World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) methods for the detection of anthelmintic resistance in nematodes of veterinary importance

The relationship between faecal egg count reduction and the lethal dose 50% in the egg hatch assay and larval development assay

The relationship between resistance detected in the faecal egg count reduction test (FECRT) and the lethal dose 50% (LD50) in the egg hatch assay (EHA) for benzimidazoles (BZs) and a larval development assay (LDA) for BZs, levamisole (LEV) and ivermectin (IVM) was examined on 13 sheep farms and 12 goat farms in Denmark. Out of 10 farms where resistance to BZs was detected according to the FECRT, nine (90%) had LD50 values above 0.5 microM thiabendazole (TBZ) (0.1 microg TBZ/ml) in the EHA, indicating resistance to BZs. However, four out of the 12 isolates susceptible to BZs in the FECRT had LD50 values higher than 0.5 microM TBZ in the EHA. For all isolates examined, LD50 values for TBZ in the LDA were lower than in the EHA. Four out of 11 and five out of 12 farms with worm populations resistant to BZs according to the FECRT and EHA respectively, had LD50 values lower than 0.5 microM TBZ in the LDA. Using the same cut-off point for resistant isolates in the LDA as in the EHA (0.5 microM TBZ), these isolates would be considered susceptible to BZs. All 10 isolates susceptible to BZs according to the FECRT and EHA and two isolates with suspect BZ resistance had LD50 values lower than 0.5 microM TBZ in the LDA. The above results indicated fairly good agreement in the detection of BZ resistance between the FECRT, EHA and the LDA. Groups of farms where resistance to LEV was detected according to the FECRT had higher mean LD50 values compared to those with LEV-susceptible or suspected resistant isolates. However, only four out of 12 farms having isolates resistant to LEV had LD50 values higher than 1.2 microM LEV (0.28 microg LEV/ml) recorded previously for a LEV-susceptible strain of Ostertagia circumcincta. This indicated discrepancies in declaring resistance to LEV between the FECRT and the LDA. Isolates from four farms where resistance to IVM was detected in the FECRT had LD50 values higher than the susceptible isolates. These were 2.5 to 7.5 times higher than those recorded previously for IVM-susceptible strains.

Multiple and multigeneric anthelmintic resistance on a sheep farm in Kenya

The anthelmintic efficacy of benzimidazoles (albendazole, fenbendazole and oxfendazole), levamisole, oral ivermectin and closantel was evaluated on a farm in Kenya using faecal egg count reduction test, larval cultures and a controlled slaughter trial. The results of this study indicated simultaneous resistance of Haemonchus contortus against benzimidazoles, levamisole and ivermectin, and of Trichostrongylus colubriformis and Oesophagostomum spp. against levamisole on the same farm. Ivermectin resistance developed to 47% within 15 months of first use. Closantel was effective against the benzimidazoles, levamisole and ivermectin resistant H. contortus.

Resistance to benzimidazoles and levamisole in nematode parasites of sheep in Nyandarua District of Kenya

The occurrence of anthelmintic resistance on 25 sheep farms in the Nyandarua District of Kenya was investigated, using the faecal egg count reduction test (FECRT), the egg hatch assay (EHA) and a larval development assay (LDA). In the FECRT, resistance to both benzimidazoles (BZs) and levamisole (LEV) was detected on four farms, resistance to LEV only on three farms and to BZs, only on two farms. Haemonchus contortus was the predominant nematode species in both pre-treatment and post-treatment faecal cultures. Out of the six farms where BZ resistance was detected in the FECRT, only isolates from one farm had an LD50 value higher than 0.5 microM thiabendazole (TBZ) (0.1 microgram TBZ/ml) in the EHA indicating resistance. Isolates from three other farms, where susceptibility to BZs was detected and four with suspected BZ resistance in the FECRT, had LD50 values higher than 0.5 microM TBZ in the EHA. The LD50 values for TBZ in the LDA for four of the six isolates with BZ resistance in the FECRT were higher than 0.5 microM (0.59-2.07) TBZ. There were disagreements in ascribing resistance for various farms, between methods of calculating and interpreting the faecal egg count reduction percentages (FECR%) based on the arithmetic mean, and those where the geometric mean eggs per gram (EPG) of faeces are used. Inclusion of pre-treatment EPG or control group EPG in the calculation of FECR% resulted in similar variations.

