Toxoplasma gondii and ocular toxoplasmosis: pathogenesis.

The protozoan Toxoplasma gondii is an obligate intracellular parasite that can infect any warm blooded animal,' as well as reptiles and fish,2' and shows a high incidence in humans, with approximately 500 million people throughout the world having antibodies to Tgondii.5 The organism is ubiquitous in nature and produces one of the most common infections ofhumans.' It is the most common cause of retinochoroiditis worldwide in humans,6 with figures ranging from 28% to 55% of all cases of posterior uveitis.7"9 Even though the causative organism was described at the beginning of the century,'01 the life cycle was only understood in 1970,12 and most of the aspects related to the interactions between the parasite and the host are still unknown. The formation of tissue cysts after an acute infection, which shows a predilection for some organs, is still poorly understood. The mechanism for the recurrences of the infection, with its most devastating consequences in the eye and the brain, has not been elucidated. Loss of visual acuity occurs as a direct consequence of lesions affecting the macula or optic nerve, or secondary to massive vitreous involvement, and also as the result of vascular changes in the retina or subretinal space. This paper tries to summarise the knowledge related to the pathogenesis of ocular toxoplasmosis, including aspects related to its transmission, clinical manifestations, and some studies on animal models that may allow a better understanding of this disease.