Craig R. Reinemeyer

The prevalence and intensity of internal parasites of horses in the U.S.A.

Fifty-five adult horses were necropsied over a 15-month period, and their worm burdens counted and speciated. Twenty-one species of Cyathostominae were recovered. Ten species: Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus longibursatus, Cyathostomum coronatum, Cylicostephanus goldi, C. calicatus, C. minutus, Cylicocyclus leptostomus, C. insigne and Cyathostomum pateratum, comprised 98.9% of the total cyathostome burdens. These same 10 species also demonstrated high prevalences in 4 previous surveys. Eight of these 10 species have been shown to be resistant to benzimidazole anthelmintics. Anthelmintic resistance apparently had little effect on the prevalence of a species, but appeared to increase its relative abundance within a mixed population. The prevalence of other internal parasites was similar to previous reports, but Strongylus vulgaris adults and arterial lesions were less common.

Comparison of efficacy and safety of paste formulations of firocoxib and phenylbutazone in horses with naturally occurring osteoarthritis

OBJECTIVE To compare efficacy and safety of paste formulations of firocoxib and phenylbutazone in horses with naturally occurring osteoarthritis. DESIGN Randomized controlled clinical trial. ANIMALS 253 client-owned horses with naturally occurring osteoarthritis. PROCEDURES Horses were treated with firocoxib (0.1 mg/kg [0.045 mg/lb], PO, q 24 h) or phenylbutazone (4.4 mg/kg [2 mg/lb], PO, q 24 h) for 14 days. Physical examinations and lameness evaluations were performed prior to treatment and after 7 and 14 days. Clinical improvement was defined as a reduction of at least 1 lameness grade or a combined reduction of at least 3 points in scores for pain during manipulation or palpation, joint swelling, joint circumference, and range of motion. RESULTS Proportion of horses clinically improved on day 14 for the firocoxib group (104/123 [84.6%]) was not significantly different from the proportion for the phenylbutazone group (103/119 [86.6%]). Proportion of horses that were improved on day 14 was significantly greater for horses treated with firocoxib than for horses treated with phenylbutazone with regard to score for pain on manipulation or palpation (P = 0.028), joint circumference score (P = 0.026), and range of motion score (P = 0.012), but not for overall lameness score or joint swelling score. No direct treatment-related adverse effects were detected during the study. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that overall clinical efficacy of a paste formulation of firocoxib in horses with naturally occurring osteoarthritis was comparable to efficacy of a paste formulation of phenylbutazone.

Small Strongyles: Recent Advances

The recent increased interest in cyathostomes can be traced to simplification of their taxonomy, improved knowledge of pathogenicity, and failures of practical control due to anthelmintic resistance. Cyathostome ova develop to infective third-stage larvae (L3) at a rate that is directly proportional to environmental temperature. Equine feces serve as a reservoir for L3, which are liberated by moderate amounts of rainfall. Third-stage larvae persist for longer periods at low temperatures, easily surviving over-winter on pastures to provide a source of infection during the following grazing season. Third-stage larvae exsheath within the host and enter the mucosa and submucosa of the cecum and large colon. Larvae develop within mucosal cysts, molt to the fourth stage, and may persist within the tissues for up to 2 1/2 years. Larvae ultimately emerge from the mucosa to become adults in the lumen. Adult populations are replenished by recently ingested larvae and by immature worms newly emerged from arrested development. The magnitude of larval and adult populations within the host displays seasonal variations, with peak numbers occurring in early spring and autumn in the United States. In typical natural infections, a small number of species comprise the majority of the cyathostome populations. Cyathostome infection may result in anorexia, weight loss, diarrhea, colic, and death. Cyathostome ova are easily detected in feces, but ova may not be present during larval cyathostomiasis. Increased concentrations of beta-globulins, hypoalbuminemia, anemia, and leukocytosis occur inconsistently. Two major problems in the treatment of cyathostome infections are anthelmintic resistance and the insusceptibility of encysted larvae to recommended dosages of most anthelmintics. The major goal of cyathostome control is prevention of environmental contamination with nematode ova. Host resistance appears to protect against cyathostome disease rather than cyathostome infection, and one manifestation of this resistance appears to be prolongation of the prepatent period.

Anthelmintic resistance in non-strongylid parasites of horses.

Since 2002, selected populations of Parascaris equorum in several countries have been reported to survive treatment with macrocyclic lactone (M/L) anthelmintics. Clinical treatment failures are characterized by negligible fecal egg count reduction, but M/L resistance has been confirmed in ascarids by controlled efficacy testing. Resistance was selected by current parasite control practices for foals, which often include exclusive and excessively frequent use of M/L dewormers, thereby minimizing refugia within the host and in the environment. Chemical control of M/L-resistant isolates can be accomplished with pyrimidine and/or benzimidazole anthelmintics, but a few M/L-resistant populations have recently exhibited resistance to pyrantel pamoate as well. Some specimens of Oxyuris equi regularly survive treatment with macrocyclic lactones, but it is uncertain whether this constitutes resistance or merely confirms the incomplete oxyuricidal efficacy of virtually all broad spectrum equine anthelmintics. Variations in other biological parameters of Oxyuris and Parascaris, specifically atypical infection of older hosts and shorter prepatent periods, have been reported anecdotally. These changes may represent genetic modifications that have evolved in parallel with resistance as a result of anthelmintic selection pressure.

