Sparganosis - Parasitic Zoonosis
A Rare Parasitic Disease in Humans Caused by a Tapeworm
Jul 31, 2009
Sparganosis is a disease caused by a cestode (tapeworm). What makes sparganosis difficult for many people to understand is that it is a zoonosis – a disease of animals occurring in a human — and that the tapeworm in this case doesn’t live in the intestine. This parasite lives in the tissues: under the skin and in various organs, including the eye, and occasionally the brain.
The Life Cycles of Parasites that Cause Sparganosis
To understand why sparganosis occurs in humans, it helps to trace the life cycle of the parasite. The adult tapeworms of these species usually live in the intestines of animals like dogs and cats, and they have life cycles very similar to the fresh water fish tapeworm of humans, Diphyllobothrium latum. Two other animal hosts are needed for the worms to reproduce successfully:
- Worm eggs pass into the environment in feces. If they arrive in fresh surface water, they develop and hatch, releasing parasitic larvae.
- Copepods living in the water become the next host (first intermediate host) when they eat the larvae. The larvae don’t die; rather they develop to a stage called the procercoid, living in the copepod’s body cavity.
- Copepods are an important food source for many animals living in and around the water, and are accidentally swallowed by many others. When fish, birds, amphibians, reptiles, wild mammals, and even humans ingest infected copepods, the procercoid larvae find themselves in a second intermediate host. If the host is suitable for them, they move into its tissues and develop to another stage —the plerocercoid, or sparganum.
- In the normal course of events, the fish, frog or other animal is preyed upon by the definitive host (the host where the adult worm lives), but if the plerocercoid finds itself in a human, this is unlikely. Instead, the larvae cause a variety of health problems depending on where they are located, and never complete their lifecycle.
How Humans Get Sparganosis
The human is a substitute for another animal in the parasite’s life cycle. This is what makes sparganosis a zoonosis – a disease of animals accidentally occurring in a human. Plerocercoid larvae can get into human bodies several different ways:
- A human sometimes inadvertently swallows an infected copepod when drinking from an untreated surface water source.
- In some cultures, the flesh of frogs, snakes, and other animals is eaten raw. The plerocercoid larva simply moves from the tissues of one host to the tissues of the other.
- Some traditional medicine uses poultices (dressings) of raw animal flesh, placing the raw meat over a wound, eye, or other body opening. Again, the larva can migrate from the dead animal into living human tissues.
Sparganosis Symptoms and Treatment
The symptoms of sparganosis depend on where the parasites are located: most often, a sparganum causes a slowly growing painful or itchy lump under the skin that may move. If the larva is located in the brain, neurological symptoms result. Similarly, when other organs and systems are infected, symptoms reflect damage to or malfunction of the particular organ or tissue. These parasitic larvae can live for decades.
In rare cases (caused by Sparganum proliferum), the sparganum proliferates, producing many spargana and spreading to multiple sites. The outlook in such cases is grim.
Antiparasitic drugs have been used to treat sparganosis but they appear to have little effect. The best treatment for the disease to date is surgical removal of the parasite(s).
Tapeworms That Cause Sparganosis
A number of different tapeworms related to D. latum, the fresh water fish tapeworm of humans, cause sparganosis in humans (but not, apparently, D. latum itself). There is confusion and uncertainty about the species involved for several reasons:
- Before the life cycles of these tapeworms were understood, plerocercoid larvae in animal flesh were thought to be a different parasite entirely. Though they, in fact, belonged to various species, all were called Spirometra mansonoides until more was known.
- The spargana of various species are virtually indistinguishable visually; thus, it is very difficult to accurately assign them to species. Newer molecular identification methods may help sort out current cases.
- The taxonomy of these tapeworms remains uncertain: scientific names continue to change and some of the adult worms such as Sparganum proliferum, remain undiscovered.
Clearly it’s relatively easy to avoid sparganosis, though cultural practices in some countries contribute to continuing incidence of the disease.
Sources
Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
"Sparganosis" Hammons, Catherine. Stanford University.
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