Faecal worm egg count analysis for targeting anthelmintic treatment in horses: Points to consider

Equine gastrointestinal nematodes are ubiquitous; in horses that graze contaminated pasture and that are not treated appropriately, large numbers of worms can accumulate, which can lead to serious clinical disease. Nematode control has traditionally followed interval treatment regimens, which involve regular anthelmintic administration to all horses based on the strongyle egg reappearance periods of each drug, usually defined around the time of licensing. Interval treatment programmes have resulted in substantial reductions in large strongyle disease, but have made major contributions to the development of anthelmintic resistance, particularly in cyathostomins. Cyathostomin resistance to 2 of the 3 available anthelmintic classes is widespread, and resistance to both classes in single populations is not uncommon. Reduced efficacy of the most commonly used macrocyclic lactone anthelmintics, as measured by shortened egg reappearance periods after treatment, is emerging in cyathostomins. Macrocyclic lactone resistance is also now commonly reported in Parascaris equorum on stud farms. Faecal worm egg counts (FWEC) are increasingly being used as part of targeted approaches to parasite control, whereby only those horses with moderate to high FWEC within a group are treated with an anthelmintic. The objective of this approach is to reduce environmental contamination, while leaving a proportion of the worm population in some horses unexposed to selection pressure for anthelmintic resistance. This article reviews recent findings in equine parasitology research that will underpin guidelines for control, with a particular focus on how to optimise the value of FWEC methodologies and anthelmintic efficacy analyses.     

Endoparasite control management on horse farms – lessons from worm prevalence and questionnaire data

Reasons for performing study: Increasing prevalence of anthelmintic resistance in equine nematodes calls for a reexamination of current parasite control programmes to identify factors influencing control efficacy and development of resistance. Objectives: To investigate if associations occur between prevalence of parasitic nematodes and management practices. Methods: German horse farms (n = 76) were investigated in 2003 and 2004. Information on farm and pasture management with respect to endoparasite control measures obtained using a questionnaire survey. Faecal examinations were performed in parallel. Results: Horses (n = 2000) were examined by faecal nematode egg counts, grouped into foals, yearlings and mature individuals for statistical analyses. Farms were categorised into 3 types, riding, stud farms and small holdings. Count regression models were used to analyse strongyle faecal egg count data. Following dichotomisation of faecal egg count (FEC) data, prevalence of strongyle and Parascaris equorum infections were assessed by logistic regression models as a function from various management factors. Yearlings on stud farms showed a 2‐fold higher risk of being positive for strongyle FEC, higher (i.e. ≥3 per year) anthelmintic drug treatment frequencies were associated with reduced strongyle infection rates only in mature individuals but not in foals or yearlings, foals on farms fertilising pastures with horse manure had a significantly higher risk of being P. equorum FEC positive and yearlings on stud farms were more often showing incomplete FECR following anthelmintic treatment compared to yearlings on other farm types. The mean yearly treatment frequencies per age group were: foals 4.52, yearlings 3.26 and mature horses 2.72 times, respectively. Conclusion and potential relevance: To delay the development of anthelmintic, resistance management should include additional nonchemotherapeutic parasite control strategies, FEC‐monitoring, controlled quarantine treatment of new arrivals and control of efficacy by the faecal egg count reduction test on a regular basis.     

Acute small intestinal obstruction associated with Parascaris equorum infection in young horses: 25 cases (1985–2004)

AIMS: To retrospectively evaluate the medical and surgical records of horses with acute small intestinal obstructions associated with Parascaris equorum infection; to describe the gastrointestinal lesions; and to determine the outcome of cases with such lesions.

METHODS: Records of 25 horses with acute small intestinal obstruction associated with P. equorum between 1985 and 2004 were reviewed to determine signalment, history, physical examination, surgical or post-mortem findings, and outcome.

