10,000 NZers may have lost vision due to toxoplasmosis

Otago Daily Times

16-05-2025

Toxoplasmosis is an infection triggered by a parasite spread by cats. Photo: RNZ
Ten thousand New Zealanders may have lost vision - sometimes permanently - due to toxoplasmosis, an infection triggered by a parasite spread by cats.
Otago University researchers estimate 40,000 people are affected by ocular toxoplasmosis - one in four seriously - but their efforts to develop new treatments are being hampered by a lack of funding.
Danielle Wilson was at work when her vision went blurry.
"I just had like a sudden onset of really sore, red eye and my vision changed, and I couldn't stand bright lights."
Luckily, she works as a nurse in the eye department at Otago University.
"I made myself an appointment to see an optometrist after work, but my colleagues being observant eye staff, all noticed my red eye so they scooped me up and took me and checked my eyes out."
Wilson, who is in her early 30s, was shocked to learn she had toxoplasmosis in her eye.
Doctors told her the parasite - commonly spread by cats - could have lain dormant in her body for years.
Treatment was intense - for the first week, she needed to take eyedrops every hour, and she was on antibiotics and courses of steroids for 18 months.
A couple of years on, she is "always nervous" it could flare up again, and is vigilant for any changes in her eye.
"Unfortunately the scar on my eye is really close to my central vision, so if the scar gets worse, I could lose that vision permanently."
NZ has high rates of infection
Toxoplasma is the leading cause of infectious eye diseases globally and a major contributor to vision loss in New Zealand.
Dr Francesc March de Ribot, Ophthalmology Consultant, Otago University Photo: Supplied
Otago University ophthalmologist Dr Francesc March de Ribot regularly sees patients with toxoplasmosis, including very young people.
One of them, a medical student, has lost central vision in one eye, which has killed his dream of becoming a surgeon.
"When there is inflammation in the retina, the vision becomes blurry, and sometimes we can see when the inflammation goes away, it leaves a scar, and the vision is never going to recover."
World-wide, up to one in three people are infected - but rates could be even higher in New Zealand: 43 percent according to one study in Waikato.
The parasite, Toxoplasma gondii, reproduces in cat guts and is spread through eggs in their faeces.
Cat poo in soil and freshwater can infect birds and mammals, including humans, with toxoplasmosis for up to two years, while the eggs survive in sea water for six months.
For unborn babies and those with weakened immune systems, toxoplasmosis can cause blindness, brain damage and even death - but for most people, it is no worse than a cold.
However, the parasite can live dormant within cysts inside the body, and make people sick when it "reactivates".
De Ribot said it was estimated to seriously affect the sight of about 10,000 New Zealanders, with 1000 recurrences a year.
"It's likely that maybe we have more incidences than other countries, and as well that some people have more difficulty in accessing eye care, so they are not detected or treated on time. So it's definitely a problem."
Research stymied by lack of funding
Otago University professor of parasitology Bruce Russell said current available treatments were not very effective.
"They don't work well against the dormant stage of the parasite. When people are found to have it, they need repeated and long-term treatments, and the current treatment doesn't stop the inflammation, which is the thing that actually destroys part of the eye. It causes retinal destruction."
The good news is Professor Russell has identified some promising compounds for new treatments - but he cannot get funding to develop them in New Zealand.
HIs latest application for a research grant - made with Dr March de Ribot and other top researchers - has just been rejected by the Marsden Fund.
They were asking for $900,000 over three years.
"We had a library of such drugs, that had already been part of a $420 million Novartis [Swiss-based multinational pharmaceutical company] target discovery programme that I was involved in, in Singapore. I try to sell this to people here, but I don't know, it doesn't seem to get through."
The Otago lab, which was testing drugs against both toxo and cryptosporidium, closed in 2023, due to lack of funding.
Most of Professor Russell's work has been farmed out overseas, with Singapore picking up the drug discovery work.
He said the developed world had largely forgotten about parasites.
"But there are a lot of hidden ones, like toxo, which we don't understand. And unfortunately I don't think we really want to try to understand, because if we knew their full impact, it would cause a lot of upset.
"For instance, we think there are probably some big impacts on mental health."
It could also explain why New Zealanders love cats and have one of the highest cat ownership rates in the world (40 percent of households), he (half) joked.
"The parasite rewires the brain of the mouse to go and find cats, so the cat can eat it, and complete the life cycle of the parasite."
Although toxoplasmosis is not a notifiable disease in New Zealand, 142 cases were reported between 2007-2016, including 15 cases of congenital toxoplasmosis (in babies infected in the womb).
It was not just a problem for people, Russell said.
"Toxoplasmosis is a killer of some of our precious native wildlife, including birds like kiwi and kākā, and our unique Hector's and Māui dolphins. It also causes big problems for sheep farmers, leading to the loss of lambs."
Doctors advise people who are pregnant or have a weakened immune system should not eat raw, undercooked or cured meats or products made from unpasteurised milk, wear gloves when emptying cat litter trays, cover children's sandpits, wear gloves to garden and wash their hands before preparing food and eating.