A survey of benzimidazole resistant nematodes in sheep in three countries of southern England

The effects of climate change on ovine parasitic gastroenteritis determined using veterinary surveillance and meteorological data for Northern Ireland over the period 1999-2009

While the influence of temperature and moisture on the free-living stages of gastrointestinal nematodes have been described in detail, and evidence for global climate change is mounting, there have been only a few attempts to relate altered incidence or seasonal patterns of disease to climate change. Studies of this type have been completed for England Scotland and Wales, but not for Northern Ireland (NI). Here we present an analysis of veterinary diagnostic data that relates three categories of gastrointestinal nematode infection in sheep to historical meteorological data for NI. The infections are: trichostrongylosis/teladorsagiosis (Teladorsagia/Trichostrongylus), strongyloidosis and nematodirosis. This study aims to provide a baseline for future climate change analyses and to provide basic information for the development of nematode control programmes. After identifying and evaluating possible sources of bias, climate change was found to be the most likely explanation for the observed patterns of change in parasite epidemiology, although other hypotheses could not be refuted. Seasonal rates of diagnosis showed a uniform year-round distribution for Teladorsagia and Trichostrongylus infections, suggesting consistent levels of larval survival throughout the year and extension of the traditionally expected seasonal transmission windows. Nematodirosis showed a higher level of autumn than Spring infection, suggesting that suitable conditions for egg and larval development occurred after the Spring infection period. Differences between regions within the Province were shown for strongyloidosis, with peaks of infection falling in the period September-November. For all three-infection categories (trichostrongylosis/teladorsagiosis, strongyloidosis and nematodirosis), significant differences in the rates of diagnosis, and in the seasonality of disease, were identified between regions.

Anthelmintic resistance: The state of play revisited

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

A small scale survey of ivermectin resistance in sheep nematodes using the faecal egg count reduction test on samples collected from Scottish sheep

Thirty-eight sheep flocks, predominantly from the south/central Scotland, were examined using a faecal egg count reduction test (FECRT) for the presence of ivermectin (IVM) resistant nematodes. Efficacies of less than 95%, 14-17 days post-treatment, were identified in 6 of 17 naturally grazing flocks where pre-treatment faecal egg counts were in excess of 150 eggs per gram. Efficacies on these IVM resistant farms ranged from 66 to 92%. One other suspected cases of IVM resistance was also identified in returned material. The larvae detected in post-treatment coprocultures from resistant flocks were from the genera Teladorsagia (4 from 6) and Trichostrongylus (2 from 6).

Use of two in vitro methods for the detection of anthelmintic resistant nematode parasites on Slovak sheep farms

A survey of the prevalence of anthelmintic resistant nematode populations was conducted on 32 sheep farms in the Slovak Republic. In vitro egg hatch test and larval development tests were used for the detection of resistance to benzimidazole anthelmintics and they were compared with in vivo faecal egg count reduction tests. There was agreement in the declaration of resistance between the faecal egg count reduction test and both in vitro tests. The presence of resistant populations was determined on two farms using egg hatch test. In both farms, the LD(50) values were higher than 0.1 microg TBZ/ml, indicating resistance. By using LD(99) values it might be possible to reveal relatively small proportion of resistant larvae in the population. The prevalence of benzimidazole resistance has not change on Slovak sheep farms during last decade.

Anthelmintic resistance – looking to the future: a UK perspective

The full extent of anthelmintic resistance in nematodes of farm animals is not known. Resistance can be detected with a faecal egg count reduction test and two in vitro tests, the egg hatch and larval development tests. The sensitivity of these two in vitro tests can be increased by using discriminating doses rather than calculating LD50 values. Only benzimidazole resistance can be detected with PCR based tests because the molecular mechanisms of resistance to levamisole and the macrocyclic lactones remain unknown. Resistance detection is important because it enables the appropriate management strategies to be put in place. The development of resistance is delayed by keeping sufficient parasites in refugia (not exposed to anthelmintic), but the necessary management details have not yet been validated in the field. It is probably too late to use combination products to delay the development of resistance, except in cattle but quarantining animals to prevent introduction of resistant helminths onto a farm is important. Dilution of resistant worms with susceptible ones is only at the preliminary research stage and the application of non-chemical methods of control to delay resistance is not yet a practical option. Extensive research is required to manage resistance, especially in the control of resistance in Fasciola hepatica.

A survey of anthelmintic resistant nematode parasites in Scottish sheep flocks

The state of anthelmintic resistance in Scottish sheep flocks was evaluated during April-August 2000 using in vitro bioassays, egg hatch assays for detecting thiabendazole (TBZ) resistance and larval development assays for levamisole, ivermectin and TBZ resistance. Anaerobic sampling kits and detailed questionnaires outlining farm demographics and current management practices were mailed to 227 Moredun Foundation member farms, replies were received from 98 farms. Kits received from 90 farms contained sufficient material to conduct one or more of the assays. A majority of the farms examined (64%) exhibited TBZ resistance but there were both farm locality and regional variations in the percentage of resistant farms. Teladorsagia was the predominant genera detected from farms. No resistance to levamisole or ivermectin was detected in any of the samples. There was no strong evidence from this survey that any of the management practices examined greatly affected TBZ resistance.

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.

Drought and flock isolation may enhance the development of anthelmintic resistance in nematodes

A survey of anthelmintic resistant nematodes was conducted in sheep and goat flocks in Greece using in vivo and in vitro tests. Faecal egg count reduction tests in Macedonia were all greater than 99% indicating very high sensitivity of the nematodes to anthelmintics. In vitro tests showed benzimidazole resistant Teladorsagia circumcincta in 17 out of 106 flocks on small islands. On the mainland there were only three cases of benzimidazole resistance out of 310 flocks and animals had recently been introduced to the flocks. Flocks on the islands are isolated and there are higher temperatures than on the more mountainous mainland, where flocks tend to intermingle. It is concluded that drought and isolation are likely to be the major factors accounting for the development of anthelmintic resistance in nematodes in the island flocks.

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.

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.

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.

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.

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.

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.

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.

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.

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.