Gongylonema pulchrum

Gongylonema pulchrum is the only parasite of the genus Gongylonema capable of infecting humans. Gongylonema pulchrum infections are due to humans acting as accidental hosts for the parasite. There are seven genera of spirudia nematodes that infect human hosts accidentally: Gnathostoma, Thelazia, Gongylonema, Physaloptera, Spirocerca, Rictularia. The G. pulchrum parasite is a nematode worm of the order Spirurida. It is a relatively thin nematode, and like other worms within its class, it has no circulatory or respiratory system. Most other Gongylonema species infect birds and mammals: there are 25 species found in mammals and 10 species found in birds. This parasite is multi-cellular, and capable of movement. They have numerous rear mucosal projections, which assumedly assist propulsion through the thin layer of skin on the inside of the human host's mouth. They also have an excretory system possessing lateral canals. This parasite eats epithelial cells. Also, very often the canals are a place of inflammation, with accumulation of exudates in them. Gongylonema also swallows these exudates.

History of discovery Gongylonema pulchrum was first named and presented with its own species by Molin in 1857. The first reported case was in 1850 by Dr. Joseph Leidy, when he identified a worm "obtained from the mouth of a child" from the Philadelphia Academy (however, an earlier case may have been treated in patient Elizabeth Livingstone in the seventeenth century). He originally described it as Filariae hominis oris, and initially considered the worm was a guinea worm (Dracunculus medinensis), but because of the unique location of the worm (buccal cavity), and the relatively short size compared to the guinea worm, the hypothesis was disregarded. There have only been around 50 reported human cases of G. pulchrum worldwide since 1864, and these infections have been widespread and globally ubiquitous. G. pulchrum infections have been notoriously and historically hard to diagnose due to symptom complaints by patients (see "Symptoms" and "Diagnosis" below). Also, morphological diagnosis of the parasite is also somewhat complicated because of the variable size of adult worms, and the tendency of the worm to be different lengths depending on what host the worm is recovered from (see "Morphology" below).

Transmission Transmission to humans is due mostly to unsanitary conditions and the ingestion of infected coprophagous insects, mostly dung beetles and cockroaches. Beyond direct ingestion of infected intermediate hosts (insects), foods can become contaminated if unsanitary conditions pervade in the production of the food- coprophagous insects are found in the food, or in the production chain. Also, contaminated water sources, again with the intermediate hosts or the infective third stage larva, can lead to transmission to humans. The infection usually occurs when someone drinks contaminated water, or consumes an infected beetle. The buccal mucosa, which is the ideal environment for the parasite, is the mucous membrane of the inside of the cheek. It is non-keratinized stratified squamous epithelium, and is continuous with the mucosae of the soft palate, the undersurface of the tongue and the floor of the mouth.

Reservoir and vectors Gongylonema pulchrum, along with most other Gongylonema nematodes, has a broad natural host range. This includes hedgehogs, cattle, dogs, cats, ruminants, rabbits, and skunks. The vector and intermediate host for Gongylonema pulchrum infections are coprophagous insects (dung beetles and cockroaches).

Incubation In humans, there can be an up to six week incubation period for worm development and symptoms may not appear until the second molting of the worm, in which the young adult worms begin migration from the esophagus to the buccal and oral palate tissue. It is this movement through the mucosa of the mouth and lips that causes patients to complain of symptoms. Gongylonema pulchrum burrows in the mucosal lining of the esophagus and other parts of the buccal cavity. There the 14 cm (5.5 in) females lay their thick shelled eggs containing first stage larvae. The larvae all possess a cephalic hook and rows of tiny spines around a blunt anterior end, so when they hatch they may further infest their hosts.

Morphology The morphology of the worm is as follows, from a 2000 Veterinary Medicine study: "The anterior end in both sexes was covered by numerous cuticular platelets. There was a pair of lateral cervical papillae. The buccal opening was small and extended in the dorsoventral direction. Around the mouth a cuticular elevation enclosed the labia, and eight papillae were located laterodorsally and lateroventrally. Two large lateral amphids were seen. On the lateral sides of the female's tail, phasmidal apertures were observed. The caudal end of the male was asymmetrically alate and bore 10 pairs of papillae and two phasmidal apertures." The average length for male worms is 29.1 mm (1.15 in), while the average length for adult females is 58.7 mm (2.31 in). The worm is highly mobile, as observed in patients’ mouths and as evidenced by the morphological design of the worm.

Life cycle

In humans, the hypothesized life cycle is as follows: Ingestion of contaminated food, water, or infected dung beetle. Infects upper esophagus, moves around and lays eggs in buccal cavity of human host, ingested eggs locate near esophagus, develop and mature into adult worms after two subsequent molting stages, migrate into buccal cavity, no eggs are ever found in human feces, which strengthens the assumption that humans are solely incidental, accidental, and dead end hosts for the Gongylonema pulchrum parasite life cycle. The G. pulchrum parasite has also been studied in vivo in rabbits. The life cycle is as follows: Infective third stage larva from naturally infected dung beetles (intermediate hosts and vectors), were orally given to rabbits. The larvae entered the upper gastrointestinal tract of the rabbits (esophagus and upper stomach), and then migrated upward into the buccal cavity- pharyngeal mucosa and tongue. A third molt took place 11 days after primary infection, and the final molt took place at 36 days after primary infection. Worms reached sexual maturity at about 8 weeks, and were found mostly in the esophagus of the rabbit. 72–81 days post primary infection, embryonated eggs appeared in the feces of the rabbits.

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