Characterisation of milbemycin-, avermectin-, imidazothiazole- and benzimidazole-resistant Teladorsagia circumcincta from a sheep flock

Understanding anthelmintic resistance in livestock using "omics" approaches

Widespread and improper use of various anthelmintics, genetic, and epidemiological factors has resulted in anthelmintic-resistant (AR) helminth populations in livestock. This is currently quite common globally in different livestock animals including sheep, goats, and cattle to gastrointestinal nematode (GIN) infections. Therefore, the mechanisms underlying AR in parasitic worm species have been the subject of ample research to tackle this challenge. Current and emerging technologies in the disciplines of genomics, transcriptomics, metabolomics, and proteomics in livestock species have advanced the understanding of the intricate molecular AR mechanisms in many major parasites. The technologies have improved the identification of possible biomarkers of resistant parasites, the ability to find actual causative genes, regulatory networks, and pathways of parasites governing the AR development including the dynamics of helminth infection and host-parasite infections. In this review, various "omics"-driven technologies including genome scan, candidate gene, quantitative trait loci, transcriptomic, proteomic, and metabolomic approaches have been described to understand AR of parasites of veterinary importance. Also, challenges and future prospects of these "omics" approaches are also discussed.

Moxidectin use in Scottish sheep flocks suggests a need for clearer product labelling and communication of updated SCOPS guidelines

BACKGROUND

Guidelines for sustainable use of moxidectin were established in 2020. This study aimed to identify how Scottish sheep farmers are using this key endectocide and estimate its effectiveness against gastrointestinal nematodes.

METHODS

Questionnaires were distributed to sheep farmers across Scotland, and analysis focused on moxidectin use in relation to Sustainable Control of Parasites in Sheep (SCOPS) guidelines. Farmers using moxidectin in their flock volunteered to submit post-treatment sheep faecal samples, which were analysed for the presence of gastrointestinal nematodes using faecal egg counts with polymerase chain reaction to determine species.

RESULTS

Despite 70% of farmers using moxidectin in 2020, knowledge levels varied: 24% of farmers included other anthelmintics when asked about moxidectin use. Moxidectin was used for a wide variety of reasons, and most farmers did not consistently follow SCOPS guidelines. Despite only 2 of 76 farmers reporting failure of moxidectin treatment, gastrointestinal nematodes were found following moxidectin treatment on five out of six farms tested and included Teladorsagia circumcincta, Cooperia curticei, Haemonchus contortus and Nematodirus sp.

CONCLUSION

Findings from this project indicate the need for improved anthelmintic product labelling and farmer support to encourage sustainable use. The presence of nematodes in treated animals is suggestive of anthelmintic resistance.

Anthelmintic Efficacy in Sheep and Goats under Different Management and Deworming Systems in the Region of Lisbon and Tagus Valley, Portugal

Infections caused by gastrointestinal parasites have been described worldwide as one of the most important issues impacting small ruminant production. The systematic administration of anthelmintic (AH) drugs without following good practice principles has led to an increase in anthelmintic resistance (AR). There is scarce information regarding AH efficacy in small ruminants in Portugal. This study aimed to characterize by in vivo methods the presence and level of AR in four farms in the region of Lisbon and Tagus Valley. All four farms kept small herds in extensive management systems and used different deworming protocols. The active substances used were fenbendazole and a combination of mebendazole plus closantel in a dosage according to the manufacturers' instructions. On day 0 (T0), fecal samples were collected from all study animals, and animals in the treatment group (n = 40) were dewormed with the AH previously chosen by the assistant veterinarian. Animals in the control group (n = 30) did not receive any AH drug. The fecal sample collection was repeated on day 15 (T15), and the control group was treated. Egg counts were performed using the McMaster method for the eggs per gram (EPG), and AR was evaluated by the fecal egg count reduction test (FECRT) with a 95% confidence level (CL). The results from this experiment indicated that the four farms presented AR with two farms to fenbendazole (FECRT of 48% and 85%) and two farms to mebendazole plus closantel (FECRT of 66% and 79%). These results indicate that the gastrointestinal parasites of the four studied Portuguese farms are resistant to benzimidazoles, which suggests an increase in AR regarding nematodes in small ruminant production systems in Portugal.