RESULTS: All horses except one were less than 12 months old. Standardbreds were over-represented in the population studied. Sixteen horses (72%) had been administered anthelmintics, including pyrantel (n=8), ivermectin (n=7), and trichlorphon (n=1), within 24 h prior to the onset of colic. Of the 25 cases reviewed, 16 had simple obstructive ascarid impactions (SOAIs), and nine had complicated obstructive ascarid impaction (COAI) including volvulus (n=6) or intussusception (n=3), both concurrent with ascarid impaction of the small intestine. Short-term survival (discharge from hospital) occurred in 79% of horses treated for SOAI, and was 64% for all horses. Long-term survival (>1 year) occurred in 33% of horses with SOAI, and the overall long-term survival was 27% for all horses. Formation of adhesions was the most frequent finding associated with death for horses that did not survive long-term.

CONCLUSIONS AND CLINICAL RELEVANCE: The incidence of anthelmintic treatment within 24 h of the onset of colic in this study population (72%) was higher than that previously reported. Resistance of P. equorum to ivermectin recently reported in Ontario may be associated with increased ascarid burdens, predisposing horses to ascarid impaction. The long-term survival of these horses was better than that reported previously.     

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.     

A survey of seasonal patterns in strongyle faecal worm egg counts of working equids of the central midlands and lowlands, Ethiopia

A study was conducted for two consecutive years (1998–1999) to determine the seasonal patterns of strongyle infection in working donkeys of Ethiopia. For the purpose 2385 donkeys from midland and lowland areas were examined for the presence of parasitic ova. A hundred percent prevalence of strongyle infection with similar seasonal pattern of strongyle faecal worm egg output was obtained in all study areas. However, seasonal variations in the number of strongyle faecal worm egg output were observed in all areas. The highest mean faecal worm egg outputs were recorded during the main rainy season (June to October) in both years in all areas. Although an increase in the mean strongyle faecal egg output was obtained in the short rainy season (March–April) followed by a drop in the short dry season (May), there was no statistically significant difference between the short rainy season and long dry season (Nov–Feb) (P > 0.05). A statistically significant difference however, was obtained between the main rainy season and short rainy season, and between the main rainy season and dry season (P < 0.05). Based on the results obtained it is suggested that the most economical and effective control of strongyles can be achieved by strategic deworming programme during the hot dry pre-main rainy season (May), when the herbage coverage is scarce and helminthologically ‘sterile’, and the arrested development of the parasites is suppose to be terminating. This could insure the greatest proportion of the existing worm population to be exposed to anthelmintic and also reduces pasture contamination and further infection in the subsequent wet season.     

Investigation of seroprevalence of <Emphasis Type="Italic">Theileria equi</Emphasis> and <Emphasis Type="Italic">Babesia caballi</Emphasis> in horses in Nigde province,Turkey

The prevalence of equine piroplasmosis caused by Theileria equi and Babesia caballi in Nigde, in central Anatolia, Turkey has remained unknown. Serum samples were obtained from a total of 125 horses and were tested for antibodies to T. equi and B. caballi using the Indirect Fluorescence Antibody Test (IFAT). Twenty-three (18.4%) horses were seropositive for equine piroplasmosis. Anti-T. equi was observed in 16 horses (12.8%) while anti-B. caballi was detected in 12 horses (9.6%). In addition, 5 serum samples were positive for both parasites. The prevalence rates of antibodies to T. equi and B. caballi for female and male horses were statistically indifferent (p = 0.19 and 0.90). The difference between the seropositivity rates to T. equi among age groups was statistically insignificant (p = 0.44) while the difference to B. caballi among age groups is statistically significant (p = 0.01). Seropositivity rates ranged from 2.9% to 25.7% for T. equi and 2.9% to 14.3% for B. caballi from the selected districts in Nigde. A statistically significant difference on seropositivity rates for the study sites was observed for only T.equi (p = 0.03). This study indicates that T. equi is higher than B. caballi in Nigde. This study was supported by the Scientific Research Projects Unit of Nigde University (FEB 2007/08).     

