The detection of anthelmintic resistance in nematodes of veterinary importance

Before revised World Association for the Advancement of Veterinary Parasitology (WAAVP) guidelines on the detection of anthelmintic resistance can be produced, validation of modified and new methods is required in laboratories in different parts of the world. There is a great need for improved methods of detection of anthelmintic resistance particularly for the detection of macrocyclic lactone resistance and for the detection of resistant nematodes in cattle. Therefore, revised and new methods are provided here for the detection of anthelmintic resistance in nematodes of ruminants, horses and pigs as a basis for discussion and with the purpose that they are evaluated internationally to establish whether they could in the future be recommended by the WAAVP. The interpretation of the faecal egg count reduction test has been modified and suggestions given on its use with persistent anthelmintics and continuous release devices. An egg hatch test for benzimidazole (BZ) resistance is described. A microagar larval development test for the detection of benzimidazole and levamisole resistance provides third stage larvae for the identification of resistant worms. The sensitivity of these two tests can be increased by using discriminating doses rather than LD(50) values. Details are given of a PCR based test for the analysis of benzimidazole resistance in strongyles of sheep and goats, horses and cattle. Although promising for ruminant trichostrongyles, quantitative determination of gene frequency using real time PCR requires further development before PCR tests will be used in the field. Apart from faecal egg count reduction tests there are currently no satisfactory tests for macrocylic lactone resistance despite the great importance of this subject. Except for treatment and slaughter trials there are no validated tests for fasciolicide resistance or for the detection of resistance in cestodes.

Canine visceral leishmaniasis: Comparison of in vitro leishmanicidal activity of marbofloxacin, meglumine antimoniate and sodium stibogluconate

The control of canine leishmaniasis largely depends on the success of treatment. Drugs currently available to treat this disease are toxic and partially effective. The curative effect of marbofloxacin, a third-generation fluoroquinolone developed for veterinarian individual treatment, was evaluated in vitro in the presence of Leishmania infantum promastigotes and dog-monocyte-derived macrophages; meglumine antimoniate and sodium stibogluconate were used as comparative treatments. We observed that the killing of Leishmania promastigotes and intracellular amastigotes by marbofloxacin was dose-dependent. We demonstrated that successful treatment of canine infected macrophages for 48 h was possible with 500 microg/ml of marbofloxacin. Leishmanicidal activity acted through a TNF-alpha and nitric oxide pathway and correlated with the generation of nitric oxide (NO(2)) production by monocytes derived macrophages from infected (23+/-5 microM) or healthy (21+/-6 microM) dogs, in comparison with NO(2) concentration in infected/non-treated macrophages (< 3 microM, P<0.01). This significant induced parasiticidal effect correlated with extensive elimination of amastigotes by macrophages derived from infected (11+/-5) and healthy dogs (6+/-2), when compared to infected/non-treated macrophages (530+/-105 and 472+/-86 amastigotes, respectively, P< 0.01). Marbofloxacin was shown to be non-toxic at 500 microg/ml in vitro and no cell apoptosis was observed. The molecule was able to induce a parasitic process after significant elimination of amastigotes in leishmania-infected dog macrophages. We propose that marbofloxacin, compared to standard chemotherapeutic agents (meglumine antimoniate and sodium stibogluconate), could be an effective and pragmatic oral route alternative to treat canine leishmaniasis.

Tracing the emergence of drug-resistance in coccidia (Eimeria spp.) of commercial broiler flocks medicated with decoquinate for the first time in the United Kingdom

Decoquinate is a quinolone coccidiostat introduced during 1967 as an in-feed prophylactic for broiler chickens. Despite early drug-resistance problems and its age, the drug is still used commercially worldwide. Decoquinate here serves as a valuable model in a field study that addresses the dynamics and economic impact of the development of coccidial resistance to potent synthetic anticoccidial drugs. The results of this unique, hitherto unpublished, study on the initial emergence of resistance of avian coccidia (Eimeria spp.) to a new drug in the field may be of strategic value in the continued use of decoquinate or the introduction of new drugs. The commercial performance of the first 3-5 crops of broilers to be medicated with decoquinate on each of six farms was monitored during 14 months in 1968-1969, supplemented by assessments of the species, population dynamics and decoquinate-resistance of coccidia isolated from each farm. During the rearing of each flock in a single shed on each farm, oocysts were counted in fresh faecal samples collected on three occasions, and the species were identified by their morphology if possible, supported if necessary by the biological characteristics of infections in chickens. E. acervulina was the most common species, followed by E. mitis, E. maxima, E. tenella and E. praecox. E. brunetti occurred rarely, and E. necatrix was not found. Decoquinate-resistance was evident in several species during the rearing of the first decoquinate-medicated crop on each farm, although clinical coccidiosis did not occur. It was concluded that inherently resistant mutants of E. acervulina, E. brunetti, E. maxima, E. tenella, and probably also E. mitis and E. praecox, were selected from field populations by 6 weeks during their first exposure to decoquinate. During up to four more subsequent crops, cycling of resistant parasites stimulated host immunity, which had no obvious adverse impact on commercial performance. There was no apparent seasonal effect. A hypothesis is proposed to explain the sudden and rapid emergence of quinolone-resistance in the coccidia, and why bird health was not thereby compromised in these circumstances.

