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Pentastomes are obligate parasites of the lower and (less often) upper respiratory tract of vertebrates (Paré 2008), a lifestyle requiring a number of special physiological adaptations (reviewed by Riley and Henderson 1999). Pentastomes are particularly prevalent in the tropics and subtropics. Around 70% of definitive hosts of pentastomes (i.e., hosts in which the parasite becomes sexually mature) are snakes; smaller numbers of species infect crocodiles and lizards and just a handful are known to use amphibians (specifically, frogs and toads), turtles, birds, and mammals as definitive hosts. Known intermediate hosts include fishes, amphibians, lizards, snakes, insects, and mammals; a few species apparently have a direct life cycle. (Abele et al. 1989 and references therein; Riley and Henderson 1999 and references therein; Paré 2008 and references therein)
Pentastomes have flat, soft bodies and range in size from less than 2 to 15 cm in length, with females much larger than males. Adults of all pentastome species are easily distinguished from other parasites based on the two pairs of retractile hooks on either side of the mouth (Paré 2008). Around 110 to 130 pentastome species have been described, including several that infest humans. (Riley 1986; Brusca and Brusca 2003; Margulis and Chapman 2010)
As noted above, the life cycle of most pentastome species is indirect, involving multiple hosts. Females produce ova (eggs) that are coughed up, sneezed, or ingested and passed with feces. Once ingested by the intermediate host, the primary larva hatches and penetrates the gut wall. When the intermediate host is consumed by the definitive host, nymphs migrate to the lungs of the definitive host, where they mature into young adults. Males travel throughout the lung tissue and fertilize females early during the infection. Males usually do not live long, so mature pentastome infections in definitive hosts may well be all female. (Paré 2008 and references therein)
Pentastomes can infect humans, causing a disease known as visceral pentastomiasis. Most cases of pentastomiasis have been reported from Africa, Malaysia, and the Middle East and less often from China and Latin America. In Europe and North America, the disease is only rarely encountered in immigrants and long-term travelers and the parasitic lesions may be confused with malignancies, leading to a delay in the correct diagnosis. (Paré 2008; Tappe and Büttner 2009; Chen et al. 2010).
In humans who accidentally serve as intermediate pentastome hosts, the infection develops when parasite ova are ingested from respiratory secretions or feces from the final hosts (dogs and some other carnivores for Linguatula, several species of large snakes for Armillifer and Porocephalus). Infection may also occur when when humans ingest raw or undercooked viscera (e.g., sheep, goat, or even camel liver) containing encapsulated infective nymphs. Most Armillifer infections are acquired when undercooked snake meat is consumed, but may also involve ingestion of water contaminated with snake feces. Humans become accidental intermediate hosts. Infective nymphs exit the digestive tract and develop in the liver, mesentery, and intestinal wall of the human host. Humans are dead-end hosts and nymphs, even if long-lived, eventually die and calcify. (Paré 2008 and references therein) According to Tappe and Büttner (2009), in asymptomatic patients no treatment is necessary, since the parasites degenerate after approximately two years. Only in symptomatic infections with numerous parasites may a surgical approach have to be considered. There is no antiparasitic chemotherapy available for pentastomiasis. (Tappe and Büttner 2009)
The phylogenetic position of the Pentastomida has long been a matter of contention. Over the years, close relationships have been suggested with platyhelminths (both cestodes and trematodes), crustaceans, nematodes, chelicerate arthropods, annelids, tardigrades, and myriapods (see historical overview in Almeida and Christoffersen 1999). Although the position of this group in the animal tree remains somewhat controversial, a range of data now indicate that this group falls within the arthropods, probably the crustacean arthropods. (Abele et al. 1989 and references therein; Lavrov et al. 2004; Chen et al. 2010; but see Almeida et al. 2008). Cambrian pentastome fossils have been collected, consistent with molecular estimates that the pentastome lineage extends back around half a billion years (Sanders and Lee 2010).