One of T researchers discovers the ‘Trojan horse’ virus hiding in the human parasite

An international team led by researchers at the University of Toronto has found a new RNA virus that they believe is fighting a common human parasite.

The virus, called Apocryptovirus odysseus, along with 18 others closely related to it, was discovered through a computational screen of human neuronal data—an effort aimed at elucidating the link between RNA viruses and neuroinflammatory disease. The virus is associated with severe inflammation in people infected with the parasite Toxoplasma gondiileading the team to hypothesize that it exacerbates the disease of toxoplasmosis.

“We found out A. odysseus in human neurons using the Serratus open science platform to search through more than 150,000 RNA viruses,” said Purav Gupta, the study’s first author, a recent high school graduate and current undergraduate student at U of T’s Donnelly Center for Cellular and Biomolecular Research. “Serratus identifies RNA viruses from public records by marking an enzyme called RNA-dependent RNA polymerase, which facilitates the replication of viral RNA. This enzyme allows the virus to reproduce and the infection to spread.”

The study was recently published in the journal Virus EVOLUTION.

The parasite T. gondii is widespread, infecting about a third of the global population. It can live in any type of non-blood cell, including neurons, forming cysts within the cells. The parasite is transmitted to nearby cells when the infected cell ruptures.

T. gondii infections often go unnoticed because they lead to symptoms only in rare cases. Regardless, toxoplasmosis deserves investigation considering how widespread it is and the potential effects it can have on pregnant women and those who are immunocompromised, Gupta said.

“We believe that the virus and the parasite work hand in hand to cause disease in the human body, where the virus hides inside the parasite, like a soldier in a Trojan horse, to enter the human brain,” Gupta said. “Our research marks the first time scientists have linked toxoplasmosis to a virus.”

Newly discovered A. odysseus found in two hypervirulent strains of T. gondii parasite, referred to as RUB and COUGAR.

RUB has been documented in French Guinea to cause severe fever and organ failure, while COUGAR has been shown in British Columbia to be associated with ocular toxoplasmosis – the leading cause of infectious blindness. The researchers found the strains in different geographic locations at different times, demonstrating their potentially far-reaching impacts.

Symptoms of toxoplasmosis can be exacerbated by a hyperactivated human immune response. The parasite carrying the virus triggers this type of response when the immune system senses the virus’s foreign RNA.

“The set of 19 RNA viruses we found are strong biomarkers for parasite infection,” said Artem Babayan, principal investigator on the study and assistant professor of molecular genetics at the Donnelly Center and Temerty School of Medicine. “It is clear now that A. odysseus the virus may be a valuable marker of disease-causing infections, such as severe toxoplasmosis, in humans or other animals. The next step is to test whether this raises the possibility that treating a parasite’s viruses can be an effective tool for treating symptoms arising from parasitic infections.

Zoonotic viruses that infect other living things in our environment to reach us are expected to cause most of the infectious diseases seen in humans, Babaian noted. “This study underscores the importance of looking beyond viruses that directly infect humans to the expanded virome,” he said.

This research was supported by the Canadian Institutes of Health Research.