Toxoplasma gondii migration within and infection of human retina

A New Ex Vivo Model Based on Mouse Retinal Explants for the Study of Ocular Toxoplasmosis

Ocular toxoplasmosis is the most prevalent clinical manifestation of T. gondii infection, which causes irreversible retinal damage. Different experimental models have been developed to study this pathology. In the present study, a new, ex vivo model is proposed to contribute to the elucidation of disease mechanisms and to possible therapeutic solutions. Ex-vivo retinal explants, prepared from mouse retinas following established protocols, were incubated with T. gondii tachyzoites maintained in Vero cells. At different times, starting at 12 h up to 10 days of incubation, the explants were analyzed with immunofluorescence and Western blot to investigate their responses to parasite infection. T. gondii invasion of the retinal thickness was evident after 3 days in culture, where parasites could be detected around retinal cell nuclei. This was paralleled by putative cyst formation and microglial activation. At the same time, an evident increase in inflammatory and oxidative stress markers was detected in infected explants compared to controls. Cell death also appeared to occur in retinal explants after 3 days of T. gondii infection, and it was characterized by increased necroptotic but not apoptotic markers. The proposed model recapitulates the main characteristics of T. gondii retinal infection within 3 days of incubation and, therefore, allows for studying the very early events of the process. In addition, it requires only a limited number of animals and offers easy manipulation and accessibility for setting up different experimental conditions and assessing the effects of putative drugs for therapy.

New advances in immune mechanism and treatment during ocular toxoplasmosis

Ocular toxoplasmosis (OT) is an intraocular infection caused by the parasite Toxoplasma gondii. OT is manifested as retinal choroiditis and is the most common infectious cause of posterior uveitis. Invasion of the retina by T. gondii leads to disruption of the blood-ocular barrier and promotes the migration of immune cells to the ocular tissues. Cytokines such as IFN-γ and IL-1β are effective for controlling parasite growth, but excessive inflammatory responses can cause damage to the host. In this review, we will discuss in detail the latest advances in the immunopathology and treatment of OT.

Ocular Toxoplasmosis

INTRODUCTION

Ocular toxoplasmosis is the leading cause of posterior uveitis worldwide, affecting individuals acrossdifferent age groups. The key to reducing vision loss includes prompt diagnosis and treatment. However, despite the prevalence of ocular toxoplasmosis, there has been little consensus regarding its pathophysiology,clinical features, diagnosis, and especially management.

METHODS

The data sources were literature reviews, including Pub Med and Medline databases. Search terms included toxoplasmosis, retinitis, vasculitis, vitritis, uveitis alone or in combination with, serum, aqueous, vitreous eye, ocular and review.

RESULTS

In this review paper, we have sought to provide an overview of the pathophysiology, epidemiology, and clinical features of the disease, both based on current literature and our own clinical experience. We have also discussed the use of serology, ocular fluid, and ophthalmic investigations that could further facilitate the diagnosis of ocular toxoplasmosis.Different management strategies have been reported worldwide, including newer approaches such as local therapy.

CONCLUSION

A better understanding of critical aspects of ocular toxoplasmosis will hopefully lead to reduced morbidity, including blindness associated with this condition.

Optical Coherence Tomography Angiography Findings in Ocular Toxoplasmosis with Multiple Recurrences

Ocular toxoplasmosis is the most common cause of posterior uveitis that is caused by Toxoplasma gondii infection. Humans can be infected congenitally or postnatally. The typical lesion of ocular toxoplasmosis is focal necrotizing retinitis with overlying vitritis, which lead to hyperpigmented retinochoroidal scar at resolution of lesion. Macula involvement can cause substantial visual impairment. The high incidence of disease reactivation may lead to greater risk of vision loss. Optical coherence tomography angiography (OCTA) is a non-invasive imaging method to visualize the vascular and density perfusion of the retina and choroid, which cannot be obtained by conventional Optical Coherence Tomography (OCT). In this case report, we present two cases of active ocular toxoplasmosis with multiple recurrences to study pathological changes in retinal and choroidal microvasculature. The findings reveal the involvement of all of the retinal layers in the choroid, with distinct changes in the deep retinal layer.

