nematode parasite Haemonchus contortus. A full length P-glycoprotein cDNA from H. contortus has been cloned and sequenced. Analysis of the sequence showed 61-65% homology to other P-glycoprotein/multidrug resistant protein sequences, such as mice, human and Caenorhabditis elegans. Expression of P-glycoprotein mRNA was higher in ivermectin-selected than unselected strains of H. contortus. An alteration in the restriction pattern was also found for the genomic locus of P-glycoprotein derived from ivermectin-selected strains of H. contortus compared with unselected strains. P-glycoprotein gene structure and/or its transcription are altered in ivermectin-selected H. contortus. The multidrug resistance reversing agent, verapamil, increased the efficacy of ivermectin and moxidectin against a moxidectin-selected strain of this nematode in jirds (Meriones unguiculatus). These data indicate that a P-glycoprotein may be involved in resistance to ivermectin and other macrocyclic lactones in H. contortus.

Efficacy of closantel, albendazole and levamisole on an ivermectin resistant strain of Haemonchus contortus in sheep

Following evidence of reduced efficacy of oral ivermectin in a field population of Haemonchus contortus on a sheep farm in Kenya, this strain of the parasite was submitted to a controlled anthelmintic test. One hundred and twenty worm-free lambs were randomly split into two groups of 60. Each lamb in the first group was infected with 5000 third stage larvae (L3) of the suspected resistant strain the remaining 60 lambs were each infected with 5000 (L3) of a H. contortus strain of known susceptibility to the major groups of anthelmintic compounds used in sheep. On day 28 post-infection, each group was subdivided according to egg counts and at random into six sub-groups of ten lambs, each of which was treated with closantel at 5.0 mg kg-1, albendazole at 5.0 mg kg-1, levamisole at 7.5 mg kg-1, oral ivermectin at 0.2 mg kg-1 or injectable (Inj.) ivermectin at 0.2 mg kg-1, or was left as an untreated control. At slaughter, 10 days later, all the anthelmintics had resulted in reduced worm burdens in animals infected with the susceptible strain (albendazole 99.0%, levamisole 99.5%, closantel, Inj. and oral ivermectin 100%). By contrast, in the lambs infected with the suspected resistant strain, closantel was 100% effective, but inj. ivermectin, oral ivermectin, albendazole and levamisole reduced worm counts by 47.6%, 24.2%, 38.5% and 41.4%, respectively. Anthelmintic resistance to the other chemical groups had been confirmed previously on this farm and although benzimidazoles and levamisole had not been used for the last 4 years, this strain was still highly resistant to albendazole and levamisole; closantel remained as the only drug evaluated which was still effective against this population of H. contortus.

Selection for high levamisole resistance in Haemonchus contortus monitored with an egg-hatch assay

To investigate the characteristics of selection for levamisole resistance in Haemonchus contortus, the consecutive nematode generations of an in vivo selection were monitored with a newly developed egg-hatch assay. The in vivo selection was started with a population not previously exposed to any anthelmintics (SHS). At first, the levamisole resistance progressed gradually in successive nematode generations by treating sheep with increasing doses of levamisole, the initial dose being 1 mg kg-1. Treatment with 5 mg kg-1 levamisole resulted, however, in a steep increase of resistance. The selection was ended after six generations, since a level of 30 mg kg-1 levamisole, which is not far from the toxic level for sheep, was reached. The final population, RHS6, was studied in a controlled test. Treatment of RHS6-infected sheep with 30 mg kg-1 levamisole caused an 80% decrease of faecal egg output, and a reduction of 34% in worm numbers. It was remarkable that only the number of female adults was reduced. RHS6 showed a reduced viability, but a fertility similar to the starting population SHS.

