Parasitic Worms: An Introduction

There appears to be an unsatisfactory amount of information, at least to me, on the internets for general information about parasitic worms. In any case, I feel like writing about parasitic worms today.

Parasitic worms, or as we call them in the world of parasitology, parasitic helminths, are metazoan animals made up of three phyla: Platyhelminths or flatworms (flukes and tapeworms); Nematoda (roundworms) and the less well known Acanthocephala (thorny headed worms). Metazoan simply means they are members of the animal kingdom and are made up of many cells.


A fluke (trematode)

Parasitic worms (helminths) are a very diverse group of animals. Adult worms can range in size from just a few hundred microns up to 10 meters in length. In terms of sex, male and female reproductive organs can be in the same worm (flukes and tapeworms) or as individual male and female worms (roundworms and acanthocephalans). Some worms have no gut others have have a gut with a single opening (no anus), while others have guts with both a mouth and an anus.

Parasitic worms also have different types of life cycles. Those with a direct life cycle have only a single host. Those with an indirect life cycle utilize more than one host to complete their life cycle. In either case, the host in which sexual reproduction of the worm takes place is called the definitive host. Hosts in which development, growth or asexual reproduction takes place are called the intermediate host. Host to host transmission occurs via eggs or larvae. Eggs are usually ingested while larvae may be ingested or they can penetrate the skin.


A tapeworm (cestode)

As noted above, worms are classified into phyla and then further into classes. In the phylum Platyhelminthes we will only concern ourselves with the class Cestoda (tapeworms) and the class (subclass?) Digenea (flatworms or flukes). The phylum Nematoda consists of many different orders that we need not really be concerned with....they are all nematodes (roundworms).


A roundworm (nematode)

Next time we'll look more specifically at the Trematodes or flukes.