Parasitism and colic.

Equids are hosts to dozens of species of internal parasites that infect no other domestic animals. Virtually all horses, especially those exposed to pasture, experience some level of parasitism continuously. Despite pathologic evidence of parasitic damage in various organs and tissues, few parasitisms are manifested systemically in well-managed horses. Contrary to conventional wisdom, only three common parasitisms of horses are likely to be manifested as colic: Strongylus vulgaris, Parascaris equorum, and Anoplocephala perfoliata. This article discusses the life cycles, pathophysiology, manifestations and clinical findings, treatment, and management of these three common parasitisms. It also discusses related aspects of several other parasitisms that are unlikely to cause colic.

SvSXP: a Strongylus vulgaris antigen with potential for prepatent diagnosis

BackgroundStrongyle parasites are ubiquitous in grazing horses. Strongylus vulgaris, the most pathogenic of the large strongyles, is known for its extensive migration in the mesenteric arterial system. The lifecycle of S. vulgaris is characterised by a long prepatent period where the migrating larvae are virtually undetectable as there currently is no test available for diagnosing prepatent S. vulgaris infection. Presence of S. vulgaris larvae in the arterial system causes endarteritis and thrombosis with a risk of non-strangulating intestinal infarctions. Emergence of anthelmintic resistance among cyathostomins has led to recommendations of reduced treatment intensity by targeting horses that exceed a predetermined strongyle faecal egg count threshold. One study suggests an apparent increase in prevalence of S. vulgaris on farms where reduced anthelmintic treatment intensity has been implemented. These issues highlight the need for an accurate and reliable assay for diagnosing prepatent S. vulgaris infection.MethodsImmunoscreening of a larval S. vulgaris cDNA library using hyperimmune serum raised against S. vulgaris excretory/secretory antigens was performed to identify potential diagnostic antigens. Immunoreactive clones were sequenced, one potential antigen was characterised, expressed as a recombinant protein, initially evaluated by western blot (WB) analysis, the diagnostic potential of the IgG subclasses was evaluated by ELISA, and the diagnostic accuracy evaluated using serum from 102 horses with known S. vulgaris infection status.ResultsThe clone expressing the potential antigen encoded a S. vulgaris SXP/RAL2 homologue. The recombinant protein, rSvSXP, was shown to be a potential diagnostic antigen by WB analysis, and a target of serum IgGa, IgG(T) and total IgG in naturally infected horses, with IgG(T) antibodies being the most reliable indicator of S. vulgaris infection in horses. Evaluation of diagnostic accuracy of the ELISA resulted in a sensitivity of 73.3%, a specificity of 81.0%, a diagnostic odds ratio of 11.69; a positive likelihood ratio (LR) of 3.85 and a negative LR was 0.33. The area under the ROC curve was 0.820.ConclusionIgG(T) antibodies to recombinant SvSXP show potential for use as an antigen for prepatent diagnosis of migrating stages of S. vulgaris with moderate to good diagnostic accuracy.

Efficacy of pyrantel pamoate against a macrocyclic lactone-resistant isolate of Parascaris equorum in horses

The expanding prevalence of Parascaris equorum populations that are resistant to macrocyclic lactone (ML) anthelmintics makes it desirable to identify dewormers which remain effective. The objective was to evaluate the efficacy of pyrantel pamoate in 14 suckling foals that had been infected orally with approximately 600 larvated eggs of a P. equorum isolate selected for ML resistance (ML-R). Seventy days after inoculation, foals were weaned, housed individually, and fecal samples were examined frequently to detect the onset of patency. Between 73 and 80 days post-inoculation, all 14 foals developed P. equorum egg counts>or=150 eggs per gram (EPG). An initial cohort of eight foals was treated orally with ivermectin paste (200 microg/kg) 84-91 days post-inoculation. Egg counts were reduced by only 47% at 2 weeks after ivermectin treatment, confirming the ML-R status of the isolate. A second cohort of six foals was not treated with ivermectin. Within each cohort, eligible foals were allocated randomly to treated (pyrantel pamoate; n=7) or untreated control (n=7) groups. Treated foals were dosed orally on Day 0 with a paste formulation of pyrantel pamoate at 13.2mg/kg. Mean ascarid egg counts of treated foals were reduced by 96.0% and 98.8% at 1 and 2 weeks post-treatment, respectively. On Day 14, foals were euthanatized and specimens of P. equorum were recovered from the gut contents, preserved in 10% formalin, and counted. Mean numbers of P. equorum adults recovered postmortem were significantly lower (P=0.0031) in foals treated with pyrantel pamoate (X=1.7; range 0-16) compared to control foals (X=63.0; range 0-320). A paste formulation of pyrantel pamoate, at a dosage of 13.2 mg/kg, was 97.3% effective against a ML-R isolate of P. equorum.

Assessment of the effectiveness of a combination product of spinosad and milbemycin oxime on the prophylaxis of canine heartworm infection.

