Toxoplasma gondii

Toxoplasma gondii is one of the most common protozoan parasites in the world. Its genus name, toxoplasma, is derived form the Greek "toxon," meaning arc or crescent shape. This is the form this parasite takes when it is living outside of cells.


The species name is derived from the rodent, Ctenodactylus gondii, from which the T. gondii was first isolated. T. gondii is classified into the phylum Apicomplexa, class Sporozoa and subclass Coccidia. It is related to the parasites Plasmodium (the agents of malaria), Cryptosporidium, Eimeria, Isospora, Neospora and Sarcocystis.

T. gondii has three forms:

1. The cysts in tissues of infected individuals;
   
2. The oocysts in the intestine of members of the cat family;
   
3. The invasive form, the tachyzoite, which is responsible for acute disease - toxoplasmosis.

The figure below is a schematic drawing of a tachyzoite showing some of its internal structures.



T. gondii is an obligate intracellular parasite -- which means that it is only able to reproduce inside cells. Humans are primarily infected with T. gondii either by the ingestion of undercooked or raw meat containing tissue cysts or by contaminated water and food from oocysts shed by infected cats. Another mode of human infection is congenital transmission, which may occur when a pregnant woman becomes infected and transmits the parasite to her fetus.

In fact, congenital transmission was the first form of transmission recognized by researchers. It was first suggested that mother to fetus transmission occurred as a consequence of an acute maternal infection or reactivation of a chronic infection in the pregnant woman. It is now known that the majority of congenital infections are due to an acute maternal infection. However, reactivation of a chronic infection is the major cause of acute toxoplasmosis and toxoplasmic encephalitis in severely immunocompromised individuals, such as patients with AIDS.

A number of precautions have been proposed to prevent infection, particularly in pregnant women. These include avoiding:

1. Ingestion of raw or undercooked meat products;
   
2. Handling raw meat;
   
3. Materials that may be contaminated with cat feces.

For more information about toxoplaxmosis and its prevention and treatment contact the Department of Immunology and Infectious Diseases at the Toxoplasma Serology Laboratory - Customer Service at 650-853-4828.

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Scientist & Staff Profiles (Articles are from previous Annual Reports; not all scientists and staff are represented here.)

• Jack Remington, M.D
  
• Fausto Araujo, DVM, Ph.D.
  

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