141 Toxoplasmosis
The agent of this disease, the Sporozoan protozoan Toxoplasma gondii, is a common parasite of many species of birds and mammals. The organism was first seen in 1908 in the tissues of a Tunisian rodent, the gundi, and fully described in 1909.
Human disease was first described in 1923 and congenital neonatal disease was reported in 1939, but the complex life cycle of the parasite was not elucidated until 1970. Serologic tests show that humans around the world harbor T. gondii, but because almost all infections are asymptomatic, very few have the disease. The protozoan is an intracellular parasite of a variety of tissues in warm-blooded vertebrates. It multiplies by binary fission in a host cell, eventually rupturing the cell and releasing parasites to attack other cells. Sexual reproduction can take place only in cats and other felines. These definitive hosts release oocysts, the stage infective for herbivores, in their feces. Asexual intracellular replication takes place in the herbivore, and, if the tissues containing T. gondii are eaten by a carnivore, asexual reproduction may also occur in their tissues. Humans can become infected by eating poorly cooked or raw meat or poultry, by ingesting oocytes from the feces of cats, or congenitally.Clinical Manifestations
Human infections are usually inapparent, although they sometimes can lie dormant for years and flare up in weakened or immunodeficient hosts. Most cases in otherwise healthy people are mild and cause vague symptoms like fever and weakness. The disease often mimics infectious mononucleosis. Chronic cases can cause diarrhea, headache, and eye damage. Rare fulminating infections cause severe symptoms and may affect the brain. Uterine transmission of toxoplasmosis often has grave consequences. Five to 15 percent of cases result in fetal death, and 18 to 23 percent result in moderate to severe brain and eye abnormalities.
Some apparently normal infants later develop severe retinal disease or mental retardation. In persons whose immune systems have been suppressed by therapy or disease, as, for example, organ transplant recipients or acquired immune deficiency syndrome victims, old or latent infections can become activated. Cerebral toxoplasmosis is a fairly common and very serious complication of AIDS.Distribution and Incidence
The highest known prevalence rate, 93 percent, was found in Parisian women who enjoy eating raw beef. By age 25, roughly 70 percent of El Salvadorians and 30 percent of white residents of New York City harbor antibodies against Toxoplasma. It is estimated that about 3,000 babies are born each year in the United States with congenitally acquired toxoplasmosis.
Diagnosis, Treatment, and Prevention
The diagnosis of toxoplasmosis is complicated by its wide range of symptoms and depends on serologic tests. Therapy is often difficult, especially in weak patients, and is ineffective for fetuses. Prevention is by thorough cooking of meat and poultry and by careful handling of cat feces. Pregnant women should not empty cat litter boxes, and children’s sandboxes should be covered when not in use.
K. David Patterson
Bibliography
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Kean, B. H. 1972. Clinical toxoplasmosis - 50 years. Transactions of the Royal Society of Tropical Medicine and Hygiene 66: 549—67.
Kean, B. H., Kenneth E. Mott, and Adair J. Russell, eds. 1978. Tropical medicine and parasitology: Classic investigations, Vol. I, 271—84. Ithaca and London.
Kennou, M. F. 1986. Propos sur Toxoplasma gondii (Nicolle et Manceaux, 1909). Archives de ITnstitutPasteur de Tunis 63: 123—31.
Wong, B., et al. 1984. Central-nervous-system toxoplasmosis in homosexual men and parental drug abusers. Annals of Internal Medicine 100: 36-42.