Nematodes - Intestinal Infections

Trichuris trichiura, also called "whipworm" for its whip-like shape. Enterobius vermicularis, also called, pinworm. Outside the GI tract, the parasites trigger the host's immune system to increase eosinophils. Ascaris lumbricoides, also called ringworm or giant ringworm. Necator americanus, also called New World Hookworm. Species of Ancyclostoma, including Old World Hookworm and others. Strongyloides stercoralis, also called threadworm. Be aware that this generalized pathway varies by species. Nematodes are introduced to host: – Ingestion of eggs, which are present in contaminated food, water, or on the hands. – Or, larvae may penetrate the skin to enter the host. The nematodes ultimately reach the intestines, where the adult forms lay eggs. In most cases, the eggs are excreted in the feces, which contaminates the soil and facilitates spread to a new host. In many cases, otherwise healthy individuals experience no or only mild symptoms when the parasite load is relatively low. We organize the species by whether they remain in the gastrointestinal tract or leave it during their larval stage and by how they are introduced to the human. Notice that there is overlap in these categories. Trichuris trichiura: If the worm burden is high, patients experience pain, abdomen distention, bloody diarrhea, malnutrition, anemia, and, rectal prolapse from the mechanical strain of diarrhea (this is especially common in children). Enterobius vermicularis: Most commonly infects children. Adult females travel to the perianal region and lay eggs in the skin folds; this most often takes place during the nighttime. Infection can cause perianal itching (pruritus). When individuals scratch the area, they pick up eggs on their hands and then transfer them to others via direct contact or fomites. A form of autoinfection occurs when eggs hatch prior to transfer and new larvae travel to the rectum. Migrate to the lungs and grow within the alveoli; this can cause peritonitis and eosinophilia. The host coughs up the parasites and swallows them, which carries them to the gastrointestinal tract where they'll lay their eggs. Ascaris lumbricoides: Enters the host via ingestion of eggs. Largest roundworm that parasitizes humans; it can grow to be between 15 and 30 cm long. Relatively common helminth infection. Serious complications occur when these large worms invade the bile duct, liver, gallbladder, or appendix. Infection can cause malnutrition and stunted growth in children; this is often due to obstruction of the gastrointestinal tract by large adult worms. Notice that this is the only nematode that leaves the gastrointestinal tract that is also introduced to the human host in egg form. Necator americanus: Adult worms have hook-like mouth parts that latch onto the intestinal mucosa and suck blood from the host. Thus, infection can cause anemia, gastrointestinal problems, and developmental impairment, as the worms steal blood and nutrients from the host. Secondary bacterial infections are more likely in hookworm infections due to the wounds caused by the "hooks." Ancyclostoma: Causes cutaneous larva migrans, which is characterized by "creeping eruptions" under the skin where the worms migrate subcutaneously. Some of these species can be introduced to the host via ingestion. Strongyloides stercoralis: Unique in that its larvae, not its eggs, are excreted in the feces; unlike the other nematodes we've learned about, Strongyloides sterocoralis eggs hatch within the host's gastrointestinal tract. When symptomatic, infection mimics peptic ulcers with inflammation, epigastric pain, vomiting, and diarrhea. In immunosuppressed individuals, autoinfection is more likely, and can lead to chronic hyper-infection and dissemination to other organs, including the heart and brain. Autoinfection is particularly common in patients on immunosuppressive therapies. Be aware that this species has an especially complicated life cycle.