The Rhabditida is a group of nematodes that is currently best known for the free-living member of the group, Caenorhabditiselegans, that is used extensively in genetic and developmental research. The best known parasite in this group is Strongyloides stercoralis which is parasitic as adults in humans, dogs, and occasionally cats. Members of the Rhabditida are unusual nematodes in that they tend towards atypical sexual differentiation. In Caenorhabditis elegans, the female, so called because of the fact that she produces eggs and has the general morphological characteristics of a female, is a true hermaphrodite in that both male and female sex cells develop within the same reproductive tract. True males also are produced as part of this life cycle. It is these features that allow self-fertilization for the perpetuation of clonal lines and the presence of males which allows the introduction of specific genetic crosses that makes this worm of such use to geneticists. Strongyloides spp. are also atypical in that the parasitic stage is a parthenogenetic female, i.e., produces offspring without fertilization. In the case of Strongyloides spp., there is also the possibility of the production of free-living male and female stages that morphologically resemble Caenorhabditiselegans more than their elongate parasitic form.
The only members of the Rhabditida which are parasitic in cats are members of the genus Strongyloides. The free-living stages, larvae and adults, have morphology that is typical of this group which includes an esophagus that is divided into three distinct portions, the corpus, the isthmus, and the bulbus, a vulva that is near midbody, and a male that does not have a bursa. The parasitic forms have atypical morphology in that the female is relatively elongate compared to the free-living form, and the esophagus is very long and cylindrical taking up from one-fourth to one-third of the total length of the digestive tract. There are two major groups within the species of Strongyloides. In one group, the parasitic parthenogenetic females have ovaries that coil about the digestive tract, and which produce embryonated eggs that are passed in the feces. In the second group of species, the parasitic parthenogenetic femals have ovaries that simply fold back on themselves, and with these species the stage passed in the feces is a first-stage larva.
Another Rhabditoid nematode that has been reported from cats is Rhabditisstrongyloides. This is typically a free-living nematode which is capable of facultative parasitism, i.e, it can live in a host when introduced under appropriate conditions.