Secernentea

Life cycle of the Human Pinworm (Enterobius vermicularis and V. gregorii)Human pinworm eggs are deposited on the human host's perianal folds (1). Self-infection occurs by transferring infective eggs to the mouth with hands that have scratched the perianal area (2). Person-to-person transmission can also occur through handling of contaminated clothes or bed linens. Enterobiasis (pinworm infection) may also be acquired from surfaces in the environment that are contaminated with pinworm eggs (e.g., curtains, carpeting). A small number of eggs may become airborne and inhaled. These would be swallowed and follow the same development as ingested eggs. Following ingestion of infective eggs, the larvae hatch in the small intestine (3) and the adults establish themselves in the colon (4). The time interval from ingestion of infective eggs to oviposition by the adult females is about one month. The life span of the adults is about two months. Gravid females migrate nocturnally outside the anus and oviposit while crawling on the skin of the perianal area (5). The larvae contained inside the eggs develop (and the eggs become infective) in 4 to 6 hours under optimal conditions (1). Retroinfection, or the migration of newly hatched larvae from the anal skin back into the rectum, may occur but the frequency with which this happens is unknown.From Centers for Disease Control Parasites and Health website.

Öesophagostomum sp. adult femalePosterior end of a female Oesophagostomum sp., showing the pointed tail.From CDC DPDx website

Male Oesophagostomum sp. Posterior end of male Oesophagostomum sp. showing the bursa. Note the spicule (SP).From CDC DPDx Website

Öesophagostomum adultMagnification of the anterior end. Note the presence of the cephalic vesicle (CV), cephalic groove (CG) and esophagus (ES).From CDC DPDx website

OesophagostomumL3 larva of Oesophagostomum sp., obtained via coproculture from the feces of a baboon (Papio ursinus) in South Africa. Note the long, thin, pointed tail. Image courtesy of the UTC Baboon Research Unit, University of Cape Town, South Africa.The definitive hosts of Oesophagostomum spp. become infected from the ingestion of infective L3 larvae, which develop in the environment. L3 larvae are longer than the L3 larvae of hookworms, measuring 710-950 µm in length. The intestine is made-up of alternating triangular-shaped cells (zigzag gut cells). The tail end has a long, thin, tapered sheath, with a considerable gap between the end of the tail and the end of the sheath.From CDC-DPDx

ÖesophagostomumPost-Mortem Pathology Plate borrowed from Brumpti and Thomas. "The Pathological Report of Oesophagostomiasis in Man."Colon and small intestine tissue samples from patient infected with Oesophagostomum. Important to note are the nodules lining the lumen of the colon (right).From: "Introduction to the parasite family Oesophagostomum" (Stanford University Human Biology 103 class)

ÖesophagostomumPost-Mortem Pathology Plate borrowed from Brumpti and Thomas. "The Pathological Report of Oesophagostomiasis in Man." Worm protruding from nodule on the lining of the colon. From: "Introduction to the parasite family Oesophagostomum" (Stanford University Human Biology 103 class)

Öesophagostomum adultHigher magnification of the anterior end. Note the presence of the cephalic vesicle (CV) and corona radiata (CR).

Öesophagostomum sp. adult in host tissueCross-section of an adult of Oesophagostomum sp. in a colon biopsy specimen from a patient from Africa, stained with hematoxylin and eosin (H&E). Image taken at 40x magnification.From CDC DPDx

ÖesophagostomumPost-Mortem Pathology Plate borrowed from Brumpti and Thomas. "The Pathological Report of Oesophagostomiasis in Man." Outside covering of encysted worm. From: "Introduction to the parasite family Oesophagostomum" (Stanford University Human Biology 103 class)

Life cycle of Trichostrongylus nematodes infecting humans Trichostrongylus eggs are passed in the stool of the definitive host (usually an herbivorous mammal) (1) and under favorable conditions (moisture, warmth, shade) larvae hatch within several days. The released rhabditiform larvae grow in the soil or on vegetation (2) and after 5 to 10 days (and two molts) they become filariform (third-stage) larvae that are infective (3). Infection of the human host occurs upon ingestion of these filariform larvae (4). The larvae reach the small intestine, where they reside and mature into adults. Adult worms inhabit the digestive tract of the definitive host and may occur as incidental infections in humans (5). (From Centers for Disease Control Parasites and Health website)

