Description: "Slender, up to 6mm long by 0.075mm wide. Cuticle finely striated. Tail short, abruptly tapered. Head with circumoral elevation which may or may not be lobed. Stoma shallow. Oesophagus long, filariform. Vulva about two-thirds of body length from anterior end, vagina very short. Oviduct short, with walls thicker than those of ovaries or uteri. Ovaries long, with reflexed loops spiralling around intestine or running parallel with it. Eggs in single row in uterus, ellipsoidal with extremely thin walls, in early cleavage when deposited and may not undergo development before leaving the host" (Rysavy and Ryzhikov, 1978).
Life Cycle: The Strongyloides life cycle is more complex than most nematodes as it involves an alternation between free living and parasitic life cycles. In the free living cycle, rhabditiform larvae are passed in feces develop into either infective filariform larvae or males and females that mate and produce more rhabditiform larvae. In the parasitic life cycle, the filariform larvae are passed in the feces and hatch. They penetrate the skin of the next host and migrate to the small intestine. From here, they develop into adults and can either pass larvae in the stool, or autoinfect the host (CDC, 2016).
Sources: Rysavy, B. and Ryzhikov, K.M. 1978. Helminths of Fish-Eating Birds of the Palaearctic Region: Nematoda. p. 46.
CDC http://www.cdc.gov/parasites/strongyloides/biology.html accessed January 22, 2016.