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.
Strongyloides tumefaciens is another species of threadworm occasionally seen in the cat.
How do cats become infected with threadworm?
Cats become infected when they come into contact with feces containing the infective threadworm larvae. These larvae penetrate the skin and migrate to the lungs via the blood. Here they migrate up the trachea and into the throat where they are swallowed. Nursing kittens can also acquire the worm via their mother's milk.
What are the symptoms of threadworm?
Cats with light infestations may show no symptoms at all. Heavier infestations may produce symptoms including:
- Occasionally respiratory symptoms
How is threadworm diagnosed?
Your veterinarian will perform a complete physical examination and obtain a medical history from you.
Diagnosis may be confirmed by microscopic examination of the feces which reveals threadworm larva.
How is threadworm treated?
Threadworm is a relatively rare infection in cats. Humans are believed to be a major source of infection for cats with S. stercoralis. 
Your veterinarian will be able to prescribe an appropriate worming medication to treat threadworm.
 The Cornell Book of Cats.