Strongyloides stercoralis, known more commonly as threadworm or pinworm, is a parasitic nematode that affects dogs as well as humans. The infection spreads through thread-like, filiform larvae which penetrate the skin. The larvae can also be transferred via ingestion. The worms go through a life-cycle that involves several stages, one of which is parasitic inhabitation of a host. The parasitic worms are all female; they reach about 2 mm in length in their adult stage in the small intestine of the host. These females produce eggs that hatch quickly into larvae and are passed out in the dog’s feces. Some larvae will grow into more infective filiform females, while others will grow into adult non-parasitic worms. These will mate and pass one life-cycle with no host before again producing parasitic larvae. Less healthy animals will often re-infect themselves with larvae that do not pass through the non-parasitic stage of the life cycle. In healthy adult dogs, the infection may produce only mild symptoms, but in puppies or dogs with a weakened immune system, it can be dangerous or even fatal. Most infections are treatable with several doses of medication.

Parasitic roundworms of the genus strongyloides (known colloquially as threadworms) can live in both dogs and humans. Veterinarians define this as strongyloidiasis infection. Dogs may be infected without showing symptoms, but for puppies and dogs with a weakened immune system, the disease can be serious and even fatal.

Take your dog to see a veterinarian immediately if you notice any of the following signs:

There are several types of strongyloides worm. The most common species found in dogs is the same as the one which affects humans, so cross-infection is possible:

Additional species of strongyloides nematodes are found in cats and farm animals.

The worms flourish in hot humid climates and are often spread through conditions of poor hygiene. Some common sources of infection include:

The parasites are easily killed by thorough washing with hot water, salt or lime and they will not survive drying out or direct sunlight. They are less common in northern climates and infections are rare among domestic pets. They can spread easily among strays, however, or large populations of dogs who don’t receive adequate care. An animal does not necessarily have to exhibit symptoms in order to spread the disease.

 

Larvae appear in the feces about a week after infection. Your veterinarian will usually be able to diagnose the worms by examining the larvae under a microscope. A sample of fresh feces will be needed, in order to ensure that there is no contamination from other free living species of worms. The larvae are separated from the feces using a straining system called the Baerman technique. A small concentrated sample is then examined under a microscope. The adult females can also usually be identified by scraping the walls of the intestine. Occasionally unhatched eggs may be found in the feces.

The veterinarian will likely perform a physical examination to evaluate the extent of the disease. Respiratory symptoms could require x-rays to determine if worms have reached the lungs (this is more common in puppies.) The vet may also discuss your dog’s recent history to determine where he became infected. If you have several dogs and one is infected, it’s a good idea to get the others checked also in order to avoid reinfection. If your dog is experiencing very severe symptoms, it suggests the infection has been progressing several weeks.

Over the counter deworming medication is not designed to treat strongyloidiasis, so you will need to see a veterinarian for diagnosis and treatment. Several medications are effective including ivermectin, diethylcarbamazine, fenbendazole, and thiabendazole. Doses are usually given the medication for a few days, and then repeated a week or several weeks later until the infection has cleared up. The exact medication and dose will be determined by your veterinarian based on your dog’s weight, breed, and the strength of the infection. Regular check-ups and feces examination will need to be done for at least six months after, even if there are no more symptoms.

 

The veterinarian may also adjust your dog’s other medications. If your dog is on any kind of immune inhibitor, such as the steroid prednisone, this will have to be discontinued as it will decrease the immune system’s natural ability to fight infection. Dogs with a weak immune system are more likely to auto-infect themselves. 

Auto-infection can lead to a more virulent form of the disease which is more likely to be fatal. Auto-infecting larvae can burrow into the walls of the colon; they may end up in the circulatory system and if they reach the lungs they can cause pneumonia. This may require additional medication, as well as IV fluids or oxygen.

There isn’t a vaccine against strongyloidiasis, so management, prevention, and prompt treatment are the best methods to stop the disease from spreading. Most dogs will recover if they receive treatment before the infection becomes too strong, but it’s a good idea to take additional steps to avoid reinfection. Thoroughly wash your dog’s bedding or kennel. If you have several dogs make sure they are all examined for signs of infection and have them treated if necessary. Protect yourself against infection by washing your hands often and wearing gloves any time you need to handle feces.

If you think your dog may have picked the disease up at a kennel, you should inform the owners immediately. Owners of kennels or shelters should ensure that surfaces are cleaned regularly, especially in warm climates where strongyloidiasis is likely to spread. Dogs with symptoms of diarrhea should be separated immediately. Mother dogs with symptoms will need to be removed from their puppies, if possible, to avoid infection them during feeding. Special care should be taken with puppies since they are more susceptible to the disease.