Treatment and prevention of intestinal parasite-associated disease.

Since 1917, only 11 new endoparasiticides have been developed for the horse, of which five chemical classes are in common use. The selection pressure of frequent administration of deworming doses for parasite control programs has been associated with the development of resistance of small strongyle parasites to the effects of benzimidazoles and pyrantel salts. Against the background of the inevitability of the occurrence of ivermectin/moxidectin resistance, responsible use of equine anthelmintics based on the clinical pharmacology of the compounds and the biology/epidemiology of intestinal parasites is a major issue for equine clinicians. The evidence base for the recommendations for treatment and control of equine intestinal parasites is sparse, however, and few robust data exist from controlled clinical trials to validate current guidelines.     

Sustainable equine parasite control: perspectives and research needs

Clinically important equine parasites are ubiquitous in managed horse populations. The traditional approach to parasite control is frequent administration of anthelmintics to all horses on a farm. However, increasing levels of anthelmintic resistance is forcing horse owners and veterinarians to shift this control paradigm. Treatment regimens involving routine deworming of all horses throughout the year are now being replaced by more sustainable approaches, which take in to account the importance of maintaining adequate parasite refugia. The selective therapy principle has been recommended for more than 15 years, but there is limited experience with this approach. The relative magnitude of the faecal egg count for an individual horse is a consistent trait, and this provides a reliable basis for selective therapy. But no studies have evaluated the consequences of selective therapy in the long-term, and such studies are strongly needed to validate this approach. Importantly, it remains unclear how selective therapy may affect the prevalence and intensity of other parasites of significant pathogenic potential (e.g. Strongylus vulgaris), which have become uncommon due to years of intensive chemotherapy. Consequently, a selective approach requires vigilant surveillance of the parasite fauna and intensity. This places a demands for reliable diagnostic tools. Also noteworthy is the fact that the majority of equine nematode parasites are more pathogenic during their larval stages, when they cannot be detected by traditional egg counting techniques. Consequently, parasite-specific diagnostic tools capable of assessing prepatent parasite burdens, and able to differentiate between strongyle species of different pathogenic potentials, would be of great value to the equine clinician. Tools for detecting infections with the tapeworm Anoplocephala perfoliata are laborious, difficult to interpret, and at present there is no established method to evaluate treatment efficacy. Thus, better diagnostic tools are needed for tapeworms as well. Biological control, especially the predacious fungi have demonstrated good potential as an adjunct for strongyle control and such a product could easily have a market in equine establishments. In summary, there is general agreement that the traditional treat-all at frequent interval approach should be abandoned, and that optimal parasite control can be maintained with far fewer anthelmintic treatments. But better diagnostic techniques and more evidence documenting the long-term consequences of selective therapy programs are needed to develop and validate systems for sustainable equine parasite control.     

Practical aspects of equine parasite control: A review based upon a workshop discussion consensus

Development of resistance of several important equine parasites to most of the available anthelmintic drug classes has led to a reconsideration of parasite control strategies in many equine establishments. Routine prophylactic treatments based on simple calendar‐based schemes are no longer reliable and veterinary equine clinicians are increasingly seeking advice and guidance on more sustainable approaches to equine parasite control. Most techniques for the detection of equine helminth parasites are based on faecal analysis and very few tests have been developed as diagnostic tests for resistance. Recently, some molecular and in vitro based diagnostic assays have been developed and have shown promise, but none of these are currently available for veterinary practice. Presently, the only reliable method for the detection of anthelmintic resistance is a simple faecal egg count reduction test, and clinicians are urged to perform such tests on a regular basis. The key to managing anthelmintic resistance is maintaining parasite refugia and this concept is discussed in relation to treatment strategies, drug rotations and pasture management. It is concluded that treatment strategies need to change and more reliance should now be placed on surveillance of parasite burdens and regular drug efficacy tests are also recommended to ensure continuing drug efficacy. The present review is based upon discussions held at an equine parasite workshop arranged by the French Equine Veterinary Association (Association Vétérinaire Equine Française, AVEF) in Reims, France, in October 2008.     

Molecular diagnosis and equine parasitology

The future implementation of improved and sustainable control strategies for the major equine parasites will be dependent on a greater insight into their basic biology, pathogenicity and epidemiology together with an enhanced ability for accurate diagnosis. This paper will provide a review of the current molecular methods under development for the detection of equine parasites and their application to current scientific questions. In particular, the strongyles are recognised as important pathogens of horses and recent advances made in the study of this parasitic group at the single species level will be addressed. The ribosomal (r)DNA region of the parasite genome has been employed to distinguish between closely related species. Molecular probes designed to this target region were used in combination with PCR technology to allow the identification of individual species within mixed infections. They have been applied to all parasite stages to look at the role of individual species in natural infection, disease and drug resistance. Similar techniques have been developed to detect other equine parasites and these will also be discussed. Further opportunities for employing existing techniques and the need for new diagnostic tools will be highlighted.     