Punctate Inner Retinal Toxoplasmosis: Case Series and Review of Literature

PURPOSE

To describe clinical and multimodal imaging characteristics of punctate inner retinal toxoplasmosis (PIRT) as an atypical presentation of ocular toxoplasmosis (OT).

METHODS

Retrospective review of OT cases with PIRT lesions and review of the literature. We describe five cases (6 eyes).

RESULTS

PIRT lesions were seen adjacent to active/healed toxoplasma retinochoroiditis. The appearance of PIRT was creamy yellowish-white, inner retinal, punctate, and sub-centimetric lesions. The depth of these lesions on optical coherence tomography was till the outer plexiform layer. Co-existing punctate outer retinal toxoplasmosis (PORT) was found in three eyes and recurrent retinochoroiditis in three. The fate of PIRT was resolution with minimal retinal thinning or progression to a full-thickness retinochoroiditis.

CONCLUSION

PIRT was noted in association with typical toxoplasma retinochoroiditis and PORT lesions, and had equal chances of resolution or progression to full-thickness lesions.

Pathophysiology of ocular toxoplasmosis: Facts and open questions

Infections with the protozoan parasite Toxoplasma gondii are frequent, but one of its main consequences, ocular toxoplasmosis (OT), remains poorly understood. While its clinical description has recently attracted more attention and publications, the underlying pathophysiological mechanisms are only sparsely elucidated, which is partly due to the inherent difficulties to establish relevant animal models. Furthermore, the particularities of the ocular environment explain why the abundant knowledge on systemic toxoplasmosis cannot be just transferred to the ocular situation. However, studies undertaken in mouse models have revealed a central role of interferon gamma (IFNγ) and, more surprisingly, interleukin 17 (IL17), in ocular pathology and parasite control. These studies also show the importance of the genetic background of the infective Toxoplasma strain. Indeed, infections due to exotic strains show a completely different pathophysiology, which translates in a different clinical outcome. These elements should lead to more individualized therapy. Furthermore, the recent advance in understanding the immune response during OT paved the way to new research leads, involving immune pathways poorly studied in this particular setting, such as type I and type III interferons. In any case, deeper knowledge of the mechanisms of this pathology is needed to establish new, more targeted treatment schemes.

Implications of TORCH Diseases in Retinal Development-Special Focus on Congenital Toxoplasmosis

There are certain critical periods during pregnancy when the fetus is at high risk for exposure to teratogens. Some microorganisms, including Toxoplasma gondii, are known to exhibit teratogenic effects, interfering with fetal development and causing irreversible disturbances. T. gondii is an obligate intracellular parasite and the etiological agent of Toxoplasmosis, a zoonosis that affects one third of the world's population. Although congenital infection can cause severe fetal damage, the injury extension depends on the gestational period of infection, among other factors, like parasite genotype and host immunity. This parasite invades the Central Nervous System (CNS), forming tissue cysts, and can interfere with neurodevelopment, leading to frequent neurological abnormalities associated with T. gondii infection. Therefore, T. gondii is included in the TORCH complex of infectious diseases that may lead to neurological malformations (Toxoplasmosis, Others, Rubella, Cytomegalovirus, and Herpes). The retina is part of CNS, as it is derived from the diencephalon. Except for astrocytes and microglia, retinal cells originate from multipotent neural progenitors. After cell cycle exit, cells migrate to specific layers, undergo morphological and neurochemical differentiation, form synapses and establish their circuits. The retina is organized in nuclear layers intercalated by plexus, responsible for translating and preprocessing light stimuli and for sending this information to the brain visual nuclei for image perception. Ocular toxoplasmosis (OT) is a very debilitating condition and may present high severity in areas in which virulent strains are found. However, little is known about the effect of congenital infection on the biology of retinal progenitors/ immature cells and how this infection may affect the development of this tissue. In this context, this study reviews the effects that congenital infections may cause to the developing retina and the cellular and molecular aspects of these diseases, with special focus on congenital OT.

