Nematodes constitute a phylum of bilaterally symmetrical tubular organisms that are present upon the earth in fairly innumerable quantities. The vast majority of nematodes are free living, being found in soils of most of the earth’s ecosystems. However, some have also developed the means of surviving within niches in other living hosts. In this relationship, nematodes have come to be recognized as important parasites of plants and animals. This chapter focuses on those which live within the feline host.

The external surface of the nematode is covered with a non-cellular surface called the cuticle. Under the cuticle is a syncytium that is called the hypodermis. Under the hypodermis is a layer of muscle cells that are arranged along the long axis of the tubular worm. The muscles of nematodes are divided into four quadrants by dorsal and ventral nerve cords and two lateral cords that are coposed of extensions of the hypodermis. Within the tubular nematode is a tubular digestive tract that extends from the mouth on the anterior end to the anus at the posterior of the worm. This digestive tract is composed of a muscular esophagus that connect to a simple intestine that is composed of a single layer of intestinal epithelial cells. The nervous system is centered in a nerve ring that encircles the esophagus and which extends into the ventral and dorsal nerve cords. Nematodes have a third body opening called the excretory pore which represents the opening of the excretory system which varies in structure in different nematode groups. The excretory pore is usaully located anteriorly on the ventral surface between the mouth and the base of the esophagus. Female nematodes have a fourth body orifice that represents the vulval opening of the female reproductive tract. The typical female digestive system consists of a vagina which leads into the uterus and ovary; most typically, the uterus is branched into two tubes and each tube is connected to its own tubular ovary. The portion of the uterus nearest the vulva is typically packed full of the stages that are to be ejected from the female, thus, the terminal uterus may contain eggs with single cells, eggs with several cell or larvae, or free larvae. Males have no additional orifices and the male reproductive tract typically consists of a single tube that opens into the rectum and which opens to the exterior through the anus. Most male nematodes have paired cuticularized structures called spicules that are inserted into the vulva of the female during copulation. Some male nematodes also possess expansions of the body on the posterior end to form a muscular bursa copulatrix which functions to grasp the female during copulation. The sperm of nematodes is amoeboid and after being introduced into the female makes its way through the uterus to the oviduct to fertilize the eggs. The entire structure of nematodes functions on the basis of a very high internal turgor pressure that is maintained in the body cavity, the pseudocoelom, in which the different tubes of the body are suspended. Thus, the esophagus functions to pull food into the body, while the muscular vulva found in some nematodes functions to keep eggs from being extruded before they reach proper maturity. In the same fashion, the digestive tract is emptied when the sphincter muscles open the anus and allow the feces to be pushed out of the body. The longitudinal muscles of the body cavity allow the nematode to flex by working against the internal pressure that is maintained. The internal pressure in nematodes can be demonstrated by a dimple pin prick which will cause the internal organs to burst out of the body through the hole made by the pin.

The life cycle of nematodes is determined in part by the external cuticle that is present. This cuticle has to be shed every time the nematode changes its external morphology. All nematodes shed their cuticle, i.e., ecdyse, and undergo morphological metamorphosis, i.e., molt, four times during their lives, and thus, there are four larval stages between the egg and the adult. There are two major groups of nematodes, the Secernentea and the Adenophorea. Typically, the stage of Secernentean nematodes that infects the vertebrate final host is the larva that has undergone two molts, i.e., the third-stage larva. The stage of Adenophorean nematodes that infects the vertebrate final host is typically a larva that has never molted, i.e., a first-stage larva. Typically, adult-like characters first appear in the fourth-stage larva, but the uterus typically does not have a vulva with a patent opening to the exterior until the nematode completes its final molt to the adult stage. In some Secernentean nematodes, the cuticle from the earlier stage or stages is maintained by the third-stage larva as a protective sheath. In other Secernentean nematodes, both molts occur within the eggshell, so the nematode that hatches from the egg is a third-stage larva.

The cat is parasitized by both Secernentean and Adenophorean nematodes. There are four orders of Secernentean nematodes parasite in the cat, the Rhabditida, the Strongylida, the Ascarida, and the Spirurida. The cat is also parasitized by a few representatives of the Adenophorean nematodes. In the Secernentea nematodes, the anus is found in a subterminal ventral position; thus, there tends to be a tail that protrudes beyond the anus in this group of nematodes. In the case of the Adenophorea, the anus is typically terminal in position, thus, there is no tail extending beyond the end of the body.