Climatic influences on development and survival of free-living stages of equine strongyles: implications for worm control strategies and managing anthelmintic resistance

Development of resistance to anthelmintic drugs by horse strongyles constitutes a growing threat to equine health because it is unknown when new drug classes can be expected on the market. Consequently, parasite control strategies should attempt to maintain drug efficacy for as long as possible. The proportion of a parasite population that is not exposed to anthelmintic treatment is described as being "in refugia" and although many factors affect the rate at which resistance develops, levels of refugia are considered the most important as these parasites are not selected by treatment and so provide a pool of sensitive genes in the population. Accordingly, treatment should be avoided when pasture refugia are small because such treatments will place significant selection pressure for resistance on worm populations. Given this new paradigm for parasite control, it has become important to identify seasons and circumstances wherein refugia are diminished. Free-living stages of equine strongyles are highly dependent on climatic influences, and this review summarises studies of strongyle development and survival under laboratory and field conditions in Northern (cool) temperate, Southern (warm) temperate and subtropical/tropical climates. In Northern temperate climates, refugia are smallest during the winter. In contrast, refugia are lowest during the summer in warm temperate and subtropical/tropical climates. Although adverse seasonal changes clearly have significant effects on the ability of free living stages of strongyle nematode parasites to survive and develop, available data suggest that climatic influences cannot effectively "clean" pastures from one grazing season to the next.     

The Domestic Livestock Resources of Turkey: Occurrence and Control of Diseases of Horses,Donkeys and Mules

Horses, donkeys, and mules have been important in Turkey for agriculture, transport, and the military for hundreds of years. Equids number more than 0.5 million in Turkey. Most horses are local types but emphasis is now on Thoroughbreds and Arabians for racing and competitions. New roles have not materialized for donkeys and mules that continue to perform their traditional activities. Disease control is assured mainly by public services acting within laws governing diseases and welfare. African horse sickness, glanders, dourine, equine infectious anemia, vesicular stomatitis, equine encephalomyelitis, anthrax, and rabies are notifiable diseases but none of the mainly equine diseases has been reported in Turkey for many years. Several zoonoses, including toxoplasmosis, brucellosis, listeriosis, and Rhodococcus infections, have been identified by serodiagnosis over wide areas, but animals carrying antibodies rarely exhibit clinical symptoms. Among other diseases in Turkish Equidae are piroplasmosis, respiratory infections, contagious equine metritis and equine influenza. Other viral and bacterial pathogens have been identified in isolated investigations. Internal and external parasites are a major cause of economic loss. Much research on equine diseases has been undertaken in the last decade of the 20th and first decade of the 21st centuries, perhaps because of Turkey's possible accession to the European Union and the goal to harmonize identification and control procedures and also because of the country's increasing participation in international equine events. This paper reviews animal health services and provides a bibliography of more than 100 references covering horse, donkey, and mule diseases in Turkey.     

Viruses associated with respiratory disease of horses in New Zealand: an update

Viruses causing or associated with respiratory disease in horses worldwide are reviewed. Results are presented from a serological survey of 121 New Zealand foals and horses that had been affected by respiratory disease, determining the prevalence of antibodies in this country to the major viruses associated with similar disease overseas. To date there is no evidence of equine influenza virus in New Zealand. Both equine herpesvirus type 1 and 2 have been frequently isolated and show high serological prevalences. Serological evidence of equine rhinovirus type 1 and type 2 is presented with a prevalence of 12.3% and 41.2% respectively observed in foal sera, and 37.7% and 84.9% in adult horse sera. Antibody reacting to equine viral arteritis virus antigen was detected in 3/121 test sera. Equine adenovirus has been isolated on occasions and has shown a 39% serological prevalence in one study reviewed. Progress in New Zealand equine virus research is discussed.     

