Introduction
Epidemiological studies have shown that toxoplasmosis is much more common than it was previously thought of.
Antibody titers are positive in about 30% to 40% of all Americans meaning that about 1/3 of the population of the US has had an encounter with this pathogen. It usually is transmitted through the cat who is a healthy carrier of this disease. Fortunately most people with a healthy immune system defend themselves unknowingly against the intruder and these people may not even know that they were exposed.
However, a fetus of a pregnant woman or a person with an immune deficiency (AIDS or a patient treated for cancer with chemotherapy) would come down with a serious toxoplasmosis infection that often will be life threatening.
Toxoplasma gondii is a protozoan parasite that can only live inside a living cell. Here is a fluorescent image and a schematic electron microscopic image of this parasite. It can multiply in two ways: sexually in the gut wall of cats, and asexually in cells of mammals or birds. Infection of man occurs through contact with cat litter, which remains infectious for 1 year in moist soil. The oocysts from the cat feces are either swallowed unknowingly or people are eating undercooked meat of pork, beef or lamb with tissue cysts. As the body produces antibodies against toxoplasmosis after exposure any tissue cysts that have been formed get walled off, but they stay potentially infectious all through the remaining life of the host.
Signs and symptoms
Acute toxoplasmosis can present similar to infectious mononucleosis with a fever, lymph gland swelling in the neck region, sore throat, muscle pains and an enlarged liver, as well as an enlarged spleen. More often though toxoplasmosis is without symptoms or only non specific lymph gland swelling. At the other end of the spectrum there is occasionally a severe disseminated form, which brings the patient to the hospital and where severe lung symptoms (a pneumonia like condition), a myocarditis (heart muscle infection), brain infection, muscle infection (polymyositis), skin rash and fever and chills are present.
In AIDS patients this more serious form is a lot more common and it likely arises out of a reactivation of dormant tissue cysts containing Toxoplasma gondii. In AIDS patients there is a preference for pneumonitis (lung tissue infection), for orchitis (testicular tissue infection in males) and CNS toxoplasmosis(brain infection). Seizures and coma are common.
Toxoplasmosis can penetrate the placenta and infect the fetus, which often will lead to stillbirths or spontaneous abortions. Late in pregnancy the baby will survive, but will have serious disease with chorioretinitis (leading to blindness), calcium deposits on X-rays of the skull , hydrocephalus and/or microcephalus as well as retardation. Survival among these children is poor.
Toxoplasmosis (Cat Litter)
Diagnostic test
As titers against T. gondii are common (see above), the physician must rely on a fourfold increase of antibodies against toxoplasmosis to be certain that the disease is active and not dormant. In AIDS patients this test is unreliable and body fluids may have to be injected into experimental mice where T.gondii can then be found 6 weeks later when they are sacrificed. This is a very specific tests, but is time consuming. In patients with CNS toxoplasmosis MRI scans will lead to the suspicion of toxoplasmosis. If further diagnosis is needed, brain biopsy can be done, but this has potential side-effects.
Treatment
Treatment for toxoplasmosis is complex and should be in the hands of an infection specialist.
Briefly, only susceptible patients such as pregnant women, immunocompromised persons and newborns need to be treated. Treatment consists of pyrimethamine in combination with sulfadiazine. The specialist may also give intramuscular leucovorin to counteract bone marrow suppression from pyrimethamine. In AIDS patients treatment has to be continued indefinitely as the recurrence rate is otherwise so high. In pregnancy another medication, spiramycin, is used instead of pyrimethamine.
Prevention
Never eat undercooked or raw meat! Washing hands after raw meat is handled, is essential to prevent infection. Do not allow pregnant women to empty the cat litter box. We don’t know whether the cat is healthy looking, but infected. For AIDS patients with CD4+ cell counts of less than 100 per microliter and positive toxoplasmosis titers, chemoprophylaxis with trimethoprim-sulfamethoxazole will not only prevent toxoplasmosis, but also Pneumocystis carinii (Ref. 1, p. 1254).
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