Pathogenesis of ocular toxoplasmosis

Ocular toxoplasmosis is a retinitis -almost always accompanied by vitritis and choroiditis- caused by intraocular infection with Toxoplasma gondii. Depending on retinal location, this condition may cause substantial vision impairment. T. gondii is an obligate intracellular protozoan parasite, with both sexual and asexual life cycles, and infection is typically contracted orally by consuming encysted bradyzoites in undercooked meat, or oocysts on unwashed garden produce or in contaminated water. Presently available anti-parasitic drugs cannot eliminate T. gondii from the body. In vitro studies using T. gondii tachyzoites, and human retinal cells and tissue have provided important insights into the pathogenesis of ocular toxoplasmosis. T. gondii may cross the vascular endothelium to access human retina by at least three routes: in leukocyte taxis; as a transmigrating tachyzoite; and after infecting endothelial cells. The parasite is capable of navigating the human neuroretina, gaining access to a range of cell populations. Retinal Müller glial cells are preferred initial host cells. T. gondii infection of the retinal pigment epithelial cells alters the secretion of growth factors and induces proliferation of adjacent uninfected epithelial cells. This increases susceptibility of the cells to parasite infection, and may be the basis of the characteristic hyperpigmented toxoplasmic retinal lesion. Infected epithelial cells also generate a vigorous immunologic response, and influence the activity of leukocytes that infiltrate the retina. A range of T. gondii genotypes are associated with human ocular toxoplasmosis, and individual immunogenetics -including polymorphisms in genes encoding innate immune receptors, human leukocyte antigens and cytokines- impacts the clinical manifestations. Research into basic pathogenic mechanisms of ocular toxoplasmosis highlights the importance of prevention and suggests new biological drug targets for established disease.

Model Systems for Studying Mechanisms of Ocular Toxoplasmosis

The most common human disease caused by infection with Toxoplasma gondii is ocular toxoplasmosis, which typically is manifest as recurrent attacks of necrotizing retinal inflammation with subsequent scarring. The multilayered retina contains specialized cell populations, including endothelial cells, epithelial cells, neurons and supporting cells, all of which may be involved in this condition. In vitro investigations of basic mechanisms operating in human ocular toxoplasmosis use cellular and molecular methods that are common to the study of many pathological processes, and the novel aspect of this research is the use of human retinal cell subsets. Most in vivo research on ocular toxoplasmosis is conducted in the laboratory mouse. Experimental models involve local or systemic inoculation of parasites to induce acute disease, or sequential systemic and local parasite inoculations to trigger recurrent disease. We present methods for in vitro and in vivo studies of ocular toxoplasmosis, including dissection of the human eye, and culture and infection of differentiated cell populations from the retina, as well as induction of mouse ocular toxoplasmosis by intraocular, or sequential systemic and intraocular, inoculations, and imaging of toxoplasmic retinal lesions.

Expression of Long Non-Coding RNAs by Human Retinal Müller Glial Cells Infected with Clonal and Exotic Virulent Toxoplasma gondii

