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Endoparasites - Damage to Host

The nature and extent of damage inflicted on the host vary tremendously for different parasites.

Types of damage may be classified into five basic categories:

Tubing a horse for control of endoparasites

Several types of damage may be classified as mechanical. The tracheal migration of the larvae of Dictyocaulus or Parascaris damages the lungs. Strongylus vulgaris larvae may cause serious damage by their migrations through blood vessels, irritating the vessel walls. Ascaris worms may obstruct bile ducts causing jaundice



Heavy infection of gut worms in foals causes diarrhoea, loss of appetite, weight loss, and loss of condition. Sometimes anaemia and oedema are also seen. These clinical signs are manifestations of parasite interference with normal digestive function. Foals commonly carry mixed infections, so damage from each species is additive. Trichostrongylus in the stomach interferes with food absorption and with the production of digestive enzymes by damaging the mucosal cells. The undigested food suppresses the appetite, a condition known as anorexia. Rapid weight loss is evident as the foal uses muscle tissue to supply vital blood proteins.



Parasites absorb food that the host has ingested for its own use. This is typical of the equine tapeworm (Anoplocephala magna) and the large roundworm (Parascaris). Growth of foals or condition may be decreased as a result of lost nutrients.



Chemical components of parasites, especially those from Parascaris, are foreign to the host and can cause allergic reactions.



Blood lost from the host must be replaced. When there is continual blood loss, body stores of iron - essential for the production of blood - become exhausted. If lost blood is not replaced, iron-deficiency anaemia occurs. Parasites cause this in two ways. First, they ingest large amounts of blood from the host. Second, some parasites have special anti-clotting agents which are released into the wounds caused when feeding. When the parasites move to other sites, bleeding continues because of these anti-clotting agents. Iron-deficiency anaemia is typical of infections by Strongylus species.

Often, the damage inflicted by a parasite is a result of a combination of these disease processes. Adult and larval Strongylus vulgaris, for example, cause mechanical, digestive, and anaemic damage to the horse. The adult large strongyles are "plug feeders": the worms ingest large plugs of intestinal lining filled with blood vessels.

If blood loss is high and protein intake is decreased, blood protein may drop to a level at which the oncotic pressure is insufficient to prevent fluid accumulation in the tissues. Oncotic pressure is the force exerted by proteins to retain fluid in the blood. Fluid collection in the abdomen, called ascites, causes the pot-bellied appearance characteristic of heavily parasitised foals. Undigested food proteins in the intestine also exert fluid-retaining pressure, contributing to diarrhoea. Normally, uptake of fluids from gut contents occurs primarily in the large intestine. If the cell damage to the intestinal lining is severe from heavy worm burdens, diarrhoea and dehydration are increased.


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