Pyrantel pamoate resistance in horses receiving daily administration of pyrantel tartrate

CASE DESCRIPTIONS

16 horses treated daily with pyrantel tartrate (2.64 mg/kg [1.2 mg/lb], PO) as part of a prophylactic anthelmintic program.

CLINICAL FINDINGS

Fecal worm egg counts (FWECs) were obtained on all 16 horses. Mean FWEC was 478 eggs/g (epg; range, 0 to 4,075 epg). Three of the 16 horses were responsible for 85% of the total fecal egg output for the herd on the day of sampling. Six horses had FWECs < 200 epg. Three horses that had arrived within 4 months of the sampling date had FWECs < 100 epg.

TREATMENT AND OUTCOME

An FWEC reduction test was initiated the day after FWECs were obtained; all horses with FWECs > 100 epg (9 horses) were treated with pyrantel pamoate (6.6 mg/kg [3 mg/lb], PO), and 14 days later, the FWEC was repeated. During the 14-day period, all horses received pyrantel tartrate (2.64 mg/kg, PO) daily. Fecal worm egg count reduction was calculated for each horse. Mean FWEC reduction for the group was 28.5% (range, increase of 21% in FWECs 14 days after treatment to a decrease of 100% in FWEC 14 days after treatment).

CLINICAL RELEVANCE

Farms should be monitored for cyathostomes resistant to pyrantel pamoate prior to use of pyrantel tartrate. Fecal worm egg counts should be monitored routinely in horses before and after treatment to ensure efficacy of cyathostome control measures.

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.

Will technology provide solutions for drug resistance in veterinary helminths?

Drug resistance in veterinary helminths affects a growing number of livestock producers on a global basis. The parasites infecting the major species of livestock are presently showing resistance in varying degrees to the commonly used classes of anthelmintics. The degree and extent of this problem especially with respect to multidrug resistance (MDR) in nematode populations is likely to increase. Finding solutions to the spread of resistance requires knowledge of the drugs' modes of action and mechanisms of resistance. This knowledge can then be applied to detect and monitor the state of resistance. Here we present a brief overview of resistance mechanisms and some of the technologies being used to study them. We also discuss some of the strategies for slowing the spread of resistance. The issue of reversal of drug resistance is analysed under consideration of recent progress in the field of MDR reversal in non-infectious diseases. Finally, we propose an application of currently available technologies that could assist in the detection and monitoring of anthelmintic resistance. Taking into account the significant complexity of the genetic mechanism of anthelmintic resistance in and between the various species, we suggest to undertake a co-ordinated effort to systematically identify anthelmintic-related single nucleotide polymorphisms (SNPs) in the most important helminth parasites. Monitoring the state of resistance in field populations could be achieved with a SNP-based protocol for genotyping the many genes known or suspected to contribute to the modes of action or mechanisms of resistance to the various classes of anthelmintics. If significant associations between genotypes and phenotypes exist within a species, then a single test with sufficient SNPs could potentially have universal applicability. These could then be explored for the development of new molecular diagnostic procedures. New classes of anthelmintics are needed, but until they are developed and available to the producers, technology can assist to achieve the goal of better sustainability in anthelmintic usage.

Efficacy of atovaquone against Babesia gibsoni in vivo and in vitro

The therapeutic efficacy of atovaquone against Babesia gibsoni was examined in three dogs experimentally infected with B. gibsoni isolated from naturally infected dogs in Aomori Prefecture, Japan. Once parasitemia reached 10%, atovaquone was administered orally (30 mg/kg twice daily for 7 days). Within 2 days of atovaquone treatment, the parasite disappeared from blood smears without any clinical side effects. Anemia and thrombocytopenia were significantly improved in all the dogs. However, a polymerase chain reaction assay revealed that a B. gibsoni marker gene was intermittently present in peripheral blood after atovaquone therapy, indicating that the organism had not been eliminated, and parasites reappeared in blood smears 33 days after the last treatment. To investigate the change in sensitivity against atovaquone, an in vitro sensitivity test was performed using peripheral blood obtained from an untreated dog that was infected with the original parasite isolate, and from two of the experimentally infected and atovaquone-treated animals (blood was collected at the time of the post-treatment recurrence of the B. gibsoni infection). Atovaquone was added to the culture medium to final concentrations of 0.1, 1, 10, 100, and 1000 nM. For the untreated parasites, complete growth inhibition occurred at 1000 nM of atovaquone, whereas the recurrent parasites were inhibited by only 39.52 +/- 8.34% and 31.31 +/- 8.14% at this concentration after 48 h of incubation. Thus, the recurring parasites were less sensitive to atovaquone than the untreated originally isolated parasites.

