Gastrointestinal Parasites of Cats

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Gastrointestinal (GI) parasites are common in cats, with prevalence rates as high as 45% depending upon the population studied. The parasites can be wormlike (e.g., stomach worms, roundworms, hookworms, tapeworms) or single-celled microscopic organisms (e.g., Isospora, Giardia, Toxoplasma). The signs associated with parasite infections are fairly nonspecific, such as a dull haircoat, coughing, vomiting, diarrhea, mucoid or bloody feces, loss of appetite, pale mucous membranes, or a pot-bellied appearance. The vomiting, diarrhea, anemia, and dehydration caused by intestinal parasites will weaken a cat, making it more susceptible to viral and bacterial infections and diseases, thus robbing your cat of good health. Some feline GI parasites have the potential to infect humans.

Wormlike Parasites

Roundworms (Toxascaris leonina and Toxocara cati) are the most common intestinal parasite of cats, with an estimated prevalence of 25% to 75%, and often higher in kittens. Adult roundworms are three to five inches long, cream-colored, and live in the cat's intestine, where they survive by eating food ingested by the host without attaching to the wall of the intestine. The adult female worm produces fertile eggs that are passed in the infected cat's feces. The eggs require several days to several weeks to develop into the infective larva stage.

Cats become infected with Toxocara cati by ingesting eggs or by eating rodents that have larvae in their tissues. Mice and other creatures that carry parasites are called transport hosts. Kittens can become infected by larvae that are passed through an infected queen's milk. In these cases, it is possible for kittens to become infected soon after birth.

Cats become infected with Toxascaris leonina by ingesting parasite eggs in the environment or larvae in the tissues of rodents. This parasite is not passed across the placenta or in the queen’s milk. It is very rare for cats less than two months of age to harbor Toxascaris leonina.

Roundworm infections are usually relatively benign, but affected kittens may show vomiting, diarrhea, constipation, and/or loss of appetite. If left untreated, roundworm infections may cause potentially life-threatening anemia (low red blood cell count) and, in extreme cases, perforation of the stomach, so infection should be taken seriously and treated aggressively. Infection is confirmed by the demonstration of parasite eggs during microscopic examination of the stool. A number of medications are effective for treating roundworm infections in cats, but reducing exposure to the feces of infected cats and prohibiting hunting are the best means of prevention. Treatment of queens prior to breeding reduces the likelihood that the parasite will infect kittens. It is important to realize that the incidence of reinfection after successful treatment if relatively high.

Human infection: Visceral larval migrans and ocular larval migrans are human diseases caused by the migration of Toxocara larvae through the tissue. Although these diseases are rare, they can be quite serious, especially when they occur in young children. They can be easily avoided by preventing ingestion of Toxocara eggs in contaminated soil or on the hands.

Hookworms (Ancylostoma and Uncinaria) are less than 1/2 inch long, slender, thread-like worms that live attached to the lining of the wall of the intestine in cats, where they feed on the blood of the host. Because of their small size, they usually are not visible in the feces of infected cats. Hookworms are long-lived and are capable of living as long as a cat. Less common than roundworm infections, the prevalence of hookworm infections among cats in North America is estimated to be between 10% and 60%.

Adult cats usually become infected with hookworms by larvae that penetrate their skin or that are ingested. Once the larvae gain entrance into the host, they migrate to the lungs and then to the intestines, where they develop into adult worms. It is uncertain whether cats can become infected by eating rodents containing larvae in their tissues, or by ingesting queen's milk that contains larvae.

While mild cases of hookworm infection may cause diarrhea and weight loss, severe parasitism can cause anemia due to blood loss from the intestines where the worms attach themselves. In these cases, a cat's feces will often appear black and tarry due to the presence of digested blood in the feces. If too much blood is lost, an affected cat can become anemic and may die if left untreated. Fortunately, like roundworms, these worms are easily diagnosed and treated. Good sanitation and daily cleaning of the litter box are keys to controlling hookworm infections.

Human infection: Hookworm larvae (Ancylostoma) can penetrate human skin when people come in close contact with contaminated soil. As they migrate under the skin, these larvae can cause a dermatitis called cutaneous larval migrans, characterized by itchiness, irritation, and the development of long, linear, track-like lesions under the skin.

Tapeworms (cestodes) have long flattened bodies that resemble a tape or ribbon. The body is comprised of a small head connected to a series of segments that are filled with eggs. The adult tapeworm lives in the small intestine with its head embedded in the mucous lining, absorbing nutrients eaten by the host. As the segments farthest from the head become fully mature, they break off and are passed in the feces. These segments can be observed near the cat's tail and rectum or in the feces. The segments are about one-quarter inch long, flat, and resemble grains of rice when fresh or sesame seeds when dry. When still alive they will usually move by increasing and decreasing in length. Microscopic examination of fecal samples may not always reveal the presence of tapeworms, because eggs are not passed singly, but as a group in the segments. Although the discovery of tapeworm segments can be quite alarming to cat owners, tapeworm infections only rarely cause significant disease in cats.

Rodents and fleas and can become infected by eating tapeworm eggs in the environment, and cats usually become infected by ingesting infected rodents or by eating infected fleas while grooming. Modern medications are highly successful in treating tapeworm infections, but reinfection is common. Controlling the flea and rodent populations will reduce the risk of tapeworm infection in cats.

Human infection: Some tapeworm species that infect cats can cause disease in humans if the eggs are accidentally ingested; but good hygiene virtually eliminates any risk of human infection.

