NZ.Merial.com : Disease Information : Dogs : Endoparasites

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:

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) 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.

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 larvae penetrating the skin of dogs. 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 anaemia occurs. Parasites 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.)

Dog tapeworms are important more for the disease they cause in intermediate hosts than for the irritation they cause in the intestines of their canine hosts. The metacestode of Taenia hydatigena develops in sheep, goats, cattle, pigs and other mammals and is known as Cycticercus tenuicollis. In the course of its migration and development, this metacestode causes extensive damage to the liver of intermediate hosts. Bleeding and impaired liver function may cause the rapid death of these animals. The mature Cysticercus tenuicollis lives in the abdominal cavity and does not harm sheep, cattle or other intermediate hosts. The life cycle of Tenia pisiformis is similar, but its importance is less since rabbits and hares are the intermediate hosts. Infected dogs would pose a problem only near commercial rabbit operations. Taenia multiceps occupies the brain tissue of intermediate hosts (sheep, goats, cattle and horses). The economic losses caused by this tapeworm are due to the presence of the metacestode which causes the disease "gid". Gid is also known as "sturdy" or "staggers" and is characterised by blindness, abnormal gaits, loss of coordination, convulsions and occasionally death. Gid persists in flocks when dogs are allowed to ingest the brains of slaughtered sheep.

Echinococcus granulosus is unique among dog cestodes in that man may serve as the intermediate host. The metacestode is the hydatid cyst. It may occur in many other animals. Damage to livestock intermediate hosts is not great from the large, fluid-filled cysts, but carcasses of infected sheep and other animals are condemned at slaughter because they provide a means of infection for dogs, which in turn may produce eggs that if ingested by a human lead to hydatid cyst disease, a serious condition that usually must be surgically corrected. Humans cannot be infected by ingesting hydatid cysts of livestock.

Dipylidium caninum is the most common tapeworm in dogs, but it poses no threat to domestic animals since its intermediate hosts are the dog and cat fleas (Ctenocephalides canis, Ctenocephalides felis), human fleas (Pulex irritans), and Trichodectes canis, a dog louse. Once a dog is infected with Dipylidium, the likelihood of reinfection is high if fleas are present. Attempts should be made to control these insect populations when dogs are treated for tapeworms.