• Mechanical
• Digestive
• Depletive
• Allergenic
• Anaemic

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:

Mechanical

Physical damage may be produced in several ways. Feeding or invading parasites may destroy tissue and cause wounds. This is accomplished in the stomach by Haemonchus placei and in the small intestine by immature rumen flukes. (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 viviparus and in the intestine with Toxocara (Neoascaris) vitulorum. The function of obstructed organs is impaired: pneumonia and lung collapse may be caused by lungworm obstruction, digestion and intestinal motility are halted when the gut is filled with adult ascarids.

Digestive

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 cattle because it destroys cells from the stomach that secrete digestive juices.

Depletive

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

Allergenic

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

Anaemic

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 Fasciola hepatica, and Bunostomum.

Bottle jaw		Drenching cattle with automatic equipment

Often, the damage inflicted by a parasite is a result of a combination of these disease processes. Infective larvae of Ostertagia ostertagi, 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, 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 (know 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 cattle not being able to utilise feedstuffs efficiently, they must replace lost blood elements simultaneously. Under such conditions, animals are less capable of growing and may even lose weight.

Larvae of the Nematodirus or Trichostrongylus irritate and damage the intestine and impair the ability of the intestine to absorb water, causing diarrhoea. Mixed infections are common but in different regions there may be fairly defined successions of species.