Toxoplasmosis is a fairly uncommon disease that affects all type of mammals and birds. When it does occur, it is a serious disease. Toxoplasmosis is caused by a single-celled protozoan parasite called Toxoplasma gondii. Although disease caused by Toxoplasma is uncommon, many animals have immunity to the disease that they acquired during a prior exposure. That is, the majority of animals are exposed to the organism and develop immunity to it without developing any disease.
Wild and domestic felines are the only animals in which Toxoplasma can fully live out their life cycle and in which sexually mature forms of the parasite occur. Because of this cats are called the definitive host of Toxoplasma. Cats are also the only true reservoir of this parasite in Nature. Cats play an important roll in transmitting Toxoplasma to other species including humans. In humans and all species other than cats, only non-sexual (asexual) stages of Toxoplasma occur. Raw or undercooked meat is a common way for the infection to be passed from animal to animal or animal to man.
The Toxoplasma organism has three stages, tachyzoite, oocyst and cyst. After eat any one of these stages in an infected small mammal or bird, the intestinal lining of cats becomes infected. Cats are the only species of animal that develop this intra-intestinal form of the parasite. The oocysts that are shed from these cells pass out of the cat with its feces. This infection persists only for a few weeks before the cat itself becomes immune to the disease. After being passed in the feces, these oocysts sporulate and become infectious for any human or animal that accidentally eats them. These sporulated oocysts are tightly encapsulated in an impervious membrane that resists drying and heat. They survives for up to a year in damp shaded soils.
While the cat is hosting this parasite, some of the Toxoplasma oocysts within the intestine burrow deeper within the lining where they divide into tachyzoites. These tachyzoites leave the intestine and spread throughout the body multiplying and causing the systemic or extra-intestinal phase of the disease. When the cat’s immune system has produced sufficient antibody against these Toxoplasma, the organisms are walled off as dormant oocysts in muscle and nerve tissue throughout the body. A few cats never produce sufficient antibody and so, chronically shed infective oocysts into their environment with their stool. These cysts remain in the host cat for the remainder of its life. In all other species the intestinal stage is lacking. These animals and man share only the extra-intestinal portion of the disease and, if they survive, harbor dormant cysts in their brain and muscle for the rest of their lives.
Most cats become infected through eating creatures that contain dormant toxoplasma cysts and most other animals become infected by ingesting prey animals or material contaminated with cat feces. In a few cases infection is passed from animal to animal and cat to cat through the womb. This is called congenital infection and it is quite uncommon.
Symptoms of Toxoplasmosis
Most cats that become infected with Toxoplasmosis show no clinical evidence of the disease. A few stressed cats, kittens and immature adults do develop transient disease related to their infection. Cats that are positive for feline leukemia or feline immunodeficiency virus are more at risk for developing the active diseases. Early in this disease, cats are listless and depressed. They may refuse to eat and they run a fever. As the disease progresses these cats often develop signs of lung congestion and pneumonia. The organism can invade the liver causing yellow mucus membranes (jaundice), vomiting and diarrhea. Inflammation of other body organs such as the pancreas and lymphatic tissue also occurs. In some cases the nervous system and eyes are attacked causing blindness, aimless meandering, walking in circles, personality changes, incoordination, seizures and loss of urine and bowel control. These central nervous system signs can be mistaken for rabies, lead or arsenic poisoning.
Diagnosis can not be based on clinical signs alone. The definitive diagnosis is based on clinical signs in the presence of high or rising immunity (antibody titer) to Toxoplasma. Titer is determined through laboratory analysis of blood serum. When an animal has died from the disease, the presence of tachyzoites in muscle and nerve confirms the diagnosis. Occasionally infected postmortem tissues are injected into white mice to reproduce the disease and confirm the diagnosis.
A high serum antibody titer in a healthy cat suggests that the cat had an infection at an earlier date and is not now shedding the organism and has fully recovered. Lack of antibody in a healthy cat suggests that the cat has not yet been exposed to the Toxoplasmosis.
A combination of two drugs, sulfadiazine and pyrimethamine, are often used to treat Toxoplasmosis. It has been recently found that the antibiotic, clindamycin, is also effective in treating cats with this disease. Clindamycin does not have the high incidence of side effects we see using pyrimethamine. Recent studies have shown that another antibiotic, Spiramycin, can reduce the likelihood of fetuses being infected with Toxoplasmosis while still in the womb. To the best of my knowledge, Spiramycin has not been approved yet for use within the United States. Physicians and veterinarians who have an interest in using Spiramycin should contact the Food and Drug Administration at (301) 827-2335.
Public Health Significance
There are estimates that about half the World’s human population has been exposed to Toxoplasma at some time in their lives. Many of these people received their infection from consuming undercooked meat or became contaminated with cat feces. In rare instances it is passed through infected dairy products. The encysted parasites in human tissue cause no problems unless the individual becomes immunocompromized. In most of these cases no signs of the disease occur.
A problem arises when woman becomes infected during pregnancy. In these cases one half to one third of the infants produced by these pregnancies become infected while still within the womb. Other sources give the figure at 40%. We call this congenital infection. It is particularly severe if it occurs during the first third of the pregnancy (the first trimester). Although transplacental infections occur in these fetuses, few of these women show signs of disease – they simply sero-convert and trap the organisms within their muscles. Children born in these circumstances may be sick at birth or disease symptoms may occur weeks to years later. Signs of the disease in these children include mental retardation, eye and nervous system disease, deafness, lung disease, fever, jaundice and rash. Toxoplasmosis during pregnancy can also result in miscarriage.
People undergoing cancer chemotherapy or immunosupression subsequent to organ transplants can also break with this disease long after the infection occurred. AIDS can also reactivate the infection. In these people signs include heart disease, lung disease, nervous system disturbances and eye disease. Mortality in this form of the disease is high.
There is no vaccine on the market to immunize cats or other animals against Toxoplasmosis.
People can guard against infection by cooking meat products well and wearing gloves when handling cat litter boxes. A temperature of 150F held for thirty minutes kills Toxoplasma oocysts in meat. If you keep cats do not let them eat wildlife or raw meat and do not feed them unpasteurized milk or milk products.
When a woman is pregnant take special added precautions to prevent contact with the oocysts by:
1) Keep cats out of garden soil, children’s sandboxes, flower beds and moist shady areas of the yard. .
2) Wear gloves when handling anything that may be contaminated with cat feces and wash your hands thoroughly with soap and water when there was any possibility contamination occurred.
3) Children’s sand boxes should be covered with a plastic or canvas tarpaulin when not being used.
4) Change your cat’s litter box frequently (daily) and let a non-pregnant member of the family do it.
5) Use boiling water and bleach to sanitize litter boxes. Remember, however, that bleach does not always kill the oocysts.
6) Do not serve rare or undercooked meats or unpasteurized dairy products. Poorly cooked pork, lamb and venison possess the most risk.
7) Have the woman and the cat’s Toxoplasma antibody titer checked. Positive titer in a healthy animal or human indicates they can not contract the disease now because they did in the past. Antibody negative cats and women can still become infected during pregnancy and so are at greater risk.
8) Do not acquire new cats during human pregnancies.
9) Minimize the contact of pregnant women with cats.
10) Wear gloves when working in the garden or with soils.
11) Wash your hands frequently with soap and water throughout the day.