Biomedical Engineering Reference
In-Depth Information
Hymenolepis nana, the dwarf tapeworm (the Greek, “nana”, means
dwarf), occurs worldwide and is the most common cestode infection in
humans. For Hymenolepis nana, humans serve as primary and secondary
hosts, and both host roles occur concurrently in the same host. Here is how it
works. Humans can become primary hosts when they eat animals (in this
case, larval fleas and beetles) that contain infectious larval cysts (so-called
cysticercoids). The larval cysts develop into adult tapeworms in the small
intestine lumen, where gravid proglottids develop. Humans become second-
ary hosts of Hymenolepis nana when they ingest gravid proglottids or eggs
dropped by a primary host. The hatched larvae penetrate into the mucosa of
the intestinal villi, where they remain, to form larval cysts within the lym-
phatic channels of the intestinal submucosa.
In the case of Hymenolepis nana, the infectious cysts (of the secondary
host) nestled in the intestinal submucosa can mature in-place, to produce
adult organisms in the small intestine. Even stranger, eggs deposited in the
small intestine, by the adult proglottids within the primary host, can encyst
in-place, in the submucosa. This means that a secondary host can assume the
role of a primary host, and a primary host can assume the role of a secondary
host.
Because a single human can serve as both the primary and the secondary
host of Hymenolepis nana, the net result is that humans infected with
Hymenolepis nana may have chronic infections, characterized by a huge
worm burden in the small intestine, composed of adult tapeworms and larval
cysts, resulting in the fecal passage of enormous number of eggs, over a long
period of time. Other tapeworms, including other species within Genus
Hymenolepis, lack this life-cycle flexibility. It is not surprising that
Hymenolepis nana is the most common tapeworm infection of humans,
worldwide (while Ascaris lumbricoides, discussed in Chapter 27, is the most
common helminth infection overall).
Rare cases of human hymenolepiasis are caused by the rodent tapeworms
Hymenolepis microstoma and Hymenolepis diminuta.
Diphyllobothrium latum uses humans as a primary host. Humans become
infested when they eat undercooked meat from an infected fish (the second-
ary host). Diphyllobothrium latum has an unusual way of passing larvae
through several intermediate hosts before infecting the final intermediate
host consumed by humans. Copepods become secondary hosts after ingesting
eggs dropped into water along with the feces of an infected primary host
(various mammals including humans). When copepods are eaten by fish,
infectious larvae persist in the larval stage (the so-called plerocercoid lar-
vae), using the fish as another intermediate host. Humans break the chain
when they ingest plerocercoid larvae that hatch and become adult tapeworms
in the human small intestine.
Spirometra species like Diphylobothrium latum belong to Class
Pseudophyllidea, and their
larvae can develop in stages, within two