Bermudagrass management in the Southern Piedmont USA. V: Gastrointestinal parasite control in cattle

Parasite-free pastures would improve cattle health and performance, resulting in possible economic return to producers. Our objective was to determine the effect of a single series of anthelmintic treatment of steers prior to stocking on Coastal bermudagrass pastures, during five consecutive summers, on the parasite burden in cattle. The site for this experiment had been conventionally cropped for several decades, with no exposure to cattle, and would be expected to be relatively free of nematode larvae. The experimental design was a randomized complete block (landscape features) with a split plot arrangement of treatments where main plots were pasture fertilization treatments (mineral, clover plus mineral, and broiler litter) and split plots were low and high forage mass. Anthelmintic treatment included pour-on ivermectin on day -21, albendazole on day -7, and injectable ivermectin 48 h prior to stocking of pastures, with the cattle remaining in drylot during the 48-h period prior to being placed on the experimental paddocks. All steers received only one series of treatments during any given year. Yearling Angus steers (Bos taurus) were managed in a put-and-take grazing system with three "tester" steers assigned to each paddock and "grazer" steers added or removed at 28-day intervals. From 1994 to 1998, steers grazed the paddocks for a 140-day period from mid May until early October each year. Fecal samples for worm egg counts were obtained on day 0 and at 28-day intervals, thereafter. On all sampling days after day 0, samples were obtained only from tester animals. Over the 5-year period, the mean eggs per gram of feces (epg) gradually increased from 0 (following treatment) to a mean of 2.2 (range from 0.7 to 3.0) by the end of the grazing season (the last sampling date) in October. Although the epg were not zero, they were below threshold levels that would allow development of a parasite burden in cattle. In traditional management systems, cattle graze parasite-contaminated pastures; therefore, parasites negatively impact growth and productivity throughout the entire grazing period. Periodic anthelmintic treatments simply give a temporary reprieve from those parasitic infections. Conceptually, using the current grazing system, it should be possible to maintain these pastures in a parasite-free status indefinitely; however, from a drug resistance perspective, it would be most applicable in sod-based rotation systems where cattle graze from two to five years before land is returned to row-crops. By removing the effect of parasites, cattle can grow without the physiological constraints that gastrointestinal parasites place on appetite, digestion, nutrient utilization, and general well being.

Drug resistance in veterinary helminths

At present, there is no effective alternative to chemical control of parasitic helminths where livestock are grazed intensively. Resistance to anthelmintics has become a major problem in veterinary medicine, and threatens both agricultural income and animal welfare. The molecular and biochemical basis of this resistance is not well understood. The lack of reliable biological and molecular tests means that we are not able to follow the emergence and spread of resistance alleles and clinical resistance as well as we need. This review summarizes some of the recent findings on resistance mechanisms, puts forward some recommendations for limiting its impact and suggests some priorities for research in this area.

Drug resistance in nematodes of veterinary importance: a status report

Reports of drug resistance have been made in every livestock host and to every anthelmintic class. In some regions of world, the extremely high prevalence of multi-drug resistance (MDR) in nematodes of sheep and goats threatens the viability of small-ruminant industries. Resistance in nematodes of horses and cattle has not yet reached the levels seen in small ruminants, but evidence suggests that the problems of resistance, including MDR worms, are also increasing in these hosts. There is an urgent need to develop both novel non-chemical approaches for parasite control and molecular assays capable of detecting resistant worms.

The changing landscape of antiparasitic drug discovery for veterinary medicine

Changes in economic imperatives in the pharmaceutical industry have led to a wave of consolidation, which has had the unintended side effect of shrinking the resource devoted to antiparasitic drug discovery in animal health companies. Scientific changes have altered the way in which drugs could be discovered in the future. New science and business models will need to be implemented to address the demand for innovative antiparasitic drugs in veterinary medicine. Novel drugs are needed to combat drug resistance and for currently non-addressed problems. At the center of the future for this field, however, lies the need for more support into the basic research on the biology of parasites.

Prevalence of anthelmintic resistant cyathostomes on horse farms

OBJECTIVE

To determine prevalence of anthelmintic resistance in cyathostome nematodes of horses in the southern United States.

DESIGN

Cross-sectional study.

ANIMALS

786 horses on 44 farms and stables in Georgia, South Carolina, Florida, Kentucky, and Louisiana.

PROCEDURE

Fecal egg count (FEC) reduction tests were performed on 44 large farms and stables. Horses on each farm were treated with an oral paste formulation of fenbendazole, oxibendazole, pyrantel pamoate, or ivermectin at recommended label dosages. A mixed linear model was fitted to the percentage reduction in FEC, accounting for differences among farms, states, ages, treatments, and treatment by state interactions.

RESULTS

By use of a conservative measure of resistance (< 80% reduction), the percentage of farms with anthelmintic-resistant cyathostomes was 97.7%, 0%, 53.5%, and 40.5% for fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate, respectively. Mean percentage reductions in FEC for all farms were 24.8%, 99.9%, 73.8%, and 78.6% for fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate, respectively. Pairwise contrasts between states for each treatment revealed that in almost all instances, there were no significant differences in results between states.

CONCLUSIONS AND CLINICAL RELEVANCE

The prevalence of resistance found in this study was higher than that reported previously, suggesting that anthelmintic resistance in equine cyathostomes is becoming a major problem. Furthermore, data from these 5 southern states, which are geographically and physiographically distinct, were remarkably similar. This suggests that drug resistance in cyathostomes is highly prevalent throughout the entire southern United States and probably nationwide.