Comparative field evaluation of divided-dosing and reduced feed intake upon treatment efficacy against resistant isolates of Teladorsagia circumcincta in sheep and goats

The effects of modifying drug administration and food intake upon the efficacy of fenbendazole against resistant isolates of Teladorsagia (Ostertagia) circumcincta were investigated using naturally infected sheep and goats. Administration of the manufacturer's recommended dose (MRD) as two 2.5 mg kg(-1) bodyweight doses divided by a 12-hour interval resulted in an apparent increase in efficacy of over 28 per cent compared with conventionally treated sheep. Withholding feed for 24 hours before treatment at the MRD resulted in respective increases of 39.7 per cent (P<0.05) and 25.2 per cent compared with conventionally treated sheep and goats. A combination of food withdrawal and divided dosing did not further increase treatment efficacy in goats. It is suggested that withholding feed, divided-dosing or combining these methods may extend the useful life of the broad spectrum class of anthelmintics.

Multiple anthelmintic resistance on a goat farm in Kenya

The anthelmintic efficacy of benzimidazoles, levamisole, rafoxanide and ivermectin was evaluated on an institutional farm in Kenya using faecal egg count reduction test, larval cultures and a controlled slaughter trial. The results of this study indicated simultaneous resistance of Haemonchus contortus against benzimidazoles, levamisole and rafoxanide and Trichostrongylus columbriformis and Oesophagostomum sp. against levamisole on the same farm. Injectable and orally administered ivermectin was effective against the benzimidazole and levamisole resistant H. contortus.

Comparative efficacy of pyrethrum marc with albendazole against sheep gastrointestinal nematodes

The efficacies of pyrethrum marc and of albendazole against experimental sheep gastrointestinal nematode infection were compared. Sheep was infected orally with 10,000 larvae (Haemonchus spp. (60.1%), Oesophagostomum spp. (13.9%), Trichostrongylus spp. (13.2%), Cooperia spp. (8.3%), Nematodirus spp. (3.5%), Strongyloides spp. (0.8%) and Ostertagia spp. (0.2%). Faecal egg count reduction in albendazole-treated sheep was 100% by day 4 following treatment, compared to 37.03%, 31.3%, 38.9% and 51.8% on days 4, 6, 8 and 10 in pyrethrum mare-treated sheep. These reductions were statistically significant on days 8 and 10 post-treatment (p < 0.05). The potential for using pyrethrins for helminth treatment is discussed.

Anthelmintic resistance in nematode parasites from goats in Spain

In a flock of cashmere goats sited in Asturias (NW, Spain) diminished efficacy was observed following treatment with netobimin and consequently a study to demonstrate the existence of anthelmintic resistance and the species of nematode involved was conducted. Results from faecal egg count reduction tests before and after treatment with netobimin or ivermectin showed efficacies of 89.4 (81.8-94.8) and 99.7% (93.9-99.9%), respectively. Teladorsagia circumcincta was found to be the dominant species involved in netobimin resistance. Effective dose (ED50) values in the egg hatch assay of 0.22 microgram thiabendazole ml-1 confirmed the existence of benzimidazole carbamate resistant nematodes. To the best of our knowledge, this is the first report on the existence of anthelmintic resistant nematodes from ruminants in Spain.

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.