Retinal infection with Toxoplasma gondii-ocular toxoplasmosis-is a common cause of vision impairment worldwide. Pathology combines parasite-induced retinal cell death and reactive intraocular inflammation. Müller glial cells, which represent the supporting cell population of the retina, are relatively susceptible to infection with T. gondii. We investigated expression of long non-coding RNAs (lncRNAs) with immunologic regulatory activity in Müller cells infected with virulent T. gondii strains-GT1 (haplogroup 1, type I) and GPHT (haplogroup 6). We first confirmed expression of 33 lncRNA in primary cell isolates. MIO-M1 human retinal Müller cell monolayers were infected with T. gondii tachyzoites (multiplicity of infection = 5) and harvested at 4, 12, 24, and 36 h post-infection, with infection being tracked by the expression of parasite surface antigen 1 (SAG1). Significant fold-changes were observed for 31 lncRNAs at one or more time intervals. Similar changes between strains were measured for BANCR, CYTOR, FOXD3-AS1, GAS5, GSTT1-AS1, LINC-ROR, LUCAT1, MALAT1, MIR22HG, MIR143HG, PVT1, RMRP, SNHG15, and SOCS2-AS1. Changes differing between strains were measured for APTR, FIRRE, HOTAIR, HOXD-AS1, KCNQ1OT1, LINC00968, LINC01105, lnc-SGK1, MEG3, MHRT, MIAT, MIR17HG, MIR155HG, NEAT1, NeST, NRON, and PACER. Our findings suggest roles for lncRNAs in regulating retinal Müller cell immune responses to T. gondii, and encourage future studies on lncRNA as biomarkers and/or drug targets in ocular toxoplasmosis.

Calling in the CaValry-Toxoplasma gondii Hijacks GABAergic Signaling and Voltage-Dependent Calcium Channel Signaling for Trojan horse-Mediated Dissemination

Dendritic cells (DCs) are regarded as the gatekeepers of the immune system but can also mediate systemic dissemination of the obligate intracellular parasite Toxoplasma gondii. Here, we review the current knowledge on how T. gondii hijacks the migratory machinery of DCs and microglia. Shortly after active invasion by the parasite, infected cells synthesize and secrete the neurotransmitter γ-aminobutyric acid (GABA) and activate GABA-A receptors, which sets on a hypermigratory phenotype in parasitized DCs in vitro and in vivo. The signaling molecule calcium plays a central role for this migratory activation as signal transduction following GABAergic activation is mediated via the L-type voltage-dependent calcium channel (L-VDCC) subtype Cav1.3. These studies have revealed that DCs possess a GABA/L-VDCC/Cav1.3 motogenic signaling axis that triggers migratory activation upon T. gondii infection. Moreover, GABAergic migration can cooperate with chemotactic responses. Additionally, the parasite-derived protein Tg14-3-3 has been associated with hypermigration of DCs and microglia. We discuss the interference of T. gondii infection with host cell signaling pathways that regulate migration. Altogether, T. gondii hijacks non-canonical signaling pathways in infected immune cells to modulate their migratory properties, and thereby promote its own dissemination.

Advances and Challenges in Understanding Cerebral Toxoplasmosis

Toxoplasma gondii is a widespread parasitic pathogen that infects over one third of the global human population. The parasite invades and chronically persists in the central nervous system (CNS) of the infected host. Parasite spread and persistence is intimately linked to an ensuing immune response, which does not only limit parasite-induced damage but also may facilitate dissemination and induce parasite-associated immunopathology. Here, we discuss various aspects of toxoplasmosis where knowledge is scarce or controversial and, the recent advances in the understanding of the delicate interplay of T. gondii with the immune system in experimental and clinical settings. This includes mechanisms for parasite passage from the circulation into the brain parenchyma across the blood-brain barrier during primary acute infection. Later, as chronic latent infection sets in with control of the parasite in the brain parenchyma, the roles of the inflammatory response and of immune cell responses in this phase of the disease are discussed. Additionally, the function of brain resident cell populations is delineated, i.e., how neurons, astrocytes and microglia serve both as target cells for the parasite but also actively contribute to the immune response. As the infection can reactivate in the CNS of immune-compromised individuals, we bring up the immunopathogenesis of reactivated toxoplasmosis, including the special case of congenital CNS manifestations. The relevance, advantages and limitations of rodent infection models for the understanding of human cerebral toxoplasmosis are discussed. Finally, this review pinpoints questions that may represent challenges to experimental and clinical science with respect to improved diagnostics, pharmacological treatments and immunotherapies.

