Detection of anthelmintic resistant nematodes in sheep in southern England by a faecal egg count reduction test

Anthelmintic resistance of Haemonchus contortus from sheep flocks in Brazil: concordance of in vivo and in vitro (RESISTA-Test©) methods

This study evaluated the resistance status of Haemonchus contortus from sheep flocks in the state of São Paulo, Brazil, through comparison between the fecal egg count reduction test (FECRT) and the larval development test (LDT). For the FECRT, 35 sheep were selected in each of five flocks and divided into groups treated with: benzimidazole, levamisole, ivermectin, monepantel and control. Feces were collected for EPG and fecal cultures. The LDT was performed using thiabendazole (TBZ), levamisole (LEV), ivermectin aglycone (IVM-A) and Zolvix (ZLV). Resistance to all drugs was detected using FECRT in 100% of the flocks, except in relation to ZLV (40% resistant and 20% suspected of resistance). LDT indicated resistance to TBZ and IVM-A in all flocks, to LEV in 80% of flocks and to ZLV in 10%. Total agreement was obtained between the two tests for TBZ and IVM (k = 1.0), while for LEV (k = 0.8) and ZLV (k = 0.9), substantial and almost perfect agreement were obtained, respectively. The concordance between the tests was significant, thus showing that it is possible to use the outcome of the LDT to predict the FECRT, and hence validating the former as a fast diagnostic test for use by sheep farmers in Brazil.

A quantitative analysis of attitudes and behaviours concerning sustainable parasite control practices from Scottish sheep farmers

Nematode control in sheep, by strategic use of anthelmintics, is threatened by the emergence of roundworms populations that are resistant to one or more of the currently available drugs. In response to growing concerns of Anthelmintic Resistance (AR) development in UK sheep flocks, the Sustainable Control of Parasites in Sheep (SCOPS) initiative was set up in 2003 in order to promote practical guidelines for producers and advisors. To facilitate the uptake of 'best practice' approaches to nematode management, a comprehensive understanding of the various factors influencing sheep farmers' adoption of the SCOPS principles is required. A telephone survey of 400 Scottish sheep farmers was conducted to elicit attitudes regarding roundworm control, AR and 'best practice' recommendations. A quantitative statistical analysis approach using structural equation modelling was chosen to test the relationships between both observed and latent variables relating to general roundworm control beliefs. A model framework was developed to test the influence of socio-psychological factors on the uptake of sustainable (SCOPS) and known unsustainable (AR selective) roundworm control practices. The analysis identified eleven factors with significant influences on the adoption of SCOPS recommended practices and AR selective practices. Two models established a good fit with the observed data with each model explaining 54% and 47% of the variance in SCOPS and AR selective behaviours, respectively. The key influences toward the adoption of best practice parasite management, as well as demonstrating negative influences on employing AR selective practices were farmer's base line understanding about roundworm control and confirmation about lack of anthelmintic efficacy in a flock. The findings suggest that improving farmers' acceptance and uptake of diagnostic testing and improving underlying knowledge and awareness about nematode control may influence adoption of best practice behaviour.

Detection of ivermectin resistance by a larval development test--back to the past or a step forward?

The aim of this study was to evaluate the potential of the larval development test for the detection of ivermectin (IVM) resistance in Haemonchus contortus of sheep. Single infections with 5000 third-stage larvae of five resistant and two susceptible isolates of H. contortus were given to sheep. Fecal samples were collected four times during patency, and the micro-agar version of the larval development test (MALDT) was performed. Three macrocyclic lactone drugs (IVM, eprinomectin and IVM aglycone) were tested. The results of the tests are presented as LC50 and LC99 values. The MALDT was well able to distinguish between susceptible and resistant isolates. Resistance factors (RF) for the LC99 values were generally higher than those obtained by comparing LC50 values. The highly resistant isolates were readily distinguishable from the susceptible isolates, particularly when using IVM aglycone and eprinomectin, with RFs above 20.

