What is a Trematode?
A parasitic trematode is a fluke, or flatworm, that lives inside another creature, deriving all off its nutritional requirements from its host. There are many species of trematodes parasitizing vertebrates, including humans. They live in the bloodstream, intestine, liver, lungs, and other organs. Some of them cause very serious disease.
Die offs of large numbers of water birds due to intestinal trematodes have been reported in the Great Lakes area for more than forty years.
Trematode Basics—Life Cycle
Trematodes have varying life cycles but most have features in common:
- Eggs usually enter the environment in urine or feces. Most trematodes require an aquatic environment for further development (one reason why fish and water birds host numerous parasitic flukes).
- The larva, or miracidium, infects the first intermediate host, almost invariably a snail, and multiplies, producing a second type of juvenile - a cercaria.
- The cercaria leaves the snail and infects a second intermediate host (some flukes do not need a second intermediate host while some also require a third). In the second intermediate host (another snail, different mollusk, a plant, a fish, or even an insect) the cercaria becomes a metacercaria.
- The second intermediate host is eaten by a vertebrate and digestive processes release the metacercaria in the intestine. Some metacercariae emerge as well developed flat worms capable of producing eggs very quickly. Others undergo further maturation and some migrate to other organs. Adults remain within this host for the rest of their lives, feeding primarily on blood and tissue.
Intestinal Flukes in Water Birds
Epidemics of trematode infection in waterfowl usually occur in the spring and fall, often when large numbers of birds visit particular areas during migration. Scaup (bluebills), coots, swans, and other birds, particularly diving ducks, are affected. Several species of intestinal flukes are involved, among them Cyathocotyle bushiensis, Leyogonimus polyoon, and Sphaeridiotrema globulus. The life cycle for these flukes likely involves first and second intermediate hosts (both are probably snails), and a metacercaria that is near maturity when its snail host is eaten.
If a water bird swallows just a few metacercariae, both the bird and the flukes survive, giving the parasites the opportunity to reproduce, and even spread to a new place. If there are large numbers of flukes, however, the bird will die, usually in less than a week and sometimes within forty-eight hours. Massive intestinal hemorrhage occurs as the flukes attach and begin to feed—anemia, a rapid drop in blood pressure, and intestinal damage kill the birds. As few as 100 flukes can be fatal: some dead birds have thousands of flukes in their intestines.
Spring and Fall Waterfowl Die Offs
Why are trematode outbreaks occurring in water birds? The success of a parasite depends on the presence of its first, second, and third hosts in its environment. In a body of water where both snail hosts coexist, activities of infected water birds in the area result in increasing levels of infection. In spring and fall, the arrival of huge flocks of migrating water birds completes picture: the visiting birds feed on the snails, and the life cycle completes itself with horrifying results. Thousands of birds die within a few days, littering shorelines and floating on the water surface.
Related Content:
Swimmer’s Itch is Caused by a Water Bird Trematode
Trematodes infecting humans:
Sources:
“Miscellaneous Parasitic Diseases.” Field Manual of Parasitic Diseases: Birds. US Department of the Interior Geological Survey
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
 |
 |
