Toxoplasmosis, a disease of cats and other mammalian species, is caused by a parasitic protozoan, Toxoplasma gondii. Protozoa are single-celled organisms that are among the simplest creatures in the animal kingdom. Although infection with Toxoplasma is fairly common, actual disease caused by the parasite is relatively rare.
THE LIFE CYCLE OF TOXOPLASMA
Cats, domestic and wild, are the definitive host (host in which the adult, or sexually mature stage, of the parasite is produced) and are the parasite's primary reservoir of infection. Domestic cats are important in transmission of Toxoplasma to other animals and human beings, which become involved only as intermediate hosts of the parasite. Consumption of raw meat tissues is another important means of transmission.
Cats acquire Toxoplasma infection by eating any of the three infective stages of the parasite: cyst, oocyst, or tachyzoite. Following ingestion of cysts in infected prey (rodents or birds), the intraintestinal infection cycle begins. This cycle occurs only in members of the cat family. The organisms multiply in the wall of the small intestine and produce oocysts, which are then excreted in great numbers in the feces for two to three weeks. Within five days the shed oocysts may sporulate, becoming infectious to other animals and to humans. Sporulated oocysts are highly resistant to environmental conditions and can survive in moist shaded soil or sand for many months.
During the intraintestinal infection cycle in the cat, some Toxoplasma organisms released from the ingested cysts penetrate more deeply into the wall of the intestine and multiply as tachyzoite forms. Very soon these forms spread out from the intestine to other body sites, starting the extraintestinal infection cycle. Eventually the cat's immune system restrains this stage of the organism, which then enters a dormant or "resting" stage by forming cysts in muscles and brain. Most cysts probably remain dormant for the life of the host. The extraintestinal infection cycle occurs not only in cats but also in the intermediate hosts (including humans).
Most healthy exposed cats shed oocysts during acute infection with Toxoplasma, but will not shed them after the acute infection. Even in those few cats that do re-excrete oocysts after another exposure to Toxoplasma, the number of oocysts shed is smaller and may even be insufficient to transmit the parasite effectively.
Ingestion of tissue cysts in infected prey or in other raw meat is probably the most common route by which cats are exposed to Toxoplasma. Congenital infection (transmission from mother to fetus) occurs in sheep, goats, and humans, but is much less common in cats.
Most cats show no clinical signs of infection with Toxoplasma. Occasionally, however, clinical disease—toxoplasmosis—occurs, kittens and young adult cats being more often affected than older animals. Lethargy, depression, loss of appetite, and fever are typical early nonspecific signs. Pneumonia, manifested by respiratory distress of gradually increasing severity, is the outstanding sign in many cats. Hepatitis (inflammation of the liver) may cause vomiting, diarrhea, prostration, and jaundice (yellowing of the mucous membranes). Inflammation of the pancreas and enlargement of lymph nodes also occur. Toxoplasmosis can also affect the eyes and central nervous system, producing inflammation of the retina or anterior ocular chamber, abnormal pupil size and responsiveness to light, blindness, incoordination, heightened sensitivity to touch, personality changes, circling, head pressing, twitching of the ears, difficulty in chewing and swallowing food, seizures, and loss of control over urination and defecation.
In some cases, coinfection with feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV) may predispose a cat to develop toxoplasmosis.
Toxoplasmosis may be strongly suspected by the history, signs of illness, and the results of supportive laboratory tests. A presumptive diagnosis may be made by demonstration of a fourfold or greater increase in antibody titers to Toxoplasma (indicating a recent infection) over a three- or four-week period in a cat showing signs suggestive of toxoplasmosis. A definitive diagnosis requires either microscopic examination of tissues or tissue impression smears for distinctive pathologic changes and the presence of tachyzoites or inoculation of suspect material into laboratory mice.
The presence of significant antibody levels in a healthy cat suggests that the cat has been previously infected and now is most likely immune and not excreting oocysts. The absence of antibody in a healthy cat suggests that the cat is susceptible to infection and thus would shed oocysts for one to two weeks following exposure.
TREATMENT AND PREVENTION
The two drugs that are most often used—pyrimethamine and sulfadiazine—act together to inhibit Toxoplasma reproduction. Treatment must be started as soon as possible after diagnosis and continued for several days after signs have disappeared. In acute illness, treatment is sometimes started on the basis of a high antibody titer in the first test. If clinical improvement is not seen within two to three days, the diagnosis of toxoplasmosis should be questioned.
Pyrimethamine may be unpalatable or toxic to some cats, even if given in small amounts. Recently, the antibiotic clindamycin has been reported to be effective in treating feline toxoplasmosis, with few side-effects observed.
No vaccine is as yet available to prevent either Toxoplasma infection or toxoplasmosis in cats, humans, or other species. Research in this area is in progress.
HUMAN HEALTH IMPLICATIONS
Although the incidence of toxoplasmosis among humans probably has not changed significantly over the years, awareness and concern about the disease have increased within the medical and veterinary communities. It has been estimated that 30 to 50 percent of the world's human population has been infected with Toxoplasma and harbors the clinically inapparent cyst form. This encysted form is important because, if given the opportunity, it can produce disease in immunocompromised patients. For this reason veterinarians are often called on to clarify the role that cats play in the transmission of Toxoplasma to humans.
