Intermediate host
Encyclopedia
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 trypanosome
s, the cause of sleeping sickness, human
s are the primary host, while the tsetse fly
is the secondary host. Cestodes (tapeworms) and other parasitic flatworm
s 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 http://www.lsc.usgs.gov/FHB/leaflets/Salmonid.asp.
In Trichinella spiralis, the roundworm that causes trichinosis
, a host has both reproductive adults in its digestive tract and immature juveniles in its muscle
s, and is therefore considered both an intermediate host and a definitive host.
Trypanosoma brucei
Trypanosoma brucei is a parasitic protist species that causes African trypanosomiasis in humans and nagana in animals in Africa. There are 3 sub-species of T. brucei: T. b. brucei, T. b. gambiense and T. b. rhodesiense.These obligate parasites have two hosts - an insect vector and mammalian host...
s, the cause of sleeping sickness, human
Human
Humans are the only living species in the Homo genus...
s are the primary host, while the tsetse fly
Tsetse fly
Tsetse , sometimes spelled tzetze and also known as tik-tik flies, are large biting flies that inhabit much of mid-continental Africa between the Sahara and the Kalahari deserts. They live by feeding on the blood of vertebrate animals and are the primary biological vectors of trypanosomes, which...
is the secondary host. Cestodes (tapeworms) and other parasitic flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s have complex life-cycles, in which specific developmental stages are completed in a sequence of several different hosts.
As the life cycles
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
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 http://www.lsc.usgs.gov/FHB/leaflets/Salmonid.asp.
In Trichinella spiralis, the roundworm that causes trichinosis
Trichinosis
Trichinosis, also called trichinellosis, or trichiniasis, is a parasitic disease caused by eating raw or undercooked pork or wild game infected with the larvae of a species of roundworm Trichinella spiralis, commonly called the trichina worm. There are eight Trichinella species; five are...
, a host has both reproductive adults in its digestive tract and immature juveniles in its muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s, and is therefore considered both an intermediate host and a definitive host.