Comparing optical coherence tomography findings in different aetiologies of infectious necrotising retinitis

Aims

To compare optical coherence tomography (OCT) features of active necrotising infectious retinitis (NIR) due to toxoplasmosis or herpesviruses and to determine distinctive OCT signs for these two causes of infectious retinitis.

Methods

OCT scans from eyes with active NIR due to varicella zoster virus (VZV), herpes simplex virus (HSV), cytomegalovirus (CMV), and toxoplasmosis (TOXO) were reviewed. All images were evaluated for the presence of previously described OCT findings in TOXO-NIR and compared with the viral group. New OCT findings were recorded and compared. Retinal and choroidal thickness were measured at the site of NIR and compared.

Results

10 eyes diagnosed with TOXO-NIR and 13 eyes affected by viral-NIR (9 CMV and 4 VZV) were analysed. All eyes showed full thickness hyper-reflectivity, disruption of the retina and a variable degree of vitritis. Among previously described OCT signs, hyper-reflective oval deposits and hypo-reflectivity of the choroid had a higher prevalence in TOXO (p=0.018 and p<0.0001, respectively). Among the new signs, hyper-reflective round deposits along the posterior hyaloid, retrohyaloid hyper-reflective spots and a disruption of the choroidal architecture were more frequent in TOXO eyes (all p<0.01). Intra-retinal oedema and hyper-reflective vertical strips within the outer nuclear layer were suggestive of a viral aetiology (p=0.045). Retinal thickness at the site of NIR did not differ between the two groups. Choroidal thickness was significantly higher in TOXO eyes (p=0.01).

Conclusions

The diagnosis of NIR is largely based on clinical and laboratory findings. OCT changes may be useful in differentiating different causes of NIR.

Ocular parasitoses: A comprehensive review

Parasitic infections of the eyes are a major cause of ocular diseases across the globe. The causative agents range from simple organisms such as unicellular protozoans to complex metazoan helminths. The disease spectrum varies depending on the geographic location, prevailing hygiene, living and eating habits of the inhabitants, and the type of animals that surround them. They cause enormous ocular morbidity and mortality not because they are untreatable, but largely due to late or misdiagnosis, often from unfamiliarity with the diseases produced. We provide an up-to-date comprehensive overview of the ophthalmic parasitoses. Each section describes the causative agent, mode of transmission, geographic distribution, ocular pathologies, and their management for common parasites with brief mention of the ones that are rare.

Use of human induced pluripotent stem cell-derived neurons as a model for Cerebral Toxoplasmosis

Toxoplasma gondii is a ubiquitous protozoan parasite with approximately one-third of the worlds' population chronically infected. In chronically infected individuals, the parasite resides primarily in cysts within neurons in the central nervous system. The chronic infection in immunocompetent individuals has been considered to be asymptomatic but increasing evidence indicates the chronic infection can lead to neuropsychiatric disorders such as Schizophrenia, prenatal depression and suicidal thoughts. A better understanding of the mechanism(s) by which the parasite exerts effects on human behavior is limited due to lack of suitable human neuronal models. In this paper, we report the use of human neurons derived from normal cord blood CD34+ cells generated via genetic reprogramming, as an in vitro model for the study T. gondii in neurons. This culture method resulted in a relatively pure monolayer of induced human neuronal-like cells that stained positive for neuronal markers, MAP2, NFL, NFH and NeuN. These induced human neuronal-like cells (iHNs) were efficiently infected by the Prugniad strain of the parasite and supported replication of the tachyzoite stage and development of the cyst stage. Infected iHNs could be maintained through 5 days of infection, allowing for formation of large cysts. This induced human neuronal model represents a novel culture method to study both tachyzoite and bradyzoite stages of T. gondii in human neurons.

