Researchers from the Massachusetts Institute of Technology say they have discovered clues as to why a malaria-related parasite, called toxoplasma gondii, can cause blindness in some people but not in others. This is according to a study published in PLOS Pathogens.

Toxoplasma gondii is a single-celled parasite that causes a disease known as toxoplasmosis.

Some people with toxoplasmosis can develop encephalitis – a swelling of the brain – or ocular toxoplasmosis – a condition that can lead to blindness.

Toxoplasmosis can be contracted a number of ways. Eating undercooked or contaminated meat can lead to infection, as well as consuming water contaminated with the parasite.

Cat feces can also contain toxoplasma gondii, so coming into contact with this while cleaning a litter tray or touching soil that has been contaminated with the feces may lead to infection.

According to the Centers for Disease Control and Prevention (CDC), more than 60 million people in the US may be infected with the parasite. But many may not know they are infected, as the human immune system is normally strong enough to stave it off.

However, those with weakened immune systems or pregnant women can experience serious health problems from the infection.

The investigators explain that strains of the toxoplasma gondii parasite that are found in North America and Europe are different from strains found in South America – which appear to cause more severe symptoms regardless of a person’s immune system strength.

To find out why, the researchers infected mouse immune cells – known as macrophages – with 29 different strains of the toxoplasma gondii parasite.

The researchers explain that macrophages are a major target of the parasite, and they also play an important role in the immune response of the host.

After the immune cells were infected, the researchers sequenced all of the cells’ messenger RNA molecules, in order to find out which genes – both in the parasite and the host’s immune cells – are activated the most during infection.

From this, the investigators found that some South American and atypical North American strains of the parasite had a type 1 interferon response. This is an immune reaction that usually occurs during viral infection.

This response prompts a strong inflammation reaction in the host’s immune cells, which the researchers believe could be the cause of the severe effects that these strains produce.

The researchers say that the parasite triggers this immune response once it has been killed by the host cell, meaning that parasite’s DNA and RNA leaks into the host cells.

Explaining this further, Jeroen Saeij, biologist at the Massachusetts Institute of Technology (MIT) and study author, says:

It’s often not the parasite that causes all the damage, but it’s actually the host immune response that’s causing most of the damage.

We think that maybe what’s happening is these parasites come in and they trigger a hyperinflammatory host immune response that might cause damage to the eyes.”

According to the researchers, toxoplasma gondii is one of very few parasites that are able to infect any warm-blooded animal. They say any parasite that can do this needs to have a large array of molecules that are able to work within any host or cell.

“However, we believe that different strains may have evolved to be able to maintain and reproduce optimally in a specific niche in nature, which may explain why different strains of toxoplasma have such varying effects in different organism,” says Mariane Melo, of MIT and lead author of the study.

For example, she explains that if a strain found in rats has adapted to that environment and stayed in that host a long time, then this could cause a fatal infection in mice, or the other way around, because “it might modulate host immune responses too much or not enough in hosts it is not optimally adapted to.”

The investigators say that further research will look to determine why specific South American strains of the toxoplasma gondii parasite are able to be so effectively killed by the host cells, and why this triggers the type 1 interferon response.

Furthermore, the research team has put all their data into a publically available database in the hope that other researchers will start to study the parasites.

Late last year, Medical News Today reported on a study detailing what toxoplasmosis does once it reaches the brain.