Resistance to benzimidazole and macrocyclic lactone anthelmintics in cattle nematodes in Argentina

In April 2003, persistent scouring and ill-thrift that was reported in calves form an intensive beef rearing operation in central Argentina despite treatments with benzimidazole and ivermectin. In order to conduct a controlled faecal egg count reduction test on this herd, 40 calves 5-8-months-old were selected on the basis that they had a nematode eggs per gram (epg) of faeces count greater than 150. Animals were divided into four groups (1-4) of 10 calves. Calves of groups 1-3 were treated, respectively, with subcutaneous injection of ivermectin (200 mcg/kg), ricobendazole (4 mg/kg) and levamisole (7.5 mg/kg), while calves of group 4 remained as untreated controls. The egg count reductions carried out 10 days later were lower than 15% in calves treated with ivermectin and ricobendazole, but 100% in animals receiving levamisole. Pooled post-treatment faecal cultures showed larval percentages of 92 and 95 for Haemonchus and 8 and 5 for Cooperia in the faeces of calves treated with ivermectin and ricobendazole, respectively. This is the first reported case of Haemonchus parasiting cattle showing simultaneous resistance to avermectins and benzimidazole type anthelmintics.

Haemonchus contortus: in vitro drug screening assays with the adult life stage

Motility and feeding assays were assessed as in vitro systems for screening of novel compounds for anthelmintic activity against adult Haemonchus contortus. The study aimed to develop an assay with the parasitic adult stage of this species that could be used in conjunction with, or as an alternative to, the free-living larval stage screens commonly used for drug discovery with many parasitic nematode species. The feeding assay showed limitations due to the apparent continuation of a significant degree of feeding in worms showing greatly reduced motility in the presence of some drugs. Hence, it appeared most likely that the feeding assay would underestimate the toxicity of these drugs. The motility assay was able to detect toxicity of known anthelmintics, including the 'slow-acting' benzimidazoles. A small-scale screening exercise used the motility assay to detect toxicity towards adult parasites in 10 compounds out of a group of 200 chemicals (selected due to known toxic effects in larval development assays). The motility assay appeared suitable for drug screening against adult H. contortus. The use of the adult stage for drug screening in this way ensures that the drug is toxic towards the parasite life stage to be targeted in vivo. A lack of activity in subsequent in vivo trials could, therefore, be most likely attributable to host pharmacokinetic factors rather than an intrinsic lack of activity of the drug towards the adult parasite.

Helminths in horses : use of selective treatment for the control of strongyles

The current level of anthelmintic resistance in the horse-breeding industry is extremely high and therefore more emphasis is being placed on studies that focus on the judicious use of anthelmintic products. The aims of the study were to: 1) establish if there is variation in the egg excretion pattern of strongyles between the different age classes of Thoroughbred horses in the Western Cape Province (WCP), 2) test if a selective treatment approach successfully reduces the number of anthelmintic treatments and maintains acceptably low helminth burdens in adult Thoroughbred horses, and 3) evaluate the efficacy of subsampling large horse herds for faecal egg counts (FECs) to monitor the strongyle burden. In 2001 the FECs of 4 adult mare, 5 yearling and 3 weanling herds from 8 different farms were compared in the WCP. Within the mare herds there were generally fewer eggexcreting individuals with lower mean FECs compared with the younger age classes. Individual faecal samples were collected every 3-4 weeks from 52 adult Thoroughbred mares from 1 farm in the WCP during a 12-month period (2002/2003). Animals with strongyle FECs 100 eggs per gram (epg ) were treated with an ivermectin-praziquantel combination drug (Equimax oral paste, Virbac). The mean monthly strongyle FEC for the entire group was 300 epg throughout the study and the number of treatments was reduced by 50 %. Resampling methods showed that an asymptote to mean FEC was reached at 55 animals for each of the pooled weanling, yearling and mare egg counts. Resampling within 4 different mare herds recorded asymptotes of between 24 and 28 animals. Subsampling entire herds for FECs therefore provided an effective approach to treatment management. This study demonstrates that selective treatment is both a practical and an effective approach to the management of anthelmintic resistance.

Anthelmintic resistance of nematodes in communally grazed goats in a semi-arid area of South Africa

A survey was conducted on the occurrence of anthelmintic resistance of nematodes in communally grazed goats in a semi-arid area in South Africa. In herds belonging to 10 small-holder goat farmers, the efficacies of fenbendazole, levamisole and rafoxanide were tested by faecal egg count reduction (FECR) tests. Efficacies of 80% were considered a threshold for anthelmintic resistance. The FECR tests showed that all drugs tested more than 80% effective in most instances, but there were notable exceptions. In 1 case, rafoxanide was only 31% effective and in another case fenbendazole was only 47% effective. The occurrence of anthelmintic resistance in this farming sector is of concern. Steps should be taken to prevent its further spread and to avoid the development of a situation as on numerous commercial sheep farms in South Africa where resistance is very common.

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.

