Physical damage may be caused in several ways. Parasites, in feeding, may destroy tissue and cause wounds. This is accomplished in the small intestine by hookworms (Ancylostoma Uncinaria -not significant in New Zealand) which suck blood. When the hookworms move to a new feeding site, bleeding occurs at the abandoned site from small blood vessels that line the wall of the digestive tract. Mechanical damage also occurs when parasites cause obstruction. classically, this occurs when masses of Toxocara or Toxacaris adults in the intestine block the passage of digestive contents. The intestine may burst and cause rapid death of the host.

Parasites absorb food that the host has ingested for its own use. This is typical of disease by tapeworms (Dipylidium caninum, Taenia spp) and roundworms (Toxocara, Toxascaris). Growth of the host is decreased as a result of lost nutrients.

Secretions and wastes produced by parasites may induce adverse reactions in cats.

Chemical components of parasites which are foreign to the host can be allergenic, especially in the case of Toxocara and Toxascaris infections. Allergy is also caused by Ancylostoma Uncinaria (not significant in New Zealand) larvae penetrating the skin of cats. This causes 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 anemia occurs. Parasites may cause anaemia 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 when feeding. When the parasites move to other feeding sites, bleeding continues because of these anti-clotting agents. Iron-deficiency anaemia is typical of infections by hookworms (Ancylostoma and Uncinaria spp - not significant in New Zealand).

Dipylidium caninum is the most common tapeworm in cats. Its intermediate hosts are the dog and cat fleas (Ctenocephalides canis, Ctenocephalides felis), and human fleas (Pulex irritans). Once a cat is infected with Dipylidium, the likelihood of reinfection is high if fleas are present. Attempts should be made to control these insect populations when cats are treated for tapeworms.