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Endoparasites - Diagnosis

The veterinarian confirms the diagnoses of most worm infections by identifying the characteristic eggs of the different species in cattle faeces. Several exceptions exist to this general rule. The filarial worms shed tiny, worm-like microfilariae larvae into surrounding tissues. Microfilariae of Stephanofilaria are found in skin lesions with adult worms, while those of Onchocerca and Parafilaria are found in tissue fluid under the skin. Larval eyeworms (Thelazia) are present in tears. Lungworm (Dictyocaulus) eggs usually hatch in the intestine, so that larvae are shed with the faeces. Lungworm (Dictyocaulus) eggs resemble those of Strongyloides, as described below, but are twice as large.

The major types of eggs which are present in cattle faeces include:

Nematodes

Toxocara (Neoascaris)

The egg of this roundworm is roughly spherical, with a thick, pitted, outer layer (fig 1).

Strongyle-type egg

"Strongyle-type" eggs are laid by many important bovine nematodes, including the stomach worms (Haemonchus, Mecistocirrus, Ostertagia, Trichostronglyus) and intestinal worms (Bunostomum, Cooperia, Nematodirus, and Oesophagostomum). 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 2). Nematodirus eggs are the largest strongyle-type eggs; however, eggs of other species in this group cannot usually be identified precisely. Species determination can be made by identifying adult worms at autopsy or by culturing the eggs in faeces until the distinctive larvae hatch.

Strongyloides

The eggs of threadworms each contain a larva when laid. The eggshell is thin and smooth, and the egg is oval in shape with blunted ends. The gullet worm (Gongylonema), of much less clinical importance, lays eggs that resemble those of Strongyloides (fig 3).

Trichuris

Whipworm eggs are brown and barrel-shaped, with a clear plug at each end (fig 4).

Cestodes

Tapeworms

Cattle tapeworms shed segments full of eggs to be passed in the faeces. The segments are rectangular and up to 3 cm. wide. Eggs are also found in the faeces because some segments break apart. The eggs are square or triangular with thick, smooth coats. Each egg contains a larva with 6 hooks known as a "hexacanth". The hooks are on a characteristic pear-shaped structure known as the "pyriform body" (fig 5).

Tremades

Flukes

Similar eggs are passed by cattle infected with Cotylophoron, Dicrocoelium, Eurytrema, Fasciola, Fascioloides, or Paramphistomum. The eggs of Dicrocoelium each contain a developed miracidium larva when passed in the faeces. At one end of fluke eggs is a cap, the "operculum", through which the larva hatches. (fig 6).

Schistosoma

Schistosome eggs are usually spindle-shaped, and may be present in the urine as well as in the faeces of infected cattle.

 

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