Around one in three people worldwide have been infected with a microscopic parasite called Toxoplasma gondii – or simply “toxo” – which can cause toxoplasmosis. After the initial infection, toxo settles into our muscles and brain, where it can remain for life.
A parasite that lives in our brain for life – that sounds worrying.
Yet, the vast majority would not even know they are infected, as it rarely poses clinical problems in healthy people who aren’t pregnant. In fact, our bodies do a remarkable job of protecting us against a barrage of infections, without us even knowing, toxo included.
So, why has toxo received so much attention? And are you at risk even if you don’t have a cat?
Do we get toxo from cats?
Cats can carry toxo and pass it on to humans through their faeces. They are the only “definitive hosts” in which the toxo parasite can reproduce.
But this doesn’t mean it’s only cat owners who are at risk.
Worldwide, eating undercooked meat that contains toxo is a major source of infection.
It’s also important to note that cats only shed toxo when they are infected for the first time and are then immune afterwards, meaning they no longer shed and pass it on. Most infected cats show no symptoms, but if they develop the clinical disease toxoplasmosis, they may lose their appetite and experience fever and lethargy.
Who is at risk?
Many people first hear about the risk of toxo infection in relation to pregnancy.
Expectant mothers are told to stay away from undercooked meat and kitty litter – two ways we can accidentally ingest toxo. The parasite can cross the placenta and infect the developing fetus, leading to blindness, congenital neurological defects and miscarriage.
Thankfully, this is very rare in Australia, estimated to be 0.17–2.3 cases per 10,000 births. The risk is considered low enough that routine screening during pregnancy is not necessary.
Some people will have previously been infected with toxo and won’t be at risk of a new infection during pregnancy. The risk is to pregnant women who have never had toxo.
In Australia, infectious disease doctors and oncologists are also trained to look for toxo.
A dormant infection can become a deadly illness if it reactivates into an acute infection which can occur if you become severely immunosuppressed, after receiving a bone marrow transplant or contracting HIV. In these rare cases, effective therapies are available, although with some significant possible side effects.
Toxo can also cause progressive blindness if parasites lodge in the retina. Retinal toxoplasmosis has been estimated to be one in 149 people in Australia. Optometrists and ophthalmologists are well trained to look for this and other eye infections.
The risk is much higher in other countries
For instance, the estimated infection rate in France is around 50%, while in Brazil up to 80% of people have been exposed. These countries report a far higher number of related health problems.
Such high infection levels are thought to do with eating undercooked meat, greater numbers of stray cats, the type of locally circulating strains, and local hotter and humid climates that allows toxo to survive in the environment.
OK, but how does toxo affect the brain?
Once infected, toxo can live in the host’s muscles or brain for life. But we have no way of knowing exactly where it is; the only tests we have show the presence of antibodies, not the parasite itself.
Interestingly, when a rodent’s brain is infected, toxo can manipulate the rodent’s behaviour, including reversing its natural aversion to cat odour.
Scientists have speculated that the parasite does this to make the infected rodent less likely to run away, and therefore more likely to be eaten by a cat. This completes the parasite’s complicated life cycle; their sexual cycle exclusively takes place inside cats.
Such a remarkable ability of toxo – a single-celled organism – to impart behavioural changes on a complex organ like the brain seems fantastical. But it has been proven true over multiple rodent laboratory studies.
But while we know toxo can affect rodents’ brain and behaviour, we don’t know how this happens, exactly why, or whether it happens outside of the laboratory.
Can toxo affect human behaviour?
In humans, researchers have found links between toxo and a wide range of brain disorders and behaviours, including risk-taking behaviour and psychiatric conditions, especially schizophrenia.
But we should view these findings very cautiously, as these are correlative studies. This means they do not prove toxo is the cause of these diseases or behaviours, just that there is a link. Further, they show only a mild increase in risk. Other published scientific papers refute some of the findings.
Correlative studies also cannot prove which came first: whether a condition such as schizophrenia, for example, changes your behaviour in ways that make a toxo infection more likely, or whether a toxo infection predisposes you to – or even causes – this condition.
This is the most contentious area of research in the field, so is continues to be a major focus for scientists.
How worried should I be?
So should the fact that one in three people carry a brain-dwelling parasite keep us awake at night?
Probably not.
For most healthy people, toxo remains a silent passenger for life, causing no illness at all. Our health-care system already manages the situations where it poses genuine risks, particularly during pregnancy and in people with weakened immune systems.
But understanding how this remarkably successful parasite hides from our immune system could lead to something we’ve never had before: treatments that completely clear dormant infection.
That would make a real difference for the people most at risk and would transform how we think about one of the world’s most common infections.
This article was originally published on The Conversation. Read the original article.
