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The Agent

Cercarial dermatitis is caused by the cercariae of several avian and bovine schistosomes, which are parasitic flukes. Schistosomes belong to the phylum Platyhelminthes that consist of flatworms. Among the distinguishing features of flatworms is the bilateral symmetry of their body. Platyhelminthes have both free-living and parasitic members.

Schistosomes are considered members of the trematode class. Trematodes only contain parasitic members. Genera that are often implicated in cercarial dermatitis are Trichobilhazia (the most common), Gigantobilharzia,Schistosomatium, Schistosoma Austrobilharzia, Bilharziella, Gigantobilharzia. Specific species that cause cercarial dermatitis are S. douthitti, ollengans, T. regenti, T. szidati, & T. ocellata (most common cause).


Morphology

Schistosome cercariae are characterized by its bifurcated tail, which aids it in skin penetration. The cercaria has a head portion that contains the penetration glands used to secrete substances that allow penetration. The tail contains glycogen stores that the cercariae use as their only source of food, for they are in a non-feeding stage. Once these are depleted, they cannot actively swim towards hosts. For this reason, cercariae are short-lived once they are outside the snail intermediate host. In fact if they do not find a suitable host, cercariae die in 5 to 8 hours. Once inside a host, the cercaria sheds its tail to become a schistosomulus, which then migrates to the blood stream of the host to mature.


courtesy of Cam.University Schistosomiasis Research Group



Life cycle

The life cycle for avian schistomoses are similar to that of human schistosomiasis. Adult flukes mate and produce eggs which are usually excretes with the bird's feces. Once in the water, the eggs hatch to give rise to miracidium. The miracidium then seeks out an intermediate host, usually a snail. In the snail, the miracidium develops into a sporocyst, which in turn develops into a daughter sporocyst or a redia. Eventually, the sporocysts and redia develop into cercariae. These exit the snail through feces and seek out new avian hosts where they can mature into adults.


courtesy of Center for Disease Control