The veterinarian diagnoses most worm infections by identifying the characteristic eggs of the different species in a horse´s faeces or by differentiation of third-stage larvae. Several exceptions exist to this general rule. Habronema eggs hatch within the female worms which are in the horse´s stomach; therefore larvae can be found in the faeces. Onchocerca and Setaria shed tiny worm-like "microfilaria" larvae into the subcutaneous tissue or blood, respectively. Pinworm eggs are usually attached around the horse´s anus. The major types of eggs that may be present in horse faeces include:

Nematodes

Lungworm

Although the adult lungworms (Dictyocaulus arnfieldi) live and lay their eggs in the bronchi, the horse coughs up the eggs, and then swallows them, so they can be found in the faces. Each egg contains a fully formed larval worm (fig 1).

Pinworm

The pinworm (Oxyuris equi) egg is oval in shape, with one flattened side and a cap on one end (fig 2).

Roundworm

The egg of the roundworm (Paradcaris equorum) is spherical, with a thick bumpy outer layer (fig 3).

Strongyle-type egg

The "strongyle-type" egg is laid by large and small strongyles, and also by Trichostrongylus. These eggs are elliptical or oval, with smooth, thin shells. When laid, the eggs have already begun to develop and are called "segmented" because the inner mass has split into a group of cells (fig 4).

Cestodes

Tapeworm

The equine tapeworm (Anoplocephala magna) egg is square, with a multilayered thick coat. Each egg contains a spherical embryo with six hooks known as "hexacanth". The hooks are on a characteristic pear shaped structure known as the "pyriform body", (fig 5).

Flukes

Similar eggs are passed by horses infected by Gastrodiscus spp and Fasciola hepatica. These eggs are thin shelled and oval, containing a mass of cells. At one end of fluke eggs is a cap, the "operculum", through which the larva hatches (fig 6).