A survey of anthelmintic resistance in nematodes of sheep in The Netherlands

The prevalence of anthelmintic resistance in nematodes of sheep was surveyed in 1994 on 70 farms in the Netherlands. An in vitro egg hatch assay, faecal egg count reduction (FECR) 14 days after treatment, and larval cultures were used as methods of investigation. Oxfendazole was tested on 69, ivermectin on 51, and levamisole on 36 farms. The median effective dose (ED50) of thiabendazole could be determined on 64 farms. On 60 farms (94%) the ED50 value was > or = 0.12 microgram ml-1, which is indicative of the presence of benzimidazole (BZ) resistance. On two farms egg output was too low to do a FECR test. Based on the results of the FECR test, BZ resistance was present on 56 farms (84%), on 2 farms there was a suspicion of resistance and on 9 farms no resistance could be found. No clear indications were found for the presence of resistance against ivermectin or levamisole. BZ resistance was demonstrated in Haemonchus contortus, Cooperia curticei, Ostertagia spp. and/or Trichostrongylus spp. No resistance was observed in species from the genus Nematodirus, Chabertia ovina and/or Oesophagostomum spp.

The strategic use of closantel and albendazole in controlling naturally acquired gastrointestinal nematodes of sheep in the Kenya highlands

The strategic use of closantel, a narrow-spectrum salicylanilide anthelmintic against bloodsucking helminths, and of albendazole, a broad-spectrum benzimidazole anthelmintic, in the control of gastrointestinal nematodes of sheep was investigated on a farm in Nyandarua District in the highlands of Kenya. Thirty Corriedale female lambs aged between 9 and 12 months were assigned to three treatment groups of 10 lambs each. The three groups were set stocked on separate paddocks for 12 months. Lambs in group 1 (strategic treatment group) were treated with closantel and albendazole at the beginning and towards the end of the long rains (April and June, respectively) and towards the end of the short rains. (December). During the intervening dry season, the lambs were treated with albendazole. Lambs in group 2 (suppressive treatment group) were kept 'worm free' by regular deworming with albendazole at 3-weekly intervals for 12 months. The third group of lambs remained untreated (control group). Gastrointestinal nematode infections and pasture infectivity were well controlled in the case of the strategic treatment group. This resulted in higher weight gains, wool production, packed cell volume, and serum albumin and protein concentrations compared with the untreated control lambs. These parameters were comparable between the strategic treatment and the suppressive treatment groups of lambs. It was concluded that worm control strategies based on the epidemiology of the parasites and the sustained anthelmintic action of closantel in combination with broad-spectrum anthelmintics can provide effective control of gastrointestinal nematodes of sheep in the study area.

Multiple anthelmintic resistance in Haemonchus contortus on a sheep farm in Kenya

Multiple resistance to albendazole, thiophanate, levamisole and orally administered invermectin was detected in an isolate of Haemonchus contortus in sheep on a farm where benzimidazole resistance had already been identified. Following a faecal egg count reduction test, this was confirmed by both critical and controlled anthelmintic tests. Different groups of sheep infected naturally or given an experimental infection with the benzimidazole-resistant isolate were treated with the recommended doses of various anthelmintics. Compared to the control group, the percentage reductions in the faecal egg counts of sheep treated with albendazole, thiophanate, levamisole and ivermectin varied between 38.2% and 79.1% and the residual worm counts between 27.3% and 57.5%. The results indicate the presence of multiple anthelmintic resistance in this isolate of H. contortus. Sheep treated with closantel showed 100% reductions in faecal egg and worm counts, indicating that this drug was very effective against the population of H. contortus on the farm.

Quality control in generic anthelmintics: is it adequate?

We became increasingly concerned about indications of possible substandard efficacy of some generic anthelmintics, particularly after P.C. van Schalkwyk (personal communication, 1990) had found some batches of imported generic products obtained from international brokers to be poorly active, despite apparently normal physical characteristics. Therefore, considering the serious consequences this would have for sheep farming, it was decided to test the efficacy of some of the generic rafoxanide products available on the South African market. One of the three commercial formulations (of highly reputable companies) tested against a known susceptible strain of Haemonchus contortus in sheep was markedly substandard, with an arithmetic mean efficacy of 66.2% (Class B, Reinecke, 1973), compared to Class A efficacy of the other two, which also differed significantly from one another (Mann-Whitney; P = 0.01). Larger differences were found between the three products against a natural infection with a partially resistant strain of H. contortus than against the susceptible strain, with corresponding arithmetic mean efficacies of 28.7% (Class X, or ineffective), 71.3% (Class B) and 87.7% (also Class B). It is concluded that the most likely reason for the observed differences is that international brokers do not disclose the sources of supply of different batches of active ingredient (with the result that the companies buying anthelmintics from them have no way of telling when a source of supply is changed); that the efficacy of such batches differs; and that efficacy testing of individual batches in some cases is inadequate. It is suggested that registering authorities should consider simplified efficacy testing of each new batch of active ingredient before it may be marketed.