Comparative studies of ivermectin and moxidectin in the control of naturally acquired cyathostome infections in horses

The control of naturally acquired cyathostome infections in horses by treatments with ivermectin and moxidectin was evaluated in three field studies. In a first study the efficacy of both drugs was assessed in a faecal egg count reduction test. Both ivermectin and moxidectin demonstrated efficacies greater than 99 per cent for up to 60 days after treatment. In a second study, the period required for strongyle eggs to reappear was estimated in horses treated either with ivermectin or moxidectin. For the horses treated with ivermectin the period varied between 10 and approximately 13 weeks, and for moxidectin between 22 and approximately 24 weeks. With both drugs strongyle eggs started to reappear in the faeces significantly earlier in foals and young horses than in adults. In a third study, two prophylactic dosing schemes involving three ivermectin treatments at intervals of eight weeks, and two moxidectin treatments 12 weeks apart, were found to be highly effective in controlling strongyle infections of horses on pasture.

A survey on resistance to anthelmintics in sheep stud farms of southern Brazil

A survey to investigate the status of anthelmintic resistance in 29 sheep studs in southern Brazil was conducted from March 1992 to December 1993. Compounds from three drug families (macrocyclic lactone, levamisole and benzimidazole) were evaluated concurrently on 22 of the 29 studs. On seven of these properties, resistance to all three families was declared or suspected; at 15 of the 22 studs, ivermectin was the only compound found to be effective in reducing faecal egg counts. Resistance to levamisole was detected on 22 of the 23 studs where it was evaluated and was suspected in the remaining one. The position of benzimidazoles was similar, resistance being declared or suspected on all 28 studs where they were tested. Results of larval cultures indicated that Trichostrongylus, Ostertagia and Haemonchus were the most prevalent nematode genera in the survey, with Trichostrongylus and Haemonchus being the genera associated with anthelmintic resistance.

Alternatives to animal experimentation in parasitology

In parasitology, several measures can contribute to the aims of 3R (Reduction, Refinement and Replacement of animal experimentation). These include legal regulations, international guidelines for evaluating antiparasitic drugs, the refinement of animal models for parasitic infections, cryopreservation and cultivation of parasites in vitro or in chicken eggs, the maintenance of arthropods by artificial feeding, and the use of immunological and molecular in vitro techniques (e.g. the production of recombinant antigens for vaccines). Considerable progress has been achieved in the development of alternative techniques but both their standardisation and validation are not far advanced. A wider acceptance and use of alternative methods will only be achieved if research can offer economic alternatives that are as effective and reliable as animal experiments. Great efforts are needed for further progress.

Faecal egg count reduction percentage calculations to detect anthelmintic resistance in Oesophagostomum spp. in pigs

The results of four alternative methods of mean faecal egg count reduction percentage (FECR%) calculations were evaluated and compared using data obtained for Oesophagostomum spp. from ten sow herds. The estimates of FECR% and 95% confidence limits obtained using the four methods were different. However, there were few discrepancies in the final decision as to whether a given herd carried drug resistant isolates or not. The methods that used geometric means were more appropriate than those that used the arithmetic mean as the measure of central tendency for eggs per gram of faeces (EPG) values. The use of geometric mean EPG values in calculations has been criticized from several viewpoints, one of which is that its use reduces the comparability of reports between laboratories. If the geometric mean is to be used as we suggest in FECR% calculations, the appropriate references, number of animals in each group, minimum and maximum EPG values and the factor added to zero EPG counts should be reported in order to improve the comparability. The difficulty in obtaining groups with similar pre-treatment EPG values in field situations suggested the inclusion of pre-treatment EPG values in the calculations as an adjustment procedure. The importance of including a non-treated control group in calculations was demonstrated during this study. Therefore, we suggest the use of geometric mean EPG values, to include pre-treatment EPG values and to include the egg counts from the control group in FECR% calculations. The interpretation of the resulting FECR estimate may be different according to the purpose for which the testing procedure is carried out, e.g. survey in detecting anthelmintic resistance, control field tests, etc.

