Although sometimes classified as arthropods because their larvae resemble those of the arthropod, Pentastomids are usually placed in their own phylum: Pentastomida. They are related to both arthropods and crustaceans, but broke off their lineages early enough, some hypothesize, to have adapted to parasitizing the dinosaurs. This is why they occur largely as a zoonosis, often infecting snakes, lizards, and other reptiles and carnivores, depending on the species (Baer 1964, cited in Drabek 1087 and Hopps 970).
There are two main genera of Pentastomids involved in human infection: Linguatula and Armillifer, with some infections by the genus Porocephalidae also reported. Two species, L. serrata and A. armillatus, account for more than 99% of all reported human cases (Hopps 972). Infection with other species is very rare and not well documented.
Life cycle of Linguatula and Armillifer, from Hopps 980. For details on life cycle, see Clinical Presenation, below.
The Pentastomids have been described as "tongue worms" because of the adult Linguatula worms' tongue-like shape. Infections are sometimes referred to as Porocephalosis or Linguatuliasis as well. Halzoun syndrome is the common name for the infection of L. serrata in the naspharyngial tract of humans. This syndrome is also known as Marrara in Sudan, after a dish of raw stomach, lung, trachea, and liver of sheep, goats, or camels, which is often responsible for the transmission of the parasite to humans.
Pentastomiasis as an animal infection was first described in 1787 by the French veterinary surgeon Chabert, who found L. serrata in the nasal cavities of several dogs and horses (Cannon 161).The taxonomy of the Pentastomids has always been problematic: In 1819, the Linguatula and several other similar genera were grouped together under the name Pentastomum. In 1848, Van Beneden placed it under genus Arthropoda, and Leuckart in 1860 under the class Arachnida. The Pentastomids were considered Arthropods until the middle of the twentieth century, when it was decided that they should be considered a separate phylum.
The first human infection by a pentastomid was described by Pruner in Cairo in 1847 (Cannon 162). Early in its history, Pentastomiasis received much attention from prominent pathologists, but subsequently its study became more sporadic. Leuckart in the 1860's, Sambon in the 1920's, and Fain in the 1960's contributed greatly to our knowledge of Pentastomiasis. Several monographic articles have been written about Pentastomiasis since then, but it still remains a rather obscure condition.
Infection with Pentastomids is mostly asymptomatic in humans. Large numbers of larvae can inhabit a human host without causing any obstruction, damage, or significant immune response. However, Pentastomids have the potential to cause bodily harm, and even, in extreme cases, to cause medical emergencies. Pentastomiasis is the general name for a collection of syndromes caused by any of the Pentastomids.
The most common one of these is infection by the primary larvae of A. armillifer. The larvae, once hatched in the intestinal tract of the human, burrow through the intestinal walls, migrate to various organs of the body, and become cysts. The cysts can be harmful as they grow into the much larger third-stage larvae if they obstruct organs as they grow. The cysted larvae can cause abdominal pain, vomiting, constipation, diarrhea, and a tender abdomen. Although people have reportedly been hosts to hundreds of larval cysts and still been asymptomatic, extremely high amounts of cysts in carefully-placed locations can be serious enough to cause death (Herzog 267). The larvae usually die and calcify within 2 years of infection of a human host (Drabek 1093).
Photo of encyted larvae, from the Bristol Biomedical Image Archive
Transverse section of a colon severely infested with encysted larvae. From Cannon 161.
If they become excysted, the third-stage larvae can migrate and cause damage by perforating organs. Lesions due to larval migration have been observed in the liver, intestinal wall, mesentery, and peritoneum. Instances of collapsed lungs and lesions of the central nervous system because of larval migration have also been reported (Hopps 984).
The second most-common Pentastomiasis syndrome is an infection with the third- stage larvae of L. serrata, a condition known as Halzoun syndrome. In the Middle East, Halzoun often occurs after religious feasts in which uncooked sheep or goats may be eaten. In Sudan, it's known as Marrara, after a dish of raw stomach, lung, trachea, and liver of sheep, goats, or camels. These dishes have the potential to cause the syndrome if they are infected with live third-stage L. serrata larvae. Once ingested, the larvae migrate to the human's nasopharyngial passage within minutes or hours and begin to cause symptoms. Symptoms include pain and itching of the throat and ears, headaches, vomiting, facial edema, and occasionally loss of hearing. Examination of nasal discharge and vomit reveals the presence of live larvae, which are about 5-10 mm long (Drabek 1090).
The symptoms usually subside within 7-10 days, but sometimes complications can occur: damage to facial nerves or perforation of the eustacean tube have sometimes been observed. Halzoun syndrome is also related to infections of Fasciola hepatica, liver flukes. (Drabek 1092). The larvae, in almost every case ever reported, don't mature, their human hosts elminating them within three weeks. However, in two reported cases, adult Linguatula developed within humans' nasopharyngial tracts, causing nosebleeds and pressure within the nose (Hopps 984). Infections of other areas, like the eyes, by third stage larvae, have also been observed.
Hypotheses about a possible relationship between Pentastomiasis and cancer, put forth because of observed correlations between the two, haven't revealed any causality (Herzog, 267).
Pentastomiasis is transmitted by animal to human through the ingestion of the organism's eggs or larvae. This can occur because of drinking water or eating food contaminated by an infected animal's feces, by handling closely an infected animal (such as collecting a snake's skin), or by eating the uncooked flesh of an infected animal.
