Toxoplasma gondii, the etiologic agent of toxoplasmosis, is one of the most common protozoan parasites of humans. T. gondii was described in 1908, both by Nicolle and Manceaux at the Pasteur Institute in Tunis from gondis (a type of small rodent) used as laboratory animals in typhus research, and by Splendore at the Hygiene Institute in Sao Paulo, Brazil, from a laboratory rabbit. Human infection was first discovered in 1924 in the eye of an infant by Janku in Czechoslovakia. This was followed, starting in 1937, by several diagnoses in infants by Wolf, Cowan, and Paige at Colombia University in New York City. After the development of a serologic test, the dye test by Sabin and Feldman in 1948, it became clear that infections in humans and animals were found worldwide and were highly prevalent in many areas. The identification of the sexual cycle of Toxoplasma in cats led to its classification as a coccidian.32 It became clear then that the infections of gondis and rabbits were linked to cats, which at the time were kept in laboratories to catch wild rodents and laboratory animals that had escaped.
LIFE CYCLE AND MODES OF TRANSMISSION
Felines, including domestic cats and wild felids, are the definitive host for T. gondii; sexual reproduction of the parasite occurs only in the feline small intestine, producing oocysts that are shed in the feces. These oocysts are unsporulated when passed and are not immediately infective, requiring between 1 and 5 days in the environment to sporulate and become infective. An intermediate host, which can be any warm-blooded animal, can ingest soil, water, or plant material contaminated with these sporulated oocysts. Shortly after ingestion, the sporozoites in the oocyst are released and transform into tachyzoites (Fig. 151-1), the rapidly proliferating intracellular form. Tachyzoites localize primarily in brain, eye, heart muscle, and skeletal muscle or other low mitotic tissues where they transform into bradyzoites. Bradyzoites multiply slowly and form cysts (Fig. 151-2) in the tissues where they may persist for years, possibly for the life of the host.32
Tachyzoites of Toxoplasma gondii in smear of mouse peritoneal fluid. (Giemsa stain, × 1200.)
Section of mouse brain showing a cyst of Toxoplasma gondii containing hundreds of bradyzoites. (Hematoxylin and Eeosin stain, × 480.)
T. gondii is adapted to spread naturally among felids by predation. Cats acquire infection by ingesting bradyzoites in fresh tissues from infected prey animals and rarely from tachyzoites or sporulated oocysts. Toxoplasma damages the brain of infected rodents, making them less neophobic and less fearful of cat odors, enhancing their chances of becoming prey to a cat and ensuring that the parasite completes its life cycle.1 Cats ...