In vitro characterization of lines of Oesophagostomum dentatum selected or not selected for resistance to pyrantel, levamisole and ivermectin

Lines of Oesophagostomum dentatum artificially selected or not selected for resistance to pyrantel, levamisole and ivermectin were used in this study. From the 10th generation of selection eggs were collected from each line and subjected to an in vitro larval development assay (LDA) and an egg hatch assay (EHPA). Significant differences were observed between an unselected line of O. dentatum and the lines selected for resistance to levamisole or pyrantel in both assays. The LDA was more sensitive than EHPA in detecting anthelmintic resistance in O. dentatum. The results obtained from the LDA confirmed side-resistance between levamisole and morantel/pyrantel. The in vitro tests failed to show significant differences between ivermectin-sensitive and resistant lines.

A survey of anthelmintic resistance in nematode parasites of goats in Denmark

The occurrence of anthelmintic resistance in 15 goat herds in Denmark was investigated using the faecal egg count reduction test (FECRT), egg hatch assay (EHA) for thiabendazole (TBZ) and a microlarval development assay (LDA) for TBZ, levamisole (LEV) and ivermectin (IVM). Resistance to both benzimidazole (BZ) and levamisole (LEV) anthelmintics was detected on six farms, and to both BZs and IVM on one farm. Resistance to BZs was also detected on two farms and to LEV on two farms. Ivermectin resistance was present on one farm. Trichostrongylus and Ostertagia spp. were the predominant nematode species in both pre-treatment and post-treatment faecal cultures. In all nine instances where BZ resistance was detected in the FECRT, the LD50 values for TBZ in the EHA were higher than 0.5 microM TBZ (0.1 microgram TBZ/ml), indicating resistance. LD50 values for TBZ in the LDA were also higher than 0.5 microM where resistance to BZs was detected in the FECRT, except two farms where LD50 values of 0.21 and 0.29 microM TBZ were recorded. LD50 values in the LDA for isolates declared resistant to LEV ranged from 0.78 to 5.62 microM LEV. For IVM, the two resistant isolates had LD50 values of 25 and 68 nM IVM. There were disagreements in the declaration of resistance between methods of calculating faecal egg count reduction percentage (FECR%) based on the arithmetic mean and those where geometric mean EPG is used. Similarly, inclusion of pre-treatment EPG or control group EPG in the calculation of FECR% influenced declaration of resistance.

Inefficacy of moxidectin and doramectin against ivermectin-resistantCooperiaspp. of cattle in New Zealand

Two cases are described in which strains of Cooperia spp. of cattle were found to be resistant to either the oral or the injectable formulations of ivermectin. Injectable moxidectin was not effective against the latter strain and doramectin was not effective against either of the ivermectin-resistant Cooperia isolates. In both cases, the anthelmintic efficacy of oral levamisole was 100%.

A questionnaire investigation on factors of importance for the development of anthelmintic resistance of nematodes in sow herds in Denmark

A questionnaire survey on the use of anthelmintics and presence of other potential risk factors for the development of anthelmintic resistance of nematodes in 136 Danish sow herds was carried out between November 1992-February 1993. Twenty one veterinarians who specialize in diseases of pigs administered the questionnaire in personal interviews during their regular visits to the respective pig farmers. In the study population, 91% of the farmers currently treated their sows with anthelmintics, while only 38% and 14% treated their weaners and fatteners, respectively. Most respondents treated their sows either before or after farrowing (40%) or once every 6 months (27%). Benzimidazole and pro-benzimidazole (Class I) anthelmintics were the most used in sows by the farmers (39%, 47%, 49%, 50%, 46%, and 47% during the years 1987, 1988, 1989, 1990, 1991 and 1992, respectively). When sows were treated, fenbendazole was the most frequently used anthelmintic during the period 1987-1992 (30%-39%). The proportion of farmers that used Class II (pyrantel and levamisole) or Class III (ivermectin and piperazine) anthelmintics during the year 1992 were 25% and 13%, respectively. Of the 70 farmers who used anthelmintics from either Class I, II or III during the study period, 44 farmers always used the same class of drugs. Eight herd owners among those 44, had changed the pharmaceutical product within the same class. Sixty nine percent of the farmers did not weight the sows, but used visual appraisal of average body.

