Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Platyhelminthes
Class Trematoda

Common name: flukes

Overview

All trematodes are parasites of molluscs and vertebrates. Generally, trematodes are endoparasites, meaning that they attach to the internal tissue of the host such as the digestive tract or the liver. Most are dorso-ventrally flattened and typically have two suckers, an oral sucker around the mouth for feeding and a ventral sucker, which they use to attach to the host. The class is divided into two subclasses; Aspidogastrea, a small group that comprises entirely aquatic species that range from one millimetre to several centimetres in length, and the Digenea, a much more diverse group that can infect both terrestrial and aquatic vertebrates.

Distribution and diversity

There are over 18,000 species of Trematoda found throughout the world. There are over 800 species known in Australia, from 410 genera in 82 families. The unknown diversity of trematodes is likely to be very high, with 20 species of trematode known to infect just one species of snail (in the case of the New Zealand mud snail, found in South-Eastern Australia).

Life cycle

A typical life cycle of a digenean trematode species will involve at least one intermediate host and a definitive host. Eggs are normally released in water, and then need to either be ingested by a host or often they hatch, the free-swimming larval form (miracidium) needing to then encounter a host within 36 hours. This initial host is typically a mollusc, such as a snail. Inside this mollusc host the trematode undergoes asexual reproduction, producing multiple copies of a second larval form (cercaria) that either migrate into a second intermediate host (often an amphibian, fish or invertebrate) or move directly to the definitive host (normally a vertebrate). Inside the definitive host, sexual reproduction occurs and eggs are laid. The Aspidogastrea have a simpler life cycle, in which only one larva emerges from the intermediate mollusc host.

Feeding

Flukes attach to an internal organ of the host with the oral sucker, feeding off of the tissue using a muscular pharynx. Where specifically in the vertebrate host�s system the parasite attaches varies amongst trematode species.

Ecology

Flukes can be economically significant in agriculture, with several species known to infect livestock. Fasciola hepatica, the liver fluke, can infect stock such as sheep and cattle and costs millions of dollars in prevention, treatment and damage each year. There are several species which infect humans, including Paragonimus westermani, the lung fluke, and blood flukes of the genus Schistosoma, but human infections in Australia are rare.