Life cycle of Wuchereria bancroftiVarious species of mosquitoes are known vectors of Wuchereria bancrofti filariasis, depending on geographic distribution. Among these are: Culex (C. annulirostris, C. bitaeniorhynchus, C. quinquefasciatus, and C. pipiens); Anopheles (A. arabinensis, A. bancroftii, A. farauti, A. funestus, A. gambiae, A. koliensis, A. melas, A. merus, A. punctulatus and A. wellcomei); Aedes (A. aegypti, A. aquasalis, A. bellator, A. cooki, A. darlingi, A. kochi, A. polynesiensis, A. pseudoscutellaris, A. rotumae, A. scapularis, and A. vigilax); Mansonia (M. pseudotitillans, M. uniformis); and Coquillettidia (C. juxtamansonia). During a blood meal, an infected mosquito introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound (1). They develop in adults that commonly reside in the lymphatics (2). The female worms measure 80 to 100 mm in length and 0.24 to 0.30 mm in diameter, while the males measure about 40 mm by 0.1 mm. Adults produce microfilariae measuring 244 to 296 μm by 7.5 to 10 μm, which are sheathed and have nocturnal periodicity, except for the South Pacific microfilariae, which lack marked periodicity. The microfilariae migrate into lymph and blood channels moving actively through lymph and blood (3). A mosquito ingests the microfilariae during a blood meal (4). After ingestion, the microfilariae lose their sheaths and some of them work their way through the wall of the proventriculus and cardiac portion of the mosquito's midgut and reach the thoracic muscles (5). There the microfilariae develop into first-stage larvae (6) and subsequently into third-stage infective larvae (7). The third-stage infective larvae migrate through the hemocoel to the mosquito's prosbocis (8) and can infect another human when the mosquito takes a blood meal (1).From Centers for Disease Control Parasites and Health website.

Life cycle of Anisakis simplex and Pseudoterranova decipiens, parasitic agents of anisakiasis in humansAdult stages of Anisakis simplex and Pseudoterranova decipiens reside in the stomachs of marine mammals, where they are embedded in the mucosa in clusters. Unembryonated eggs produced by adult females are passed in the feces of marine mammals (1). The eggs become embryonated in water, and first-stage larvae are formed in the eggs. The larvae molt, becoming second-stage larvae (2a), and after the larvae hatch from the eggs, they become free-swimming (2b). Larvae released from the eggs are ingested by crustaceans (3). The ingested larvae develop into third-stage larvae that are infective to fish and squid (4). The larvae migrate from the intestine to the tissues in the peritoneal cavity and grow up to 3 cm in length. Upon the host's death, larvae migrate to the muscle tissues, and through predation, the larvae are transferred from fish to fish. Fish and squid maintain third-stage larvae that are infective to humans and marine mammals (5). When fish or squid containing third-stage larvae are ingested by marine mammals, the larvae molt twice and develop into adult worms. The adult females produce eggs that are shed by marine mammals (6). Humans become infected by eating raw or undercooked infected marine fish (7). After ingestion, the anisakid larvae penetrate the gastric and intestinal mucosa, causing the symptoms of anisakiasis.From Centers for Disease Control Parasites and Health website.

Life cycle of the nematode (roundworm) Toxocara canis, the most common cause of Toxocariasis in humansToxocara canis completes its life cycle in dogs, with humans acquiring the infection as accidental hosts. Unembryonated eggs are shed in the feces of the definitive host (1). Eggs embryonate and become infective in the environment (2). Following ingestion by dogs (3), the infective eggs hatch and larvae penetrate the gut wall. In younger dogs, the larvae migrate through the lungs, bronchial tree, and esophagus; adult worms develop and oviposit in the small intestine (4). In older dogs, patent infections can also occur, but larval encystment in tissues is more common (an infection is "patent" when direct evidence of the organism can be detected, e.g., in the patient’s feces or blood, regardless of whether symptoms have appeared). Encysted stages are reactivated in female dogs during late pregnancy and infect by the transplacental and transmammary routes the puppies (5), in whose small intestine adult worms become established (6). Puppies are a major source of environmental egg contamination. Toxocara canis can also be transmitted through ingestion of paratenic hosts ("transport hosts"): eggs ingested by small mammals (e.g. rabbits) hatch and larvae penetrate the gut wall and migrate into various tissues where they encyst (7). The life cycle is completed when dogs eat these hosts (8) and the larvae develop into egg-laying adult worms in the small intestine. Humans are accidental hosts who become infected by ingesting infective eggs in contaminated soil (9) or infected paratenic hosts (10). After ingestion, the eggs hatch and larvae penetrate the intestinal wall and are carried by the circulation to a wide variety of tissues (liver, heart, lungs, brain, muscle, eyes) (11). While the larvae do not undergo any further development in these sites, they can cause severe local reactions that are the basis of toxocariasis. The two main clinical presentations of toxocariasis are visceral larva migrans and ocular larva migrans. Diagnosis is usually made by serology or the finding of larvae in biopsy or autopsy specimens.From Centers for Disease Control Parasites and Health website.