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What are threadworms?
Also known as pinworms, threadworms (Strongyloides) are small worms which live in the small intestine, they are around 1/8 to 1/5 of an inch long.  There are several dozen species of threadworms, cats are most commonly affected by Strongyloides stercoralis. Threadworm is a rare infection in cats, and humans are believed to be a major source of infection for cats with S. stercoralis. 
Cats become infected when they come into contact with feces containing the infective threadworm larvae. The larvae penetrate the skin and migrate to the lungs via the blood, they move up the trachea and into the throat, the cat then swallows them.
Nursing kittens can also acquire the worm via their mother’s milk.
Most infected cats will be asymptomatic, however, cats with heavy infestations may develop diarrhea and occasionally respiratory symptoms.
Your veterinarian will perform a complete physical examination and obtain a medical history from you.
Microscopic examination of the feces for the presence of larva can confirm diagnosis.
Your veterinarian will be able to prescribe an appropriate worming medication to treat threadworm.
 The Cornell Book of Cats.