The facts of pinworm infection (enterobiasis or oxyuriasis), infestation with Enterobius vermicularis, are well understood. Most basic accounts, however, don’t include many of the interesting things we know about these little worms.
Pinworm Life Cycle Facts
Odd facts about the life cycle of pinworms help us understand them better:
- Eggs of E. vermicularis may sometimes come complete with another human parasite hidden inside. Scientists suspect they transmit Dientamoeba fragilis, an intestinal protozoan.
- Male and female E. vermicularis worms reproduce sexually, but their union produces only female larvae. Male larvae are a product of parthenogenisis – the development of an unfertilized egg.
- It’s thought that females that have crawled out through the anus to distribute eggs may sometimes crawl back in again. In addition, larvae that hatch on the perianal skin may migrate into the body by the anal route. This could keep an infection going without the patient having to swallow more eggs.
- Cats and dogs don’t have their own pinworm species, and they don’t catch human pinworms. There is, however, the potential for household pets to carry pinworm eggs from one person to another on their fur.
Unusual Pinworm Infections
Most pinworm infections are acquired when people inadvertently swallow eggs transferred from soiled hands. Pinworms, however, are tough and versatile:
- Eggs of E. vermicularis survive away from the human body for about a week if there is sufficient moisture in the environment. Light and easily carried in air currents, they have been found in dust in homes and schools.
- When eggs are breathed in through the nose, or deposited in the nostrils with fingers, they sometimes hatch there, resulting in a pinworm infestation in the nose.
- Female pinworms that have crawled out onto the perianal skin sometimes migrate into the vagina and even up the fallopian tubes to the ovaries. The symptoms that result depend largely on the location of the worm(s).
History of Pinworm in Medicine
It’s thought that pinworms have been with humans for a very long, perhaps throughout human evolution:
- It’s not known who first discovered pinworms but, because the females are visible to the naked eye, it’s assumed that prehistoric people were aware of the worms.
- Because E. vermicularis requires only one host and eggs are passed directly from person to person, the parasite can thrive anywhere there are humans. As early humans spread out around the globe, they left some parasites behind, but they took their pinworms with them.
- Eggs of E. vermicularis have been found in coprolites (preserved human stools) that were left in North American caves about ten thousand years ago.
- Because modern DNA technology can now identify the exact species of a pinworm egg, and E. vermicularis infects only humans, the parasites’ presence in a coprolite can be used to confirm that the specimen is indeed from a human.
Do Other Animals Have Pinworms?
There are many species of pinworm other than E. vermicularis.
- Scientists think that pinworms may have been parasites of arthropods (the group that includes insects, spiders, scorpions, crabs etc.) before they adapted to humans and other vertebrates. Species of pinworm still infect insects and millipedes.
- Many species of animal, including birds, reptiles, amphibians, and mammals have their own pinworm species.
- Dogs and cats don’t have pinworms, and pinworms are rare in fish.
- The pinworm species that infects humans has not been found in any other animal.
Related Content:
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.
“Human Intestinal Parasites in the Past: New Findings and a Review.” Gonçalves, Marcelo Luiz Carvalho, Adauto Araújo, and Luiz Fernando Ferreira. Mem Inst Oswaldo Cruz, 98(Suppl.1): 103-118, 2003.
Medical Parasitology 5th ed. Leventhal, Ruth and Russell F. Cheadle. Philadelphia: F.A. Davis, 2002.
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