Metacercaria

Metacercaria

The term metacercaria refers to a developmental stage in the life cycle of certain parasitic flatworms, specifically those belonging to the class Trematoda. These organisms are commonly known as flukes. The metacercaria stage is crucial for the transmission of the parasite to its definitive host, where it will mature into an adult fluke.

Life Cycle of Trematodes

The life cycle of trematodes typically involves multiple hosts and several distinct developmental stages:

1. Eggs: The life cycle begins when eggs are released into the environment, often through the feces of the definitive host.

2. Miracidium: The eggs hatch into free-swimming larvae called miracidia, which must find and penetrate a suitable intermediate host, usually a snail.

3. Sporocyst and Redia: Inside the snail, the miracidium transforms into a sporocyst, which then produces rediae. These stages multiply asexually, increasing the number of larvae.

4. Cercaria: The rediae give rise to cercariae, which are motile larvae that leave the snail to find a second intermediate host or encyst in the environment.

5. Metacercaria: The cercariae encyst to form metacercariae, which are the infective stage for the definitive host. This stage can occur in a second intermediate host or on vegetation, depending on the species.

6. Adult Fluke: Once ingested by the definitive host, the metacercaria excysts in the host's digestive tract, migrates to its target organ, and matures into an adult fluke.

Characteristics of Metacercariae

Metacercariae are typically encysted forms that are resistant to environmental conditions. They are often found in the tissues of the second intermediate host, such as fish, amphibians, or crustaceans, or on aquatic vegetation. The cyst wall provides protection until the metacercaria is ingested by the definitive host.

Importance in Disease Transmission

Metacercariae are significant in the transmission of trematode infections to humans and animals. For example, the metacercariae of the liver fluke, Fasciola hepatica, are found on aquatic plants and can cause fascioliasis when ingested by humans or livestock. Similarly, the metacercariae of the lung fluke, Paragonimus westermani, are found in crustaceans and can cause paragonimiasis.

Diagnosis and Control

Diagnosis of trematode infections often involves identifying eggs in the feces of the definitive host. Control measures include preventing contamination of water sources, controlling snail populations, and cooking intermediate hosts thoroughly to kill metacercariae.

Also see

• Trematoda
• Fasciola hepatica
• Paragonimus westermani
• Life cycle of parasites