Anthelmintic resistance in sheep flocks in Ontario, Canada

Gastrointestinal nematodes (GIN) are a significant constraint to pasture-based sheep production worldwide. Anthelmintic resistance (AR) has been reported in most sheep-raising areas in the world, yet little is known about the AR status in Canada. This study was conducted to determine the frequency of AR in GIN in sheep flocks in Ontario, Canada. Forty-seven sheep flocks were enrolled in the study, and their level of parasitism was monitored monthly throughout a grazing season by analyzing owner-acquired fecal samples from 15 grazing lambs per flock. When the mean GIN fecal egg count (FEC) reached a threshold of 200 eggs per gram (epg), oral ivermectin was supplied to producers to check ivermectin efficacy; the reduction in mean FEC 14 days after ivermectin treatment was calculated. 'Drench failure' was defined as a reduction in mean FEC of <95%. In those flocks with apparent drench failure, researchers performed a Fecal Egg Count Reduction Test (FECRT), dividing sheep into 4 treatment groups (n=10-15): control (i.e. untreated), ivermectin, and, if sufficient numbers of animals - fenbendazole and levamisole. AR was defined as a reduction in mean FEC <95% and a lower 95% confidence interval <90%. Larval cultures were performed on pooled post-treatment FECRT samples. Larval Development Assays (LDAs) to detect the presence of resistance to thiabendazole and levamisole were performed prior to the ivermectin drench check on pooled owner-acquired fecal samples that reached the 200 epg threshold. Approximately 89% (42/47) of the farms reached the FEC threshold of 200 epg; 93% (39/42) of these farms performed an ivermectin drench check, and 88% (34/39) of these farms had drench failure. The FECRT was performed on 29 of the 34 farms. Resistance to ivermectin, fenbendazole and levamisole was demonstrated on 97% (28/29), 95% (19/20) and 6% (1/17) of the farms tested, respectively, with considerable variability in resistance levels among farms. Haemonchus sp. was the most commonly cultured parasite from post-treatment fecal samples. LDA results for 21 farms were available; of these, 14% (3/21) and 62% (13/21) had low and high levels of thiabendazole resistance, respectively, while none of the farms exhibited resistance to levamisole. Amongst these tested farms, resistance to both ivermectin and benzimidazoles was very common. These findings strongly suggest that AR, particularly in Haemonchus sp., is a serious problem in these sheep flocks. Thus, marked changes in GIN management need to be instituted immediately to mitigate a worsening situation.

Comparative use of faecal egg count reduction test, egg hatch assay and beta-tubulin codon 200 genotyping in small strongyles (cyathostominae) before and after benzimidazole treatment

A survey on benzimidazole (BZ) resistance in small strongyles was performed on three farms in the tenth region in Chile. Samples from a total of 100 horses were tested using the faecal egg count reduction test (FECRT), the egg hatch assay (EHA) and an allele-specific PCR for the detection of beta-tubulin isotype 1 genes coding for phenylalanine (phe) or tyrosine (tyr) at codon 200. In the past, BZ-type drugs have been used within anthelmintic campaigns on all the three farms. This has predictably led to a high degree of BZ resistance at the Valdivia and Riñihue farms and to a lesser degree at the Frutillar farm, as demonstrated by all the three tests. The FECRT indicated resistance in every farm by faecal egg count reductions (FECR) of 27% (S.D. +/- 33), 26.5% (S.D. +/- 26.9) and 83.9% (S.D. +/- 22.8) for the Valdivia, Riñihue and Frutillar farms, respectively. With the EHA, the following mean LD(50) values were found before and after treatment with fenbendazole (FBZ): 0.093, 0.141 and 0.066 microg TBZ/ml and 0.149, 0.158 and 0.091 microg TBZ/ml, respectively, for the Valdivia, Riñihue and Frutillar samples. The corresponding LD(96) values were 0.222, 0.263 and 0.188 microg TBZ/ml before treatment and 0.316, 0.322 and 0.221 microg TBZ/ml after treatment, indicating BZ resistance in all the cases. Genotyping was performed on more than 1700 single larvae, at least 10 per faecal sample, for 98 pre- and 66 post-treatment samples. Despite a general trend toward higher percentages of phe/tyr and tyr/tyr individuals following treatment, no statistically significant difference was found between these two and the phe/phe genotype percentages. However, a significantly negative correlation was detected between the LD(50) values and the phe/phe percentages and there was a positive correlation between the FECRT results and the phe/phe percentages. Thus, there seems to be a difference in the significance of the codon 200 polymorphism in the mechanisms of BZ resistance in small strongyles of the horse and sheep trichostrongyles.