Effectiveness of Anthelmintic Treatments in Small Ruminants in Germany

Simple Summary

Parasitic worms are a major threat to the health and production of sheep and goats worldwide, particularly because many worms have become resistant to commonly applied drugs. This problem is well known around the world, but the situation is currently not well studied in Germany. This study was therefore performed to evaluate the effectiveness of treatments against infection with gastrointestinal nematodes. Faecal samples from animal groups were examined before and after treatment, the worm eggs in the feaces were counted, and the reduction in egg excretion following treatment was calculated. Eggs of Haemonchus contortus (barber’s pole worm) were stained to differentiate them from other gastrointestinal nematodes. Treatments were chosen and carried out by farmers together with their local veterinarian. Additional information was collected by questionnaires to check if the treatments were performed correctly. Reduced effectiveness was observed for all available drugs, but some showed better treatment success than others. The barber’s pole worm frequently survived treatments by most products. The high percentage of treatment failures is highly concerning, and it is necessary to develop and/or apply alternative methods of worm control to prevent the deterioration of this situation.

Abstract

Widespread anthelmintic resistance is a concern for small ruminant health and production worldwide. The current situation regarding anthelmintic efficacy is, however, not very well studied in Germany. Thus, a nationwide field study was undertaken to assess the effectiveness of 253 treatments performed in 223 small ruminant flocks by faecal egg count reduction test (FECRT) using pooled samples and a modified McMaster method. The percentage of Haemonchus contortus and non-Haemonchus eggs was determined by fluorescence microscopy following peanut agglutinin–fluorescein isothiocyanate staining. Treatments were chosen and performed by farmers together with their local veterinarian, and potentially confounding factors for FECRT results were addressed as far as possible by rigorous inclusion criteria. Reduced effectiveness was observed for treatments with all examined anthelmintic classes, but treatments with benzimidazoles and moxidectin showed significantly poorer results than monepantel, a closantel and mebendazole combination, and levamisole. Low case numbers precluded reliable assessment of avermectins. Unsuccessful treatments were frequently associated with the survival of H. contortus, but this was also observed for non-Haemonchus genera. The results are highly concerning, and sustainable approaches to parasite control are urgently needed to prevent further deterioration of this situation.

Progress in the development of subunit vaccines for gastrointestinal nematodes of ruminants

The global increase in anthelmintic resistant nematodes of ruminants, together with consumer concerns about chemicals in food, necessitates the development of alternative methods of control for these pathogens. Subunit recombinant vaccines are ideally placed to fill this gap. Indeed, they are probably the only valid option for the long-term control of ruminant parasitic nematodes given the increasing ubiquity of multidrug resistance in a range of worm species across the world. The development of a subunit multicellular parasite vaccine to the point of practical application would be a groundbreaking step in the control of these important endemic infections of livestock. This review summarizes the current status of subunit vaccine development for a number of important gastrointestinal nematodes of cattle and sheep, with a focus on the limitations and problems encountered thus far, and suggestions as to how these hurdles might be overcome.

Characterisation of ivermectin and multi-drug resistance in two field isolates of Teladorsagia circumcincta from Irish sheep flocks

Ivermectin resistant Teladorsagia circumcincta were isolated from two Irish sheep farms. Parasite naïve lambs were artificially challenged with the isolates in order to test their sensitivity to benzimidazole (BZ), levamisole (LEV) and ivermectin (IVM) by means of a faecal egg count reduction test (FECRT). Both isolates were found to be resistant to all three anthelmintics with reductions of 47% (C.I. 10-68), 92% (C.I. 74-97) and 50% (C.I. 24-68) for the Farm A isolate and of 85% (C.I. 59-94), 89% (C.I. 69-96) and 73% (C.I. 49-86) for the Farm B isolate for BZ, LEV and IVM, respectively. A controlled efficacy test (CET) was undertaken to confirm resistance to IVM and to determine the efficacy of moxidectin (MOX). In agreement with the FECRT, IVM resistance was confirmed for both isolates, with reductions in worm burden of 78% (C.I. 67-85) and 80% (C.I. 45-93) for the Farm A and B isolates respectively. Both isolates were found to be MOX susceptible, with reductions in worm burden of 97% (C.I. 91-99) and 100% for the Farm A and B isolates respectively. Additionally, in vitro tests confirmed the results of the FECRT; results from the larval development assay (LDA) confirmed that both isolates were resistant to BZ and LEV with development observed at the discriminating dose for each drug, while the larval migration inhibition assay (LMIA) confirmed IVM resistance and indicated MOX susceptibility in both isolates. These results represent the first confirmed cases of IVM resistance on Irish sheep farms as well as the first report of multiple anthelmintic resistant gastrointestinal nematodes in Ireland.