Contact with oocyst-contaminated soil is probably the major means by which many different species—rodents, ground-feeding birds, sheep, goats, pigs, and cattle, as well as humans living in developing countries—are exposed to Toxoplasma. In the industrialized nations most transmission to humans is probably due to eating undercooked infected meat, particularly lamb and pork (in many areas of the world, approximately 10 percent of lamb and 25 percent of pork products contain Toxoplasma cysts). The organism may also on occasion be present in some unpasteurized dairy products, such as goat's milk.
Congenital infection is of greatest concern in humans. About one-third to one-half of human infants born to mothers who have acquired Toxoplasma during that pregnancy are infected. In general, Toxoplasma infection of the fetus is least common (but disease is most severe) if the maternal infection occurs during the first trimester of pregnancy. Fetal infection is most common (but disease is least severe, often without symptoms) if the maternal infection occurs during the third trimester. The vast majority of women infected during pregnancy have no symptoms of the infection themselves.
It has been estimated that Toxoplasma is responsible for over three thousand human congenital infections in the United States each year, most of which are symptomless. Among symptomatic individuals, symptoms may be present at birth, or may first appear weeks, months, or even years later (the majority of clinical cases appearing at puberty, for example, are the result of congenital, rather than recent, infection). Ocular and central-nervous-system disturbances, deafness, fever, jaundice, rash, and respiratory disease, in varying combinations, are among the more common clinical manifestations in these patients. In immunocompromised persons—those undergoing immunosuppressive therapy (e.g., for cancer or organ transplantation) or those with an immunosuppressive disease such as AIDS—enlargement of the lymph nodes, ocular and central nervous-system disturbances, respiratory disease, and heart disease are among the more characteristic symptoms. In these patients—especially those with AIDS—relapses of the disease are common, and the mortality rate is high.
Tissue cysts can be destroyed by thoroughly cooking meat to an internal temperature of 70°C (158°F) for at least 15 to 30 minutes. Freezing and thawing, salting, smoking, or pickling will not reliably destroy cysts in meat. Restricting the access of pet cats to rodents and birds and offering them only cooked meat, commercially prepared cat food, and pasteurized dairy products should prevent most transmission. (Nor should humans eat uncooked meat or unpasteurized dairy products.) Scavenging can be discouraged by placing secure lids on all garbage cans.
Because excreted oocysts are highly resistant to environmental conditions and millions may be present in a single stool, contamination of garden soil, flower beds, children's sandboxes, cats' litter boxes, and other areas of loose, moist soil where cats defecate may be extensive. Under such conditions transmission of oocyts to humans can be minimized by the following measures:
- Avoid contact with potentially contaminated soil, or wear rubber gloves during contact, and follow by washing hands vigorously and thoroughly with soap and water.
- Cover children's sandboxes to prevent contamination by cats.
- Dispose of feces from litter boxes daily or every other day to remove oocysts before they sporulate and become infective.
- Disinfect potentially contaminated litter boxes with scalding water or with dry-heat sterilization (55°C, 131°F).
- Chemical disinfection does not reliably destroy oocysts.
Specific Recommendations for Pregnant Women
A pregnant woman (or one who contemplates pregnancy) can minimize exposure to Toxoplasma by taking the following measures:
- Exclude rare or undercooked meat and unpasteurized dairy products from the diet.
- A healthy adult cat is actually less likely to shed Toxoplasma than any other mammal. However, kittens are extremely likely to shed. A pregnant woman should not adopt or handle any cat under 1 year of age.
- Test household cats for antibodies to Toxoplasma. Assuming that a cat is healthy, a positive antibody test indicates that the animal is most probably immune and not excreting oocysts and thus would be an unlikely source of infection. A healthy antibody-negative cat is most probably susceptible to infection and would shed oocysts for one to two weeks after exposure to Toxoplasma. If possible, the cat should be tested before the woman becomes pregnant.
- Have herself tested for antibodies, preferably before becoming pregnant. A positive test would indicate past infection that will not be transmitted to the fetus. The presence of antibodies also lessens the likelihood that congenital transmission would occur should she be exposed again to the parasite during pregnancy. An antibody-negative woman would thus be at greater risk of transmitting Toxoplasma to the fetus should she become infected during pregnancy.
- Protect cats from infection (or reinfection) by preventing access to birds, rodents, uncooked meat, and unpasteurized dairy products.
- Avoid handling litter boxes. Even if a cat is antibody-positive and hence most likely immune, there exists a potential for reshedding of oocysts (although in much smaller numbers than during the initial infection). For safety, litter boxes should be changed daily or every other day by another person to eliminate any potential for accidental infection.
- Avoid handling free-roaming cats, because the fur or paws could be contaminated with oocysts, which might be transmitted by hand-to-mouth contact. Any cat allowed indoors should be kept off the bed, pillows, blankets, or other furnishings the woman uses.
- Avoid handling any cat showing signs of illness.
- Wear rubber gloves if working with garden soil. Uncooked vegetables, whether grown in a home garden or supplied commercially, should be washed thoroughly before ingestion, in case they have been contaminated by cat feces.
- Make a habit of vigorously and thoroughly washing hands with soap and water after contact with soil, cats, unpasteurized dairy products, or uncooked meat or vegetables.