The genus Strongyloides is composed of small parasitic worms with parthenogenetic adult females that live within the mucosa of the intestinal epithelium of their hosts. The parasitic stages are characterized by the possession of a muscular esophagus that is about one-third of the length of the body; this type of esophagus is called a filariform esophagus. The parasitic females fall into two major groups, those with ovaries that simply fold over at their distal ends to produce a hair-pin shape and which produce eggs that hatch before they leave the intestine, and those with ovaries that distally spiral around the intestine and which produce eggs that are passed in the feces (Little, 1966). For species of both types of Strongyloides, there are also free-living stages that do not resemble the adults in that they appear to be shorter and stockier. These free-living stages have a short esophagus that is divided into three distinct sections, i.e., the corpus, isthmus, and bulbus; this type of esophagus is called a rhabditiform esophagus. When eggs or larvae passed in the feces undergo development into free-living stages, the first-stage rhabditiform larvae that are passed in the feces or hatch from the eggs develop through four feeding larval stages to produce free-living adult males and females. The free-living adults produce eggs that hatch, and typically rhabditiform first and second-stage larvae develop into infective filariform third-stage larvae. On rare occasions the free-living cycle may repeat itself one more time. Infective filariform larvae also can develop directly from the stages passed in the feces of the host. When the cycle involves the development of free-living stages, it is termed a heterogonic cycle. When the cycle involves the direct development of infective larvae, it is called a homogonic cycle.
Cats are hosts to three species of Strongyloides. Strongyloidesfelis is a species with hair-pin turns to the ovaries that is found in the small intestine of the infected cat. Strongyloidesplaniceps is a species with spiral ovaries that is also found in the small intestine of the feline host. Strongyloidestumefaciens is a parasite that has been recovered on a few occasions from the nodules in the mucosa of the large intestine of the feline host. Strongyloides stercoralis, a parasite of the small intestine of humans and dogs, will produce infections in experimentally infected cats (Sandground, 1926), but it has not been observed in naturally infected cats.
Little MD. 1966. Comparative morphology of six species of Strongyloides (Nematoda) and redefinition of the genus. J Parasitol 52:69-
Sandground JH. 1926. The role of Strongyloides stercoralis in the causation of diarrhea. Some observations on the condition of dogs and cats experimentally infected with this parasite. Am. J. Trop. Med. 6: 421-432.