Benzimidazole-resistant beta-tubulin alleles in a population of parasitic nematodes (Cooperia oncophora) of cattle

Three anthelmintic classes with distinct mechanisms of action are commercially available. Selection of nematode populations resistant to all these drugs has occurred, particularly in trichostrongyloid parasites of sheep. Anthelmintic resistance in cattle parasites has only recently been recognized and appears to be less pronounced, even though very similar species infect both hosts. To understand the bases for differences in the rate of resistance development in sheep versus cattle parasites, it is important to first demonstrate that the same kinds of resistance alleles exist in both. The benzimidazoles (BZ), which have been used for more than 40 years, were chosen as an example. BZ-sensitive (BZ(S)) and BZ-resistant (BZ(R)) nematodes that parasitize sheep have been distinguished at the molecular level by a single nucleotide change in the codon for amino acid 200 of a beta-tubulin gene, a switch from TTC (phenylalanine) to TAC (tyrosine). PCR primers were designed to completely conserved regions of trichostrongyloid beta-tubulin genes and were used to amplify DNA fragments from Haemonchus contortus (cDNA from a BZ(S) and a BZ(R) library) as positive controls. The technique was then extended to the cattle parasites, Cooperia oncophora and Ostertagia ostertagi (from genomic DNA). Sequence analysis proved the presence of amplified BZ(S) alleles in all three species and BZ(R) alleles in the BZ(R) population of H. contortus. Based on these data, nested PCR primers using the diagnostic T or A as the most 3' nucleotide were designed for each species. Conditions for selective PCR were determined. To demonstrate feasibility, genomic DNA was recovered from individual H. contortus L(3) larvae from both BZ(S) and BZ(R) populations. Genomic DNA was also isolated from >70 individual adult male C. oncophora collected from a cattle farm in New Zealand with reported BZ resistance. Allele-specific PCR discriminated among heterozygotes and homozygotes in both species. This method could find utility in studying the molecular epidemiology of BZ resistance in cattle parasites and for defining the variables that limit the development and spread of anthelmintic resistance in this host.

Isolation and selection of ionophore-tolerant Eimeria precocious lines: E. tenella, E. maxima and E. acervulina

Eimeria parasites were isolated from Nanhai Guangdong province (southern China) and studied in chickens in wire cages to evaluate their drug resistance against commonly used ionophores: monensin (100 mg/kg of feed), lasolacid (90 mg/kg), salinomycin (60 mg/kg), maduramicin (5 mg/kg) and semduramicin (25 mg/kg). Chinese Yellow Broiler Chickens were infected with 40,000 crude sporulated Eimeria oocysts at 15 days of age and prophylactic medication commenced a day prior to infection. Drug resistance was assessed for each ionophore drug by calculating the anticoccidial index (ACI) and percentage optimum anticoccidial activity (POAA) based on relative weight gain, rate of oocyst production and lesion values. Results revealed that Nanhai Eimeria oocysts comprising of E. tenella, E. maxima and E. acervulina, were resistant to monensin, sensitive to both salinomycin and lasolacid and partially sensitive to maduramicin and semduramicin. By selection for early development of oocysts during passage through chickens, the prepatent time of E. tenella, E. maxima and E. acervulina were reduced by 49, 36 and 22 h, respectively. The precocious lines are less pathogenic than the parent strains from which they were selected and conferred a satisfactory protection for chickens against coccidiosis. These ionophore-tolerant precocious lines could have wider applications in the development of anticoccidial vaccines for sustainable control of coccidiosis.

Identification of foals infected with Parascaris equorum apparently resistant to ivermectin

During September 2002, routine fecal examinations performed on 16 Thoroughbred foals residing on a farm outside Toronto, Ontario, Canada, revealed low to moderate numbers of Parascaris equorum eggs in feces from 9 of the 16. All foals were then treated with ivermectin at a dose of 220 to 280 microg/kg (100 to 127 microg/lb), p.o., and fecal egg counts were repeated 12 days later. Fecal P. equorum egg counts increased between the first and second fecal examination in 7 foals, were unchanged in 1, and decreased in 5. Fecal samples were collected 13 days after treatment from 21 additional foals that had been treated with ivermectin at the same dose, and P. equorum eggs were detected in 12 of the 21. For all 37 foals, high P. equorum egg counts (> or = 100 eggs/g of feces) 12 to 13 days after ivermectin treatment were significantly more likely in foals that had been regularly treated with ivermectin since birth and permanently resided on the farm, compared with foals that had been treated with other anthelmintics or had an unknown deworming history. Collectively, these data suggested that P. equorum in these foals was resistant to ivermectin administered at the recommended dose.

Evaluation of prevalence and clinical implications of anthelmintic resistance in gastrointestinal nematodes in goats

OBJECTIVE

To determine prevalence of resistance to all anthelmintics that are commonly used to treat gastrointestinal nematodes (GINs) in goats.

DESIGN

Prospective study.

ANIMALS

777 goats.

PROCEDURE

On each farm, goats were assigned to 1 of 5 treatment groups: untreated controls, albendazole (20 mg/kg [9.0 mg/lb], p.o., once), ivermectin (0.4 mg/kg [0.18 mg/lb], p.o., once), levamisole (12 mg/kg [5.4 mg/lb], p.o., once), or moxidectin (0.4 mg/kg, p.o., once), except on 3 farms where albendazole was omitted. Fecal samples were collected 2 weeks after treatment for determination of fecal egg counts (FECs), and percentage reductions were calculated by comparing data from anthelmintic-treated and control groups. Nematode populations were categorized as susceptible, suspected resistant, or resistant by use of guidelines published by the World Association for the Advancement of Veterinary Parasitology.