Anthelmintic resistance detection methods

The development of species and populations of parasitic helminths with resistance to one or more anthelmintics is an increasing problem world-wide. The majority of currently available anthelmintics used to control parasitic nematodes of cattle and sheep belong to only three main groups, the benzimidazoles, imidazothiazoles and the avermectins/milbemycins. The successful implementation of helminth control programmes designed to limit the development of resistance in nematode populations depends to some degree on the availability of effective and sensitive methods for its detection and monitoring. A variety of in vivo and in vitro tests have been developed for the detection of nematode populations resistant to the main anthelmintic groups, but each suffers to some degree from reliability, reproducibility, sensitivity and ease of interpretation. This review covers those tests that have been reported and described and highlights some of their strengths and weaknesses.

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.

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.

Nematode control practices and anthelmintic resistance on British sheep farms

Nearly 23 per cent of 3000 randomly selected sheep farmers returned questionnaires on strategies to control the development of anthelmintic resistance. Twenty-five per cent stated that they regularly checked the accuracy of their drenching gun, and 52 per cent stated that they weighed a few animals and treated the whole flock like the heaviest. The mean annual frequency of dosing lambs was 4.39 compared with 2.43 for ewes. Forty-eight per cent stated that they changed the anthelmintic group annually. Seventeen per cent always treated sheep brought on to their farm with ivermectin. However, only 7 per cent had had their flocks tested for anthelmintic-resistant nematodes. The responses suggest that most sheep farmers are not actively seeking to prevent the development and spread of anthelmintic-resistant nematodes.

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.

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.

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.

In vitro characterization of anthelmintic susceptibility of field isolates of the pig nodular worm Oesophagostomum spp., susceptible or resistant to various anthelmintics

A larval development assay (LDA) and an egg hatch paralysis assay (EHPA) were used to measure the sensitivity to anthelmintics of eggs and larvae of nodular worms (Oesophagostomum spp.) in pigs. The tests were carried out using in vivo defined resistant and susceptible isolates of Oesophagostomum dentatum, O. quadrispinulatum and Oesophagostomum spp. For measurement of pyrantel/morantel and levamisole sensitivity the LDA was found able to distinguish between susceptible or resistant isolates of Oesophagostomum. The EHPA was able to detect levamisole resistance, but the test failed to show differences in response to pyrantel between pyrantel susceptible and resistant lines. The possible routine application of LDA and EHPA in the diagnosis of anthelmintic resistance in Oesophagostomum spp. is discussed.

Potential for misinterpretation of the faecal egg count reduction test for levamisole resistance in gastrointestinal nematodes of sheep

The faecal egg count reduction test (FECRT) is widely used to assess the presence or absence of levamisole resistance in gastrointestinal nematodes in sheep flocks. However, in a recent study the FECRT appeared to falsely indicate the presence of levamisole resistance. One possible explanation for this result could have been that the anthelmintic failed to completely remove all immature stages of levamisole-susceptible strains, which then developed into egg laying adults and gave misleading results in the FECRT. Artificial infection of lambs with levamisole-susceptible, benzimidazole-resistant strains of Ostertagia circumcincta, Haemonchus contortus and Trichostrongylus colubriformis was followed by treatment with levamisole when the infections were between 2 and 10 days old. Levamisole was 84%, 88% and 98% effective against these immature stages of three species, respectively. The positive control, ivermectin, was more than 99% effective against all three species. The percentage faecal egg count reduction on samples taken 11 and 20 days post-treatment with levamisole was 80% and 78%, respectively. These results demonstrate that, due to the maturation of immature stages, the FECRT based on faecal eggs counts taken 11 or more days after treatment may give results indicative of levamisole resistance despite the absence of levamisole resistant strains of gastrointestinal nematodes.