Characterising Toxoplasma gondii virulence: role of host and pathogen

Lead Research Organisation: Moredun Research Institute
Department Name: Disease Control


Toxoplasma gondii is a parasite of cats that can infect all warm-blooded animals, including humans where it can cause severe disease in immune compromised people, such as AIDS or cancer patients, and in children who were infected in utero. The World Health Organisation and the Centre for Disease Control recognise toxoplasmosis as one of the most important foodborne diseases worldwide and a leading cause of death amongst foodborne illnesses. The disease also impacts the livestock sector where it is a major cause of abortion in sheep. There is considerable variation between different strains of the parasite with some being more pathogenic than others, particularly in South America where, worryingly, cases of severe disease have been reported in healthy, immune competent individuals. The high pathogenicity of some strains has sparked much research interest on T. gondii virulence to further our understanding of the host-parasite relationship as well as predict disease outcome.

Although T. gondii is recognised as a major foodborne pathogen, there is a significant lack of data on the role of retail meat in the transmission of this parasite - something which was recently highlighted as a knowledge gap by the European Food Safety Authority. Meat consumption in Brazil is amongst the highest in the world so it is crucial to determine the role of meat in transmission of T. gondii particularly since more pathogenic strains dominate in this region. This project will address the knowledge gap by conducting the first ever comprehensive study of retail meat in Brazil, investigating incidence, viability and genetic diversity of T. gondii in different retail meat samples in São Paulo. Through collaboration between Brazil and the UK, it will be possible to assess the virulence of T. gondii isolated from meat products in a host-specific system to help determine the risk of different isolates to public and veterinary health.

While the mouse has proven to be an extremely important model for research into T. gondii virulence, it remains unclear how these results extrapolate to other hosts, such as humans and sheep. Given the lack of evidence of a correlation between the severity of disease in mice compared to other animals, there is an urgent need for a more relevant, host-specific system for identifying host and pathogen factors involved in determining virulence and predicting disease outcome. This project aims to address this significant knowledge gap by investigating the host-pathogen interactions during acute infection of mouse, sheep and human cells with virulent and non-virulent strains of T. gondii in vitro. The project will also assess the applicability of 3D "mini guts" as in vitro host-specific models for investigating virulence, offering a unique and exciting experimental system which mimics the primary site of infection thereby reducing the reliance on experimental animals.

Overall, this project will allow us to further our understanding of foodborne infections and parasite virulence. The development of a host-specific in vitro system to characterise virulence of T. gondii will offer a platform to investigate the mechanisms of virulence which will not only allow for the prediction of disease outcome in specific hosts, it will also aid vaccine design, a more specific treatment regime as well as the development of new drug compounds.

Technical Summary

Toxoplasma gondii is a major foodborne pathogen as well as a significant cause of ovine abortion worldwide. Research into its genetic diversity has revealed an abundance of atypical strains in South America where there are more cases of severe toxoplasmosis. Despite T. gondii being an important foodborne pathogen, there is little data on the role of meat products in transmission. Meat consumption in Brazil is extremely high so it is crucial to determine its role in T. gondii transmission particularly given the high incidence of atypical strains in this region. The mouse has been an invaluable tool in investigating T. gondii virulence; however, given the lack of evidence of a correlation between virulence in mice versus higher animals, there is an urgent need for a more relevant, host-specific system for identifying host and pathogen factors involved in virulence and predicting disease outcome. This project aims to address the knowledge gaps and assess the role of meatborne transmission as well as characterise virulence in a host-specific system. We will 1) use molecular, in vitro and in vivo techniques to assess the incidence, viability, genetic diversity and virulence of T. gondii in different retail meat products in São Paulo; 2) compare virulent and non-virulent strains of T. gondii using whole genome sequencing to identify markers for virulence; 3) analyse parasite invasion, proliferation and parasite and host-cell gene expression during acute infection of murine, ovine and human cells infected with virulent and non-virulent strains of the parasite; 4) determine the applicability of in vitro 3D host-specific gut organoids (enteroids) for characterising virulence of T. gondii in relevant hosts. Outputs from this project will greatly improve our understanding of foodborne transmission and T. gondii virulence and will provide an in vitro host-specific platform to enable vaccine design and drug discovery in a relevant system.

Planned Impact

Toxoplasmosis is a disease with global impact, recognised as one of the most important foodborne diseases worldwide and a major cause of production loss in livestock. Toxoplasma gondii can cause serious disease in the developing foetus, if a pregnant woman has a primary infection, resulting in brain damage, eye and/or auditory disease and occasionally death. T. gondii is also one of the most important causes of infectious abortion in sheep and goats worldwide. There is considerable strain variation as regards virulence of T. gondii, which seems to have a geographical distribution, with strains occurring in South and Central America being much more pathogenic even in immunocompetent individuals compared to the strains occurring in North America and Europe. There are currently no vaccines for use in humans and limited therapeutic options.

Our research proposal is focussing on improving our understanding of foodborne transmission as well as differences in virulence of geographically distinct strains looking at both host and pathogen factors and developing relevant in vitro systems. Improving our knowledge of the key factors involved in determining pathogen virulence will have significant impact on both public and livestock health.

Although the incidence of toxoplasmosis may be low in comparison to other diseases, the life-long consequences of congenital infection can be substantial. Due to the high impact on disability-adjusted life-years (DALY's), toxoplasmosis is one of the most economically important foodborne diseases worldwide. Furthering our understanding of the virulence of T. gondii will lead to better control and diagnostic strategies to reduce transmission to humans and hence reduce the negative impact on public health, such as loss of human life, learning disabilities and visual and auditory disorders.

T. gondii is a significant cause of abortion in sheep costing the UK livestock industry £12.4 million per year. Once an animal has aborted it is usually immune and will not abort again. However, there have been reports of abortions in previously immune sheep re-infected with an atypical strain suggesting a lack of absolute protective immunity. Understanding the virulence of atypical strains will significantly aid future vaccine development and control of this disease in sheep, thus benefiting the livestock industry.

The data arising from the project will be relevant to several different stakeholder groups and our impact plan involves events and activities designed to maximise the benefits of the research outputs to those who may gain value from them. In addition to the publication of original research papers and conference abstracts we will host a workshop on virulence of T. gondii as a satellite meeting attached to an international scientific conference to enable discussions with other scientists and academic beneficiaries. As T. gondii is a major cause of abortion in sheep we will develop and deliver roadshow meetings with farmers in the UK and Brazil to discuss prevention and control of toxoplasmosis in sheep. These roadshows will be developed with the Moredun Foundation and the National Sheep Association (UK) and the Brazilian Sheep Breeders Association (Brazil).

The impact of T. gondii strain variation will be of interest to public health practitioners and in collaboration with our partners in Brazil we will prepare a visual booklet using imagery and a comic strip format to communicate the main transmission routes, impact and current knowledge on prevention and control of T. gondii. To accompany the booklet we will produce a short animated film with subtitles in English and Portuguese.

Information about T. gondii in relation to food safety will be communicated to relevant policy colleagues both in the UK and Brazil and we will develop a public engagement and educational activity called "The beast within us" aimed at school children to raise awareness about the transmission and impact of T. gondii.


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Plaza J (2020) Detection of in retail meat samples in Scotland. in Food and waterborne parasitology