This phylum is composed of dorsoventrally flattened worms.
Class Trematoda
This group of worms is characterized by two ventrally located sucker attachment structures.
S. mansoni are somatic endoparasites and the causative agent of schistosomiasis. They are found in areas of Africa, especially areas that house large populations of freshwater snails which serve as the parasites' intermediate host. Snails release cercaria, the human infective stage, into the water which then enter the body by penetrating the skin. Adult worms reside in the blood vessels of the hepatic portal system where the male and female live permanently in copula. The pathogenecity of infection is due to the resulting eggs produced, not from the adult worms. From the liver, the eggs must make their way through the body's tissues so that they may enter the lumen of the intestines and pass with the feces. To do so, they secrete proteolytic enzymes and are hooked which enables them to work their way to the lumen. The presence of the eggs has a high degree of immunopathology in which the immune system becomes over stimulated. Blood cells infiltrate to infected areas, granulomas are produces around the eggs, and calicification of the veins in the liver can occur. The resulting disorder is Symmer's Clay Pipestem Fibrosis which causes extreme enlargement and ascites of the liver and spleen.
Class Cestoidea
This group of worms is an entirely parasitic group that lacks a mouth and gut structure.
E. granulosus are true tapeworms consisting of long, segmented, ribbon-like bodies which resides in the intestinal tract of their definitive hosts. Here they attach to the intestinal mucosal cells by their scolex holdfast. This species of tapeworm displays both sylvatic and domestic life cycles in which the definitive host consists of carnivores, such as wolves or dogs, that become infected through trophic transmission from the deer or sheep intermediate hosts which contain hydatid cysts. Extreme pathology is exhibited within the intermediate host from the unilocular hydatid cysts that develop. The cysts consist of multiple brood capsules with multiple protoscolices. They are tumor-like in that each cyst buds multiple daughter cysts and increases over time. Humans can become infected when ingesting feces contaminated with E. granulosus eggs. Once infected, the human host becomes the intermediate host and will develop hydatidosis where cysts can occur in the liver, brain, bones, kidney, lungs, muscles, or spleen.
Phylum Nematoda
This phylum consists of elongate, unsegmented ecdysozoans with a thick external cuticle. They include free-living predators and scavengers, as well as parasites of most terrestrial land and animal species.
Hookworms
This group of Nematodes are hooked shaped at their anterior end.
A. duodenale, also known as the human hookworm, is generally considered an old world hookworm, but cases have occurred in North America as well. It has a number of different ways in which it infects its human host. Primarily people become infected when the juvenile worm is stepped on in the soil and is able to penetrate the skin. Once in the circulatory system, the worm travels to the lungs where it uses the hosts ciliary movement to carry itself up towards the trachea and is coughed up and swallowed. In the small intestines the worms molt into their adult forms and begin producing eggs which are passed in the feces. Vertical transmission in which the juvenile stage is passed on to a child through breast milk or the placenta is another method this specie uses. Symptoms and pathology of an infection are density dependent and include anemia, abdominal pain, loss of appetite, and geophagy. Symptoms are usually more severe in people who are malnourished or immunocompromised. In heavy infections, greater than 500 worms, symptoms can include severe protein deficiency, potbelly, delayed puberty, mental dullness, heart failure, and death.
Filarial Worms
Filarial worms are somatic endoparasites that are a major cause of human disease. They are characteristically long, this, and thread-like in form.
O. volvulus, better known as Riverblindness, is a rural disease, endemic in Africa, Arabia, and tropical Americas where their vector host, the Black Fly, breeds. When the flies feed, they release the juvenile worm stage, or microfilariae, that remain in the tissue fluid. The larvae invade subcutaneous tissues eliciting an inflammatory response causing dermatitis and lesions to the infected areas. Adult worms also live subcutaneously near bite wounds where they become encapsulated in host collagen material, forming fibrous nodules or onchocercomas. If microfilariae enter the sclera of the eye, they become trapped and eventually die. In response to the dead worms, the host's immune system increases vascularization and fibrosis which results in keratosis and scarring of the cornea. This scarring of the cornea ultimately results in blindess of the infected person. In infected areas, blindness can affect up to 40% of adults in a population.
Pinworms
Pinworms consist of small worms and are characterized by the female which has slender, pointed posterior end.
Enterobius vermicularis
E. vermicularis is a human pinworm and is prevalent in just about every country on the planet. It is estimated that between 20-30% of elementary students in the United States are infected. A person becomes infected when the pinworm eggs are ingested. Eggs can be found on people's carpets, bedding or clothing; many daycares are considered focis of infection. Once inside the intestines, the eggs hatch into adult worms. Female adults deposit their eggs on the skin of the perianal area causing perianal itching and pain. As a result, autoinfection, where an infected person reinfects themselves, is common among this species. Symptoms of this kind of infection include perianal itching, nervousness, restlessness, and irritability, ulceration of intestinal mucosa, and granulomas in the female reproductive tract which occur from wandering worms.