Flukes: Trematoda

Flukes with an indirect life cycle begin life as eggs in a primary host and then pass with the host's feces (FEE-seez) or waste into water or onto land. After the eggs hatch, the larvae move to another host, called the intermediate host, which is often a mollusk. Mollusks (MAH-lusks) are animals with a soft, unsegmented body that may or may not have a shell. The flukes change form, exit the host, and move to another intermediate host, which is frequently another mollusk, a fish, or an amphibian, and change form again. The life cycle continues when a new primary host eats the second intermediate host, at which point the fluke infects the primary host. Primary hosts often are mammals and birds.

In some species the first-stage larvae do not feed. For this reason, they must find a first intermediate host very quickly, usually within one or two days of hatching. In moving from the first to the second host, most flukes use environmental cues, such as light or water turbulence, to seek the new host. Some species also follow a chemical trail. In some species, however, the larvae seem to stumble upon rather than track their hosts. Some flukes with an indirect life cycle skip the second intermediate host and invade the primary host directly. Others live on plants rather than in a secondary host. The primary host then becomes infected by eating the fluke-infested vegetation.

Flukes with an indirect life cycle use asexual and sexual reproduction. Asexual (ay-SEK-shuh-wuhl) means without, and sexual means with, the uniting of egg and sperm and the transfer of DNA from two parents. When the first-stage larvae reach their destination within the first intermediate host, the asexual phase begins when the larvae lose their hairlike fibers and change into another form of larvae. The new larvae produce more of the same type of larvae or a transformed type. With asexual reproduction, the number of invading flukes can multiply very quickly inside the first intermediate host.

In the first intermediate host, the asexually produced larvae transform into free-living young flukes. The young flukes swim to the second intermediate host, which is typically prey for the primary host. Once on or in the second intermediate host, the young flukes transform again. It is only after the flukes finally enter the primary host and become adults that they use sexual reproduction either by mating with other flukes or by fertilizing (FUR-teh-LYE-zing) themselves. Fertilization (FUR-teh-lih-ZAY-shun) is the joining of egg and sperm to start development. Almost all flukes make both eggs and sperm. Blood flukes have separate sexes, and the adult females and males mate with each other.

Flukes with a direct life cycle use only sexual reproduction. The entire life cycle occurs in one host, usually a mollusk. Although predators may eat the host species and temporarily harbor the worms, the worms can survive for only a short time in the predator's digestive tract and cannot reproduce or develop there.