Threadworms in dogs

Written by cheyenne cartwright
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Threadworms are parasites that can infest dogs, cats and people. The threadworm species that infects dogs is called Strongyloides tumefaciens. Threadworms got their name because they can grow to two millimetres in length, but they rarely get much wider than .035 millimetres.

Life Cycle

Writing for Doctors Foster and Smith, a pet-supply and information company, veterinarian Holly Nash explains that the threadworm is unusual because there are no adult male threadworms, only females. Threadworms take two forms: the parasitic larvae, which must find a host to survive, and the free-living worm that can live outside a host.

Female threadworms live in the host animal's intestines, where they lay eggs that require no fertilisation to hatch into larvae. The host then passes the larvae out of the body in its faeces, where the larvae may either develop into free-living worms or into parasitic larvae.

The parasitic larvae enter a new host by burrowing into its skin. They move from there to the lungs and up the trachea, where the host animal swallows them. When the larvae reach the intestinal tract, they develop into worms, and the cycle begins again.

Illnesses Caused by Threadworms

In adult dogs, threadworms usually cause, at worst, only mild intestinal distress, including diarrhoea. In severe cases, however, threadworms can infest the dog's tissues to the extent that mother dogs may pass larvae on to their puppies through their milk.

Threadworm infestations in puppies can be fatal, causing diarrhoea, loss of weight and appetite, weakness and dehydration. Since they spread easily from animal to animal, threadworms can be a particularly serious problem in kennels or pet stores where many dogs are in close contact.


A vet diagnoses a threadworm infection by examining a stool sample for larva under a microscope.

Treating Threadworm Infestations

Dr. Nash writes that thiabendazole is the only FDA-approved treatment for threadworms in dogs. Some recent experiments suggest that fenbendazole and ivermectin also are effective treatments, but the FDA has not yet approved either drug for this use. All these medications are given orally, and none appear to kill off larvae that has infested the dog's tissues.

Preventing Infestations

Threadworm larvae can't survive in cold, dry conditions, so they aren't a serious threat in colder climates. For warmer places, Dr. Nash recommends keeping your dog clean and dry and your yard clean of dog faeces to help reduce the risk the dog will contract threadworms. Since threadworms can infect humans, wear gloves while you're cleaning up after your dog and wash your hands thoroughly when you've finished the job.

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