Biomedical Engineering Reference
In-Depth Information
from excised parts. A tiny fragment of planaria (Schmidtea mediterranea),
several hundredths the size of the original organism, can regenerate into a
full-sized planaria. The pathogenic species of Class Platyhelminthes, though
lacking the full regenerative abilities of the planaria, have a great capacity to
modify their life cycles to enhance their parasitism.
The flatworms that cause human disease fall into two subclasses: Class
Cestoda (the tapeworms) and Class Trematoda (the flukes).
Platyhelminthes
Cestoda (tapeworms)
Cyclophyllidea
Taeniidae
*Echinococcus
*Taenia
Dipylidiidae
*Dipylidium
Hymenolepididae
*Hymenolepis
Pseudophyllidea
Diphyllobothriidae
*Diphyllobothrium
*Spirometra
The generalized life cycle of cestodes applies to all members of the class
and accounts for the pathologic manifestations of these infections.
Adult tapeworms live in the intestinal tract of an animal (the primary
host), absorbing nutrients from partially digested food and other products
in the bowel lumen. The adult tapeworm is composed of a scolex (the
“head”), which attaches to the intestinal wall, followed by a neck, followed
by the proglottids, that line up, one after the other, forming a “tape”. The
proglottids drop off from the end of the tape, and pass out of the organism
in feces.
By the time the proglottid has dropped into the environment, it is gravid
with infective eggs. The gravid proglottids are ingested by animals that eat
grasses or food that has been contaminated by the proglottids. Animals
infected by proglottids are the intermediate, or secondary, hosts. Eggs hatch
out juvenile forms that migrate through the secondary host, eventually stop-
ping to encyst in muscles and other organs. The cysts cause illness, in the
secondary host, commensurate with their number, size, and anatomic
locations.
Tapeworm infections for which humans are the primary host, are caused,
in almost all cases, by eating undercooked tissues from an infected animal
that is a secondary host. The disease that results consists of adult tapeworm
disease, wherein the tapeworm, attached to the gut wall, lives off nutrients in
the intestinal lumen.
