Thorny-Headed Worms: Acanthocephala

When they attach themselves to the intestinal wall of their hosts, thorny-headed worms can cause serious, sometimes fatal, internal damage. In most cases, the more worms there are, the more serious is the damage.

Female and male thorny-headed worms mate in the intestines of their primary hosts. Fertilization (FUR-teh-lih-ZAY-shun), the joining of egg and sperm to start development, takes place inside the female's body. She releases the fertilized (FUR-teh-lyzed) eggs into the host's intestine, and they leave the host in its feces (FEE-seez). Outside the host, the eggs are eaten by intermediate hosts. The larvae hatch, bore into the intermediate host's intestinal wall, and develop there through two more stages. When a primary host, such as a bird, eats the intermediate host, such as an insect, the larvae enter the primary host and develop into adults.

Some species of thorny-headed worms have transport hosts. These hosts are vertebrates that eat intermediate hosts containing final-stage larvae but in which the larvae cannot develop into adult worms. The larvae do not die but stay in the transport host until it is eaten by a suitable primary host.