**2. Etiologic organism:** *Toxoplasma gondii*

*T. gondii* was discovered since 1908 simultaneously by 2 groups of researchers. Firstly, Charles J. H. Nicolle (1866-1936) and Louis H. Manceaux (1865-1943) from the Pasteur Institute in Tunisia isolated a new parasite from the African rodent**,** *Ctenodactylus gundi*, and differentiated it from *Leishmania*. Secondly, the Italian researcher namely Alfonso Splendore (1871-1953) who worked at Sao Paulo, Brazil identified this protozoan from the liver of rabbit (Dubey et al., 1970; Sukthana, 2006). *T. gondii* was named a year later by Nicolle and Manceaux according to its bow-like shape (Toxoplasma is from a Greek word: toxos means bow or arc; plasma means life) and gondii may result from a misspelling of the scientific

Toxoplasmic Encephalitis 283

fissions, occurs resulting in many tachyzoites causing cell rupture and subsequently they invade other enterocytes producing the active stage of infection (Figure 1). In intermediate host tachyzoites will shortly transform to be inactive bradyzoites and reside silently in tissue cyst for the whole life of the infected host. Thus the chronic stage of infection occurs. On the contrary, tachyzoites in definitive host modify to be macrogametocyte and microgametocyte and sexual reproduction occurs producing immature oocysts which when shed with cat's fecaes, need about 2-5 days to develop in the environment until mature as infective stage. *Toxoplasma* is transmitted to human by 3 routes. The most common two are ingesting contaminated food or water with mature oocysts and consuming undercooked, infected meat where bradyzoites harbor in tissue cysts. The least frequent route is transplacental transmission which occurs only when the mother acquires primary infection during pregnancy. A European multicenter study including cities in Western Europe identified the consumption of undercooked meat as the strong risk factor for acquiring a *T. gondii* infection, whereas in Central and South America it is related with large numbers of stray cats that have access to the outdoor environment of which the climate favours and prolongs the survival of oocysts (Sukthana, 2006). Toxoplasmosis transmitted by cat excreta is not straight forward in Southeast Asia. Due to the religious belief, Malaysian and Indonesian Muslims prefer cats to dog as pet, while lots of stray cats are left in Buddhist temples in Thailand. Those settings should promote cat's excreta as a strong risk factor in that region. Noteworthy, human *Toxoplasma* seroprevalence in Thailand is much lower than in those two countries (21.9% vs. 44.8% and 58%) and correlated with cat seroprevalence (Konishi et al., 2000; Nissapatorn et al, 2004a; Sukthana, 2006). This might be due to high humidity and more rainfall in the latter countries suggesting that ground temperature is an important

About 20% to 40% of individuals with AIDS develop TE from the reactivation of a latent *T. gondii* infection when the CD4 cell count falls below 100/mm3 (Luft & Remington, 1992; Sukthana, 2000; Ajzenberg, 2009). The incidence of TE is thus directly proportional to the prevalence of antibodies to *Toxoplasma* in any given population. Before the advent of HIV/AIDS epidemic in 1981, toxoplasmosis was occasionally reported in immunocompromised patients, mostly in those with malignancies of the reticuloendothelial system and cardiac transplant recipients with lesions mostly outside the CNS. But, TE has become one of the commonest causes of focal brain lesions in Western Europe and North America due to AIDS pandemic (Luft & Remington, 1988). Since then, more and more TE cases in HIV/AIDS individuals were diagnosed worldwide. Nearly three decades from that point, nowadays to get a clearer picture of its epidemiology and clinical course, herein, three periods could be divided as: 1) TE during the beginning of AIDS pandemic period (1980s), 2) TE during prophylaxis period (1990s) and 3) TE during HAART period (1997-present).

At the beginning of the 1980s, more and more of the mysterious cases with severe manifestations and fatal outcomes presented in homosexual men, hemophiliacs and Haitian. HIV was subsequently identified as the cause that impaired host immunity causing acquired immunodeficiency syndrome (AIDS) and opportunistic infections. In November 1982, the Centers for Disease Control (CDC) reported 19,744 AIDS cases from the United States, with

**3.1 TE during the beginning of AIDS pandemic period (1980s)** 

determinant of oocyst survival.

**3. Epidemiology and risk factor** 

name of its original host, the gundii (Ferguson, 2009a). The first congenital case of toxoplasmosis was described in 1923 and the first adult case was diagnosed in 1940 (Frenkel & Fishback, 2000). It was not until 1969 when its life cycle was completely known with cats and other felids as the only definitive host in which sexual reproduction takes place to produce infective oocysts. Human, warm-blooded domestic animals, birds, and rodents including wild and marine mammals are intermediate hosts that harbor tissue cysts in their bodies (Hutchison et al., 1969; Dubey et al., 1970; Dubey, 2007; Ferguson, 2009b). *T. gondii*  was classified as coccidian belonging to the phylum Apicomplexa which is an intracellular organism (Dubey et al., 1970).