The definitive hosts of Armillifer are snakes, lizards, and other reptiles. The parasites live in the reptiles' upper respiratory tracts and lay eggs that are passed to intermediate hosts through respiratory secretions, saliva, or feces. Intermediate hosts can be rodents, humans, or other mammals.
Linguatula infect mammalian carnivores as a definitive host, and are passed to intermediate hosts trhough nasal secretions. Intermediate hosts can include humans and other mammals, but are often herbivores such as sheep and goats that have ingested plants contaminated with parasite eggs.
The time of larval stage development varies across species; however, it generally takes only a short amount of time (several hours) for ingested Pentastomid eggs to hatch and become primary larvae. The larvae may then remain encysted for weeks or months (Hopps 978). In the case of Halzoun syndrome, the third-stage larvae migrate almost immediately to the nasopharyngial tract and begin to cause symptoms within a few hours.
Pentastomids vary in size and shape according to their species. They are usually colorless or yellow. Females are much bigger than males, and can reach up to 130 mm long and 10 mm wide. Males grow up to 30 mm long (Guerrant). They have nonsegmented body cavities and a lack of circulatory and respiratory systems, although they have simple digestive and nervous systems. Although their name suggests otherwise, they have only one mouth; however, because their mouths are sometimes surrounded by four finger-like protrusions, they appear to have five. These mouths are surrounded with a chitinous ring, and there are two pairs of sinister-looking hooks in the head area.
Adult Linguatula are flat and resemble tongues. The Armillifer have pseudoannulations, which give them a "string-of-beads" appearance. Both Linguatula and Armillifer have distint larval stages, the first of which looks like an arthropod, with specialized piercing apparatus for penetrating through host tissues (Self 65). The secondary and tertiary nymphal stages look like smaller adult worms.
An adult Linguatula
An adult female Armillifer with two adult males (Pictures from Despommier et al.)
Because Pentastomiasis is mostly asymptomatic, it's often not detected until autopsy. X-rays often reveal calcified nymphs, crescent-shaped bodies, distributed throughout the body. Because Pentastomes do not trigger massive specific immune reactions, serologic tests are harder to perform, although one has been developed and is in use in France. The test involves using gel immunodiffusion and indirect immunoflourescence, and requires high amounts of antibody (Drabek 1093).
An x-ray reveals tiny, cashew-shaped calcified cysts. From Despommier et al.
Pentastomiasis is only treated when it becomes a serious medical condition. In these cases, surgical removal of the larvae is performed. For Halzoun Syndrome, clearing of breathing passageway is sometimes necessary (Strickland).
Pentastomiasis is found mostly in tropical and subtropical areas. It's been reported relatively frequently in equatorial Africa, the Middle East, and Southeast Asia, and less often in the Americas and Southeastern Europe. A. armillatus is typically found in West Africa, where it infects the respiratory tracts of pythons and other reptiles. L. serrata is found mostly in the Middle East, and has also been found to be relatively common in the nasopharyngial passageways of dogs in certain parts of Europe. Infections have also been reported in the Americas (Drabek 1089).
Improved sanitation and food sterilization techniques will limit the spread of Pentastomiasis. Screening of dogs in heavily affected areas, like the Middle East, could also be helpful in prevention. Education about the parasite, too, will alert people to the risks of eating uncooked foods and handling wild reptiles.
A case study in Chicago, from Applied Radiology Online
2 photographs (1 2) of encysted larvae, from the Bristol Biomedical Image Archive.
A page about reptile parasites for concerned owners
A page of links to helpful parasite information
1. Cannon, D. A. "Linguatid Infestation of Man." Annals of Tropical Medicine, Vol. 36, No. 4, Dec. 1942. pp. 160 - 167.
2. Despommier, Dickson D., Gwadz, Robert W., Hotez, Peter J. Parasitic Diseases, 3rd ed. Springer-Verlag, New York, 1994.
3. Drabick, JJ. "Pentastomiasis." Reviews of Infectious Diseases, vol. 9, no. 6, Nov-Dec, 1987. pp. 1087-94.
4. Guerrant, Richard L., Walker, David H., and Weller, Peter F. Tropical Infectious Diseases. Churchill Livingstone, Philadelphia, 1999.
5. Herzog, U; Marty, P; Zak, F. "Pentastomiasis: case report of an acute abdominal emergency." Acta Tropica, vol 42, no. 3, Sept, 1985. pp. 261-71.
6. Hopps, Howard C. et al. "Pentastomiasis." In Pathology of Protozoal and Helminthic Diseases. Marcial-Rojas, Raul A., editor. The Williams and Wilkins Company, Baltimore, 1971.
7. Self, J. Teague. "Biological Relationships of the Pentastomida; A Bibliography of the Pentastomida." Experimental Parasitology, Vol. 24, No. 1, Feb. 1969. pp. 63 - 119.
8. Strickland, Thomas G. Hunter's Tropical Medicine, 7th ed. W.B. Sauders Company, 1991.
9. Zaman, Viqar and Keong, Loh Ah. Handbook of Medical Parasitology, 2nd ed. Churchill Livingstone, London, 1989.
Title image is from Zaman et al., page 217. Thanks to Patrick Iber for his help on this page.