Published: 01 Oct 2020 | Last Updated: 01 Oct 2020 11:00:28

A Royal Veterinary College (RVC) study, published in Trends in Parasitology, suggests that the human infection burden of Toxoplasma gondii (T. gondii), a globally distributed parasitic infection, have been greatly underestimated.

The research highlights the considerable number of human psychiatric and behavioural conditions that T. gondii may cause, including some cases of schizophrenia, bipolar disorder, obsessive compulsive disorder and epilepsy.

The zoonotic parasite is found in every continent and is estimated to infect up to a third of the world’s human population, via its two main post-natally-acquired transmission routes (i.e. infection at any age after birth) of eating infected undercooked or raw meat, or of ingesting parasites shed in the faeces of infected cats via contaminated soil, food or water. Infection results in the formation of parasite cysts in the brain, eyes and other tissues, which potentially last for life.

If the parasite is acquired during pregnancy, transmission to the foetus may cause either abortion or stillbirth, or if the baby is brought to term, a range of abnormalities and development problems (termed congenital toxoplasmosis).

T. gondii shares a family tree with other commonly known Apicomplexan parasites, including malaria parasites and is more commonly known for its danger to pregnant women and those with weakened immune systems. However, the study, which involved collecting and analysing published data that examined links between T. gondii infection and human disorders, highlighted that otherwise healthy T. gondii–infected people have a higher risk of developing a range of autoimmune, psychiatric and behavioural conditions, including an increased risk of addiction, suicide attempts and even traffic accidents.

By using information on the incidence of different human disorders worldwide, the researchers estimated that globally each year T. gondii infection potentially causes between 150,000 and 335,000 cases of schizophrenia, between 2.1 and 10.2 million non-fatal traffic accidents, and between 0.5 and 2.9 million non-fatal suicide attempts. By comparison, worldwide each year there are approximately 190,000 cases of congenital toxoplasmosis.

These findings highlight the considerable global health impact that T. gondii may cause and suggest the burden of toxoplasmosis has been considerably underestimated.

Gregory Milne, Lead Author and PhD student under the supervision of Dr Martin Walker and Professor Joanne Webster, at the RVC said: “While it has been suspected since as early as the 1950s that T. gondii infection might cause neurological diseases like schizophrenia, the scale of the problem has often not been truly appreciated.

“I am excited to share our study, which confirms that T. gondii poses a significant burden on human health worldwide and demonstrates the sheer diversity of conditions that can result from T. gondii infection. 

“My hope is that our paper stimulates future research in this field.”

Professor Fuller Torrey, founder and former director of the Stanley Medical Research Institute and founder of the Treatment Advocacy Center, said: “The public health consequences of T. gondii infections have been seriously underestimated. In this paper Milne et al. draw together what is known about these consequences, focusing especially on the neuropsychiatric complications that may follow such infections. The paper is an important alert regarding the need for more research.”

Research Reference

Milne, G., Webster, J.P. & Walker, M. (2020). Toxoplasma gondii: an underestimated threat? Trends in Parasitology (in press) www.cell.com/trends/parasitology/fulltext/S1471-4922(20)30221-X

Notes to Editors