Parasites

Take-Home Messages

• Perform fecal egg count reduction tests (FECRT) annually to ensure effective dewormers.

• Recognize that no anthelmintic can eliminate all parasitic stages.

• Conduct fecal egg counts (FEC) once or twice a year to stratify horses into low, medium, and high shedders to reduce pasture contamination.

• Deworm all horses baseline (once or twice a year) and target selected horses more often based on FEC.

• Avoid using FEC to diagnose disease in horses; no correlation between FEC and disease-causing parasites has been established.

• Discontinue fixed interval deworming (e.g., every 2 months) and stop rotating anthelmintic classes blindly.

1. Goals of Parasite Control

• Aim to keep horses healthy and reduce clinical illness risk, rather than completely eradicating all parasites, to prevent accelerated drug resistance.

2. Introduction

2.1 Cyathostomins (Small Strongyles)

• Most common internal parasites of horses, often not associated with disease.

• Acute larval cyathostominosis can lead to severe illness but is rare in the U.S.

2.2 Large Strongyles

• Includes the pathogenic Strongylus vulgaris, responsible for thromboembolism and severe peritonitis.

• Historically reduced in the U.S. due to effective treatment.

2.3 Anoplocephala perfoliata (Tapeworms)

• Prevalent in grazing horses; associated with ileal impactions and spasmodic colic.

2.4 Parascaris spp. (Roundworms)

• Most significant in foals, can lead to intestinal impaction and severe complications.

2.5 Strongyloides westeri (Threadworms)

• Common in foals, typically not pathogenic; transmitted lactogenically from mares.

2.6 Oxyuris equi (Pinworms)

• Historically affects young horses; signs include tail rubbing and skin irritation.

2.7 Gasterophilus spp. (Bots)

• Mildly pathogenic; commonly found but rarely cause clinical issues.

2.8 Habronema and Draschia spp. (Stomach Worms)

• Vector-borne; causes summer sores but rarely leads to severe disease.

2.9 Onchocerca cervicalis (Neck Threadworm)

• Adult worms resistant to treatment; microfilariae can cause dermatitis but respond to some treatments.

3. Parasite Diagnostics

3.1 Diagnosing Parasitic Disease

• Not straightforward; positive fecal samples do not necessarily indicate disease.

• Diagnosis requires pattern recognition of clinical signs (e.g., diarrhea, colic).

3.2 Fecal Egg Counts

• Numerous techniques exist; FEC does not correlate directly with parasite burden or clinical disease.

3.3 Strongyles FEC

• Used to evaluate shedding status and treatment efficacy.

3.4 Ascarids FEC

• Limited to specific age ranges; qualitative detection is more common.

3.5 Tapeworms

• Diagnostic challenges due to mode of egg shedding; necessitates modified FEC techniques.

3.6 Pinworms

• Detected via perineal scraping (Scotch tape test) rather than fecal exam.

3.7 Neck Threadworms

• Detected through skin biopsy; not commonly used in practice.

4. Anthelmintic Treatment Evaluation

4.1 Resistance Testing

• Revised guidelines for testing resistance using FECRT, focusing on egg count thresholds for efficacy determination.

4.2 Interpretation of FECRT

• Analyze average egg counts pre and post-treatment to assess efficacy and resistance.

4.3 Guideline Thresholds and Group Sizes

• Specific thresholds based on historic efficacy data; detailing recommended group sizes depending on count.

4.4 Treatment Check with Few Horses

• Even with fewer than recommended horses, testing can indicate treatment efficacy but risks variability in reliability.

5. Anthelmintic Resistance Status

5.1 Tapeworms

• Emerging treatment failures reported; consider screening for efficacy.

5.2 Pinworms

• Resistance reported; testing for efficacy is challenging.

5.3 Other Parasites

• Lack of responsiveness to treatments in some cases reported; scientific backing is lacking for these claims.

6. Strongyle Egg Reappearance Periods

6.1 Shortened ERP: Interpretation

• Shortened ERP may not indicate resistance but could reflect life cycle adaptations.

6.2 Determining ERP

• Guidelines for estimating ERP following treatments through surveillance of egg counts.

7. Larvicidal Treatment

• Limited efficacy reported for currently used medications; no clinical benefits proven.

8. Methods of Parasite Control

8.1 Environment-Based Approaches

• Focus on manure management to prevent infections.

8.2 Environmental Control

• Temperature impacts on parasite larvae; manure management critical for control.

8.3 Combination Deworming

• Limited data for horses; evidence for ruminants suggests potential benefits.

8.4 Alternative Remedies

• Unproven efficacy in non-drug dewormers marketed without regulations.

9. Recommendations for Parasite Control Programs

• Target non-cyathostomin parasites and implement two-tiered strategies based on transmission and egg shedding patterns.

• Implement a targeted approach that considers the specific parasites affecting the horse population and the local environment.

• Adopt a two-tiered strategy that differentiates between high and low transmission potential parasites, prioritizing those that pose the greatest risk to health.

• Utilize egg shedding patterns to inform deworming schedules, with the aim to minimize the development of drug resistance.

• Regularly monitor and reassess parasite control effectiveness, adjusting strategies based on the latest data and individual horse needs.

• Educate horse owners about the importance of maintaining hygiene, pasture management, and the proper timing of deworming treatments to enhance overall effectiveness of parasite control measures.