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 egg-excreting 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 > or =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.     

Effectiveness of oxibendazole against benzimidazole-resistant strongyles in horses

Twenty-eight horses with a residual burden of strongyle eggs in the faces after treatment with mebendazole (MBZ) paste were treated with a suspension of either MBZ or oxibendazole (OBZ). Fecal samples were collected before and 14 days after these treatments. The number of strongyle eggs/g (epg) of feces for each horse was estimated using the Cornell-McMaster dilution and the Cornell-Wisconsin double centrifugation procedures. The epg for each horse was transformed using log (× + 1) and in an analysis of variance of the reduction in egg count for each horse on the logarithmic scale, there was a highly significant difference between the treatments. The mean epg was increased in the MBZ-treated horses and reduced in the OBZ-treated horses, but the reduction was only by 82% with an upper confidence limit of 89%. Subsequently, the horses were retreated with MBZ and OBZ suspensions without significant reduction in the mean epg for OBZ-treated horses.     

A survey on the prevalence of internal parasites of equines in macedonia and Thessalia-Greece

A 3-year survey was conducted to determine the incidence of endoparasitic infection of equines in several regions of Macedonia and Thessalia, Greece. Three hundred fecal and 252 blood samples were examined. Fecal examination was carried out using. Teleman and Faust techniques while blood was examined according to the modified Knott technique. The Graham test was also performed. The prevalence of infection found was 62.4% for horses, 75.7% for donkeys and 89.2% for mules. Strongyles (large and small) were the most common parasitic species found, with a corresponding prevalence of 42.5% and 45.6% for horses, 73.0% and 37.8% for donkeys, and 89.2% for mules. Other parasites found were: Eimeria leuckarti (2.6% in horses and 8.1% in donkeys), Anoplocephala perfoliata (0.4% in horses), Parascaris equorum (1.7% in horses), Oxyuris equi (4.1% in horses and 8.7% in mules), Strongyloides westeri (2.2% in horses, 5.4% in donkeys and 10.8% in mules), Dictyocaulus arnfieldi (0.9% in horses and 2.7% in donkeys) and Setaria equina (2.2% in horses and 2.7% in donkeys).     

Effectiveness of current anthelmintic treatment programs on reducing fecal egg counts in United States cow-calf operations

During the United States Department of Agriculture (USDA) National Animal Health Monitoring System’s (NAHMS) 2007–2008 beef study, producers from 24 states were offered the opportunity to evaluate their animals for internal parasites and for overall responses to treatment with anthelmintics. A lapse of 45 d was required between initial sampling and any previous treatments. Choice of anthelmintic (oral benzimidazoles, and both injectable and pour-on endectocides) was at the discretion of the producer so as not to alter the local control programs. Fresh fecal samples were collected from 20 animals, or from the entire group if less than 20, then randomly assigned to 1 of 3 participating laboratories for examination. Analyses consisted of double centrifugation flotation followed by enumeration of strongyle, Nematodirus, and Trichuris eggs (the presence of coccidian oocysts and tapeworm eggs was also noted). Where strongyle eggs per gram (epg) exceeded 30, aliquots from 2 to 6 animals were pooled for egg isolation and polymerase chain reaction (PCR) analysis for the presence of Ostertagia, Cooperia, Haemonchus, Oesophagostomum, and Trichostrongylus. Results from 72 producers (19 States) indicated that fecal egg count reductions were < 90% in 1/3 of the operations. All operations exhibiting less than a 90% reduction had used pour-on macrocyclic lactones as the anthelmintic treatment. While some of these less than expected reductions could have been the result of improper drug application, PCR analyses of the parasite populations surviving treatment, coupled with follow-up studies at a limited number of sites, indicated that less than expected reductions were most likely due to anthelmintic resistance in Cooperia spp. and possibly Haemonchus spp.     