Effect ofToxoplasma gondiiinfection on the junctional complex of retinal pigment epithelial cells

Ocular toxoplasmosis is the most frequent cause of uveitis, leading to partial or total loss of vision, with the retina the main affected structure. The cells of the retinal pigment epithelium (RPE) play an important role in the physiology of the retina and formation of the blood–retinal barrier. Several pathogens induce barrier dysfunction by altering tight junction (TJ) integrity. Here, we analysed the effect of infection byToxoplasma gondiion TJ integrity in ARPE-19 cells. Loss of TJ integrity was demonstrated inT. gondii-infected ARPE-19 cells, causing increase in paracellular permeability and disturbance of the barrier function of the RPE. Confocal microscopy also revealed alteration in the TJ protein occludin induced byT. gondiiinfection. Disruption of junctional complex was also evidenced by scanning and transmission electron microscopy. Cell–cell contact loss was noticed in the early stages of infection byT. gondiiwith the visualization of small to moderate intercellular spaces. Large gaps were mostly observed with the progression of the infection. Thus, our data suggest that the alterations induced byT. gondiiin the structural organization of the RPE may contribute to retinal injury evidenced by ocular toxoplasmosis.

Experimental Models of Ocular Infection with Toxoplasma Gondii

Ocular toxoplasmosis is a vision-threatening disease and the major cause of posterior uveitis worldwide. In spite of the continuing global burden of ocular toxoplasmosis, many critical aspects of disease including the therapeutic approach to ocular toxoplasmosis are still under debate. To assist in addressing many aspects of the disease, numerous experimental models of ocular toxoplasmosis have been established. In this article, we present an overview on in vitro, ex vivo, and in vivo models of ocular toxoplasmosis available to date. Experimental studies on ocular toxoplasmosis have recently focused on mice. However, the majority of murine models established so far are based on intraperitoneal and intraocular infection with Toxoplasma gondii. We therefore also present results obtained in an in vivo model using peroral infection of C57BL/6 and NMRI mice that reflects the natural route of infection and mimics the disease course in humans. While advances have been made in ex vivo model systems or larger animals to investigate specific aspects of ocular toxoplasmosis, laboratory mice continue to be the experimental model of choice for the investigation of ocular toxoplasmosis.

Infection and characterization of Toxoplasma gondii in human induced neurons from patients with brain disorders and healthy controls

Toxoplasma gondii is a protozoan parasite capable of establishing persistent infection within the brain. Serological studies in humans have linked exposure to Toxoplasma to neuropsychiatric disorders. However, serological studies have not elucidated the related molecular mechanisms within neuronal cells. To address this question, we used human induced neuronal cells derived from peripheral fibroblasts of healthy individuals and patients with genetically-defined brain disorders (i.e. childhood-onset schizophrenia with disease-associated copy number variations). Parasite infection was characterized by differential detection of tachyzoites and tissue cysts in induced neuronal cells. This approach may aid study of molecular mechanisms underlying individual predisposition to Toxoplasma infection linked to neuropathology of brain disorders.

From the blood to the brain: avenues of eukaryotic pathogen dissemination to the central nervous system

Infection of the central nervous system (CNS) is a significant cause of morbidity and mortality, and treatments available to combat the highly debilitating symptoms of CNS infection are limited. The mechanisms by which pathogens in the circulation overcome host immunity and breach the blood-brain barrier are active areas of investigation. In this review, we discuss recent work that has significantly advanced our understanding of the avenues of pathogen dissemination to the CNS for four eukaryotic pathogens of global health importance: Toxoplasma gondii, Plasmodium falciparum, Trypanosoma brucei, and Cryptococcus neoformans. These studies highlight the remarkable diversity of pathogen strategies for trafficking to the brain and will ultimately contribute to an improved ability to combat life-threatening CNS disease.

