Cats are parasitized by all four superfamilies of nematodes that make-up the order Strongylida. This group of nematodes forms a fairly cohesive group of worms whose males are characterized by the possession of a copulatory bursa. Also, all four groups typically have a muscular esophagus. The females of these groups tend to lay eggs that contain four to eight cells when they are passed in the feces, although some produce eggs that contain larvae. The longitudinal muscles that appear under the hypodermis of the worms tend to be few in number in each quadrant. The vulva may be located anywhere from midbody to just anterior to the anus.
The anterior end of these nematodes typically contains characters that can be used to assign these nematodes to one of the four superfamilies. The Ancylostomatoidea (the hookworms) have a large buccal cavity that typically is armed with anteriorly placed teeth or cutting plates. The Strongyloidea (the strongyles) have a large buccal cavity that is often delineated anteriorly by a set or two of fine fibrous teeth which produce the leaf crown or corona radiata of the buccal capsule. There may be large teeth present in the base of the buccal capsule of these worms. The Trichostrongyloidea (trichostrongyles) tend to have very small buccal capsules that may or may not contain a single tooth called a lancet. Adult hookworms are typically parasites of the small intestine, adult strongyles are typically parasites of the large intestine, and adult trichostrongyles are typically parasites of the stomach or small intestine. The fourth superfamily, the Metastrongyloidea (the metastrongyles) is typically found associated with parasitism of the lungs or vascular system, and like the trichostrongyles, the metastrongyles tend to have a small buccal cavity. The metastrongyles also tend to have a copulatory bursa that is quite reduced, sometimes to the point of being very difficult to recognize.
The typical like history of the hookworms, strongyles, and trichostrongyles is direct, i.e., there is no intermediate host involved in the life cycle. Eggs are passed in the feces that develop and hatch in the soil to produce larvae. The larvae then develop to an infective third-stage larva which often maintains the second-stage larval cuticle as a protective sheath. Infection with hookworms is often through the skin, infections with strongyles and trichostrongyles is typically through the ingestion of the larvae. In the case of metastrongyles, the stage passed in the feces is typically a first-stage larva that requires a molluscan intermediate host (snail or slug) for the development of the larva to the infective third-stage larva. In the case of the metastrongyles of carnivores, the larva in the mollusk typically can infect a small vertebrate paratenic host where the larvae persist in the tissues until they are ingested by the carnivore. Another possibility is that the larvae are capable of infecting the next host in the first stage.