Control of endoparasites in horses with a gel containing moxidectin and praziquantel

A gel formulation containing moxidectin (20 g/kg) and praziquantel (125 g/kg) reduced the geometric mean faecal strongyle egg count in horses to below 100 eggs per gram of faeces (epg) for at least 12 weeks despite their being exposed continuously to reinfection from pasture grazed by treated and untreated horses. The geometric mean egg count of horses treated with a proprietary paste containing abamectin (3.7 g/kg) and praziquantel (46.2 g/kg) increased steadily from six weeks after the treatment, peaking at over 820 epg after 12 weeks. Relative to the efficacy of the abamectin/praziquantel treatment, the reduction in mean faecal egg count compared with the pretreatment counts was significantly (P<0.05) better in the horses treated with moxidectin and praziquantel from eight weeks after the treatment. Both products eliminated tapeworms from horses in a non-invasive modified critical trial.     

Prevalence and Significance of Parasites of Horses in Some States of Northern Nigeria

This study was conducted to determine the prevalence and significance of parasites of horses in northern Nigeria. Blood and faecal samples were randomly collected from 243 horses from different stables in some states of northern Nigeria for laboratory analyses. Fifty-seven horses (23.5%) were found infected with parasites. The hemoparasites detected, 21 (8.6%), include Theileria equi, Babesia caballi, Trypanosoma vivax and Trypanosoma evansi. The endoparasites encountered, 29 (11.9%) were Strongylus spp., Strongyloides spp., Oxyuris equi, Parascaris equorum, Paragonimus spp. and Dicrocoelium spp., 3 (1.2%) was Eimeria spp. Four horses (1.6%) had mixed infection of hemo- and endoparasites. This preliminary finding shows that parasitism is a problem in the horse stables examined, and calls for proper stable hygiene, routine tick control and regular deworming programme.     

Lessons learned from a strangles outbreak on a large Standardbred farm

Streptococcus equi subspecies (ssp.) equi infection (strangles) remains one of the most frequently diagnosed and costly infectious diseases of horses. Large breeding herds, where a disease outbreak competes for personnel and financial resources needed for foaling management, present a special challenge for equine practitioners. A 15‐month outbreak involving 62 clinical cases of strangles occurred on a large Standardbred breeding farm (average population of 1400 horses). Sixteen asymptomatic horses were found to be PCR (polymerase chain reaction)‐positive for S. equi ssp. equi. During the outbreak, serological samples from 48 clinically normal horses were found to be seropositive for S. equi ssp. equi, confirming herd‐wide exposure. After several clinical cases of strangles had been diagnosed, an intranasal S. equi ssp. equi vaccine was administered to clinically normal horses (n = 558) considered to be at risk of exposure. Strangles complications included 7 fatalities (none in vaccinated horses) and 6 cases of purpura haemorrhagica (4 in vaccinated horses). Midway through the outbreak, injectable, sustained release ceftiofur crystalline free acid (CCFA), given as an initial dose followed by a second dose 4 days later, was used exclusively for systemic antimicrobial treatment of clinically affected and PCR‐positive horses. This antimicrobial regimen coincided with a reduction in disease incidence and eventual resolution of the outbreak. Two horses with persistent guttural pouch infection were endoscopically confirmed as carrier horses. The herd history demonstrated that a strangles outbreak will often result in asymptomatic carrier horses and that identification and treatment of these horses are necessary to eliminate long‐term sources of infection. Ceftiofur crystalline free acid was found to be a suitable antimicrobial due to its activity against S. equi ssp. equi and the efficiencies associated with twice parenteral dosing during a 10‐day treatment period. Occurrence of purpura in 4 vaccinated horses suggests that vaccination should be reserved for healthy seronegative horses and avoided during an active outbreak.     

