Capillaria philippinensis

This one is near and dear to my heart, as it was first discovered in the Philippines, and is frequently found in Thailand! I actually first learned about these this summer at the Parasitology Museum at Siriraj Hospital in Thailand, which is where this blurry picture of the fish most likely to carry helminths comes from:

Even though they have Philippines in the name, they're found in all areas where fresh fish is eaten, though they do tend to be most common in the Philippines. Fish eating birds are their definitive hosts, which once again means that they do not want to be living inside of us! In the natural life cycle of Capillaria philippinensis, the adult worms live in the intestines of birds, their eggs are passed out with feces, these hatch in water, and then they are eaten by progressively larger and larger fish until they get back to a bird. (By the way, a host in which a parasite exists but does not progress in life stages is called a paratenic host, which is what most of the larger fish are!)

Unlike most species which cause problems for humans because they don't want to be in humans, Capillaria philippinensis can progress from a larva (which is the point at which it is swallowed) to an adult worm. This is where the real problems begin with this helminthic infection. Since it adapts to living in humans, it creates a cycle of auto-infections within the human bowel. This means that worm loads can become very high within the intestine, leading to abdominal pain, persistent permanent diarrhea, and intestinal crypts. It can lead to deaths because of bowel obstructions, diarrhea, and malabsorption of nutrients.  

Adult worms are 28-47mm, and inhabit the small intestine. Eggs are peanut shaped, and rather small. They have a thin shell. They exhibit flattened bipolar prominences, and they are typically unembryonated when passed.

Photo Credit: Wikipedia.com

Diagnosis here really relies on knowing the patients history, such as living in/traveling to areas where this parasite is prevalent and eating a lot of raw fish. Also, unlike Anisakiasis, which presents within a day of fish ingestion, Capillaria philippenensis takes 3-6 weeks to mature to adults in the intestine.

Treatment must be given over a longer course of time, since the parasite can exist in many stages within the intestine, and most anti-helminthics only target one life stage. It is important to stop the cycle of auto-infection early, as worm burdens can get very high very quickly.