Analysis of putative inhibitors of anthelmintic resistance mechanisms in cattle gastrointestinal nematodes

Effects of the cytochrome P450 inhibitor piperonyl butoxide and the P-glycoprotein inhibitor verapamil on the efficacy of ivermectin and thiabendazole were studied in vitro in susceptible and resistant isolates of the cattle parasitic nematodes Cooperia oncophora and Ostertagia ostertagi. The effects of combined use of drug and piperonyl butoxide/verapamil, respectively, were investigated in the Egg Hatch Assay, the Larval Development Assay and the Larval Migration Inhibition Assay. The effects of piperonyl butoxide and verapamil as inhibitors of thiabendazole and ivermectin responses were particularly marked for larval development, where both inhibitors were able to completely eliminate all differences between susceptible and resistant isolates. Even the lowest concentrations of anthelmintics used in combination with inhibitors caused complete inhibition of development. Differences and/or similarities among responses in different isolates were only obtained in the two other assays: in the Egg Hatch Assay piperonyl butoxide caused a shift in concentration-response curves obtained with thiabendazole to the left for all isolates tested, changing relative differences between isolates. In contrast, an effect of verapamil in the Egg Hatch Assay was only apparent for benzimidazole-resistant isolates. In the Larval Migration Inhibition Assay only ivermectin was tested and piperonyl butoxide shifted the concentration-response curves for all isolates to the left, again eliminating differences in EC50 values between susceptible and resistant isolates. This was not the case using verapamil as an inhibitor, where curves for both susceptible and benzimidazole-resistant isolates shifted to the left in Ostertagia isolates. In Cooperia the picture was more complex with ivermectin-resistant isolates showing a larger shift than the susceptible isolate. Single nucleotide polymorphisms in the β-tubulin isotype 1 gene were investigated. Significantly increased frequencies of resistance-associated alleles were observed for the codons 167 and 200 in one benzimidazole-resistant isolate but not in an isolate selected for benzimidazole resistance at an early stage of selection.

Improving liveweight gain of lambs infected by multidrug-resistant nematodes using a FECRT-based schedule of treatments

The aim of this study was to compare the liveweight gain of lambs, infected by multidrug-resistant nematodes, treated by conventional schemes of helminth control or using a schedule based on fecal egg count reduction test (FECRT). The flock was selected after a FECRT (experiment 1) which revealed a parasite population resistant to benzimidazoles, imidazothiazoles, macrocyclic lactones (ivermectin), salicylanilides, nitrophenols, and organophosphates. Despite the parasite resistance to ivermectin (an avermectin), the moxidectin (a milbemycin) was effective against the gastrointestinal nematodes (PR > 90%). In experiment 2, 48 suckling lambs were distributed in four randomized blocks (G1, G2, G3, and G4) by previous body weighings. G1 was kept as untreated control; G2 was treated following a FECRT-based schedule with drugs chosen based on fecal analysis (first drench with moxidectin, second drench with a combination of moxidectin and levamisole, and third drench with praziquantel, an anti-cestode drug); G3 and G4 received three drenches with ivermectin or disophenol, respectively. Body weighings and fecal analysis of these lambs were performed every 2 weeks over a 98-day period. An effective control of gastrointestinal nematodes was obtained with two nematicidal drenches following the FECRT-based schedule of treatments. On the other hand, eggs per gram of feces (EPG) counts were no different among untreated control, G3, and G4. Lambs treated using the FECRT-based schedule had the greatest liveweight gain among the groups tested. Additionally, liveweight gain was no different among the groups G3, G4, and G1. The FECRT-based schedule of anthelmintic treatments was beneficial regarding productivity and sustainability of helminth control in lambs infected by multidrug-resistant nematodes.

Anthelmintic resistance in Northern Ireland (I): prevalence of resistance in ovine gastrointestinal nematodes, as determined through faecal egg count reduction testing

The prevalence of anthelmintic resistance in Northern Ireland sheep flocks was evaluated between July and October 2011. Sampling kits were sent to 172 flock owners and returns were received from 91. Within this survey population, 27 flock owners used benzimidazole products, 10 used levamisole products, 15 used avermectin products, 26 used milbemycin products and 4 flock owners used the amino acetonitrile derivative, Monepantel. The remaining 9 flock owners used combination drenches (broad spectrum wormer plus fasciolicide). However, 15 sets of samples were ineligible for faecal egg count reduction testing due to either too low an egg count or insufficient faecal volume. Treatment efficacy below 95%, indicating significant resistance, was detected in 81% (n=24) of flocks tested for benzimidazole resistance; in 14% (n=1) of flocks tested for levamisole resistance; and in 50% (n=7) and 62% (n=13) of flocks tested for avermectin and milbemycin resistance, respectively. Monepantel resistance was absent in all (n=3) flocks tested. Combination products (broad spectrum nematocide plus flukicide) containing levamisole were entirely effective, while treatment efficacy below 95% was detected in 60% (n=3) of flocks where the nematocide in the combination product was a benzimidazole. Where parasite identification based on coproculture was completed, Trichostrongylus was the dominant genus detected in all cases post-treatment, indicating the occurrence of anthelmintic-resistant Trichostrongylus spp. populations. Benzimidazole efficacy was highest in treating Trichostrongylus spp. (51%) and lowest when treating Teladorsagia spp. Levamisole was 100% effective in treating Cooperia, but ineffective (0%) in treating Trichostrongylus spp. Avermectin efficacy was highest when treating Haemonchus contortus (100%) and Teladorsagia spp. (73%), with a marginally lower efficacy against Trichostrongylus spp. (71%). Moxidectin efficacy was 33% against Trichostrongylus spp., 68% against Teladorsagia spp., 97% against Cooperia spp. and 100% against Haemonchus contortus infections.