Phenothiazine in the origin of benzimidazole resistance in population-B equine strongyles

Clinical trials on a central Kentucky Thoroughbred Farm (B) during 1960-1961 involved nine test treatments of mares, yearlings and weanlings (n = 124), and compared efficacies of phenothiazine (PTZ), piperazine (PPZ), mixtures of PTZ + PPZ and thiabendazole (TBZ) for (1) percent reduction and (2) complete clearance or graded reduction of strongyle egg per gram (epg) counts. Three grades of PTZ (N.F. green, micronized purified and 2-3 microns purified) at the dose rate of 55 mg kg-1 were ineffective in six tests; reduction of strongyle epg count were low grade (26%) and complete clearance was found in only 1 of 32 horses. Piperazine alone at 88 mg base kg-1 was efficacious in two tests; epg counts were effectively reduced (98%), but complete clearance was found in only 2 of 10 horses. Several mixtures of PTZ + PPZ with various dosages of PTZ (27.5-55 mg kg-1) plus PPZ base (88 mg kg-1) were quite effective in six tests, as epg counts were reduced by 96% and complete clearance occurred in 25 of 38 horses. Four trials with TBZ at dose rates between 13 and 44 mg kg-1 in 1961 reduced epg counts by 98%, but cleared only 15 of 36 horses. These data indicated: (1) a large segment of the Farm B small strongyle population was resistant to PTZ at the outset of these observations in 1960; (2) TBZ-resistant individuals were also present in this population when TBZ was first used in 1961. During the ensuing 4-year study (1962-1965) on Farm B mares (n = 15 per year) and yearlings (n = 15-17 per year), biweekly epg and larvae per gram (lpg) counts were carried out to compare the efficacy of three treatment programs: (1) monthly doses of TBZ at 26 mg kg-1; (2) monthly doses of TBZ at 13 mg kg-1; (3) bimonthly doses of a mixture of PTZ + PPZ and carbon disulfide (CS2) at dose rates of 27.5 + 88 and 78 mg kg-1, respectively, or a mixture of PTZ + PPZ + trichlorfon (TCF) at dose rates of 27.5 + 88 + 40 mg kg-1, respectively. Emergence of a TBZ-resistant segment of the small strongyle infections was evaluated by three criteria: (1) annual mean strongyle epg counts; (2) efficacy of individual treatments for epg reductions (%); (3) complete clearance or graded reductions of strongyle epg counts.4+ PTZ-resistant small strongyles. Initial use of TBZ in 1961 produced effective epg reductions, but complete clearance data revealed the presence of TBZ-resistant small strongyles at the outset...     

Evaluation of exclusive use of ivermectin vs alternation of antiparasitic compounds for control of internal parasites of horses.

A study for about a 30-month period was done to compare strongyle control programs, using per os treatments of ivermectin (IVE) paste exclusively or alternation of 4 antiparasitic paste compounds: IVE, oxfendazole (OFZ), oxibendazole (OBZ), or pyrantel pamoate (PRT). Every 8 weeks, 1 group of horses (barn C; n = 14 to 16) was given IVE paste exclusively, and a second group (barn E; n = 16) was given the 4 antiparasitic pastes on an alternating schedule. Worm eggs and larvae per gram of feces (epg and lpg, respectively) values were determined every 2 weeks during the investigation. This study in grazing horses (mares and fillies), naturally infected with internal parasites, was conducted during the period between Oct 22, 1987 and Feb 8, 1990, with an additional observation on Mar 28, 1990. For barn-C horses, treated exclusively with IVE (200 micrograms/kg of body weight) 14 times, 2-week posttreatment mean strongyle epg and lpg (small strongyle) values were reduced 99 to 100%. Mean strongyle epg and lpg (small strongyle) values for each 2-week sample period remained low (less than 20) throughout the study period, except for 1 moderate transient increase in July 1988. For the entire study period, the aggregate mean strongyle epg value was 12 and the lpg value was 6. Two-week posttreatment mean strongyle epg and lpg (small strongyle) values for barn-E horses, treated alternately with therapeutic (approx) dosage of IVE (200 micrograms/kg; 4 times), OFZ (10 mg/kg; 5 times), OBZ (10 mg/kg; 4 times), or PRT (6.6 mg base/kg; 2 times), varied within and between compounds.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of febantel used concurrently with piperazine citrate in horses

