A secondary host or intermediate host is a host that harbors the parasite only for a short transition period, during which (usually) some developmental stage is completed. For trypanosomes, the cause of sleeping sickness, humans are the primary host, while the tsetse fly is the secondary host. Cestodes (tapeworms) and other parasitic flatworms have complex life-cycles, in which specific developmental stages are completed in a sequence of several different hosts.
As the life cycles of many parasites are not well understood, sometimes the "more important" organism is arbitrarily defined as definitive, and this designation may continue even after it is determined to be incorrect. For example, sludge worms are sometimes considered "intermediate hosts" for whirling disease, even though it is known that the parasite causing the disease reproduces sexually inside them
In Trichinella spiralis, the roundworm that causes trichinosis, a host has both reproductive adults in its digestive tract and immature juveniles in its muscles, and is therefore considered both an intermediate host and a definitive host.
A paratenic host is similar to an intermediate host, only that it is not needed for the parasite's development cycle to progress. There are also reservoir hosts. A reservoir can harbor a pathogen indefinitely with no ill effects. A single reservoir host may be reinfected several times. The difference between a paratenic and reservoir host is that the latter is a primary host, whereas paratenic hosts serve as "dumps" for non-mature stages of a parasite which they can accumulate in high numbers.
A dead-end host is an intermediate host that does generally not allow transmission to the definite host, thereby preventing the parasite from completing its development. For example, humans are dead-end hosts for Echinococcus canine tapeworms. As infected humans are not usually eaten by dogs, foxes etc., the immature Echinococcus - although it causes serious disease in the dead-end host - is unable to infect the primary host and mature.
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