Causal Agents:
The nematode (roundworm) Capillaria philippinensis causes human intestinal capillariasis.  Two other Capillaria species parasitize animals, with rare reported instances of human infections.  They are C. hepatica, which causes in humans hepatic capillariasis, and C. aerophila, which causes in humans pulmonary capillariasis.

Life Cycle:

Typically, unembryonated eggs are passed in the human stool and become embryonated in the external environment ; after ingestion by freshwater fish, larvae hatch, penetrate the intestine, and migrate to the tissues .  Ingestion of raw or undercooked fish results in infection of the human host .  The adults of Capillaria philippinensis (males: 2.3 to 3.2 mm; females: 2.5 to 4.3 mm) reside in the human small intestine, where they burrow in the mucosa .  The females deposit unembryonated eggs.  Some of these become embryonated in the intestine, and release larvae that can cause autoinfection.  This leads to hyperinfection (a massive number of adult worms) .  Capillaria philippinesis is currently considered a parasite of fish eating birds, which seem to be the natural definitive host .

Capillaria hepatica has a direct life cycle that requires only one host.  Adult worms invade the liver of the host (usually rodents, but may also be pigs, carnivores and primates, including humans), and lay hundreds of eggs in the surrounding parenchyma .  The eggs are not passed in the feces of the host, and remain in the liver until the animal dies and decomposes , or is eaten by a predator or scavenger .  Eggs ingested by such an animal are unembryonated, are not infectious, and are passed in the feces, providing an efficient mechanism to release eggs into the environment .  Cannibalism has been reported as an important role in transmission among rodent populations.  Eggs embryonate in the environment , where they require air and damp soil to become infective.  Under optimal conditions, this takes about 30 days.  The cycle continues when embryonated eggs are eaten by a suitable mammalian host .  Infective eggs hatch in the intestine, releasing larvae.  The larvae migrate via the portal vein to the liver.  Larvae take about four weeks to mature into adults and mate.  Humans are usually infected after ingesting embryonated eggs in fecal-contaminated food, water, or soil .  Occasionally in humans, larvae will migrate to the lungs, kidneys, or other organs.  The presence of C. hepatica eggs in human stool during routine ova-and-parasite (O&P) examinations indicates spurious passage of ingested eggs, and not a true infection.  Diagnosis in humans is usually achieved by finding adults and eggs in biopsy or autopsy specimens.

Geographic Distribution:
Capillaria philippinensis is endemic in the Philippines and also occurs in Thailand.  Rare cases have been reported from other Asian countries, the Middle East, and Colombia.  Rare cases of human infections with C. hepatica and C. aerophila have been reported worldwide.