Whipworms are an uncommon parasite of cats in the United States. Adult whipworms reside in the large intestine of infected cats and do not usually cause serious disease, although heavy infestations may cause diarrhea in affected cats.

Stomach Worms
Ollanulus tricuspis and Physaloptera species are worms that can inhabit the feline stomach. Ollanulus infections occur only sporadically in the United States and are more common in free-roaming cats and those housed in multiple-cat facilities. Cats become infected by ingesting the parasite-laden vomit of an infected cat. Chronic vomiting and loss of appetite, along with weight loss and malnutrition may be seen, although some infected cats show no signs of disease. Diagnosing Ollanulus infections can be difficult, and a veterinarian must rely on detecting parasite larvae in the vomit. Effective treatment is available, and avoiding exposure to another cat's vomit is the most effective means of controlling infection.

Physaloptera infections are even more rare than Ollanulus infections. Adult female worms attached to the stomach lining pass eggs that are subsequently ingested by an appropriate intermediate host, usually a species of cockroach or cricket. After further development within the intermediate host, the parasite is capable of causing infection when a cat ingests the insect or a transport host, such as a mouse, that has eaten an infected insect. Cats infected with Physaloptera may experience vomiting and loss of appetite. Diagnosis relies upon microscopic detection of parasite eggs in the stool, or seeing the parasite in the vomit. Effective treatment exists, and infection can be prevented by limiting exposure to intermediate and transport hosts.

Human infection: Neither Ollanulus nor Physaloptera are capable of causing disease in humans.

Protozoan Parasites

Isospora sp. (coccidia) are microscopic single-celled organisms causing the disease coccidiosis. Virtually all cats become infected with Isospora felis during their life. Cats usually become infected with this parasite by eating the cyst (thick-walled, egg-like stage) that has been passed in the feces and has matured in the soil. The cysts can be infective within six hours of being excreted in the feces. Cats may also become infected by eating either flies or cockroaches that carry infective Isospora cysts.

Isospora infections usually cause no problems in adult cats, but the parasite can cause significant disease in kittens. In infected kittens, the coccidia may destroy the lining of the intestine and cause diarrhea which often contains mucous. Infected kittens may also demonstrate vomiting and/or a decreased appetite. Serious infections may develop in crowded environments. Good sanitation and hygiene will help control coccidia, but accurate diagnosis, which relies upon demonstration of cysts in the feces, can only be achieved with your veterinarian's assistance. Isospora of cats cannot cause disease in humans.

Giardia are single-celled organisms that parasitize the small intestine of cats. The prevalence of feline Giardia infection (giardiasis) is estimated to be less than 5% but can be much higher in some environments. Cats become infected by ingesting Giardia cysts present in the feces of another infected animal, usually a littermate. Giardiasis is more common in multiple-cat households and catteries due to its mode of transmission, and the infection rate is greater in cats less than one year old.

Giardia cysts are very resistant to freezing, and chlorination of municipal water does not destroy the cysts. After ingestion of Giardia cysts, it takes five to 16 days before the cat will show signs of diarrhea. Acute or chronic, and continuous or intermittent diarrhea is the most common sign of infection, although the majority of Giardia-infected cats are free of disease. They do, however, remain a source of infection to other cats. The cat probably requires several exposures to the organism before infection actually occurs.

Diagnosis of giardiasis depends upon microscopic identification of cysts in the stool or upon identification of DNA and/or Giardia proteins in the stool using more advanced molecular biological and/or antibody-based techniques. For accurate diagnosis, several fecal samples may need to be evaluated since cysts are not continuously shed in the stool. Effective medications are available to treat giardiasis in cats, but resistance is common. Eliminating Giardia infections from households of cats may be difficult and depends on proper treatment and sanitation.

Human infection: It is uncertain whether species of Giardia that infect cats are contagious to humans or vice versa, although recent studies suggest the possibility that cats can transmit the infection to humans. Careful hygiene will eliminate the risk of accidental ingestion of cysts.

Infection with Toxoplasma parasites is fairly common, but actual disease caused by this parasite is relatively rare in cats. Cats can become infected by Toxoplasma by eating cysts (the dormant stage of the organism) in infected prey or in other raw meat. The organism multiplies in the small intestines and in approximately two to three weeks, the spores, called oocysts, are excreted in the infected cat's feces. These oocysts take approximately 1-5 days to become capable of infecting another organism after being shed in the feces, highlighting the importance of regular, daily cleaning of the litter box in the control of the spread of toxoplasmosis. (For more detailed information on this parasite, see Toxoplasmosis.)

Human infection: Toxoplasma can be transmitted to humans, although the majority of otherwise-healthy people infected with this organism show few, if any, signs of disease. The exception to this is immunocompromised individuals and pregnant women, both of whom should be very careful to avoid exposure to infective toxoplasma oocysts.


Treatment may require administering one or more dosages of the medication prescribed by your veterinarian. Whenever using medications, be sure to carefully follow the directions provided by your veterinarian.

Parasite reinfections are very common, but can be prevented. Parasite control begins with good sanitation procedures, including daily removal of feces, washing the litter box with a disinfectant (e.g., diluted household bleach) on a regular basis, avoiding overcrowded conditions, avoiding diets with raw meats, and controlling intermediate hosts (fleas, ticks, and rodents). Good parasite control is the key to a healthier cat.