An update on cyathostomins: Anthelmintic resistance and worm control

Intestinal nematodes are an important cause of equine disease. Of these parasites, the Cyathostominae are the most important group, both in terms of their prevalence and their pathogenicity. Cyathostomin infections are complex and control is further complicated by ever‐increasing levels of resistance to some of the commonly used anthelmintics. There are no new equine anthelmintics under development, so it is imperative that the efficacy of any currently‐effective drug classes be maintained for as long as possible. It is believed that the proportion of refugia (i.e. the percentage of parasites not exposed to a drug at each treatment) is one of the most crucial factors in determining the rate at which anthelmintic resistance develops. It is important, therefore, that levels of refugia be taken into account when designing nematode control programmes for horses. This can be assisted by knowledge of the local epidemiology of the infection, supplemented by faecal egg count analysis to identify those animals that are making the major contribution to pasture contamination. This type of rational nematode control requires equine veterinary surgeons to get involved in designing and implementing deworming programmes. The advice given must be based on a combination of knowledge of cyathostomin biology and epidemiology as well as an awareness of the parasite population's current drug sensitivity and a sound history of husbandry at the establishment. As anthelmintic resistance will be the major constraint on the future control of cyathostomins, researchers are now actively investigating this area. Studies are underway to develop tests that will enable earlier detection of anthelmintic resistance and an assay that will help identify those horses that require anthelmintic treatments targeted at intestinal wall larvae.     

Resistance to benzimidazole anthelmintics in small strongyles (Cyathostominae) of horses in Denmark.

This study was undertaken to establish whether anthelmintic resistance was present in nematode parasites of horses in Denmark. Sixteen horse farms were selected for faecal egg count reduction (FECR) tests to measure the efficacy of the anthelmintic used. Resistance to benzimidazole anthelmintics was found on 13 of the 16 farms, with FECR values ranging from 80.0% to -101.3%. On the remaining 3 farms FECR was 100.0%, 99.3% and 97.2%. Results of a questionnaire study on anthelmintic usage, parasite control measures and management practices showed that horses in this study were treated on average 7.1 times/year. Horse owners changed between preparations of drugs but almost only within the same class of anthelmintics. Nine owners gave an anthelmintic treatment to purchased horses before they were introduced on the farm. On 14 farms, the same paddock was grazed every year and the average stocking rate was estimated to be 2.4 horses/ha. Strategies to avoid development of anthelmintic resistance are discussed and recommendations of parasite control on horse farms are presented.     

Advances in equine immunology: Havemeyer workshop reports from Santa Fe,New Mexico,and Hortobagy,Hungary

The horse has been human kind's most important partner throughout history. Similarly, in the field of immunology, many critical scientific advances have depended on the horse. Equine immunology today is an active and important field of study, with a focus on control of many common infectious diseases and immunopathologic conditions of broad comparative interest. In 2001 two major equine immunology workshops were held, in Santa Fe, USA, and in Hortobagy, Hungary, with major sponsorship from the Havemeyer Foundation. This report summarizes the scientific themes and foci of those meetings.     

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.     

Molecular evidence for transplacental transmission of Theileria equi from carrier mares to their apparently healthy foals

The intra-erythrocytic parasite Theileria equi is one of two tick-transmitted causative agents of equine piroplasmosis. Piroplasms of T. equi can be transmitted across the equine placenta and once a horse is infected, it appears to remain a lifelong carrier, since anti-theilerial drugs suppress but do not eliminate the parasite. Carrier mares may transmit the organism to their offspring and this may result in abortion or neonatal piroplasmosis, but observations by some researchers suggest that foals may be born as carriers yet remain apparently healthy. Using a T. equi-specific oligonucleotide probe, we have determined that transplacental transmission occurs early in equine foetal development and that carrier mares may give birth to healthy carrier foals. Investigation of parasite levels and the effect of maternal colostrum on the newborn suggests that colostral T. equi antibody may act to suppress parasitaemia in the newborn, reducing the incidence of clinical neonatal piroplasmosis.     

Strongylus vulgaris associated with usage of selective therapy on Danish horse farms-Is it reemerging?

Nematodes belonging to the order Strongylida are ubiquitous in grazing horses, and the large strongyle Strongylus vulgaris is considered the most pathogenic. This parasite was originally described widely prevalent in equine establishments, but decades of frequent anthelmintic treatment appears to have reduced the prevalence dramatically. Increasing levels of anthelmintic resistance in cyathostomin parasites have led to implementation of selective therapy to reduce further development of resistance. It has been hypothesized that S. vulgaris could reoccur under these less intensive treatment circumstances. The aim with the present study was to evaluate the occurrence of S. vulgaris and the possible association with usage of selective therapy. A total of 42 horse farms in Denmark were evaluated for the presence of S. vulgaris using individual larval cultures. Farms were either using a selective therapy principle based on regular fecal egg counts from all horses, or they treated strategically without using fecal egg counts. A total of 662 horses were included in the study. Covariate information at the farm and horse level was collected using a questionnaire. The overall prevalence of S. vulgaris was 12.2% at the individual level and 64.3% at the farm level. Farms using selective therapy had horse and farm prevalences of 15.4% and 83.3%, respectively, while the corresponding results for farms not using selective therapy were 7.7% and 38.9%. These findings were found statistically significant at both the horse and the farm level. Stud farms using selective therapy were especially at risk, and occurrence of S. vulgaris was significantly associated with the most recent deworming occurring more than six months prior. The results suggest that a strict interpretation of the selective therapy regimen can be associated with an increased prevalence of S. vulgaris. This suggests that modifications of the parasite control programs could be considered on the studied farms, but it remains unknown to which extent this can be associated with increased health risks for infected horses.     