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.

Attempts to immunize sheep against Teladorsagia circumcincta using fourth-stage larval extracts

A ConcanavilinA (ConA)-binding fraction of a detergent-soluble membrane extract from Teladorsagia circumcincta (formerly Ostertagia circumcincta) fourth-stage larvae was isolated, and two vaccine trials were conducted with this preparation in groups of 7 worm-free sheep. All groups were challenged with a total of 5000 T. circumcincta larvae from 1 week after the final immunization and protection assessed by comparing the egg and worm counts, and length of developing worms, of the immunized groups with their respective controls. Immunization with the ConA-binding antigen induced high-titre serum antibody responses in both trials. However, no significant reduction in either egg count or worm burdens was observed in the vaccinated groups in either trial. It was concluded that detergent-soluble, ConA-binding extracts prepared from T. circumcincta fourth-stage larvae did not contain significantly protective antigens, despite the fact that an extract prepared in a similar manner from Ostertagia ostertagi had previously significantly protected calves against homologous challenge.

An inconvenient truth: global worming and anthelmintic resistance

Over the past 10-15 years, we have witnessed a rapid increase in both the prevalence and magnitude of anthelmintic resistance, and this increase appears to be a worldwide phenomenon. Reports of anthelmintic resistance to multiple drugs in individual parasite species, and in multiple parasite species across virtually all livestock hosts, are increasingly common. In addition, since the introduction of ivermectin in 1981, no novel anthelmintic classes were developed and introduced for use in livestock until recently with the launch of monepantel in New Zealand. Thus, livestock producers are often left with few options for effective treatment against many important parasite species. While new anthelmintic classes with novel mechanisms of action could potentially solve this problem, new drugs are extremely expensive to develop, and can be expected to be more expensive than older drugs. Thus, it seems clear that the "Global Worming" approach that has taken hold over the past 40-50 years must change, and livestock producers must develop a new vision for parasite control and sustainability of production. Furthermore, parasitologists must improve methods for study design and data analysis that are used for diagnosing anthelmintic resistance, especially for the fecal egg count reduction test (FECRT). Currently, standards for diagnosis of anthelmintic resistance using FECRT exist only for sheep. Lack of standards in horses and cattle and arbitrarily defined cutoffs for defining resistance, combined with inadequate analysis of the data, mean that errors in assigning resistance status are common. Similarly, the lack of standards makes it difficult to compare data among different studies. This problem needs to be addressed, because as new drugs are introduced now and in the future, the lack of alternative treatments will make early and accurate diagnosis of anthelmintic resistance increasingly important.

Small ruminant resistance against gastrointestinal nematodes: a case of Haemonchus contortus

Gastrointestinal nematode (GIN) infections are a common constraint to small ruminant industry throughout the world, and among those, haemonchosis has its own significance. Control of GIN primarily relies on the use of anthelmintics, but this approach has become less reliable due to the development of resistance in GINs against commonly used anthelmintics and an increased consumer demand for environmentally friendly animal products. These issues have stimulated investigations to find alternative sustainable control strategies, which are less reliant on anthelmintic input. One of such strategies is breeding of small ruminants for their resistance to the GINs. The susceptibility and resistance of animals to GIN infections varies within and between breeds. Various parasitological, biochemical and immunological parameters are employed to evaluate natural resistance status of animals both in natural pasture and artificial infections. The immune mechanisms responsible for resistance are not completely understood, but it has a significant effect in inherited resistance. Relatively resistant or tolerant animals show better local and generalised immune response as compared to susceptible. Immune response against GINs is influenced by many physiological factors. Determination of specific genes linked with host resistance will provide a valuable approach to find out the molecular mechanism of host resistance to GINs. Resistance has been reported to reduce pasture contamination, which in turn reduces re-infection and thus the requirement of the frequent anthelmintic treatments. The efficiency of control can be increased through objective and accurate identification of genetically tolerant individuals by natural and artificial infections with GINs. Complete resistance is the ultimate solution, but this has generally been ignored as a commercial reality. This paper reviews the published reports on natural resistance in small ruminants and discusses the prospects of developing small ruminants, which could be resistant to GINs.