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Strongyloides (Threadworms) in Cats
Veterinary & Aquatic Services Department, Drs. Foster & Smith
Digestive System Parasites
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Strongyloides are parasites of carnivores and man. Strongyloides tumefaciensis the feline intestinal threadworm and S. stercoralis can infect cats and dogs. It is thought that each species of host, e.g., cat or dog, is infected by a different strain or variety of the parasite. However, we do know that Strongyloides stercoralis can pass from man to dog, and dog to man. Strongyloides are common in the southern Gulf states of the United States.

The name threadworm comes from the fact that although it is long in parasite standards (2 mm), it is only 0.035 mm wide   a thread. It is unusual in several respects. It has two forms: a parasitic form, and a form that is called 'free-living,' meaning it can live and reproduce just fine outside of a host. Another unique aspect of Strongyloides is that the parasitic worms are only females. Have we piqued your curiosity?

What is the extraordinary life cycle of the intestinal threadworm?

Different Strongyloides species infect horses, pigs, ruminant animals like cows and deer, reptiles, and wild animals. They even exist in Africa where they infest zebras and baboons.

The female worm lives in the intestine of the host where it lays eggs. Remarkably, the eggs can develop even though they have not been fertilized by a male. In fact, there are no adult male worms. Either the eggs are passed in the feces, or hatch into larvae in the intestine and then are passed out. These larvae can either develop into infective parasitic larvae or into free-living worms of either sex. The parasitic larvae enter a new host by ingestion or penetrating the skin. They then migrate to the lungs, travel up the trachea and are swallowed. The free-living larvae mate, but do not produce more free-living larvae, only infective larvae that must enter a host to survive.

What determines whether the larvae passed in the feces develop into parasitic or free-living forms?

It is believed that the severity and length of infection, the species, age of the host, and the status of the host's immune system affect the development of the larvae.

Some migrating larvae may remain in the tissues of a dog. In a bitch, these larvae can make their way to the mammary glands, and infection can be passed directly to her puppies through her milk. This is one reason why puppies can have severe infections at such a young age.

Do the intestinal threadworms cause disease in animals?

S. tumefaciens generally does not result in disease in cats, but in some individuals, small white nodules can develop in the colon. If this occurs, chronic diarrhea may develop.

Most infections in dogs are inapparent or cause only mild diarrhea. Infections in young puppies can become extremely serious and ultimately fatal. They may show diarrhea or respiratory signs including severe pnuemonia.

What are the signs and symptoms of Strongyloides infestations in humans?

An inflammation of the skin may develop where the larvae entered. A cough or other respiratory difficulties may occur when the larvae migrate through the lungs.

The intestinal illness caused by Strongyloides infections in humans can range from very mild to fatal. Most cases do not show any signs, but in persons whose immune systems are not functioning adequately, severe illness can result. Usually, the disease is chronic and causes abdominal pain, diarrhea, nausea, vomiting, weight loss, weakness, and sometimes constipation. Sometimes bacterial infections can take hold because of the intestinal damage.

In some humans, especially those with suppressed immune systems, the larvae that hatch from the eggs in the intestine may stay there and develop into adults. This can greatly increase the number of worms in the person and the severity of disease. In children and others who may have poor hygiene, persons may re-infect themselves if their hands become contaminated with their own fecal material, and they in turn contaminate their food or place their fingers in their mouths. The methods of infection are called 'autoinfection,' which means the person is the source of their continued infection. Because of autoinfection, some persons have been known to remain infected for up to 35 years.

How is an infestation with intestinal threadworms diagnosed?

A diagnosis can be made when the eggs, or more commonly, the larvae are found in the feces through microscopic examination. Often, the moving larvae are best seen by just smearing a small amount of feces on a microscope slide and examining it. The solutions used for a routine fecal examination will deform the larvae and make them indistinguishable. A special procedure can be performed on a stool sample to concentrate the larvae and make them easier to find. This is called the Baermann technique.

In the Baermann technique, a funnel is fitted with rubber tubing at the bottom, and the tubing is clamped off. The funnel is filled with water, and a fecal sample wrapped in a loosely-woven cloth is placed in the water. The larvae will migrate from the sample, through the cloth, and into the water. After several hours, because of gravity, the larvae will sink, and can be found in the water right above the clamp. The clamp is gently released and the first drop of fluid is placed on a microscope slide and examined.

How are intestinal threadworm infections treated?

Until recently, thiabendazole was the most common drug used to treat infections with Strongyloides, although it is not FDA approved in cats. Fenbendazole and ivermectin are now recommended treatments, although, they too, are not FDA approved.

It has also been suggested that ivermectin (Ivomec 1% solution) could be given at an oral dose of 0.8 mg/kg (four times the usual dose). This treatment may need to be repeated. Ivermectin has not been approved for this use in cats or dogs.

In general, these medications are not very useful for eliminating the larvae that may be encysted in the tissues.

What control methods are effective against Strongyloides?

Strongyloides larvae are killed by cold temperatures and in dry environments. It is imperative that animals be kept in dry and clean environments. As with other parasites spread by feces, the yard and litter boxes should be kept clean. Good hygiene measures (use of gloves and handwashing) should be used by persons who may have contact with feces. Cats should not be allowed to use a sandbox or garden as their toilet area.

 
References and Further Reading

Georgi, JR; Georgi, ME. Canine Clinical Parasitology. Lea & Febiger. Philadelphia, PA; 1992;160-165.

Griffiths, HJ. A Handbook of Veterinary Parasitology. University of Minnesota Press. Minneapolis, MN; 1978;92.

Sherding, RG; Johnson, SE. Diseases of the intestine. In Birchard, SJ; Sherding, RG (eds.) Saunders Manual of Small Animal Practice. W.B. Saunders Co. Philadelphia, PA; 1994;698.

Sousby, EJL. Helminths, arthropods and protozoa of domesticated animals. Lea & Febiger. Philadelphia, PA; 1982;168-172.

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