Pachysentis

Pachysentis is a genus in Acanthocephala (thorny-headed worms, also known as spiny-headed worms) that parasitize primates and carnivorans. They are distributed across Africa, the Middle East, and the Americas. Pachysentis species attach themselves to the inner lining of the gastrointestinal tract of their hosts using their hook-covered proboscis. Their life cycle includes an egg stage found in host feces, a cystacanth (larval) stage in an intermediate host such as the Egyptian cobra, and an adult stage where cystacanths mature in the intestines of the host. This genus appears identical to the closely related Oncicola apart from a greater number of hooks on the proboscis. There are eleven species assigned to this genus, although P. septemserialis is of uncertain taxonomic status. The female worms range from 12 mm long and 1.6 mm wide in P. lauroi to 50 mm long and 4 mm wide in P. dollfusi. Virtually all of the length is the trunk, with a short proboscis. There is pronounced sexual dimorphism in this species as females are around twice the size of the males.

Taxonomy
Phylogenetic analysis has been conducted on one of the eleven species in the genus, P. canicola, and supporting its independent status in the family Oligacanthorhynchidae. Phylogenetic analyses have also been conducted on Oncicola, a genus morphologically nearly identical to Pachysentis apart from the number of hooks on the proboscis, and have also placed it in the family Oligacanthorhynchidae. The type species for Pachysentis is P. canicola. In 1972, Schmidt reclassified six species from the Prosthenorchis genus and one species from the Oncicola genus into the genus Pachysentis''.

Description
Pachysentis looks identical to the closely related Oncicola apart from the number of hooks on the proboscis. Species of Oncicola have 36 or fewer hooks whereas species of Pachysentis have more. The probosces of Pachysentis species are not quite spherical and contain 42 to 102 hooks arranged into 12 longitudinal rows of 3 to 12 hooks each. Depending on the species, the rows may be regularly or irregularly alternating and straight or crooked. Hooks have tips with or without barbs, and the larger hooks with complex heads (manubria) and roots with the remaining spines being rootless. The trunk is around twice as long as wide with the anterior half usually wider than the posterior half. The testes are in tandem with at least one located before the middle of the worm. There are eight cement glands compactly arranged each with a single giant nucleus used to temporarily close the posterior end of the female after copulation. The eggs have a sculptured outer membrane. Species can be distinguished based on the number and arrangements of proboscis hooks, whether these hooks are barbed, the arrangement of the cement glands, host, and the length of lemnisci (bundles of sensory nerve fibers).

Species
The genus Pachysentis Meyer, 1931 contains eleven species, although P. septemserialis is of uncertain taxonomic status.


 * Pachysentis angolensis (Golvan, 1957)

P. angolensis was found infesting the side-striped jackal (Canis adustus). The proboscis has a total of 42 hooks without barbs in 12 regularly alternating rows of three and four hooks (six rows of each). The eight cement glands are organized in pairs. This species is named after Angola, the country where it was first discovered.


 * Pachysentis canicola Meyer, 1931

P. canicola was found infesting the red fox (Vulpes vulpes) in Bushehr, Bushehr Province, Iran, the golden jackal (Canis aureus) also in Iran, captive maned wolves (Chrysocyon brachyurus) in a zoo in Texas, and the striped skunk (Mephitis mephitis) found nearby. It was also found infesting the western diamondback rattlesnake (Crotalus atrox). It is the type species of the genus Pachysentis. The species name canicola is derived from the Latin word for canine which is the type of creature (family Canidae) that this species infests.


 * Pachysentis dollfusi (Machado-Filho, 1950)

P. dollfusi was found infesting the intestines of the common brown lemur (Eulemur fulvus) in a Brazilian zoo but originally from the island of Madagascar. It is thus unknown if the worm originates from Brazil or Madagascar. The proboscis has 48 barbed hooks arranged into six rows of four hooks each followed by another six rows of four hooks each. The cement glands are in uniform pairs. It is synonymous with Prosthenorchis spirula Travassos 1917. It is named after the parasitologist Robert-Philippe Dollfus.


 * Pachysentis ehrenbergi Meyer, 1931

P. ehrenbergi was found infesting the red fox (Vulpes vulpes) in Egypt and in an intermediate host, the Egyptian cobra (Naja haje). The proboscis is armed with 102 barbed hooks arranged into six rows of nine each followed by six rows of eight hooks each. P. ehrenbergi was reported infesting the body cavity of 5.4% of a sample of African five-lined skinks (Trachylepis quinquetaeniata) in the Qena Governorate of Egypt, however the measurements and morphological description do not match either the original description by Meyer in 1931 or the reexamination of original specimens by Gomes and colleagues in 2019. Juvenile P. ehrenbergi have been reported infesting the long-eared hedgehog (Hemiechinus auritus) in the Faran Oasis, South Sinai, Egypt. Juvenile trunk length is reported to be 3.22–4.16 mm by 0.87–1.04 mm, the proboscis length to be 0.42–0.60 mm in length by 0.45–0.68 mm in width, the proboscis sheath to be 0.79–1.0 mm by 0.37–0.52 mm (whereas the measurements given by Meyer in the original 1931 description is larger, at 1.3 mm in length and the proboscis measuring 0.8 mm in length by 0.9 mm in width). The anterior proboscis hooks in the juveniles are reported to be large, from 0.078–0.086 mm long, and the posterior hooks smaller, from 0.052–0.062 mm.