Duration of anthelmintic efficacy of doramectin and ivermectin injectable solutions against naturally acquired nematode infections of cattle

A comparison of persistent efficacy of doramectin injectable (D) and ivermectin injectable (I) was investigated under field conditions with treated permanent principal (PP) and interval-grazed principal (IGP) calves. The experiment was initiated on October 13, 1992 (day 0). Cattle used were crossbred beef heifers of 185 kg average weight and 8 to 10 months old. By random allotment, 66 calves were divided into two groups of 15 PP-D and PP-I calves for each treatment and two groups of 15 IGP-D and IGP-I calves for each treatment. Three extra or replacement calves were allotted for each group. Permanent principal calves in three replicates of five cattle per treatment grazed continuously on nematode-contaminated replicate pastures from day 0 to day 70. At 2-week intervals, i.e., days 0 to 14, 14 to 28, 28 to 42, 42 to 56 and 56 to 70, one IGP-D and one IGP-I calf was grazed with each of the respective PP-D and PP-I calf replicates and necropsied 21 days after removal from pasture. All respective PP calves and IGP calves were treated with doramectin at 200 micrograms kg-1 or ivermectin at 200 micrograms kg-1 by s.c. injection on day 0. After the day 0-14 interval, all IGP-D calves had zero egg counts. From the day 14-28 interval through the next three grazing intervals, the arithmetic mean egg counts of IGP-D calves were 18, 90, 281 and 31; those of IGP-I calves were 30, 226, 74 and 185. This suggested a persistence effect of approximately 2 to 4 weeks. In PP-D calves, egg counts reached a mean maximum at day 56 of only five eggs per gram, while counts of PP-I calves reached a peak of 40 on day 42. From the day 14-28 interval and through all subsequent intervals, arithmetic mean total worm counts from IGP-I calves were 58 to 73% greater than those in IGP-D tracers. A maximal total worm count of 4159 was observed in IGP-D calves of the day 42-56 interval; total worm counts in IGP-I calves from the day 14-28 interval through the day 42-56 interval were: 5420, 6739 and 9979, respectively. Haemonchus and Cooperia were higher in prevalence than Ostertagia in both treatments. Results of PP-D egg counts and total worm burdens in IGP-I calves indicated a high level of doramectin persistent activity for approximately 4 to 5 weeks and an advantage over persistence activity of ivermectin.

Comparison of the persistent activity of ivermectin, abamectin, doramectin and moxidectin in cattle in Zambia

The persistent efficacy of four commercially available macrocyclic lactones (ML) in maintaining reduced faecal egg counts in cattle grazing naturally infested pastures was evaluated in 44 zebu animals aged 1-2 years in Zambia. The study started in February (rainy season) when the strongyle egg output was increasing. Four days before the start of the trial, all animals were treated with a double dose of oxfendazole. They were then divided into five groups which were again treated on day 0. Groups A, D, I and M received 0.2 mg kg-1 of abamectin, doramectin, ivermectin and moxidectin, respectively. Animals of group C received albendazole (7.5 mg kg-1). Faecal samples were collected twice a week for egg counts and larval differentiation. Faecal egg counts in the C group increased from day 21 onwards and plateaued from day 42 between 180 and 380 eggs per gram. The main genera found in cultures were Cooperia (90%) and Haemonchus (7%). Faecal egg excretion in groups M, A, D and I started on day 35, 42, 42 and 45, respectively. Subsequently and until day 84, average counts in these four groups were always significantly lower than in group C. Compared with albendazole, all four ML gave over 95% reduction in cumulative faecal egg counts for 42 days after treatment. The percentage efficacy was still over 84% by day 84 when an average cumulative egg count of 11320 eggs per gram faeces was calculated in group C. In addition, there was no significant difference in efficacy between the four ML groups at any of the sampling dates. During the trial no significant difference in weight gain between any of the groups was observed.

How long before resistance makes it impossible to control some field strains of Haemonchus contortus in South Africa with any of the modern anthelmintics?

