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Chapter 45

Toxoplasmosis

Toxoplasmosis is a parasitic infection due to the protozoan Toxoplasma gondii, which occurs worldwide, most commonly in the tropics, less frequently in the desert and polar regions. In 1908, Nicolle and Manceaux were working at the Pasteur Institute in Tunis studying leishmaniasis and typhus and for the first time described the T. gondii organism. They took the generic name from the Greek word "toxon", meaning an arc or bow, which describes the characteristic shape of the parasite: the specific name came from the North African rodent, gondi, which they were using in their research. They were but a week ahead of Splendore, in Brazil, who published his description after recognizing Toxoplasma in a laboratory rabbit. The organism was first isolated from man by Janku, who in 1923 discovered it in the eye of a congenitally infected infant in Prague, Czechoslovakia. The congenital pattern of infection was fully described in both children and adults between 1937 and 1939, and in the same years acquired infection was recognized. It was not for another 10 years, in 1949, that serological tests were developed and showed that the infection could be asymptomatic and that it was present worldwide. It took another 20 years, until 1969-70, before the significance of cats was recognized and the full biological life cycle was unraveled.

When the first edition of this book was published (1981), the primary concern of radiologists was the congenital form of toxoplasmosis, seen as hydrocephalus and scattered intracranial calcification in the newborn. Acquired toxoplasmosis was occasionally recognized as a serious illness in patients who were immunosuppressed following transplant surgery or with a chronic debilitating illness such as lymphoma. Now AIDS has altered the clinical significance and natural history of this infection. Nevertheless, the majority of those infected will be asymptomatic and unaware that they harbor the parasite.

Synonyms

Fr: Toxoplasmose. Ger: Toxoplasmose. Sp: Toxoplasma.

Definition

Toxoplasmosis is infection with the coccidian protozoan, Toxoplasma gondii. It is generally classified as a two-host coccidian in the subfamily Toxoplasmatidae, in the family Sarcocystidae. Some authorities have described six species within the Toxoplasma genus, with only T. gondii infecting humans. Others describe T. gondii as the sole species, the others being in the Hammondia. There is, however, agreement that only T. gondii infects humans.

Geographic Distribution

Toxoplasmosis infects man and other warm-blooded animals on every continent, only Antarctica having escaped so far. Birds play an important role in the spread of infection from one geographic region to another, and perhaps because of the migratory pattern of birds, its incidence varies markedly from one region to another, and within individual countries. It is most prevalent in warm and humid climates and population studies have shown an incidence of nearly 100% in countries such as Tahiti and the lowlands of Guatemala. Focal regions of increased infection are found in flooded or irrigated land, but toxoplasmosis is seen most frequently in areas where cats are numerous and where there is sufficient moisture and shade to favor the survival of oocysts. Regional differences are shown by a 43% incidence in Costa Rica, 31% in southern Africa, and only 2% in India. Working in slaughterhouses carries an occupational hazard, and those who eat raw or poorly cooked pork, lamb, and beef are at particular risk. Fortunately these incidence figures are based on antibody studies and not on clinical ill health.

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Copyright: Palmer and Reeder