The Common Culprits


The modified McMaster’s fecal egg count is a quantitative method used for examining strongyle and ascarid populations where large numbers of eggs are encountered. Eggs from other parasites, including tapeworms, are less informative, and typically are just noted as being present or absent. Therefore, here are our primary targets:



The eggs of large strongyles are indistinguishable from those of small strongyles. All members of the order Strongylida (including strongyles in horses and Trichostrongyles in ruminants) will produce eggs that are very similar in appearance. This makes identification through the use of egg morphology difficult, if not impossible. Often times they are identified simply as Strongyle-type eggs.

Identification of equine strongyle eggs:
•    Elliptical or oval shaped
•    Smooth, thin shell wall
•    Measures approximately 90 × 50 μm
•    When passed in the feces they contain 8 to 16 cell morula (solid ball of embryonic cells) visible inside





Unlike stronglyes, thick-shelled Ascarid eggs are resistant to freezing and drying and can therefore survive for long periods in the environment. The outer layer is sticky and ensures that eggs will be present almost everywhere in a horse’s environment.

Identification of equine ascarid eggs:
•    Round
•    Clear to dark brown
•    Thick shelled
•    Single celled egg
•    Measures approximately 90-100 μm