RESULTS

Resistance to albendazole was found on 14 of 15 farms, and resistance to ivermectin, levamisole, and moxidectin was found on 17, 6, and 1 of 18 farms, respectively. Suspected resistance to levamisole and moxidectin was found on 4 and 3 farms, respectively. Resistance to multiple anthelmintics (albendazole and ivermectin) was found on 14 of 15 farms and to albendazole, ivermectin, and levamisole on 5 of 15 farms. Mean overall FEC reduction percentages for albendazole, ivermectin, levamisole, and moxidectin were 67, 54, 94, and 99%, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Anthelmintic resistance in GINs of goats is highly prevalent in the southern United States. The high prevalence of resistance to multiple anthelmintics emphasizes the need for reexamination of nematode control practices.

Management of drug-resistant cyathostominosis on a breeding farm in central North Carolina

REASONS FOR PERFORMING STUDY

Possible anthelmintic resistance on a breeding farm where a rapid rotation anthelmintic programme had been implemented for 9 years was investigated. Cyathostomins resistant to fenbendazole and pyrantel were documented by faecal worm egg count reduction test (FWECRT).

OBJECTIVES

To 1) manage small strongyle transmission in a herd of horses in which resistance to both pyrantel pamoate and fenbendazole was identified and thereby reduce the risk of clinical disease in the individual animal, 2) monitor the change in resistance patterns over time and 3) monitor the efficacy of ivermectin over the study period.

METHODS

Targeted ivermectin treatment of horses on the farm was instituted for mature horses with faecal worm egg counts (FWEC) > 200 eggs/g (epg) and for horses < age 2 years with FWEC > 100 epg.

RESULTS

Over a 30 month period, targeted ivermectin treatment achieved acceptable control in mares, as judged by FWEC, and improved control of patent cyathostome infection in consecutive foal crops. Egg reappearance time (ERT) after treatment with ivermectin was < 8 weeks in mares and foals more frequently in the second year of the study than in the first year. Numbers of anthelmintic treatments were reduced by 77.6 and 533% in the mare and foal group, respectively.

CONCLUSIONS

Targeted ivermectin treatment may be an economically viable method of managing multiple drug resistant cyathostominosis.

POTENTIAL RELEVANCE

Use of ivermectin should be monitored closely for development of resistance.

Resistance to trypanocidal drugs--suggestions from field survey on drug use in Kwale district, Kenya

A household survey was conducted in 21 villages of Kwale district, Kenya, to assess farmers' trypanocidal drug use characteristics for treatment of bovine trypanosomosis and their relationship to drug effectiveness. Descriptive statistical tools were used to summarize the farmers' drug use patterns. The chi-square test was the done to establish the relationship between proper drug use and recovery. The results indicate that the farmers had considerable knowledge about trypanocidal drugs with 82% (n = 65) having used these drugs within 6 months preceding the survey. Cases of incorrect drug use were reported. This study established that there was no significant relationship between correct drug use and recovery of the treated animals, suggesting the presence of drug resistance in Kwale district.

Development of a mouse model to determine the systemic activity of potential flea-control compounds

Probe studies were performed to determine if the cat flea (Ctenocephalides felis), the most common ectoparasite of companion animals, will feed on laboratory mice and, if so, to incorporate this into a small animal assay to detect systemically active compounds. Consequently, a protocol was developed which incorporated acepromazine maleate to temporarily sedate various strains of mice and allow fleas a window of time to feed undisturbed. For validation of the model, CD-1 mice were dosed per os with seven known insecticides at 30, 10 and 1mg/kg. Mice were sedated with 0.0125 ml acepromazine maleate intraperitoneally, and infested with fleas. After 2h, fleas were removed, one-third were examined immediately to confirm the occurrence of feeding, and 77% were found to have ingested a blood meal. The remaining fleas were incubated for 24h to determine mortality. Nitenpyram, the active ingredient in Capstar, was highly active (>94%) at 1mg/kg. Selamectin, the active ingredient in Revolution, was very active (86%) at 10mg/kg, but inactive at 1mg/kg. Fipronil, the active ingredient of Frontline Topspot, was very active (83%) at 30 mg/kg, moderately active (54%) at 10mg/kg and inactive at 1mg/kg. Cythioate, the active ingredient in Proban, and nodulisporic acid, a recently discovered oral insecticide, were moderately active (64 and 55%, respectively) at 10mg/kg, but both were inactive at 1mg/kg. Lufenuron and ivermectin exhibited no efficacy at any level tested. These findings suggest that this mouse model can effectively identify systemic flea-control leads and, subsequently, reduce the use of large animals in research.

Assay of nematocidal activity of isoquinoline alkaloids using third-stage larvae of Strongyloides ratti and S. venezuelensis

We examined the effects of isoquinoline alkaloids in vitro in an effort to identify a treatment for Strongyloides stercoralis larva migrans in humans. Infective third-stage larvae of S. ratti and S. venezuelensis were used as model nematodes for S. stercoralis. Nematocidal activity was evaluated by the 50% paralysis concentration (PC(50)). Most of the tested isoquinoline alkaloids had activity for S. ratti and S. venezuelensis. We then evaluated in vitro cytotoxicity, which was the 50% inhibition concentration (IC(50)) of the compounds using HL60 tissue-culture cells. Three of the compounds (protopine, D-corydaline, and L-stylopine) which exhibited strong nematocidal activity, showed little cytotoxicity. In addition, we examined the relationship between nematocidal activity and cytotoxicity using the PC(50)/IC(50) ratio. A ratio equivalent to or lower than that calculated for the currently prescribed strongyloidosis treatments, ivermectin, albendazole and thiabendazole, was observed for allocryptopine, protopine, dehydrocorydaline, D-corydaline, L-stylopine, and papaverine. In contrast, the PC(50)/IC(50) ratios for protopine, D-corydaline, and L-stylopine were substantially more favorable. Therefore, protopine, D-corydaline, and L-stylopine were identified as potential effective treatments for strongyloidosis.