 * Pachysentis gethi (Machado-Filho, 1950)

P. gethi was originally described in 1950 by Machado-Filho infesting tayra (Eira barbara) in Pará and Rio de Janeiro, Brazil but this remained the only record until it was rediscovered in 2016 infesting the wild lesser grison (Galictis cuja), also in Rio de Janeiro, with different measurements. The proboscis has 42 hooks without barbs arranged into six rows of four hooks followed by six rows of three hooks. The eight cement glands occur in pairs. The species is named after Dr. Geth Jansen.

P. lauroi has been found infesting the South American coati (Nasua nasua) in Mato Grosso do Sul, Brazil. The proboscis has 48 barbed hooks arranged into six rows of four hooks followed by six rows of four hooks. The eight cement glands are clustered. It was found in the small intestine. The species is named after Lauro Travassos, a parasitologist who studied Brazilian Acanthocephala.
 * Pachysentis lauroi Gomes, Amin, Olifiers, Bianchi, Souza, Barbosa & Maldonado, 2019


 * Pachysentis lenti (Machado-Filho, 1950)

P. lenti has been found infesting the white-headed marmoset (Callithrix geoffroyi) in Espírito Santo, Brazil. The proboscis has 48 hooks without barbs arranged into six rows of four hooks followed by six rows of four hooks. It is named after Herman Lent, a Brazilian parasitologist.


 * Pachysentis procumbens Meyer, 1931

P. procumbens has been found infesting the red fox (Vulpes vulpes)	in Argo, Egypt. The proboscis has 90 hooks without barbs arranged into six rows of seven hooks followed by six rows of eight hooks. The species name is from the Latin procumbent, meaning "lying prostrate".


 * Pachysentis procyonis (Machado-Filho, 1950)

P. procyonis was found infesting the intestines of a crab-eating raccoon (Procyon cancrivorus) in Rio de Janeiro, Brazil. The proboscis has 42 hooks without barbs arranged into six rows of four hooks followed by six rows of three hooks. Distinguishing features include eight clustered cement glands and very short lemnisci that do not reach the anterior testis. The species is named after the genus of the host (Procyon) it was found infesting. It is the only known parasite of the crab-eating racoon in Brazil.


 * Pachysentis rugosus (Machado-Filho, 1950)

P. rugosus has been found to infest the large intestine of Azaras's capuchin (Sapajus cay) in Rio de Janeiro, Brazil, and the tufted capuchin (Sapajus apella), also in Brazil. The proboscis is armed with 42 hooks without barbs arranged into six rows of four hooks each followed by six rows of three hooks each. This species can be identified by its clustered cement glands and long leminisci that reach the anterior testis. The species name rugosus is Latin for 'wrinkled'.


 * Pachysentis septemserialis (Machado-Filho, 1950)

P. septemserialis is considered by Gomes (2019) to have an uncertain taxonomic status due to differences between the paratypes morphological characteristics and the original description, the similarity in hosts (primates of the family Callitrichidae including the black tamarin (Saguinus niger), and the absence of samples or measurements of adult males. Specifically, the original description of one paratype described the lack of a collar at the base of the proboscis whereas a collar was observed (suggesting affiliation with the genus Prosthenorchis). A second discrepancy from another paratype is the number of hooks; 12 longitudinal rows of four hooks with total of 48 hooks were observed but contradicts the seven rows of seven hooks with a total of 49 hooks given in the original description. The name septemserialis refers to the seven rows in series. Morphologically, new observations suggest it is synonymous with P. lenti.

Distribution
The distribution of Pachysentis species is determined by that of its hosts. Pachysentis species have been found in South America (Brazil), Africa (Egypt and Angola), Asia (Egypt and Iran).

Hosts
The life cycle of an acanthocephalan consists of three stages beginning when an infective acanthor (development of an egg) is released from the intestines of the definitive host and then ingested by an arthropod, the intermediate host. The intermediate hosts of most Pachysentis species are not known. When the acanthor molts, the second stage called the acanthella begins. This stage involves penetrating the wall of the mesenteron or the intestine of the intermediate host and growing. The final stage is the infective cystacanth which is the larval or juvenile state of an Acanthocephalan, differing from the adult only in size and stage of sexual development. The cystacanths within the intermediate hosts are consumed by the definitive host, usually attaching to the walls of the intestines, and as adults they reproduce sexually in the intestines. The acanthor is passed in the feces of the definitive host and the cycle repeats.

Pachysentis species exclusively parasitize primates and carnivorans as their primary host. Paratenic hosts (hosts where parasites infest but do not undergo larval development or sexual reproduction) for Pachysentis have been found for P. ehrenbergi (Egyptian cobra, Naja haje) and P. canicola (western diamondback rattlesnake, Crotalus atrox). There are no reported cases of any Pachysentis species infesting humans in the English language medical literature.