zombie parasite?
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I had a ‘zombie parasite’ living in the back of my eye. Chances are, you’ve hosted one too
I had a parasite living in my eye
VIDEO CREDIT: Alex Cairns/Stuff
For years, I had a squatter living in me. I don’t know when they took up residence, and I don’t know how long they hung around. They may even still be there, hiding.
What I do know is that they left a permanent mark of their occupation, right at the back of my left eye.
At some stage in my life, I became host to a delightfully sounding parasite called Toxoplasma gondii– T. gondii for its street name.
I’m certainly not alone in having this uninvited lodger. Roughly a third of the world’s population is estimated to have been exposed to the little bugger.
Oh, did I mention it’s also been dubbed a “zombie parasite”?
Otago University ophthalmologist, associate professor and retina specialist Dr Francesc March de Ribot explains what we’re dealing with.
What is it?
T. gondii is a parasite that can infect almost all warm-blooded animals. And sorry, feline lovers, but cats are its definitive hosts – the animals in which the parasite reproduces.
“They become infected by eating infected rodents, birds or other small animals and then the toxoplasma gets in their body, living in their intestines and spreading around,” explains March.
Basically, it reproduces in a cat’s gut and is shed in its poo.
Delightful.
How many people in New Zealand have been exposed
Numbers here vary because toxoplasmosis isn't routinely captured through national disease notification, says March.
University of Otago researchers estimate nearly 40% of New Zealanders have been exposed to Toxoplasma gondii, with about 7% – or 364,000 people – experiencing ocular involvement.
An estimated 10,000 people have visual impairment from ocular toxoplasmosis, with about 1000 recurrent episodes each year.
“For comparison, a global meta-analysis put prevalence in the general population at about 2%.”
How does it spread?
People can accidentally host the parasite after coming into contact with Tinkerbell’s contaminated droppings.
March says this can happen by swallowing oocysts – the parasite’s egg-like form, shed in cat faeces – via contaminated food, flies, dirty hands or unwashed fruit and vegetables.
“Cleaning litter trays and gardening in soil that cats use are common routes.”
It can also come from eating cysts in undercooked meat.
And it can cross the placenta from a pregnant woman to her baby if she is newly infected during pregnancy.
One thing it cannot do is spread from person to person through everyday contact.
What are the symptoms?
For most people, nothing at all. Some may develop a mild, flu-like illness with fever, tiredness, muscle aches and swollen lymph glands. These symptoms usually settle, and most healthy people never know they have been infected.
There are two higher risk groups though - people with weakened immune systems, such as those with HIV or taking immunosuppressant medicines, and babies infected before birth.
“Congenital toxoplasmosis ranges from mild to severe and can cause eye and brain damage or miscarriage,” says March.
The parasite can remain in the body for life as dormant tissue cysts. In most healthy people, this causes no problems, but infection can reactivate in those whose immune systems become weakened.
What is ocular toxoplasmosis?
Globally, it is an important cause of infectious eye disease, says March.
“Ocular toxoplasmosis is the infection of the retina. It can follow congenital or acquired infection.”
Symptoms can include blurred or reduced vision, floaters, light sensitivity, eye pain and a dark patch in vision.
It can recur, and each flare can leave scarring that permanently damages vision, particularly if it affects the macula or optic nerve.
What’s the treatment?
For most healthy people, the body’s immune system deals with the infection and no treatment is needed.
There are antiparasitic medicines for people who are more seriously affected.
With ocular toxoplasmosis, treatment depends on whether the infection is active and factors including the size and location of the lesion. Antiparasitic treatment, sometimes alongside steroids to control inflammation, may be used.
So, back to me – is my parasite still in me?
It seems there may be bad news.
“Your immune system usually can’t fully clear Toxoplasma. Instead, the active form is suppressed and converts into a dormant form inside tiny tissue cysts. These typically persist for life, most often in the brain, muscles and retina.”
The scar in my eye is the healed damage caused by earlier inflammation – not the parasite itself. Some dormant cysts may sit around the edge of the scar.
So it hasn’t necessarily moved somewhere else.
“It was likely already present in a dormant state, and the eye was simply where it caused trouble.”
Can it come back?
Short answer: yes. Longer answer: still yes.
“Recurrence in the eye is fairly common, often at the border of an old scar, and can happen months or years later,” says March.
The good news is that many people have one episode of inflammation and never experience another.
So, back to that ‘zombie parasite’ comparison
I’ll let March explain.
“The name is a media nickname that highlights the ‘mind-control’ involvement of Toxoplasma.
“Toxoplasma gondii is a parasite that lives inside the bodies of animals, where it forms cysts in tissues such as the eyes and the brain. In these organs it can cause damage, including vision problems and neurological effects.
“Some studies have linked infection with mental health conditions such as depression, as well as other psychiatric and behavioural changes.”
He adds these findings show an association, but it has not been proven that the parasite directly causes these conditions.
“Cats are its final host, the only place where it can complete its life cycle. To get there, the parasite alters the brain of infected mice so that they lose their natural fear of cats and may even seek them out.
“This makes the mouse more likely to be caught and eaten, which returns the parasite to the cat and allows its life cycle to continue.”
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