This paper describes an exceptional spectrum of multiple anthelmintic resistance in two strains of Haemonchus contortus in South Africa, one from Howick in KwaZulu-Natal, and the other from Badplaas, in Mpumalanga. Apparently for the first time, a helminth strain is described with resistance to compounds from all five of the modern anthelmintic groups used for nematode control in sheep; also, two strains of H. contortus show resistance to the two substituted phenols, dinitrophenol and nitroxynil. Only closantel at 5 mg kg-1 of nine compounds tested appears to have undiminished efficacy against the Howick strain, but even in the case of closantel, the residual activity and minimal effective level need to be tested before it can be concluded that its efficiency is unaffected. The exceptional resistance of the Howick strain is demonstrated by the fact that sequential daily drenching of sheep infected with the strain, with levamisole at 18 mg kg-1, oxfendazole at 20 mg kg-1, levamisole at 20 mg kg-1 and a mixture of fenbendazole at 10 mg kg-1 plus trichlorfon at 132 mg kg-1 on the fourth day, failed to clear sheep of the infection. There are strong indications that side-resistance occurs between dinitrophenol and nitroxynil, on the one hand, and the salicylanilides, on the other, and it is suggested that, before long, strains of H. contortus will be found with high levels of resistance to all the currently available anthelmintics.

Nematocidal efficacy of eprinomectin, delivered topically, in naturally infected cattle

OBJECTIVE

To assess the nematocidal efficacy of eprinomectin in naturally infected cattle.

ANIMALS

62 (31 eprinomectin-treated and 31 control) beef mixed-breed or Holstein cattle, either 6 to 11 or 48 to 96 months old.

PROCEDURE

Cattle were housed 21 to 27 days before treatment to allow parasites to reach maturity. Animals were grouped by sex, ranked by weight, and randomly assigned to treatment group. Fecal flotation was done to identify cattle with intestinal nematode infections. Treatment groups were: 1--eprinomectin topical vehicle (1 ml/10 kg) and 2--eprinomectin topical solution (1 ml/10 kg). Cattle were euthanatized by replicate on day 14 or 15, and standard procedures were used to recover of pulmonary, abomasal, small intestinal, and large intestinal nematodes.

RESULTS

Eprinomectin efficacy across all trials was 100% against adult Trichostrongylus axei, Haemonchus placei, Oesophagostomum radiatum, and Dictyocaulus viviparus, as well a fourth-stage larval Oes radiatum, Ostertagia ostertagi, Nematodirus helvetianus, and Cooperia spp. Efficacy against adult O ostertagi, Cooperia oncophora, C punctata, C surnabada, C spatulata, N helvetianus, Trichuris sp, and Trichuris fourth-stage larvae was 99.9 and 99.8, 99.6, 98.9, 98.3, 99.7, 97.8, and 84.3%, respectively. All results were significant (P < 0.01) except those for C spatulata. Adverse reactions were not observed.

CONCLUSION AND CLINICAL RELEVANCE

Eprinomectin is a safe and effective nematocide against naturally acquired nematode infections in cattle when administered at a dosage of 500 micrograms/kg. Milk and meat withholding is not necessary when using this product.

Haemonchus contortus: the uptake and metabolism of closantel

Closantel is an anthelmintic which associates with plasma albumin and is useful for the control of sheep parasites, such as Haemonchus contortus, that ingest blood. However, the utility of closantel for parasite control has been threatened by the emergence of resistance. The mechanisms of resistance are unknown. A closantel-resistant and a closantel-susceptible isolate of H. contortus were compared with respect to the distribution and metabolism of closantel. Neither strain appeared to metabolise closantel in vitro or in vivo. Following treatment of infected sheep with radioactively labelled closantel, isotope levels in closantel-resistant adult H. contortus were significantly lower than in susceptible worms. This reduced accumulation of drug could contribute to closantel resistance by mechanisms such as reduced feeding, failure to dissociate the drug-albumin complex in the gut or increased efflux of closantel from resistant worms.

