The term "threadworm" is sometimes used to refer to any worm that lives inside of a dog's belly. More commonly, however, when "threadworm" is used in relation to our companion dogs, it refers to a tiny internal parasite that lives inside of a dog's small intestine, which is the connection between the stomach and the anus – the outer passage of digestive contents. Threadworms are fairly host-specific, which means that they are picky eaters and prefer to live and feed on particular species of animals.
The threadworms that infect domestic dogs are in the genus Strongyloides. The particular species of threadworm that infects dogs, people and many other mammals is Strongyloides stercoralis. These parasites thrive in hot, humid, subtropical areas, such as the Gulf Coast and southeastern United States. The disease caused by threadworms is called "strongyloidosis."
Threadworms pose a direct health hazard to people, which makes strongyloidosis a zoonotic disease. This means that the infective "bugs" can be transmitted directly through ticks from dogs to people, and vice versa. Threadworm larvae are motile and can penetrate both canine and human skin. People with threadworm infections, especially newborn babies, often develop debilitating disease. People with weakened or suppressed immune systems should be extremely careful to avoid any contact with fresh dog feces.
Causes of Threadworm Infection (Strongyloidosis)
Dogs can become infected with threadworms through a number of different routes. Most threadworm eggs and larvae are passed out of infected dogs in their stool (in their "poop"). Other dogs can become infected by licking or eating fertile eggs, or the larvae hatched from them. Infective threadworm larvae are motile, or mobile, meaning that they are capable of moving independently. In the environment, these larvae have a unique ability to penetrate the unbroken skin of their mammalian hosts; this is called "transdermal transmission." This occurs with greater frequency in areas with poor sanitation and hygiene, such as some high-density kennels, rescue facilities and pet stores, where fecal material is allowed to build up. Hot temperatures and high humidity increase the risk of transdermal transmission of threadworm larvae. Infection can also be transferred to newborn puppies through their mother's milk, especially if the bitch becomes infected late in her pregnancy or while she is lactating. This is called "transmammary transmission".
Fertile eggs that are not expelled in fecal matter can hatch inside the dog's gastrointestinal tract, causing what is known as "auto-infection of the host." This can happen in the large intestine (colon), where threadworm eggs in unexpelled stool hatch into motile larvae, which then penetrate the large intestinal lining and become re-routed back to the dog's small intestine. This can lead to persistent infection and re-infection of the same animal, without any contact with outside sources of the parasite.
Regardless of the route of transmission, once they enter a host, threadworm larvae migrate through the bloodstream and tissues of the throat (pharynx), windpipe (trachea) and lungs. Eventually, they end up in the dog's small intestine, where they mature into adults. At maturity, adult threadworms are only about 0.7 to 2.2 millimeters in length. They are tiny.
Preventing Threadworm Infection (Strongyloidosis)
Because threadworms usually are transmitted through a fecal-oral or a fecal-skin route, the best way to prevent infection is to keep dogs away from the stool of infected dogs. Especially in tight housing situations, the facilities should be cleaned and disinfected daily. Routine de-worming protocols may also be helpful in preventing the spread of these parasites.
Symptoms of Threadworm Infection (Strongyloidosis)
Despite their small size, threadworms can cause severe illness in dogs. The symptoms of threadworm infection may include one or more of the following:
Skin inflammation (dermatitis; rash)
People can develop these symptoms as well. Young puppies are at great risk of developing threadworm infection due to the possibility that the bugs can be transmitted to the puppies from their mother's milk. Newborns also have underdeveloped or immature immune systems. Puppies living in crowded conditions are especially vulnerable. Human infants are also extremely susceptible to threadworm infection. The diarrhea seen in newborns can be especially severe, ranging from watery to soft or semi-formed. It may or may not contain blood (hemorrhagic diarrhea), mucous (mucoid diarrhea), or both. In some cases, affected animals may be constipated, although this is much less common.
Diagnosing Threadworm Infection (Strongyloidosis)
In most cases, threadworm infections in dogs go undetected, because most infected dogs do not show any observable symptoms of disease or distress. When signs do show up, the diagnosis is made by finding Strongyloides stercoralis larvae in fresh fecal samples that are examined microscopically. A number of different diagnostic techniques are available to check feces for evidence of threadworms at the larval stage. These procedures are not complicated and can be done at most local veterinary clinics. Multiple fresh stool samples and examinations may be required, because threadworm larvae are shed irregularly and at times are passed in very low numbers. It is important to evaluate fresh fecal samples; old stool may contain the larvae of other internal parasites, such as hookworms, which can look a lot like Strongyloides stercoralis.
Treating Threadworm Infection (Strongyloidosis)
Threadworms usually can be treated with standard de-wormers on an outpatient basis. Another name for de-wormers is "anthelmintics." Some medications that reportedly are effective against Strongyloides stercoralis include fenbendazole (Panacur), thiabendazole and ivermectin, among others. Not all of these are labeled for this use. These drugs can have potentially severe side effects. For example, ivermectin is not recommended for use in dogs that have tested positive for heartworm. In addition, many dogs are hyper-sensitive to ivermectin, including Collies, Shetland Sheepdogs and some other herding breeds. Ivermectin should not be used in those breeds.
The dog's attending veterinarian is the only one who should recommend an appropriate de-wormer or other treatment in any given case and advise the owner on the correct dosage and duration of treatment. If the dog has become dehydrated as a result of a threadworm infection, its veterinarian may advise that it be supplemented with intravenous fluids until proper hydration has been reestablished.
The prognosis for most dogs infected with threadworms is usually very good, provided that appropriate treatment is administered for the proper length of time. Most veterinarians recommend repeating fresh fecal examinations monthly for up to 6 months, to be sure that the infection has been completely cleared. Unfortunately, the outlook for young animals that develop pneumonia and/or severe bloody diarrhea is guarded.