Toxoplasma gondii: Mode of Infection and Effect on Neurological Cells

By [Alexander S. McQuiston]

Introduction

The left shows a bradyzoite cyst with the dense cyst wall surrounding the bradyzoites. The right shows tachyzoites inside the parasitophorous vacuole. Bradyzoites will slowly reproduce asexually in the cyst and tachyzoites will rapidly reproduce asexually CDC.

Toxoplasma gondii is one of the most common cyst parasites around the world due to its ability to infect almost all warm blooded organisms, including humans (Gaskell et al. 2009). Some estimates of T. gondii infections are reported to be as high as a third of the world’s population (Tenter et al. 2000; Weiss et al. 2009). More specifically, T. gondii is an intracellular heteroxenous parasite belonging to the apicomplexan phylum. The apicomplexan phylum is a large group of parasitic protists that contain specialized organelles required for target host invasion starting at the apical pole (Dubey et al. 1998; Weiss 2011)). Infection of T. gondii causes toxoplasmosis in both healthy and immunocompromised organisms. When the parasite infects healthy organisms the disease is normally asymptomatic, but if the parasite infects an immunocompromised organism, such as an AIDS patient, diseases like encephalitis can occur (Carruthers et al. 2007). Along with medical diseases, T. gondii has also been hypothesized to cause a variety of neurological disorders such as schizophrenia (Weiss et al. 2009).

T. gondii has both intermediate and definitive hosts. The intermediate hosts include a wide variety of warm blooded animals from rodents to humans, but only has one definitive host, felines (cats) (Tenter et al. 2010). T. gondii is also capable of both asexual and sexual reproduction. When the parasite is in an intermediate host it reproduces asexually, and when the parasite is in its definitive host it is able to reproduce sexually. The ultimate goal of T. gondii is to infect its definitive host so it can sexually reproduce and then go on to infect other organisms (Webster et al. 2007). An interesting way T. gondii has been able to facilitate its transmission from an intermediate host to its definitive host is by invading target host cells and then manipulating host cellular behavior and physiological processes (Prandovsky et al. 2011; Webster et al. 2007).

How T. gondii Infects Host Cells

This displays a tachyzoite invading a target cell. The apical pole is already inside the host, but the rest of the cell has not been encapsulated. Constriction can be seen in the middle of the tachyzoite which makes invasion slightly more difficult NATURE.

T. gondii's Effect on the Brain

Tyrosine and phenylalanin hydroxylase catalytic reactions. Phenylalanine hydroxylase converts phenylalanine in to tyrosine, and tyrosine hydroxylase converts tyrosine to L-DOPA. Tyrosine hydroxylase controls the rate limiting step in this catalysis. PLoSONE.