THE NEMATHELMINTHES OR ROUNDWORMS

Nemathelminthes - Nematoda Includes numerous free-living and parasitic species. They range in length from 2mm (Stronglyoides stercoralis) to a more than a meter ( Drancunculus medinensis) the sexes are usually separate. The male is smaller than the female and commonly has a curved posterior end and in some species, copulatory spicules and a bursa.

Morphology and Physiology The adult nematode is an elongated cylindrical worm, primarily bilaterally symmetrical. The anterior ends maybe equipped by hooks, teeth, plates, setae, and papillae for purposes of abrasion, attachment and sensory response The body wall is consist of: A. an outer, hyaline, noncellular cuticle B. subcuticular epithelium C. muscle cells layer

The cuticle has various surface markings and spines, bosses or sensory papillae. The thin, syncytial , subcuticular layer is thickened into four longitudinal cords – dorsal, ventral, and two lateral that project into the body cavity and separate the somatic muscle cells into four groups. Theses cords carry longitudinal nerves and often lateral excretory canals. The body wall surrounds a cavity, within which lie the digestive, reproductive and parts of the nervous and excretory system. It is line by delicate connective tissues and a single layer of muscle cells

Alimentary tract is a simple tube extending from the mouth to the anus which opens on the ventral surface a short distance from the posterior extremity. The mouth is surrounded by lips or papilae and in some species is equipped by teeth or plates. It leads into a tubular or funnel-shaped buccal cavity expanded for sucking. The esophagus lined with an extension of the buccal cuticle has striated muscular wall, a triradiate lumen and associated esophageal gland

The midgut is a flattened with a wide lumen that follows a straight course from the esophagus to the rectum. In the female it leads into a short rectum lined with cuticle. In the male it joins with the genital duct to form the common cloaca which opens through the anus. There is no circulatory system. The fluid of the body cavity contains hemoglobin, glucose, proteins, salts and vitamins and fulfills the function of the blood.

The nervous system consist of a ring or commissure of connected ganglia surrounding the esophagus. From this commissure six nerve trunks pass forward to the head and circumolar region, and six nerve trunks connected commissures extend posteriorly. The excretory system consist of two lateral canals that lie in the lateral longitudinal cords. The lateral canals join in a bridge from which the terminal duct leads to a ventral pore in the region of the esophagus. Nematodes possess only longitudinal muscle which produce their typical sinuous movement.

Reproductive organs MALE – situated in the posterior third of the body as a single coiled or convoluted tube, the various parts of which are differentiated as testis, vas deferens, seminal vesicle and ejaculatory duct. FEMALE – either a single or a bifurcated tube, differentiated into ovary, oviduct, seminal receptacle, uterus, ovejector, and vagina. The daily output of a gravid female ranges from 20 to 200,000 eggs

Position maintenance Ancylostoma – oral attachment to the mucosa Trichuris – anchorage with their attenuated ends Stronglyoides – penetration of the tissues Ascaris – retention in the folds of mucosa and pressure against it.

Methods of obtaining food Ancylostoma – sucking with ingestion of blood Trichuris – ingestion of lysed tissues and blood embedded worms Ascaris – feeding on the intestinal contents Filarial worms – ingestion of nourishments from the body fluids

Life span The female Trichinella spiralis is passed from the intestine 4 to 16 weeks Enterobius vermucularis has a life span of 1 to 2 months Ascaris lumbricoides may live for about a year Hook worms have been observed to persist for about 14 years

Life cycle Larvae either resume their parasitic existence of again develop into mature free-living worms that produce further generations. Some species have an intermediate host in which the larva passes through a cyclic development. The location of the adult parasite in the host , to a large extent, governs the escape of the eggs and the character of the life cycle. During larval development, nematodes pass through several molts or ecdyses, both inside and outside the host . Nematodes with few exceptions do not multiply in humans,thus differing from many other pathogenic organisms

Pathogenicity Since nematodes can rarely multiply in humans, the number of parasites presents, or the intensity of infection. Is a critical factor in determining the amount of damage to the host. The local reaction to adult worms in the intestine are generally minimal; there maybe some irritations, some degree of invasion to the intestinal wall or mucosal damage from blood sucking. Tissue reaction to nematode parasites can involve both immediate hyper sensitivity, or allergic reactions as well as delayed-type cell mediated reactions with glaucoma and giant cell formation

Resistance and Immunity Inability of nematode parasites to infect a host may be due to some innate, preexisting incompatibility that renders the host resistant or the immunity acquired from previous exposure to the parasite. A variety of immunologic reactions that take place at the surface membrane of the nematodes are complement activation, neutrophil interaction to generate chemotactic factors for eosinophils, and direct attack of larval nematodes by eosinophils.

Production of antibodies of the immunoglobin class IgE, which fix to mast cells and mediate immediate hypersensitivity reactions is a prominent feature of many nematode infections. Immunopathology can be manifested y allergic reactions, such as urticarial skin eruptions during accute trichosis or visceral larva migrans, or the bronchopasm and cough of tropical pulmonary eosinophilia or filariasis. Immunopathologic tissue damage to the skin and eye is a prominent feature of onchocerciasis.