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Feces: parasitology - McMaster worm egg count

pequis

Introduction

  • This technique uses a specific counting chamber (McMaster slide) to examine microscopically a known volume of a fecal suspension for helminth (worm) eggs.
  • It is widely used.
  • If a known weight of feces and a known volume of flotation fluid are used to prepare the suspension, then the number of worm eggs per gram (epg) of the feces can be calculated by multiplying the number of eggs under the marked areas in the slide chamber by a simple conversion factor.

Uses

  • The demonstration and counting of worm eggs in fecal samples.
  • Used as part of a parasite control program   Parasite control programs  .
  • Used to identify worm resistance to anthelmintic by means of fecal egg count reduction (FEC) test.
  • Used in investigation of horses with weight loss/diarrhea.

Advantages

  • Allows the number of worm eggs per gram of feces to be calculated.
  • Allows identification of horses with high adult worm burdens, which are likely to shed large numbers of eggs on pasture, and guides targeted treatment.
  • Horses with low or zero egg counts can be identified, avoiding unnecessary anthelmintic treatment.

Disadvantages

  • Only patent infections are identified; numbers of eggs reflect the presence of adult worms in the intestinal lumen. Most pathology due to strongyle infection results from migrating or encysted larvae, which will not be identified by egg counts, despite potentially large numbers of immature worms/larvae being present.
  • The number of eggs in a fecal sample does not always indicate the number of worms present in the animal from which the feces came, ie no direct linear correlation between actual worm burden and the FEC. However, horses with FEC <500 epg have significantly lower intestinal worm burdens that horses with FEC >500 epg.
  • Not all eggs from the various different species of helminths are the same weight, and the heavier eggs will not float as well in lower specific gravity solutions. This will decrease their count.
  • Strongyle eggs cannot be differentiated to genus or species level by this technique, despite there being approximately 60 different species with varied pathological potentials.
  • The concentration of epg will aslo be influenced by the volume of feces passed each day, intestinal ingesta passage rate, and the way the eggs are distributed through the feces.
  • Individual FECs are highly variable, with variation of +/- 50% reported. However, studies show good level of repeatability of FECs in individual horses over time, allowing consistently high/low egg shedders to be identified.
  • Owners are not always aware of the significance of interpretation of the FEC and assume the aim is to achieve a zero egg count, rather than to identify those horses with high burdens or evidence of anthelmintic resistance. They also may not be aware that the FEC will not readily identify tapeworm or larval strongyle burdens, nor pinworm.

Requirements

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Procedure

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Aftercare

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Lester H E & Matthews J B (2014) Faecal worm egg count analysis for targeting anthelmintic treatment in horses: points to consider. Equine Vet J 46 (2), 139-145 PubMed.
  • Anderson U V et al (2013) Recent advances in diagnosing pathogenic equine gastrointestinal helminths: the challenge of prepatent detection. Vet Parasitol 192 (1-3), 1-9 PubMed.
  • Nielson M K (2012) Sustainable equine parasite control: perspectives and research needs. Vet Parasitol 185 (1), 32-44 PubMed.

Other sources of information

  • Gibbons L M, Jacobs D E, Fox M T & Hansen J (2105) McMaster Egg Counting technique. In: The RVC/FAO Guide to Veterinary Diagnostic Parasitology. The Royal Veterinary College, UK. Website: www.rvc.ac.uk/Review/Parasitology/EggCount/Principle.htm. Last Accessed 5th November 2015.

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