Intermediate and definitive hosts

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In parasitology, an intermediate host is an organism inside which a parasite does not sexually reproduce. It may grow or reproduce asexually, but there is no exchange of genetic material between individuals. The definitive host is the host in which the parasite reproduces sexually. For example, the broad fish tapeworm Diphyllobothrium latum develops in fish for part of its life cycle but only reproduces sexually when it is in a mammalian digestive tract, so its intermediate host is the fish and its definitive host is the mammal.

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[1].

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.