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Endoparasites - Damage to Host

The nature and extent of damage inflicted on the host very tremendously for different parasites. Types of damage may be classified into five basic categories:



Physical damage may be produced in several ways. Feeding parasites may destroy tissue and cause wounds. This is accomplished in the abomasum by Haemonchus contortus and in the small intestine by immature rumen flakes (Paramphistomum cervi). As a result, bleeding occurs from small blood vessels that line the wall of the digestive tract.

Mechanical damage also occurs when parasites cause obstruction. Such obstruction may occur in the small airways of lungs infected by Dictyocaulus filaria. The function of obstructed organs is impaired; pneumonia and lung collapse may be caused by lungworm obstruction. Another example of mechanical damage is the malfunction caused by pressure on the liver or lungs from Echinococcus granulosus hydatid cysts. Tissue under excessive pressure is not well nourished with blood and may become inactive.



Parasites may destroy host cells and tissues by enzymatic digestion. Small pieces of the intestinal wall, for example, are ingested by immature rumen flukes. (Paramphistomum cervi). Ostertagia is a very serious parasite of sheep because it destroys cells from the stomach that secrete digestive juices.



Parasites absorb food that the host has already ingested for its own use. This is typical of tapeworms (Moniezia expansa). Growth of the host is decreased as a result of lost nutrients.



Chemical components of parasites are foreign to the host and can cause allergic reactions. Allergy is also caused by Bunostomum larvae penetrating the skin of sheep. The result is an intense itching, particularly of the feet.



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. In addition, 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 Haemonchus contortus and Fasciola hepatica.

Often, the damage inflicted by a parasite is a result of a combination of these disease processes. Infective larvae of Ostertagia circumcincta, for example, cause mechanical, toxic, and possibly allergic reactions in the wall of the abomasum, causing cells to die rapidly. Continuous erosion means that replacement tissue formed does not have time to mature completely. Body fluid leaks through the damaged gut wall into the abomasum. Food cannot be properly digested by the eroded tissue, so absorption is impaired. The end result is diarrhoea. Also, the fluid lost from blood into the gut contains much albumin, which is largely responsible for the ability of the circulatory system to reabsorb water from body tissue. In ostertagiasis the decreased blood albumin causes fluid accumulation in tissues, a condition called oedema. The most common sites for the accumulation of fluid are under the lower jaw(known as bottle jaw) and in the abdomen (known as ascites).

Blood albumin is also lowered in liver fluke (Fasciola) infections because mechanical irritation to the bile ducts of the liver allows fluid leakage. Diarrhoea is not usually seen, but bottle jaw does develop.

Loss of blood proteins and the inability to use nutrients place stress on the host. In addition to infected sheep not being able to utilise foodstuffs efficiently, they must replace lost blood elements simultaneously. Under such conditions, animals are less capable of growing and may even lose weight.


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