Tongue Worms: Pentastomida
TONGUE WORMS: PentastomidaTONGUE WORM (Linguatula serrata): SPECIES ACCOUNT
As their common name suggests, many adult pentastomids (pen-tah-STOH-mids) have long, wormlike bodies in the shape of a tongue. They measure up to 5.9 inches (150 millimeters) in length. The head has a small mouth underneath with a hook on each side. The hooks can be withdrawn inside the head. Their long, fluid-filled bodies are ringed, but not segmented. The soft external skeleton (exoskeleton) is whitish or clear. Because they live inside their food and are surrounded by everything they need, tongue worms do not have or need circulatory, respiratory, or excretory systems. They move like a maggot (fly larva), inching along by contracting their muscles and shifting body fluids inside to force the body forward or backward. Adult females are much larger than males and are usually filled with hundreds of thousands to millions of eggs.
Tongue worms are found on all continents, but most species are found in the warmer tropics and subtropics.
Adult tongue worms live in respiratory systems of vertebrates (birds, reptiles, and mammals). The larvae (LAR-vee) of tongue worms develop in the tissues and organs of hosts different from those of the adults. Larval hosts include arthropods, birds, reptiles, and mammals.
Both the adults and the larvae of tongue worms are internal parasites. Internal parasites spend most their lives inside other animals, where they eat their tissues and fluids. Most species feed on blood, but tongue worms eat the tissues and linings inside the nose and its sinuses (SIGH-nes-ehs). Sinuses are openings, or channels, inside the head that are connected to the nose.
BEHAVIOR AND REPRODUCTION
Tongue worms usually need to complete their life cycle in more than one host. Larval hosts are called intermediate hosts, while the hosts of adults are called definitive hosts. An intermediate host accidentally swallows the eggs as they eat. The larvae hatch and infect the tissues and organs of the intermediate host. They molt, or shed their exoskeletons, several times before reaching the infective stage of their life cycle. The infective stage is a dormant, or resting, stage, and the larvae are surrounded by a non-living, protective covering called a cyst (sist). The larvae break out of the cyst when another animal, the definitive host, eats the intermediate host and its cysts. In most species, the larvae first burrow into the lining of the intestines or stomach of their definitive host. Later, they move into the body cavity before tunneling into the lung.
Both males and females are required for reproduction. Mating occurs when the males and females are about the same size. Females store the sperm in their bodies. Mated females increase in size as their bodies fill up with developing eggs. Hundreds of thousands to millions of eggs are produced and released continuously. Eggs released in the lungs are coughed up by the definitive host, swallowed, and then passed out of the body with the solid waste. The eggs of species that live in the nose and its sinuses are sneezed out or swallowed and passed out of the body with the solid waste.
ARE TONGUE WORMS REALLY CRABBY?
For many years, tongue worms were not considered arthropods at all and were placed in a phylum near velvet worms and water bears. Recent studies, based on the structures of the genetic material, sperm, and larvae of tongue worms, show they are crustaceans. They might be related to fish lice, but not all scientists agree. They have been on Earth for 500 million years, long before their modern vertebrate hosts. Ancient fishlike animals may have been their original hosts.
TONGUE WORMS AND PEOPLE
Five species of tongue worms are known to infect people. In four of these, people are only an accidental intermediate host. Human infection with these species is usually the result of eating uncooked snake meat. However, both the larvae and adults of Linguatula serrata can infect humans.
No species of tongue worms are considered threatened or endangered.
Physical characteristics: Tongue worms are long, tongue-shaped worms. The males are 0.71 to 0.79 inches (18 to 20 millimeters) in length, while the females are 3.15 to 4.72 inches (80 to 120 millimeters).
Geographic range: They are found worldwide, especially in warm tropical regions. Because they are found throughout the world, no distribution map is provided.
Habitat: Adult tongue worms live in the noses of dogs, foxes, coyotes, wolves, and cats (definitive hosts). They rarely infect people. The larvae infest the tissues and organs of rabbits, horses, goats, and sheep (intermediate hosts).
Diet: Adults eat the tissues and secretions lining the nasal passages and sinuses. The larvae feed on blood and lymph (limf) of the hosts. Lymph is a yellowish body fluid filled with white blood cells.
Behavior and reproduction: Both males and females are required for mating. Males only mate with females that are close to their own size. The bodies of females may contain up to five hundred thousand eggs at a time and can produce several million eggs in their lifetime. Fertilized eggs are sneezed out or swallowed and passed from the body with solid waste. Intermediate hosts, such as rodents, cattle, sheep, and goats, accidentally swallow the eggs with their food. The eggs hatch into four-legged larvae that travel through blood vessels into the lungs and the digestive organs. The larvae must molt six to eight times before they become infective cysts. Definitive hosts, dogs and their relatives, eat the infested flesh. When swallowed, the larvae go directly into the air passages and sinuses associated with the nose inside the head. There they develop into adults.
Tongue worms and people: People living in the Middle East, India, Africa, southeast Asia, and the East Indies are sometimes infected with the larvae. Infections are usually the result of eating raw glands of cattle, sheep, and goats that have the larvae. These glands are considered a special treat in these parts of the world. People may not be aware that they have an infestation or suffer from irritation in their nose and throat. Deaths have been reported due to blocked air passages. Larval infestations also occur when the eggs are accidentally swallowed. There is no cure for infestations in dogs or humans, but most single infestations disappear after a year.
Conservation status: Tongue worms are not considered threatened or endangered by the World Conservation Union (IUCN). ∎
FOR MORE INFORMATION
Mehlhorn, H. Parasitology in Focus. Berlin: Springer Verlag, 1988.
Martin, J. W., and G. E. Davis. "An Updated Classification of the Recent Crustacea." Science Series, Natural History Museum of Los Angeles County 39 (2001): 1-124.
Riley, J. "The Biology of Pentastomids." Advances in Parasitology 25 (1986): 46-128.
Arthropod Oddments. http://www.kendall-bioresearch.co.uk/arthrpod.htm#penta (accessed on March 18, 2005).
External Parasitic Diseases of Dogs and Cats.http://www.ivis.org/special_books/carter/carter7/chapter_frm.asp?LA=1 (accessed on March 18, 2005).