Fifty horses from a herd known to have benzimidazole-resistant small strongyles were treated with febantel (6 mg/kg), combinations of febantel (6 mg/kg) and piperazine citrate (25 or 55 mg base/kg), thiabendazole (44 mg/kg), or placebo (0.6 ml of water/kg). Pretreatment and 7-day posttreatment fecal examinations were done. Fecal cultures, strongyle egg per gram (epg) counts, sugar flotation fecal examinations, and in vitro testing for benzimidazole resistance were performed. Results of fecal examinations before treatment were similar in all horses, and results of testing were positive for benzimidazole resistance. Horses treated with febantel and piperazine at all dosages had significantly lower mean strongyle epg counts and greater percentage reduction in mean strongyle epg counts (99.7% to 99.9%) 7 days after treatment, compared with those determined for horses treated with febantel, thiabendazole, or placebo. Adverse reactions to treatment were not observed.     

<Emphasis Type="Italic">Theileria</Emphasis> (<Emphasis Type="Italic">Babesia</Emphasis>) <Emphasis Type="Italic">equi</Emphasis> and <Emphasis Type="Italic">Babesia caballi</Emphasis> Infections in Horses in Galicia,Spain

The control of equine piroplasmosis is becoming increasingly important to maintain the international market open to the horse industry. The purpose of this study was to demonstrate the occurrence of equine piroplasmosis (Theileria equi and Babesia caballi) in Galicia, north-west Spain, and to compare haematological and serum biochemistry parameters between non-parasitaemic horses and horses parasitaemic with T. equi and B. caballi. Sixty serum samples (control group) were taken from healthy horses pastured on two farms, and examined for evidence of equine T. equi and B. caballi infection by indirect fluorescent antibody test (IFAT). Of the 60 samples, 24 (40%) and 17 (28.3%) samples were positive for T. equi and B. caballi, respectively. Twelve (20%) samples were positive for both parasites. Haematology and serum biochemistry were compared between controls and a series of 36 horses clinically affected by T. equi (25) or B. caballi (11). Compared with the healthy group, there was a 43% and 37% decrease in the haematocrit for T. equi and B. caballi infection, respectively. Parasitaemic horses presented an intense anaemia and serum biochemistry signs of liver damage. The anaemia was more severe in T. equi-infected than in B. caballi-infected horses. Our results suggest that equine piroplasmosis is widespread in the region and is a cause for concern.     

Seasonal patterns of strongyle infections in grazing sheep under the traditional production system in the region of Trikala, Greece

Ten 4-month-old female sheep of the Karagouniko dairy breed were used to monitor the seasonal fluctuations of strongyle infections in sheep during the first year of grazing under the traditional production system in the region of Trikala, Greece, where control of nematodes is currently based exclusively on the frequent use of anthelmintics which might contribute to the appearance of anthelmintic resistance. The sheep grazed communal pasture plots and did not receive any anthelmintic treatment during the entire study period. Faecal egg counts (epg), plasma pepsinogen levels, blood values, and genera of parasitic strongyles recovered from coprocultures were recorded monthly. Mean epg for strongyle-type eggs were significantly higher during summer. Mean plasma pepsinogen levels were significantly higher during spring and summer. The genera of parasitic strongyles recovered from the faecal cultures were Haemonchus, Ostertagia, Trichostrongylus, Cooperia, Nematodirus, Oesophagostomum, and Chabertia. The percentages of larvae for Haemonchus, Ostertagia, Cooperia, and Oesophagostomum were significantly higher during winter. The percentages of larvae for Trichostrongylus were significantly higher during summer. During summer, levels of RBC, HGB, and HCT were depressed and levels of MCV, MCH, and MCHC were elevated. Mean epg for strongyle-type eggs had a significant inverse correlation with RBC, HGB, HCT, and positive correlation with MCH, MCHC, and percentages of eosinophils in differential leucocyte counts. The seasonal pattern of infection observed in the present study indicates that it is possible to decrease the number of treatments to one per year, thus, reducing the possibility for the appearance of anthelmintic resistance.     

