**2.1 Life cycle and mode of transmission of** *T. gondii*

There are 3 infective stages of *T. gondii* i.e. 1) oocysts produced and shed by cat and felid animals, 2) an active, rapidly dividing tachyzoite form and 3) an inactive dormant bradyzoite harbored in tissue cysts. Tachyzoite and bradyzoite, sized 2 x 9 µm, cannot be differentiated by light microscope, while the mature oocyst is an oval shape, sized 9 x 13 µm containing 2 sporocysts with 4 sporozoites in each.

Fig. 1. Life cycle of *Toxoplasma gondii*: Cat and felids are only definitive host where *T. gondii* completes its life cycle. Intermediate hosts are human, warm-blooded domestic animals, bird and rodent including wild and marine mammals. Immature oocysts develop in the environment before becoming infected mature oocysts. Acute stage of infection occurs in both definitive and intermediate hosts, but turns to be chronic and develops as a tissue cyst in the intermediate host. Sexual reproduction occurs in cat producing immature oocysts.

When mature oocysts are ingested by hosts, sporozoites are released after exposure to gastric enzymes and invade enterocytes, whereas in asexual multiplication, multiple

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

There are 3 infective stages of *T. gondii* i.e. 1) oocysts produced and shed by cat and felid animals, 2) an active, rapidly dividing tachyzoite form and 3) an inactive dormant bradyzoite harbored in tissue cysts. Tachyzoite and bradyzoite, sized 2 x 9 µm, cannot be differentiated by light microscope, while the mature oocyst is an oval shape, sized 9 x 13 µm

Fig. 1. Life cycle of *Toxoplasma gondii*: Cat and felids are only definitive host where *T. gondii* completes its life cycle. Intermediate hosts are human, warm-blooded domestic animals, bird and rodent including wild and marine mammals. Immature oocysts develop in the environment before becoming infected mature oocysts. Acute stage of infection occurs in both definitive and intermediate hosts, but turns to be chronic and develops as a tissue cyst in the intermediate host. Sexual reproduction occurs in cat producing immature oocysts.

When mature oocysts are ingested by hosts, sporozoites are released after exposure to gastric enzymes and invade enterocytes, whereas in asexual multiplication, multiple

organism (Dubey et al., 1970).

**2.1 Life cycle and mode of transmission of** *T. gondii* 

containing 2 sporocysts with 4 sporozoites in each.

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 determinant of oocyst survival.