Three separate randomized, blinded, vehicle-controlled studies were conducted to determine the effectiveness of a single treatment and consecutive monthly treatments of a combination flavored tablet product containing spinosad and milbemycin oxime (MBO) in the prevention of the establishment of heartworm infections in dogs challenged with recent field isolates of the heartworm (HW), Dirofilaria immitis. For each study, dogs were allocated randomly based on pre-treatment body weights to treated or control groups of 10 animals each. Dogs were infected once with infective HW larvae, on Day-30, using either a Michigan isolate or a Georgia (MP3) isolate of D. immitis. Treatments of beef-flavored chewable tablets were administered in two studies one time either on Day 0 or Day 15, and in one study twice (Days 0 and 30, or Days 15 and 45) or 3 times (Days 0, 30 and 60). For the combination product groups, dosages were in the range of 30-45 mg/kg (13.6-20.5mg/lb) for spinosad and 0.5-0.75 mg/kg (0.2-0.34 mg/lb) for MBO. Necropsies for heartworm counts were completed following euthanasia on Day 120 or Day 123. A single treatment with the combination product of spinosad and MBO 30 or 45 days post-inoculation with infective HW larvae was completely effective (100%) in preventing establishment of the Michigan D. immitis isolate, but efficacy against the Georgia MP3 isolate was incomplete, with geometric mean reductions in HW counts relative to vehicle treated controls of 99% reduction of the 30 day infection and a 98.9% reduction of the 45 day old infection. Against this same MP3 isolate, 3 consecutive monthly treatments provided complete prevention (100%) against establishment of D. immitis infections. The combination product of spinosad and MBO provides effective control of canine heartworms. A single treatment at 30 days post infection showed high but incomplete effectiveness against a heartworm isolate that had been shown to be partially refractory to treatment with marketed monthly heartworm preventives. Three consecutive monthly treatments provided complete control, providing support to the recommendation that heartworm prophylaxis should be maintained year round for optimal effectiveness.

CURRENT CONCERNS ABOUT CONTROL PROGRAMS IN TEMPERATE CLIMATES

Efforts to control cyathostome nematodes in temperate climates have not been uniformly successful, as evidenced by contemporary reports of clinical disease and widespread anthelmintic failure. Indeed, effective control is confounded by numerous factors, including ignorance of local transmission patterns, confusing or conflicting control recommendations, and use of ineffective anthelmintics or regimens. Most of these problems are universal rather than being climatically unique, and it is important to recognize that they are mere symptoms of larger and more comprehensive problems. The most basic problem in managing cyathostomes is inadequate knowledge of biological factors that influence control recommendations. These include population dynamics, arrested development, host resistance, and regional bionomics. Another hindrance is the absence of objective criteria to define effective control. Target fecal egg counts are arbitrary and subjective, and parameters to measure the subclinical impact of cyathostome infection in well-managed horses have not been identified. The final general challenge to effective parasite control is motivation and education of equine practitioners and horse owners, who currently receive much of their parasite control information in the form of biased advertising. Ultimately, more effective equine parasite control will result from collaborative efforts of basic research, clinical application, and improved information transfer.

Efficacy of pyrantel pamoate and ivermectin paste formulations against naturally acquired Oxyuris equi infections in horses

In recent years, numerous veterinary practitioners have reported anecdotal episodes in which anthelmintic treatment did not appear to deliver the expected efficacy against equine pinworms (Oxyuris equi). Anthelmintic resistance has not been demonstrated formally in equine pinworms, so a clinical study was designed to evaluate the efficacy of paste formulations of pyrantel pamoate or ivermectin against naturally acquired infections with O. equi. Twenty-one horses (>4 months to 15 years of age) with patent, naturally acquired pinworm infections were blocked by source of origin and allocated randomly to one of three treatment groups: horses (n=7) assigned to Group 1 were treated orally with pyrantel pamoate paste at a dosage of 13.2 mg/kg (2x label dosage), Group 2 horses (n=7) were untreated controls, and horses (n=7) assigned to Group 3 were treated orally with ivermectin paste at a dosage of 200 microg/kg. Fourteen days after treatment, horses were euthanatized, necropsied, and large intestinal contents were processed for recovery of adult pinworms. In addition, duplicate 1% aliquots of intestinal contents from the cecum, ventral colon, dorsal colon, and small colon were collected, preserved, and examined for recovery and enumeration of fourth-stage larval O. equi. Anthelmintic efficacy against pinworms was evaluated by comparing the post-treatment worm counts of Groups 1 and 3 to those of control animals. Mean numbers of O. equi adults recovered postmortem were significantly decreased by both pyrantel pamoate (P=0.0366) and ivermectin (P=0.0137) treatment, with respective efficacies of 91.2% and 96.0%. In addition, both products demonstrated >99% efficacy against fourth-stage O. equi larvae. The current study demonstrated acceptable adulticidal and larvicidal efficacy of both pyrantel pamoate and ivermectin paste formulations against O. equi and did not support the existence of macrocyclic lactone or pyrimidine resistance in the pinworm populations evaluated.