Physical damage may be produced in several ways. Feeding parasites may destroy tissue and cause wounds. Larvae migrating through tissues cause damage to the affected organs. Lungs are damaged by Ascaris and Metastrongylus larvae. Stephanurus larvae cause adhesions and harm to various abdominal organs. Mechanical damage also occurs when parasites cause obstruction. Such obstruction may occur in the small airways of lungs infected by Metastrongylus apri and in the intestine with Ascaris suum. The function of obstructed organs is impaired: pneumonia and lung collapse may be caused by lungworm obstruction, and digestion is halted when the gut is filled with adult ascarids.

Parasites may destroy host cells and tissues by enzymatic digestion. Small pieces of the intestinal wall, for example, are ingested by adult nodular worms (Oesophagostomum).

Parasites absorb food that the host has ingested for its own use. This is typical of the large roundworm (Ascaris) and the nodular worm (Oesophagostomum). The red stomach worm Hystrongylus rubidus sucks blood. Growth of the host is decreased as a result of lost nutrients.

Chemical components of parasites, especially those from ascarids, are foreign to the host and can cause allergic reactions. Allergy is also caused by Strongyloideslarvae penetrating the skin of pigs. 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. Firstly, they ingest large amounts of blood from the host. Secondly, 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 Oesophagostomum dentatum and Trichuris suis.

Lung damage often produces severe coughing. Migrating Strongyloideslarvae may cause muscle stiffness and soreness. Both of these cause discomfort, resulting in loss of appetite and rapid loss of condition.