Real-time imaging of Toxoplasma-infected human monocytes under fluidic shear stress reveals rapid translocation of intracellular parasites across endothelial barriers

Peripheral blood monocytes are actively infected by Toxoplasma gondii and can function as 'Trojan horses' for parasite spread in the bloodstream. Using dynamic live-cell imaging, we visualized the transendothelial migration (TEM) of T. gondii-infected primary human monocytes during the initial minutes following contact with human endothelium. On average, infected and uninfected monocytes required only 9.8 and 4.1 min, respectively, to complete TEM. Infection increased monocyte crawling distances and velocities on endothelium, but overall TEM frequencies were comparable between infected and uninfected cells. In the vasculature, monocytes adhere to endothelium under the conditions of shear stress found in rapidly flowing blood. Remarkably, the addition of fluidic shear stress increased the TEM frequency of infected monocytes 4.5-fold compared to static conditions (to 45.2% from 10.3%). Infection led to a modest increase in expression of the high-affinityconformation of the monocyte integrin Mac-1 (CD11b/CD18), and Mac-1 accumulated near endothelial junctions during TEM. Blocking Mac-1 inhibited the crawling and TEM of infected monocytes to a greater degree than uninfected monocytes, and blocking the Mac-1 ligand, ICAM-1, dramatically reduced crawling and TEM for both populations. These findings contribute to a greater understanding of parasite dissemination from the vasculature into tissues.

Tightly regulated migratory subversion of immune cells promotes the dissemination of Toxoplasma gondii

While the spread of Toxoplasma gondii within the infected human or animal host is associated with pathology, the pathways of dissemination have remained enigmatic. From the time point of entry into the gut, to the quiescent chronic infection in the central nervous system, Toxoplasma is detected and surveyed by immune cells that populate the tissues, for example dendritic cells. Paradoxically, this protective migratory function of leukocytes appears to be targeted by Toxoplasma to mediate its dissemination in the organism. Recent findings show that tightly regulated events take place shortly after host cell invasion that promote the migratory activation of infected dendritic cells. Here, we review the emerging knowledge on how this obligate intracellular protozoan orchestrates the subversion of leukocytes to achieve systemic dissemination and reach peripheral organs where pathology manifests.

Epidemiology, pathophysiology, and the future of ocular toxoplasmosis

Despite large advances in the field of ocular toxoplasmosis, large gaps still exist in our knowledge concerning the epidemiology and pathophysiology of this potentially blinding infectious disease. Although ocular toxoplasmosis is considered to have a high health burden, still little is known about its exact prevalence and how it affects the quality of life. The epidemiology of toxoplasmosis depends on local habits throughout the globe, and changes are likely in view of increased meat consumption in developing countries and demands for higher animal welfare in the Western world. Water is increasingly seen as an important risk factor and more studies are needed to quantitate and control the role of water exposure (drinking, swimming). Tools are now becoming available to study both the human host as well as parasite genetic factors in the development of ocular toxoplasmosis. Further research on the role of Toxoplasma strains as well as basic studies on parasite virulence is needed to explain why Toxoplasma associated eye disease is so severe in some countries, such as Brazil. Although genetic analysis of the parasite represents the gold standard, further developments in serotyping using peptide arrays may offer practical solutions to study the role of parasite strains in the pathogenesis of Toxoplasma retinochoroiditis. More research is needed concerning the pathways whereby the parasite can infect the retina. Once in the retina further tissue damage may be due to parasite virulence factors or could be caused by an aberrant host immune response. Local intraocular immune responses are nowadays used for diagnostic procedures. Future developments may include the use of Raman technology or the direct visualization of a Toxoplasma cyst by optical coherence tomography (OCT). With the availability of ocular fluid specimens obtained for diagnostic purposes and the development of advanced proteomic techniques, a biomarker fingerprint that is unique for an eye with toxoplasmosis may become available. It is hoped that such a biomarker analysis may also be able to distinguish between acquired versus congenital disease. Recently developed mouse models of congenital ocular toxoplasmosis are extremely promising with regard to disease pathogenesis, diagnosis, and treatment.