Common helminth infections of donkeys and their control in temperate regions

Roundworms and flatworms that affect donkeys can cause disease. All common helminth parasites that affect horses also infect donkeys, so animals that co‐graze can act as a source of infection for either species. Of the gastrointestinal nematodes, those belonging to the cyathostomin (small strongyle) group are the most problematic in UK donkeys. Most grazing animals are exposed to these parasites and some animals will be infected all of their lives. Control is threatened by anthelmintic resistance: resistance to all 3 available anthelmintic classes has now been recorded in UK donkeys. The lungworm, Dictyocaulus arnfieldi, is also problematical, particularly when donkeys co‐graze with horses. Mature horses are not permissive hosts to the full life cycle of this parasite, but develop clinical signs on infection. In contrast, donkeys are permissive hosts without displaying overt clinical signs and act as a source of infection to co‐grazing horses. Donkeys are also susceptible to the fluke, Fasciola hepatica. This flatworm can be transmitted, via snails and the environment, from ruminants. As with cyathostomins, anthelmintic resistance is increasing in fluke populations in the UK. A number of the anthelmintic products available for horses do not have a licence for use in donkeys, and this complicates the design of parasite control programmes. As no new equine anthelmintic classes appear to be near market, it is important that the efficacy of currently effective drugs is maintained. It is important that strategies are used that attempt to preserve anthelmintic efficacy. These strategies should be based on the concept that the proportion of worms in a population not exposed to anthelmintic at each treatment act as a source of ‘refugia’. The latter is an important factor in the rate at which resistance develops. Thus, it is imperative that parasite control programmes take into account the need to balance therapy to control helminth‐associated disease with the requirement to preserve anthelmintic effectiveness.     

Anthelmintic resistance in equine parasites - detection, potential clinical relevance and implications for control

During the past two decades anthelmintic resistance in equine parasites has been found in the group of small strongyle species (cyathostomins) and in the ascarid species Parascaris equorum. The ubiquitous nature and possible severe consequences of disease with these nematodes make them the prime targets of current worm control programmes. Traditional control strategies mainly rely on the strategic application of anthelmintics, currently represented by three major drug classes: benzimidazoles (BZ), the tetrahydropyrimidine pyrantel (PYR) and macrocyclic lactones (ML). Following decades of routine and frequent anthelmintic applications, many cyathostomin populations on horse farms in industrialised countries must be considered as resistant to BZ anthelmintics. However, to date no published cases of cyathostomin disease specifically associated with anthelmintic resistance were reported. Possibly this is due to the generally subclinical and unspecific symptoms associated with cyathostomin infections. Nevertheless, exclusive reliance on the ML drug class may increase the threat of clinical disease due to drug-resistant cyathostomins. More recently, P. equorum has been reported as having developed resistance against ivermectin and moxidectin, two representatives of the ML-class. These anthelmintics are currently the most frequently used drug class in horses. This nematode species is mainly found in foals and in younger horses due to the development of immunity following exposure to infection. Infection with P. equorum can result in clinically drastic consequences such as obstruction and/or penetration of the small intestine, the latter usually leading to death. In conclusion, on horse farms the efficacy of anthelmintic treatments should be examined routinely for each drug class. Several factors can influence the rate at which anthelmintic resistance develops; high frequency of treatment being one of the most important. Modern control strategies should therefore attempt to significantly reduce anthelmintic treatments. Several pasture and farm management practices found to be negatively associated with nematode and anthelmintic resistance prevalence will be discussed in the review presented here.