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Toxoplasma gondii as a Parasite in Food: Analysis and Control, Page 1 of 2
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Common parasitic protozoal infections are frequently transmitted by food containing fecally contaminated soil or water, which may carry the environmentally resistant oocyst stage of the parasites Cyclospora cayetanensis, Cryptosporidium spp., Giardia spp., Toxoplasma gondii, or Sarcocystis spp. However, both T. gondii and Sarcocystis can also be transmitted by consumption of a cyst stage of the parasite which is present in the meat of infected animals; currently, the extent of Toxoplasma and Sarcocystis infection due to foodborne transmission is unknown ( 1 – 5 ). Differences in the definitive and intermediate hosts exist between these pathogens which impact their abundance and geographical distribution in the environment ( Table 1 ). As an example, because Giardia and Cryptosporidium spp. oocysts are excreted in large numbers by cattle, other ruminants, and a wide variety of other species (more than 109 per day), oocysts are very commonly found in the environment, while oocysts of Toxoplasma, which are exclusively excreted by felids, are restricted to areas inhabited by wild and domestic cats ( 6 , 7 ).
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Life cycle of Toxoplasma gondii.
Tachyzoites of Toxoplasma gondii. Bars = 10 µm. (A) Individual (small arrows), binucleate (large arrow), and divided (arrowhead) tachyzoites. Impression smear of lung. Compare size with red blood cells and leukocytes. Giemsa stain. (B) Tachyzoites in a group (large arrow) and in pairs (small arrows) in section of a mesenteric lymph node. Note that organisms are located in parasitophorous vacuoles and some are dividing (arrowhead). Hematoxylin and eosin stain.
Stages of Toxoplasma gondii. Bars in A–D = 20 µm, in E–G = 10 µm. (A) Tachyzoites in impression smear of lung. Note crescent-shaped individual tachyzoites (arrows) and dividing tachyzoites (arrowheads) compared with size of host red blood cells and leukocytes. Giemsa stain. (B) Tissue cysts in section of muscle. The tissue cyst wall is very thin (arrow) and encloses many tiny bradyzoites (arrowheads). Hematoxylin and eosin stain. (C) Tissue cyst separated from host tissue by homogenization of infected brain. Note tissue cyst wall (arrow) and hundreds of bradyzoites (arrowheads). Unstained. (D) Schizont (arrow) with several merozoites (arrowheads) separating from the main mass. Impression smear of infected cat intestine. Giemsa stain. (E) A male gamete with two flagella (arrows). Impression smear of infected cat intestine. Giemsa stain. (F) Unsporulated oocyst in fecal float of cat feces. Unstained. Note double-layered oocyst wall (arrow) enclosing a central undivided mass. (G) Sporulated oocyst with a thin oocyst wall (large arrow) and two sporocysts (arrowheads). Each sporocyst has four sporozoites (small arrow), which are not in complete focus. Unstained.
Transmission electron micrograph of a tachyzoite of Toxoplasma gondii in a mouse peritoneal exudate cell. Am, amylopectin granule; Co, conoid; Dg, electron-dense granule; Fp, finger-like projection of tachyzoite plasmalemma; Go, Golgi complex; Hc, host cell cytoplasm; Im, inner membrane complex; Mi, mitochondrion; Mn, microneme; Nu, nucleus; Pl, plasmalemma; Pv, parasitophorous vacuole; Rh, rhoptry; Sm, subpellicular microtubule; Tv, tubulovesicular membranes. Bar = 1 µm.
Toxoplasma gondii stages in in vitro and in vivo preparations. (A) Tachyzoites in culture of human foreskin fibroblast cells. Giemsa stain. Bar = 25 µm. (B) Rosettes of tachyzoites in human foreskin fibroblasts. Immunohistochemical stain with antitachyzoite-specific antibody. Smear. Bar = 10 µm. (C) Tachyzoites in a cytospin smear of pleural fluid from a cat with pneumonia. Giemsa stain. Compare the size of tachyzoites (arrow) with host cells. Giemsa stain. Bar = 10 µm. (D) Tachyzoites (arrows) and tissue cysts (large arrow) in section of mouse brain. Immunohistochemical stain with T. gondii–specific antibody. Bar = 10 µm.
Tissue cysts of Toxoplasma gondii. Bar = 10 µm. (A) Two tissue cysts (arrows). Note the thin cyst wall enclosing bradyzoites. Impression smear of mouse brain. Silver impregnation and Giemsa stain. (B) A tissue cyst freed from mouse brain by homogenization in saline. Note the thin cyst wall (arrow) enclosing many bradyzoites. Unstained. (C) A large tissue cyst in a section of rat brain 14 months postinfection. Note the thin cyst wall (arrow). Hematoxylin and eosin stain. (D) A small tissue cyst with intact cyst wall (arrow) and four bradyzoites (arrowheads) with terminal nuclei adjacent to it. Section of mouse brain 8 months postinfection. Hematoxylin and eosin stain. (E) A tissue cyst in a section of mouse brain. Note Periodic acid Schiff (PAS)–negative cyst wall (arrow) enclosing many PAS-positive bradyzoites (arrowheads). The bradyzoites stain bright red with PAS but they appear black in this photograph. PAS hematoxylin stain. (F) An elongated tissue cyst (arrow) in a section of skeletal muscle of a mouse. PASH stain.
Transmission electron micrograph of a tissue cyst in the brain of a mouse 6 months postinfection. Note the thin cyst wall (opposing arrows), numerous bradyzoites each with a conoid (C), and electron-dense rhoptries (R). Bar = 3.0 µm.
Protozoans of food safety importance and their hosts
Host tissues invaded by protozoan parasites
Oocyst characteristics of protozoans of food safety importance
Clinical signs in humans a
Diagnosis and treatment a