The Organism

 T. gondii belongs to the group of sporozoans and has a worldwide distribution, infecting a wide range of animals and birds. The normal final hosts are strictly cats and members of the family Felidae. These are the only hosts in which the oocyst-producing sexual stage of Toxoplasma can develop.

Organisms (either sporozoites from oocysts or brady zoites from tissue cysts) invade the mucosal cells of the cat’s small intestine, where they form schizonts or gametocytes. After sexual fusion of the gametes, oocysts develop, exit from the host cell into the gut lumen of the cat, and pass out via the feces. The environmentally resistant oocysts become infective after 1–5 days. When oocysts are ingested by the cat, the parasites repeat their asexual and sexual cycle. If oocysts are ingested by intermediate hosts such as certain birds, rodents, or mammals, including humans, the parasites can establish an infection but reproduce only asexually. In this latter case, the oocyst opens in the human’s or animal’s duodenum and releases the sporozoites, which pass through the gut wall, circulate in the body, and invade various cells, especially macrophages, where they form trophozoites, multiply, break out, and spread the infection to lymph nodes and other organs. These rapidly multiplying crescentic cells (tachyzoites) initiate the acute stage of disease. Subsequently, they penetrate nerve cells, especially of the brain and eye, where they multi ply slowly (as bradyzoites) to form quiescent tissue cysts, initiating the chronic stage of disease. The tissue cysts (zoitocysts or pseudocysts) are infective when ingested by cats (resulting in the intestinal sexual stage and oocyst production); when they are eaten by other animals, more tissue cysts are produced (asexually).

Pathology and Pathogenesis

The organism in humans produces either congenital or postnatal toxoplasmosis. Congenital infection, which develops only when nonimmune mothers are infected during pregnancy, is usually of great severity; postnatal toxoplasmosis is usually much less severe. Most human infections are asymptomatic. However, fulminating fatal infections may develop in patients with AIDS, presumably by alteration of a chronic infection to an acute one. Varying degrees of dis ease may occur in immunosuppressed individuals, resulting in retinitis or chorioretinitis, encephalitis, pneumonitis, or various other conditions.

The tachyzoite directly destroys cells and has a predilection for parenchymal cells and those of the reticuloendothelial system. Humans are relatively resistant, but a low-grade lymph node infection resembling infectious mononucleosis may occur. When a tissue cyst ruptures, releasing numerous bradyzoites, a local hypersensitivity reaction may cause inflammation, blockage of blood vessels, and cell death near the damaged cyst.

Congenital infection leads to stillbirths, chorioretinitis, intracerebral calcifications, psychomotor disturbances, and hydrocephaly or microcephaly. In these cases, the mother was infected for the first time during pregnancy. Prenatal toxoplasmosis is a major cause of blindness and other con genital defects. Infection during the first trimester generally results in stillbirth or major CNS anomalies. Second- and third-trimester infections induce less severe neurologic dam age, though they are far more common. Clinical manifestations of these infections may be delayed until long after birth, even beyond childhood. Neurologic problems or learning difficulties may be caused by the long-delayed effects of late prenatal toxoplasmosis.

Epidemiology

 Avoidance of human contact with cat feces is clearly important in control, particularly for pregnant women with negative serologic tests. Since oocysts usually take from 1 to 5 days to become infective, changing cat litter daily (and its safe disposal) can prevent transmission. An equally important source of human exposure is raw or undercooked meat, in which infective tissue cysts are frequently found. Humans (and other mammals) can become infected either from oocysts in cat feces or from tissue cysts in raw or undercooked meat. USDA recommends freezing meat to 0°F for several days or cooking meat to 145°F (poultry to 165°F) and allowing the meat to rest for 3 minutes to prevent the risk of toxoplasmosis. Kitchen cleanliness, handwashing after touching raw meat, thoroughly washing fruits and vegetables, and avoidance of cats and cat litter are essential during pregnancy. Serologic screening for immunoglobulin IgG and IgM antibodies to Toxoplasma is available (Guerrant et al, 2006; Montoya and Remmington, 2008).

اخر الاخبار

اخبار العتبة العباسية المقدسة

المجمع العلمي يقيم برنامجًا قرآنيًّا رمضانيًّا في جامعة الكوفة

الحلقة 21 من جائزة العميد القرآنية تشهد تأهل القارئ محمد حسن زاده من أفغانستان إلى المرحلة النهائية

موكب أهالي كربلاء يحيي ذكرى شهادة الإمام علي (عليه السلام) عند مرقده الطاهر

لليوم الثالث.. شعبة الخطابة تنظم مجالس العزاء لإحياء ذكرى شهادة الإمام علي (عليه السلام)

المزيد

الآخبار الصحية

خمسة مؤشرات صحية مهملة قد تنقذ حياتك

متى تكون الحمى طبيعية؟

التدخين الإلكتروني يهدد قلبك.. دراسة جديدة تكشف مخاطر ارتفاع ضغط الدم

دراسة تكشف فائدة بعض أنواع الشاي في محاربة الاكتئاب

المزيد

الآخبار التكنلوجية

الجهاز المناعي يظهر دورا غير متوقع في صحة العظام

ابتكار من جامعة الشارقة منخفض التكلفة للحماية من الزلازل

اكتشاف تأثير غير متوقع للكاكاو على وظائف الدماغ

باحثون يرصدون أثر تغيّر المناخ في دم البشر

المزيد

مواضيع ذات صلة

Trichuris trichiura (Whipworm— Intestinal Nematode)

Enterobius vermicularis (Pinworm—Intestinal Nematode)

Plasmodium Species (Blood Sporozoa)

Leishmania Species (Blood Flagellates)

Trypanosoma cruzi (Blood Flagellate)

Trypanosoma brucei Rhodesiense and Trypanosoma brucei gambiense (Blood Flagellates)

Cryptosporidium (Intestinal Sporozoa)

Entamoeba histolytica (Intestinal and Tissue Ameba)

Giardia lamblia (Intestinal Flagellate)

Classification of Parasites

Toxoplasmosis

The Lung Flukes