Anthelmintic resistance of trichostrongylids in sheep in the highveld of Zimbabwe

A survey was conducted on the occurrence of anthelmintic resistance of trichostrongylids on commercial sheep farms in the highveld of Zimbabwe. On ten farms the efficacy of fenbendazole, levamisole and rafoxanide was tested by a faecal egg count reduction test. Benzimidazole resistance was additionally examined using an egg hatch assay with thiabendazole. Results of the faecal egg count reduction tests and larval differentiations showed fenbendazole resistance of Haemonchus sp. on all investigated farms. Resistance of Haemonchus sp. against rafoxanide was demonstrated on all farms with reliable egg counts. Levamisole resistance of Haemonchus sp. was found on most farms but 2 farms showed an efficacy of 100%.

Persistent efficacy of abamectin and doramectin against gastrointestinal nematodes of cattle

OBJECTIVE

To assess the persistent activity of injectable formulations of abamectin and doramectin against gastrointestinal nematodes of cattle.

DESIGN

Controlled slaughter study assessing residual efficacy.

PROCEDURE

Nematode-free calves were treated with abamectin or doramectin (each at a dose of 200 micrograms/kg) and infections then induced with repeated doses of infective larvae of Trichostrongylus axei, Haemonchus placei, Ostertagia ostertagi and Cooperia species. The duration of challenge ranged from 14 to 28 days. The calves were slaughtered at either 38/39 or 45/46 days after the treatments and nematodes recovered from the gastro-intestinal tract.

RESULTS

Significant reductions in numbers of O ostertagi occurred for both abamectin and doramectin treatments (> 93%) relative to counts in untreated calves, when challenge was administered up to 21 days after treatment. For T axei and Cooperia spp significant reductions occurred when the challenge occurred for 14 days after treatment (99%). Although differences from untreated animals were not significant, the results for H placei suggested high efficacy (> 85%) for up to 21 days for doramectin and up to 28 days for abamectin.

CONCLUSIONS

There was no significant difference between abamectin and doramectin for any parasite at any challenge point, indicating that there is equivalent persistent activity of doramectin and abamectin against important gastrointestinal nematodes of cattle.

Comparative efficacies of closantel, ivermectin, oxfendazole, thiophanate and levamisole against thiabendazole resistant Haemonchus contortus in sheep

Forty-nine sheep artificially exposed to a thiabendazole (TBZ) resistant isolate of Haemonchus contortus were assigned to 7 groups of 7 animals each and used to conduct a controlled anthelmintic trial. One group of sheep served as untreated infected controls and 6 groups were treated as follows: closantel, 5.0 mg kg-1; ivermectin, 0.2 mg kg-1; oxfendazole, 5.0 mg kg-1; thiophanate, 50 mg kg-1, levamisole, 7.5 mgkg-1 and thiabendazole, 66 mg kg-1. Eggs per gram of faeces were determined on days 21, 24 and day 34 (10 days post-treatment) after infection and all animals were necropsied for residual worm counts. The calculated efficacies of the treatments against H. contortus as indicated by worm reduction were closantel (100%), ivermectin (99.3%), oxfendazole (35.2%), thiophanate (56.7%), levamisole (98.6%) and thiabendazole (24.3%). The data therefore indicate that the TBZ-resistant isolate of H. contortus used was highly resistant to the 2 benzimidazole (BZ) anthelmintics, oxfendazole and thiophanate. This is the first report in Kenya of a field strain of H. contortus resistant to thiophanate.

Novel sources of anthelmintics

The G-series of naturally occurring compounds from Eucalyptus species contain representatives that are endoperoxides and Mannich bases. Mannich bases have been shown to be effective in killing protozoa in vitro. Previous studies of the role of cells of the immune system reveal that helminth parasites are susceptible to free radical attack. A 3-year research project studied the anthelmintic potential of endoperoxides and/or Mannich bases derived from G-compounds. Results from 1 series of synthetic analogues which show a positive relationship between chemical structure and anthelmintic activity are reported here. A G-Mannich base with an aromatic ring side chain as well as a long aliphatic side chain was the most active analogue in the series with an LD 50 < 0.1 mM in an in vitro larval motility assay. This work showed that Mannich bases are a potential source of compounds for parasite control. More work on formulation and pharmacokinetics is required to advance these compounds to the threshold of commercial production.