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.

Evidence for multiple anthelmintic resistance in sheep and goats reared under the same management in coastal Kenya

Four experiments, two with sheep and two with goats, were carried out to determine the efficacy of ivermectin, fenbendazole, levamisole, closantel and some of their combinations by faecal egg count reduction tests. In the first experiment, injectable ivermectin, oral ivermectin, fenbendazole and levamisole were tested in 6-month-old lambs, and their reduction percentages were 77%, 13%, 42% and 92%, respectively. In the second experiment, with yearling sheep, the reduction percentages were 35% for injectable ivermectin, 32% for fenbendazole, 99% for levamisole, 48% for closantel, 92% for injectable ivermectin combined with fenbendazole, 99% for injectable ivermectin combined with levamisole, and 100% for fenbendazole combined with levamisole. In the study with 18-month-old goats given the same dose rates as those recommended for sheep, the reduction percentages were 73% for injectable ivermectin, 25% for fenbendazole, and 78% for levamisole. Another group of 14-month-old goats was treated with dose rates 1.5 times those recommended for sheep and the reduction percentages were 93% for levamisole, 92% for injectable ivermectin, and 97% for a combination of levamisole and ivermectin. In all experiments with sheep and goats the gastrointestinal nematode parasites identified by larval cultures were Haemonchus contortus, Trichostrongylus spp. and Oesophagostomum spp. The gastrointestinal nematodes of both sheep and goats on this farm are resistant to ivermectin and fenbendazole, whereas levamisole is still effective in sheep, but not in goats. The results are discussed in relation to the farm as a source of breeding stock to smallholder farmers and its potential to spread anthelmintic resistance.

Studies on multispecific resistance of gastrointestinal nematodes to benzimidazoles on dairy-goat farms

Multispecific resistance to benzimidazoles was studied in three selected farms. These farms had bred dairy goats for more than 15 years. The helminths were introduced with the goats at the establishment of the farms which afterwards remained isolated. Nematode resistance could then be related to their own management practices. Faecal egg count tests and egg hatch assays were performed to assess intensity of resistance. The generic (infective larvae in faecal cultures) and specific richness (adult worms) were assessed. The resistant species were Trichostrongylus colubriformis, Teladorsagia circumcincta, Haemonchus contortus and Oesophagostomum venulosum. Faecal egg count reduction tests and egg-hatch assays did not match exactly. Faecal larval counts after treatments gave a distorted picture of multispecific resistance: Haemonchus and Oesophagostomum were very largely over represented. The number of species found in the three farms was relatively low compared with other reports in goat farms of the area. This reduction of diversity might also be due in part to characteristics of breeding management and history (use of permanent pasture and introduction of goats at the establishment of farm).

World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) second edition of guidelines for evaluating the efficacy of anthelmintics in ruminants (bovine, ovine, caprine)

The first edition of the W.A.A.V.P. anthelmintic guidelines for ruminants was published in 1982. Since then improved parasitological procedures have been developed, new therapeutic and prophylactic products have appeared requiring different test methods, and registration authorities are requesting more detailed record keeping and data validation. This second edition addresses these developments and fulfills the original goal of publishing guidelines for high quality, scientifically valid testing standards for trials that would be accepted as proof of efficacy by registration authorities regardless of country of origin. This second edition includes updated guidance on standard parasitological procedures, dose titration, dose confirmation and clinical trials, and provides guidelines for evaluating products for efficacy against anthelmintic resistant parasites, persistence of activity and prophylactic activity. Tests for efficacy against nematodes, trematodes and cestodes are included.

