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.
