Most endoparasite infections are diagnosed by identifying the eggs or larvae in sheep faeces. Several exceptions exist to this general rule; in these infections no evidence is usually present in the faeces. The eyeworm, Thelazia, sheds larvae into the tears. Elaeophora adults in blood vessels produce microfilariae, which can be observed in blood samples under the microscope. Skrjabinema lays eggs on the skin around the anus of sheep. The fluke, Fascioloides magna, may wander through the sheep without reproducing. Taenia use the sheep as an intermediate host. The final diagnosis of Taenia or Fascioloides is usually made during a postmortem examination.

The major types of parasite eggs and larvae which may be present in sheep faeces includes:

Nematodes

Trichuris

The eggs of this species are barrel-shaped, with a clear plug at each end. They may be differentiated on the basis of size; Trichuris eggs are about 75 microns long (fig 1).

Lungworm

The lungworm (Dictyocaulus, Muellerius and Protostrongylus) lay eggs that hatch in the airways. The larvae are coughed up and swallowed; larvae may often be found in both the faeces and the nasal mucus. The larvae may be differentiated by their microscopic anatomy.

Strongyle-type eggs

"Strongyle-type" eggs are laid by many important ovine nematodes, including the abomasul worms (Haemonchus, Ostertagia and 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. Nematodirus eggs are the largest strongyle-type eggs, but eggs of the species in the group cannot usually be identified precisely. Species determination can be made by identifying adult worms at necropsy or by culturing the eggs in faeces until the distinctive larvae hatch (fig 2).

Strongyloides and Gongylonema

Differentiation of these nematodes is important because Strongyloides causes serious damage, while Gongylonema is insignificant. Their eggs are very similar; each egg is smooth-shelled and contains a larva when laid. Gongylonema eggs will not hatch unless eaten by an intermediate host, but Strongyloides eggs will hatch within hours if kept in the faeces in a warm place (fig 3).

Cestodes

Tapeworms

Tapeworms segments, or proglottids, filled with eggs can be found in the faeces. If the proglottids were damaged, individual eggs may be present. Moniezia segments look like cooked rice grains; the eggs of this cestode are square or triangular, and each contains a hexacanth larva (fig 4).

Trematodes

Fluke

Fluke eggs are oval and have a smooth shell with a cap or operculum at one end. The damaging Fasciola lays eggs which are quite similar to those of Paramphistomum. Fasciola eggs have yellowish shells with indistinct opercula, whereas eggs of the rumen fluke are clear, with distinct opercula. Eggs of Dicrocoelium, the lancet fluke, are much smaller in size than those of the other flukes, and each lancet fluke egg contains a developed miracidium larva when passed in the faeces (fig 5).