Occurrence of anthelmintic resistant nematodes on sheep farms in England and goat farms in England and Wales

A survey in 1992 showed that 44 per cent of the sheep farms tested in the south west and 15 per cent of those in the north east of England had parasitic nematode worm burdens which were resistant to benzimidazole anthelmintics, and that 65 per cent of the non-dairy goat farms tested in England and Wales had resistant worms. Ostertagia circumcincta was the main species in sheep and Haemonchus contortus in goats. The resistance to benzimidazoles was diagnosed by a combination of an egg hatch assay and a larval development test. A simplified faecal egg count reduction test was used to investigate sheep and goat farms for signs of ivermectin resistance but no positive cases were detected. The initial identification of levamisole resistance on one farm, both from a faecal egg count reduction test and a positive larval development test, was confirmed by a 'dose and slaughter' trial.

Interaction between fenbendazole and piperonyl butoxide: pharmacokinetic and pharmacodynamic implications

The effect of the cytochrome P450 inhibitor, piperonyl butoxide on the pharmacokinetics and anthelmintic efficacy of the benzimidazole compound fenbendazole was studied in sheep and goats. Pretreatment of goats with the inhibitor caused a greater than three-fold increase in the relative bioavailability of fenbendazole and fenbendazole sulphoxide. A pharmacokinetic dose titration study was carried out in sheep with fenbendazole (5 mg kg-1) and piperonyl butoxide administered orally at 0, 15, 31, 63, 125 and 250 mg kg-1. The AUC of fenbendazole and the sulphoxide were significantly increased when fenbendazole was co-administered with piperonyl butoxide at dose rates equal to or higher than 31 mg kg-1. Peak plasma concentrations (Cmax) and mean residence time (MRT) were also significantly increased. The efficacy of the combination was assessed in sheep against two species of benzimidazole-resistant abomasal nematodes; Ostertagia circumcincta and Haemonchus contortus. The percentage reduction in the total number of O. circumcincta worms was 7.9% (fenbendazole) and 97.8% (fenbendazole-piperonyl butoxide). For H. contortus, the percentage reduction was 84.8% (fenbendazole) and 99.0% (fenbendazole-piperonyl butoxide). The in-vitro S-oxidation of fenbendazole and fenbendazole sulphoxide was studied using microsomal preparations from rat liver. Piperonyl butoxide inhibited significantly the sulphoxidation and sulphonation of fenbendazole. It was concluded that piperonyl butoxide inhibited the oxidative conversion of fenbendazole into inactive metabolites and this resulted in a potentiated anthelmintic action.

Anthelmintic resistance in India

Five sheep farms located in different geo-climatic regions were surveyed for resistance in gastrointestinal nematodes to albendazole, levamisole and ivermectin. Resistance to albendazole and levamisole was evident on all the farms. Albendazole reduced faecal egg counts by 0-73% and levamisole by 0-61%. No eggs were detected after treatment with ivermectin.

The effect of level of feed intake on the pharmacokinetic disposition and efficacy of ivermectin in sheep

The kinetic disposition of orally administered [3H]-Ivermectin (IVM) was examined in sheep in which the feed intake was maintained at either 800 or 400 g/day. The [3H]-metabolites were almost completely associated with particulate digesta in the rumen. In the low feed intake group the digesta flow was slower than in sheep on high feed intake. This resulted in an extended residence time and greater availability of IVM and its metabolites. The anthelmintic efficacy of IVM was then examined in sheep in which feed intake was reduced from 800 g/day to 400 g/36 h prior to and 36 h after IVM administration. In sheep with reduced intake 97% of IVM-resistant Haemonchus contortus were removed, compared with 53% in sheep maintained on high feed intake.

The effects of closantel treatment on the ultrastructure of Haemonchus contortus

H. contortus were recovered from sheep 0-14 h after intramuscular treatment with closantel. Ultrastructural examination revealed that mitochondria were more electron dense and contained swollen cristae compared with untreated controls. Following treatment, the basal channels in the intestine became prominent and there was vesicle formation in all organs examined. In contrast, closantel-resistant H. contortus appeared normal after drug treatment. It is likely that closantel affects membrane associated processes responsible for fluid and ion homeostasis as well as mitochondrial function. Untreated H. contortus were maintained in balanced salt solution for 12 h which caused lesions indicative of fluid imbalance, but at 23 h there were serious structural abnormalities.

