Introduction to Helminths
The helminths are a large group of multicellular worms. They range in lengths from 1mm (tiny roundworms) up to 25m (longest tapeworm). Yes. You read that correctly. 25 meters. That is an 82 foot long tapeworm! You might be asking your self why we are studying something this big in a MICRObiology class. The study of helminths is appropriate because diagnosis of helminth infection typically involves examination of clinical samples for microscopic evidence of helminth eggs (all the pictures above are of microscopic stages of helminth lifecycles).
Most helminths are free-living (don't need a host), but there are three categories of parasitic helminths that we will discuss here. Parasites need a host, and we will focus on helminths that use humans as their hosts, at least for part of their life cycle. Many of these helminths we will be talking have a huge impact on large swaths of the worlds population. There are some estimates that as many as 15-25% of the worlds population are infected with some sort of helminthic parasite. Many of these inflict tremendous disability and damage. Many of them could be controlled or eliminated if the resources and political will were there.
All helminths are divided into three categories, which you will read about in a moment. You will learn the basic structures and life cycles of each of the three categories today, but let's discuss some of the things they all have in common first.
You can choose to read through the "textbook" version below
or watch this video.
Note that there are some extra details like really specific species of worms or the drugs we use to treat--you aren't expected to know that level of detail! Again, the video is just going to better for some students to watch first, and then refer to the text below later.
Life Cycle
There are three basic stages in a worm's life cycle:
- Egg: If a helminth infection is suspected, the diagnosis is often carried out by microscopy of eggs, because most parasitic worms release many eggs and each have unique characteristics.
- Larvae (can be free-living or parasitic)
- Adult worm: Adult animals are usually large enough to be seen with the naked eye (82 foot tape worm, remember?!). The adult worm is typically the stage that reproduces and produces fertilized eggs.
Reproduction
- Some worms have both male/female sex organs, which allows for self-fertilization (hermaphroditic) while others have distinct male and female organisms, where one of each is needed to reproduce (dioecious).
- Most worms don't proliferate (make a bunch of worms) inside hosts, but instead produce many eggs that are then "voided" in feces, sputum, blood, or some other bodily excretion from the host to infect others.
Transmission
Worms can be transmitted through a variety of mechanisms, some of which you've heard of when discussing other pathogens.
- Fecal/oral: eating or drinking substances contaminated with feces that has eggs or larvae
- Predator/prey: eating an infected animal
- Transdermal: through the skin (there are some of these things that will BURROW THROUGH YOUR SKIN... ick.)
- Vector: the eggs or larvae are transmitted through contact with or a bite from an insect
Infectious Form
The infectious form of the worm depends on the species. Sometimes the eggs are infective and sometimes it's the larvae.
Hosts
These are parasitic helminths, so they must have hosts, at least for some part of their lifecycle. Some of them have multiple hosts. There are two kinds of hosts that you need to identify.
- Definitive host: this is the host that supports the adult/sexually mature form of the helminth. All of the worms we are looking at in this lab are human pathogens, so humans are the definitive host
- Intermediate host: this host is required for the worm to carry out its life cycle, but the helminth is in an immature version (typically the larvae) when it is inside the intermediate host.
Use what you learned about helminths on this page to answer the next two questions about a devastating helminth called Schistosoma.
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