A survey of equine parasite control practices in Tennessee

A weighted, random sample of 130 horse owners in Tennessee was selected from subscribers to an equine newsletter published by the Agricultural Extension Service. Data about the participants' farms, horses, parasite control practices, and sources of information regarding equine parasite control were obtained through a telephone survey. The response rate was 98%. The typical respondent kept 4 horses on 2 pastures totaling 15 acres, and did not rotate pastures as a parasite control measure. The majority (83%) of horse owners indicated that they administered anthelmintics according to a regular schedule; 9% delayed deworming until the onset of weight loss. Proportions of owners deworming the various classes of horses at least 1, 2, 3, or 4 times annually were as follows: foals (0 to 6 months) 100, 60, 15, and 0%, respectively; weanlings (6 to 12 months) 100, 91, 32, and 0%, respectively; yearlings (1 to 2 years) 100, 100, 86, and 51%, respectively; and adults (greater than 2 years) 100, 94, 56, and 24%, respectively. Median treatments of yearlings and adults were evenly spaced at 6-, 4-, or 3-month intervals when the owners dewormed 2, 3, or 4 times annually, respectively. Mean intervals between treatments of foals and weanlings were less regular. Of the horse owners who dewormed 2 or more times annually, most (53 to 80%) used the same product exclusively for all treatments. In all age classes of horses, ivermectin paste (46 to 63%) was the most common product used, followed by tube deworming (23 to 38%) and use of benzimidazoles (4 to 16%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Equine protozoal myeloencephalitis caused by Neospora hughesi in an adult horse in Saskatchewan

A protozoal parasite identified as Neospora hughesi was found in inflammatory lesions in the central nervous system of a Canadian-born adult horse presented with neurological signs. This is believed to be the first case of equine protozoal myeloencephalitis (EPM) caused by Neospora hughesi in a horse outside of the United States.     

A survey of helminth control practices in equine establishments in Ireland

We report the results of a survey to assess the attitudes of horse owners and managers of equine businesses in Ireland to the control of helminth parasites, and the extent to which veterinary practitioners are involved in providing advice on parasite control of horses.Replies to our questionnaire indicated that there is a high level of awareness on the need for helminth control, typically leading to several doses of anthelmintic being administered to each animal every year. While a majority of respondents (61%) were concerned about the issue of anthelmintic resistance, only a few were of the opinion that it was a problem on their establishment. Measures to minimise the development of resistance, such as decreasing the frequency of dosing, optimising non-drug-based control strategies and assessing the need for dosing by faecal egg counts (FECs), were not widely employed in a strategic manner. Just over half (54%) of respondents indicated that they devised worming control programmes based on veterinary advice, and a large majority did not perceive any need for modification of their current worm control programmes.We concluded that there is considerable scope for increased veterinary involvement in the design and implementation of control programmes for helminths in horses, and that veterinarians do not always provide advice in accordance with "best practice" in helminth control.     

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.     

COMPUTED TOMOGRAPHIC ANATOMY OF THE DISTAL EXTREMITY OF THE HORSE

X-ray computed tomography (CT) is an imaging tool that is becoming increasingly available for use in veterinary medicine. Advantages, including depiction of detailed cross-sectional anatomy, improved contrast resolution, and computer reformatting, make it a potentially valuable diagnostic technique. Veterinary application of CT has been primarily limited to use in small animals. Current reports describe the technique for use in the horse, but there are no published studies correlating serial CT images with equine cross-sectional anatomy. A study of the distal extremity of the horse was undertaken to facilitate interpretation of images produced using CT. Transverse CT images of the distal forelimb of equine cadavers were evaluated relative to gross anatomic dissection. Resolution of bone architecture in transverse plane images was satisfactory, but soft-tissue' resolution, although satisfactory for visualization of major structures, was relatively poor, probably due to lack of interposed fat and insufficient differences in physical density and atomic number. Thus it appears in the equine distal limb that CT may be most useful for evaluation of complex bone abnormalities.