Multiple anthelmintic resistance in Haemonchus contortus on a sheep farm in India

Multiple resistance to benzimidazoles (fenbendazole, albendazole and mebendazole) in a strain of Haemonchus contortus in sheep was detected on a farm where fenbendazole resistance had already been identified. Following a faecal egg count reduction test, this was confirmed by both critical and controlled anthelmintic tests. Different groups of sheep infected naturally or given an experimental infection with the fenbendazole-resistant strain were treated with the recommended doses of various anthelmintics. Compared to the control group, percentage reductions in faecal egg counts of sheep treated with fenbendazole, albendazole, mebendazole, levamisole and morantel varied between 56% and 81% and worm counts between 71% and 86%. The results indicate the presence of multiple anthelmintic resistance in this strain of H. contortus on this farm. Sheep treated with ivermectin and closantel showed 100% reductions in faecal egg and worm counts, suggesting high efficacy of these drugs against the population of H. contortus on this farm.

Effectiveness of the ivermectin sustained-release bolus in the control of bovine nematodosis

OBJECTIVE

To evaluate the nematocidal effectiveness of the ivermectin sustained-release bolus throughout its 135-day delivery period.

DESIGN

Twenty-four naturally infected calves were randomly allocated to 1 of 3 equivalent experimental groups: group-T1 calves were untreated controls, group-T2 calves each received a sustained-release bolus on trial day 0, and group-T3 calves were rendered nematode-free and used at 35-day intervals during the study as tracers. One contaminated pasture was used for all principal calves for the 135-day grazing interval of the study. Calves of groups T1 and T2 were also artificially administered mixed infective nematode larvae at intervals during the grazing period, after which, all calves were confined to concrete for 21 days prior to necropsy.

ANIMALS

All calves were approximately 6 months old on trial day 0, weighed from 136 to 216 kg, and were of mixed breeding and sex.

PROCEDURE

At intervals during the study, feces from all calves were analyzed for nematode egg counts, and all calves were weighed and examined for bolus retention (T2 calves only). For nematode recovery, all calves were necropsied 21 to 22 days after removal from the contaminated pasture.

RESULTS

Parasitic populations of Haemonchus, Ostertagia, Trichostrongylus, Cooperia, Bunostomum, and Oesophagostomum spp were significantly reduced in cattle treated with the ivermectin sustained-release bolus.

CONCLUSION

The nematocidal activity of the ivermectin sustained-release bolus proved highly effective, with > 98% efficacy for all nematode species present.

Duration of the persistent activity of moxidectin against Haemonchus contortus in sheep

Weaned lambs were infected with Haemonchus contortus 35, 28, 21, 14 and 7 days after treatment with moxidectin at 0.2 mg/kg and 35 and 14 days after treatment with ivermectin at the same dose rate. Worm counts 14 days after infection showed that moxidectin prevented the establishment of over 99% of infective larvae for 28 days and reduced the establishment rate at 35 days by 96%, relative to ivermectin. There was no difference in the protective efficacy of ivermectin at 14 or 35 days. The persistence of moxidectin is likely to provide advantages in nematode control, particularly when used as a strategic early summer treatment or as a pre-lambing treatment to ewes. Implications of the persistent activity of moxidectin for the development of resistance during the decay phase are discussed.

Efficacy of pyrantel tartrate against experimental infections with Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis in goats

The efficacy of pyrantel tartrate was evaluated in goats against induced infections with Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis. All the strains were of sheep origin and tested for susceptibility to pyrantel tartrate in sheep at the standard dose rate (20 mg kg-1) prior to the infection of goats. Fifteen French Alpine female goats were inoculated with the three nematode species. On Day 25 post-infection, goats were randomized into an untreated control group and two pyrantel treatment groups (20 mg kg-1 bodyweight once, and 40 mg kg-1 bodyweight as two doses 24 h apart). The goats were killed and processed for worm recovery 10 days after treatment. The two dose rates achieved high and similar levels of efficacy (> 96%) against Haemonchus contortus and Teladorsagia circumcincta. Against Trichostrongylus colubriformis, however, pyrantel tartrate was less effective at both dose rates as worm reductions ranged from 55 to 62%.