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.

Selection for resistance to macrocyclic lactones by Haemonchus contortus in sheep

An anthelmintic-sensitive Haemonchus contortus strain was selected for moxidectin and ivermectin resistance concurrently for 22 generations. Treatment with 0.002 mg moxidectin/kg BW or 0.02 mg ivermectin/kg BW produced >99% efficacy against the susceptible parent strain passaged for 22 generations without any anthelmintic exposure. However, to obtain similar efficacy the moxidectin-selected and the ivermectin-selected strains of H. contortus required 0.05 mg moxidectin/kg BW or 0.4 mg ivermectin/kg BW. These results indicate that development of resistance to one macrocyclic lactone, simultaneously results in resistance to another macrocyclic lactone. However, rates of resistance development differ between compounds and occurs more slowly with moxidectin than with ivermectin.

Characteristics of a flubendazole resistant isolate of Oesophagostomum dentatum from Germany

Two controlled tests were performed to investigate the benzimidazole resistance of a nodular worm isolate "GIBZ" from a pig breeding farm in Germany. In Trial I, groups of five pigs, artificially infected with Oesophagostomum larvae isolated from that farm were treated with flubendazole at a single dose of 5 mg kg(-1) bodyweight (BW) or remained untreated. In Trial II, three groups of three pigs each infected with larvae after a further laboratory passage of this isolate were treated with flubendazole either at a single dose of 5 mg kg(-1) BW or at a divided dose of 1.5 mg kg(-1) BW daily for 5 consecutive days, or with fenbendazole at a single dose of 5 mg kg(-1) BW, the fourth infected group remained untreated. The respective doses of anthelmintics were mixed with a small amount of feed and administered to individual pigs in both trials. Fecal egg counts before and after treatment and post-mortem worm burdens 7 days after (last) treatment were examined to assess the anthelmintic efficacies. Only infections with Oesophagostomum dentatum were found in both trials. In Trial I, the mean worm count reduction by flubendazole was 30% as compared to the untreated controls. In Trial II, flubendazole administered at a single or divided dose reduced the mean worm burden by 0 and 85%, respectively, whereas fenbendazole was 100% effective. These results establish resistance to flubendazole in the isolate "GIBZ" of O. dentatum. The failure to reveal side resistance to fenbendazole may be explained by that the currently recommended dose rate of this compound is supra-optimal for porcine nodular worms.

Changes in properties of adenosine transporters in Trypanosoma evansi and modes of selection of resistance to the melaminophenyl arsenical drug, Mel Cy

Resistance to arsenical drugs in trypanosomes has been linked to changes in adenosine uptake. The transport of melaminophenyl arsenicals into Trypanosoma brucei was shown to be mediated by an unusual adenosine nucleoside transporter, P2 (Carter and Fairlamb, 1993), and the loss of this transporter is associated with resistance to melaminophenyl arsenicals in these parasites. To further understand the mechanisms of arsenical resistance, we generated several lines of Mel Cy-resistant T. evansi from a drug-sensitive isolate using both in vivo and in vitro selection methods. Uptake of the melaminophenyl arsenical, Mel Cy on the P2 transporter was studied in the drug-sensitive as well as Mel Cy-resistant parasites, by means of inhibition of Mel Cy-induced lysis of trypanosomes, in an in vitro lysis assay. Adenosine uptake was also investigated using competition inhibition assays. Our study shows that T. evansi, TREU 1840, possesses the P1/P2 adenosine transport system as reported in T. brucei and T. equiperdum. However, in T. evansi, the P2 transporter is the larger transport process instead of the P1. The P2 transporter in T. evansi mediated the uptake of Mel Cy in the drug-sensitive parasites. The P2 was retained in all the arsenical-resistant T. evansi lines studied. However, the activity of the transporter was reduced to different extents in the different-resistant lines. The residual P2 activity related well to the levels of drug resistance in each line, suggesting that P2 activity could be an important marker for arsenical resistance. Furthermore, important differences were observed between the in vivo- and the in vitro-selected arsenical-resistant parasites suggesting that there may be differences in resistance phenotypes selected on the field.

Persistent efficacy of doramectin topical solution against induced infestations of Bovicola bovis and Solenopotes capillatus

A study was conducted in two locations, Wyoming and Wisconsin, USA, to evaluate the persistent efficacy of doramectin topical solution at a dose rate of 500 microg/kg body weight against artificially induced infestations of Bovicola bovis and Solenopotes capillatus on cattle. At each location, lice-free beef calves were individually housed and randomly allocated to treatment. Both B. bovis and S. capillatus were transferred from untreated donor animals to doramectin-treated cattle at the end of 35, 63, 91 or 126 day post-treatment periods. Cattle treated with a saline pour-on served as the control. Based on the geometric means of lice counts 2 weeks following transfer, the persistent efficacy of a single treatment with doramectin topical solution against induced infestations of B. bovis was 100.0, 100.0, 99.5, and 100.0% at post-treatment days of 35, 63, 91, and 126, respectively. Persistent efficacy against induced infestations of S. capillatus, for the same intervals, were 100.0, 94.9, 86.3, and 74.9%.