Efficacy of moxidectin against multiple resistantOstertagiaspp. in lambs

Moxidectin was demonstrated to have a high efficacy in lambs against Ostertagia spp. which were resistant to albendazole, levamisole and ivermectin in goats. Moxidectin reduced the number of eggs in faeces by 99.6% and the number of worms found at post-mortem dissection of the lambs by 99.9%. Of the adult worms found in abomasa, 91% were identified as Ostertagia circumcincta and 9% as Ostertagia trifurcata.

In vivo metabolism of aminopyrine by the larvae of the helminth Heligmosomoides polygyrus

The in vivo N-dealkylation of [13C-2]-labeled aminopyrine by the L1-L2 larvae of Heligmosomoides polygyrus was demonstrated by the use of a sensitive gas chromatography-mass spectrometry method. This is the first evidence for the possible existence of a cytochrome P-450-dependent activity in helminths.

Anthelmintic resistance in goats in peninsular Malaysia

Ninety-six randomly selected farms, located throughout peninsular Malaysia, were surveyed for goat nematodes resistant to benzimidazoles (BZ). On 33 farms BZ resistance was demonstrated by means of an egg hatch assay. Haemonchus contortus was found to be the main species involved in anthelmintic resistance. There was a positive association between the frequency of anthelmintic treatments on a farm and the presence of benzimidazole resistance. To assess the value of the egg hatch assay, faecal egg count reduction (FECR) tests were also performed on 20 farms. On six farms the LD50 of thiabendazole (TBZ) was less than 0.10 micrograms ml-1 and the FECR higher than 95% and on ten farms with an LD50 TBZ of over 0.10 micrograms ml-1 a FECR of less than 95% was measured. On four farms the FECR was less than 95%, although the egg hatch assay showed LD50 TBZ values of less than 0.10 micrograms ml-1 and on two of these three farms a controlled efficacy test confirmed the presence of BZ resistant H. contortus. From these results it can be concluded that the egg hatch assay underestimated the true incidence of benzimidazole resistance. Levamisole resistance was detected with a FECR test on two of ten farms investigated.

Multiple and multigeneric anthelmintic resistance on a sheep farm in Malaysia

The anthelmintic efficacy of benzimidazoles, levamisole, closantel, ivermectin and moxidectin was evaluated on an institutional farm in Malaysia using faecal egg count reduction tests, controlled slaughter trials and an in vitro egg hatch assay. The results of this study indicated simultaneous resistance of Haemonchus contortus against benzimidazoles and ivermectin and of Trichostrongylus colubriformis against benzimidazoles and levaminsole on the same farm. Moxidectin was effective against the ivermectin resistant H. contortus.

Treatment of multiple resistant field strain of Ostertagia spp. in Cashmere and Angora goats

A multiple resistant field strain of gastro-intestinal nematodes was detected in Cashmere and Angora goats imported from New Zealand. Different treatments with three types of broad spectrum anthelmintics (albendazole, levamisole/tetramisole and ivermectin) at various dose rates are described. Satisfactory effect in Angora goats was not achieved until all three anthelmintics were administered simultaneously at twice the normal sheep dose. Post mortem worm counts and identification revealed that the resistant population consisted of 89% Ostertagia circumcincta, 6% Ostertagia ostertagi and 5% Ostertagia trifurcata. However, egg hatch assay carried out seven months later detected the presence of resistant nematodes again.

Anthelmintic resistance in Haemonchus contortus from sheep in Malaysia

Albendazole, oxfendazole, fenbendazole, levamisole, closantel, ivermectin and febantel were administered to sheep on four farms and their efficacy assessed by faecal egg count reduction test. High level of resistance of Haemonchus contortus was found to benzimidazoles (albendazole, oxfendazole, fenbendazole) on all farms and to febantel on the one farm where it was tested. No resistance to closantel and levamisole was observed. Resistance to ivermectin was absent on the three farms examined under this study, but has been reported on the fourth farm in earlier work. It is concluded that anthelmintic resistance to benzimidazoles and the probenzimidazole, febantel, is a serious and widespread problem in H. contortus in sheep in Malaysia.

