Biomedical Engineering Reference
In-Depth Information
Tapeworm infections for which humans are a secondary host are caused
by eating food that is contaminated by proglottids and eggs dropped by a pri-
mary host. The disease that results consists of larval cysts growing in human
tissues.
The six genera of cestodes that infect humans are: Hymenolepis,
Echinococcus, Taenia, Dipylidium, and Diphyllobothrium and Spirometra.
Echinococcus species use humans as secondary hosts. This means that
humans become infected when they ingest the eggs or the proglottids that
were passed into the environment in the feces of primary hosts (usually dogs
and other carnivores). The subsequent disease, echinococcosis, also known
as hydatid disease (from Greek, “hydatid”, meaning watery cyst), results
from the growth of larval cysts (cysticercoids) within human organs.
Because humans are seldom eaten by potential primary hosts, the human is a
dead-end host for the organism. The severity of the disease is determined by
the size and locations of the cysts. The most deadly forms of the disease pro-
duce large cysts in the central nervous system.
Several different species of Genus Echinococcus cause human disease:
Echinococcus granulosus, Echinococcus multilocularis, Echinococcus vogeli,
and Echinococcus oligarthus.
Taenia solium (the pork tapeworm) and Taenia saginata (the beef tape-
worm) use humans as the primary host for adult tapeworms. The resulting
disease is often referred to as taeniasis. Humans become infected when they
eat undercooked meat from infected animals (pork for Taenia solium, and
beef for Taenia saginata).
Taenia solium can also use humans as a secondary host. When humans
ingest eggs or proglottids (from soil contaminated with the feces of infected
pigs), the hatched larvae may produce cysts (cysticercoids) throughout the
body. This disease is known as cysticercosis. When cysts occur in the brain,
it is referred to as neurocysticercosis.
Thus, Taenia solium can cause two separate diseases in the human popu-
lation: taeniasis when the human is a primary host; and cysticercosis when
the human is the secondary host.
Dipylidium caninum uses humans as a primary host. Humans become
infected when they accidentally ingest fleas or lice (the secondary hosts)
whose tissues are infected with cysticercoids. The fleas and lice are carried
by dogs. After ingestion by humans (usually children), the cysts develop into
adult tapeworms in the intestine. One of the names given to this adult tape-
worm infestation is “double-pore tapeworm disease”. This name derives
from the characteristic anatomic feature of Class Dipylidiidae wherein geni-
tal pores appear on both sides of proglottids.
Hymenolepis nana, the last-to-be-described human tapeworm of Class
Cyclophyllidea, has a complex life cycle that can only be understood in
terms of its ability to combine the primary and secondary phases of its life
cycle within a single host organism.