Molecular approaches to studying benzimidazole resistance in trichostrongylid nematode parasites of small ruminants

Molecular techniques are of growing importance in the study of anthelmintic resistance in trichostrongylid worm populations. A knowledge of the genetic determinants of benzimidazole (BZ) resistance has made it possible to construct a molecular tool for genotyping individual worms, in respect of mutation of the beta-tubulin gene responsible for BZ resistance. This tool offers new possibilities in the diagnosis of BZ resistance, and also in the study of anthelmintic use and other breeding management factors that can affect the selection of BZ-resistant alleles in worm populations. New molecular methods have also made it possible to study the origin and diversity of BZ-resistant alleles in trichostrongylid populations. The results demonstrate the value of a multidisciplinary approach to the study of anthelmintic resistance, combining molecular, ecological and epidemiological techniques.

Evaluation of beta-cyclodextrin against natural infections of cryptosporidiosis in calves

The effectiveness of beta-cyclodextrin, excipient used in pharmaceutical industry, in the treatment of natural infection by Cryptosporidium parvum in suckling calves, was evaluated. Administration of the drug at a dose of 500 mg/kg body weight for 3 consecutive days from birth (prophylactically) or following confirmation of the infection (therapeutically) decreased the severity of diarrhoea and shortened the duration of oocyst shedding.

Ethno-veterinary medicine: screening of Nigerian medicinal plants for trypanocidal properties

Trypanosoma congolense and T. brucei bloodstream form parasites were propagated axenically in suitable standard media at 34 degrees C. The effects of 33 plant extracts, fractions and pure compounds were evaluated on two clones of T. brucei and drug-sensitive and multi-drug-resistant clones of T. congolense. The cytotoxic activity of the trypanocidal extracts was also evaluated on calf aorta endothelial cells in vitro. Of the extracts tested, 22% killed T. congolense IL 1180 at a concentration of 100 microg/ml while 18% killed 90-100% of T. brucei ILTat 1.4 at the same concentration. However, 6% of the active extracts killed 93% of a dyskinetoplastid form of T. brucei IL Tat 1.1, indicating that the intact kinetoplast is a target of some of the compounds tested. Of the 12 extracts that displayed activity against drug sensitive trypanosomes, 66.7% had trypanocidal activity on a multi-drug-resistant clone, T. congolense IL 3338. The extracts of Eugenia uniflora, Acacia artaxacantha, Terminalia ivorensis, T. superba and Alchornea cordifolia had median lethal concentrations of between 13 and 69 microg/ml on both the drug-sensitive, IL 1180 and multi-drug-resistant clone, IL 3338. The median lethal doses of the active plant extracts on the calf aorta endothelial cells varied between 112 and 13750 microg/ml while the calculated selective indices ranged between 0.71 and 246.8 indicating bright prospects for the development of some of these extracts as potential trypanocidal agents.

Anthelmintic resistance on an organized sheep farm in India

Monitoring anthelmintic resistance in strongyle nematodes by the faecal egg count reduction test and a commercial larval development assay on an organized sheep farm in the semi-arid area of Rajasthan revealed the emergence of resistance to benzimidazoles and rafoxanide and a potential risk of the development of levamisole/tetramisole resistance. A benzimidazole/levamisole combination, avermectins and closantel were each found to be efficacious.

Antimicrobic and anthelmintic resistance

Antimicrobial and anthelmintic resistance are growing issues for the equine practitioner. The development of antimicrobial or anthelmintic resistance is a source of significant concern because of increased frequency of treatment failures and increased treatment costs. In addition, antimicrobial resistance may have important consequences for public health. Only through judicious use can the efficacy of antimicrobials and anthelmintics be prolonged. This article discusses the development of resistance and suggestions for control.

EFFECTS OF SIX FASCIOLICIDES AGAINST Fascioloides magna IN WHITE-TAILED DEER

Thirty-three white-tailed deer (Odocoileus virginianus) of various ages, both sexes, and in good physical condition were captured for anthelmintic evaluation of six compounds against the large American liver fluke, Fascioloides magna. Based on fluke mortality, hexachlorophene administered at the rate of 12 to 26 mg/kg of body weight was lethal to 5 of 10 mature flukes in seven deer. Nitroxynil at 11 to 24 mg/kg inhibited egg production, but did not kill mature flukes in eight deer. Rafoxanide at 12 to 25 mg/kg killed 6 of 8 (75%) immature flukes in eight deer, but was not effective against 17 mature flukes. Clioxanide at 16 to 38 mg/kg, diamphenethide at 255 to 280 mg/kg, and hexachloroethane at 463 to 629 mg/kg were not effective against F. magna in four, two and four deer, respectively. There was no indication that treatment with fasciolicides at the higher dose rates was more efficacious than at the lower dose rates. Detection of fluke eggs in the feces was a reliable method for diagnosing the presence of mature F. magna in deer prior to treatment, but was not reliable for measuring fasciolicidal activity of all compounds tested.