Anthelmintic profile of the cyclodepsipeptide PF1022A in in vitro and in vivo models

A novel cyclodepsipeptide of fungal origin, PF1022A, recently was reported to have anthelmintic activity. To supplement published reports and determine potential utility of PF1022A as a ruminant anthelmintic, the compound was examined in in vitro and in vivo models. Assays used measured motility of Haemonchus contortus (intrinsic drug potency), ATP levels (parasite death), and activity against H. contortus, Ostertagia ostertagi, and Trichostrongylus colubriformis in the jird (spectrum, potency, and efficacy by various routes). The potency of PF1022A in reducing motility is greater than commercial anthelmintics. Examination of ATP levels in PF1022A-paralyzed H. contortus indicates that worms are not killed, suggesting the compound acts as a neurotoxin in nematodes. In the jird, PF1022A has activity orally against each of the parasites studied and at doses comparable to all commercial anthelmintics, except the macrocyclic lactones which are more potent. Unfortunately, for some nematode species, parenteral delivery is ineffective at realistic doses.

Characterisation of an avermectin resistant strain of Australian Haemonchus contortus

A strain of Haemonchus contortus (CAVR) isolated in Australia was found to be resistant to ivermectin (IVM) with 0.4 mg kg-1 of the anthelmintic failing to significantly reduce worm burdens. Resistance to IVM was sex-influenced in the CAVR strain with adult males showing a greater sensitivity to IVM. Cross resistance to moxidectin was evident with approximately 15% of the population surviving a dose of 0.1 mg kg-1. The free-living stages of the CAVR isolate had a reduced sensitivity to avermectin (AVM) inhibition of development and motility. Similar structure-activity patterns and resistance factors were obtained for a series of related AVMs as inhibitors of larval development and L3 motility in CAVR and White River II, an IVM-resistant H. contortus isolate from South Africa. Further, both isolates were found to be 3 times more sensitive to paraherquamide than a susceptible H. contortus isolate. This suggest that the same resistance mechanism is operating in both isolates. The CAVR strain is susceptible to the benzimidazoles, levamisole and closantel.

Levamisole resistance in Haemonchus contortus selected at different stages of infection

Resistance to the anthelmintic levamisole is common amongst most genera of trichostrongylid nematodes of sheep, but is relatively rare in the species Haemonchus contortus. The level of resistance in at least one strain of this species varies during the life cycle. Investigation of this phenomenon, especially in parasitic stages of the parasite may reveal information on resistance mechanisms and factors affecting the selection of resistance. Compared with adult worms of the same resistant strain, immature worms are more susceptible to levamisole both in in vitro contraction assays and an in vivo efficacy experiment. To ascertain if immature parasitic worms of a susceptible strain could be selected for resistance, 3 lines of worms were selected with levamisole for 9 generations. Lines were selected with the same dose rate (2 mg kg-1) at the 4th and 24th day of infection. In addition, because levamisole is less active against immature worms a further line was selected at the 4th day with a higher dose (8 mg kg-1) but similar efficacy to the selection at day 24. In in vitro assays the line selected at day 4 with the lower dose of levamisole showed no resistance compared with an unselected line. The other 2 lines developed high levels of resistance indicating that resistance can develop after selection at either stage. In addition these data directly show that selection pressure is important in determining the rate of development of resistance.

Efficacy of oral moxidectin against benzimidazole-resistant isolates of gastrointestinal nematodes in sheep

The efficacy of orally administered moxidectin was determined against four benzimidazole-resistant nematode isolates. At the start of the trial, 30 lambs were each infected experimentally with 20,000 third stage larvae (5000 Haemonchus contortus, 7000 Teladorsagia circumcincta, 3000 Trichostrongylus colubriformis and 5000 Cooperia curticei); 28 days later they were allocated randomly to three groups of 10: one untreated group, one group treated orally with fenbendazole (5 mg/kg bodyweight) and one group treated orally with moxidectin (0.2 mg/kg). Samples of faeces were taken five and 10 days after treatment and the lambs were killed 10 days after treatment. Fenbendazole reduced the average number of nematode eggs in faeces by 95 per cent and the average number of worms by 25 to 45 per cent according to the species. The efficacy of moxidectin against these benzimidazole-resistant isolates was 100 per cent. No adverse reactions to either of the drugs were observed.