The effects of anthelmintics on ovine larval nematode parasite migration in vitro

The anthelmintics ivermectin, levamisole, morantel tartrate and thiabendazole all inhibited, in vitro, the motility of third stage larvae (L3) of Trichostrongylus colubriformis. The bioassay, based on the inhibition of L3 migration from agar gels, yielded sigmoid dose-response curves for ivermectin, levamisole and morantel tartrate, but not thiabendazole. The concentration of levamisole giving 50% inhibition of migration (EC50) was determined for Cooperia curticei, Haemonchus contortus, Nematodirus spathiger, Ostertagia circumcincta, Trichostrongylus axei, T. colubriformis and T. vitrinus. EC50s differed between species but within species the EC50s for ensheathed and exsheathed L3 were similar except for N. spathiger which showed significantly higher EC50 for the ensheathed L3. No difference between EC50s for levamisole-resistant and susceptible strains of T. colubriformis were found. Similarly, morantel-resistant and susceptible strains of T. colubriformis could not be differentiated in this bioassay. The inhibition of L3 motility by known anthelmintic compounds in this bioassay suggests that the bioassay could be used as a screen for potential new anthelmintics.

Anthelmintic resistance in Australian sheep nematode populations

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

World Association for the Advancement of Veterinary Parasitology (W.A.A.V.P.) guidelines for evaluating the efficacy of anthelmintics for dogs and cats

Guidelines have been designed to assist in the planning, operation and interpretation of studies for the assessment of the efficacy of drugs against helminth parasites of dogs and cats. The advantages, disadvantages and application of critical and controlled tests are presented. Information is also provided on the selection of animals, housing, feeding, dose-titration, confirmatory and clinical trials, record keeping and necropsy procedures. These guidelines should assist both investigators and registration authorities involved in the evaluation of anthelmintics to employ comparable and standard procedures and will have the added benefit of minimising the numbers of animals needed for such tests.

The development of anthelmintic resistance in ruminant livestock

Despite the enormous advances over the last 50 years in the chemotherapeutic control of nematode parasites, the economic importance of these parasites to the grazing livestock industries remains as great as ever. Added to this, the emergence and rapid development of resistance to the new broad spectrum anthelmintics in the important nematode species now looms large as a major international threat. This particularly is of concern to the sheep industry, especially in the major sheep-raising countries of the Southern Hemisphere, but there are clear signs that the problem in Western Europe and North America is rapidly escalating. Resistance is also a serious concern in the intensive goat industry, but at present appears to occur only on isolated, individual cattle properties. To obtain a perspective of the development of anthelmintic resistance in relation to husbandry practices, this review focuses on the problem in the sheep industry in relation to its development, detection and management.

Criteria for diagnosing anthelmintic resistance by the faecal egg count reduction test

This study examines the possible outcome of adopting criteria recommended by the Australian Working Party for defining anthelmintic resistance by the faecal egg count reduction test, namely a mean percentage reduction of less than 95%, together with a lower 95% confidence limit of less than or equal to 90%. Based on an analysis of 313 veterinary diagnostic cases submitted for faecal egg count reduction testing in sheep and goats over a 7-year period, it was found that lower confidence limits of 90% or less were always associated with mean percentage reductions of less than 95%. These results suggest that if anthelmintic resistance is to be defined on the basis of mean faecal egg count reductions of this magnitude, then little practical purpose is likely to be served by further consideration of the lower confidence limits of these estimates.

Detection of anthelmintic resistance by culture in vitro of parasitic stages of ovine nematodes

Exsheathed L3 larvae of Haemonchus contortus were cultured at 38.5 degrees C in a complex medium supporting growth to adult worms. Although there was a clear difference in response between susceptible and benzimidazole/ivermectin/closantel resistant H. contortus using thiabendazole, the test failed to differentiate satisfactorily between the strains using ivermectin or closantel.