Malaika Arora

One in three people is infected with the Toxoplasma parasite – and the clue could be in our eyes

Toxoplasma gondii is probably the most successful parasite in today’s world. This microscopic creature can kill any mammal or birdand people on all continents are infected. Once infected, a person carries Toxoplasma for life. So far, we have no drug that can eradicate the parasite from the body. And there is no vaccine approved for use in humans.

Around the world, an estimated 30-50% of people are infected with Toxoplasma – and in Australia infections may be on the rise. An overview of studies conducted at blood banks and pregnancy clinics across the country in the 1970s estimated the infection rate at 30%. However, a recent community survey in Western Australia found that 66% of people were infected.

The disease caused by this parasite can cause scarring on the back of the eye. Our new study looked for signs of disease in otherwise healthy people and found that a significant number carried the Toxoplasma mark.

We don’t just get it from cats

The cat is the primary host for Toxoplasma. cats catch the parasite when they eat infected prey. Then, for a few weeks, they pass large numbers of parasites in their feces in a form that can survive in the environment for a long time, even in extreme weather. When the excrement is ingested by livestock during grazing, parasites settle in the muscle and survive there after the animals are slaughtered for meat. Humans can become infected by eating this meat, eating fresh produce or drinking water contaminated by cats. It is also possible for a woman who is infected for the first time during pregnancy to transmit the infection to her unborn child

Although infection with toxoplasma is very common, the most important health statistic is the rate of disease caused by the infection, which is called toxoplasmosis.

Yes, cats spread Toxoplasma. But they are not alone to blame. (Photo: Unsplash/Daria Shatova)

How it affects the eye?

Toxoplasma really likes the retina, the multilayered nerve tissue that lines the eye and generates vision. Infection can cause recurrent attacks of the retina inflammation and permanent scars on the retina. This is known as ocular toxoplasmosis.

Contrary to what is written about ocular toxoplasmosis, medical research shows that this condition usually affects healthy adults. In the elderly or people with a weakened immune system, or when contracted during pregnancy, it can be more serious. A bout of active inflammation causes “floaters” and blurred vision. When the inflammation develops into scarring, there may be permanent vision loss.

In a study of patients with ocular toxoplasmosis seen at a large ophthalmic clinic, we measured reduced vision below driving level in more than 50% of the eyes, and 25% of the eyes were irreversibly blind.

How many eyes?

Ophthalmologists and optometrists are well versed in the treatment of ocular toxoplasmosis. But the magnitude of the problem is not widely recognized, even by the medical community. The number of Australians with ocular toxoplasmosis had never been measured until now.

We wanted to investigate the prevalence of ocular toxoplasmosis in Australia, but we knew it would be challenging to get funding for a large questionnaire of this neglected disease. So we used the collected information for a different purpose: As part of the Busselton Healthy Aging Study, retinal images were taken from more than 5,000 baby boomers (born 1946–64) living in Busselton, Western Australia. The photos were collected to look for other eye diseases, macular degeneration and glaucoma

By screening these retinal images, we estimate the prevalence of ocular toxoplasmosis at one in 150 Australians. This may seem surprisingly common, but it fits the way people catch Toxoplasma.

In addition to domestic cats, Australia has huge populations of wild cats. And Australia is home to a lot of farmland, including more than 50% of the world’s organic farming area. Most importantly, many Australians like to eat their red meat rare, which puts them at real risk.

toxoplasma Toxoplasma likes the retina at the back of the eye very much and can leave a scar. (Photo: Unsplash/Marc Schulte)

How the condition is treated?

To diagnose ocular toxoplasmosis, a retinal examination is necessary, ideally with the pupils dilated. The retinal lesion is easy to spot, because of the way Toxoplasma activates retinal cells to produce certain proteins, and an ophthalmologist or optometrist can immediately recognize its appearance. A blood test is often also done to determine the diagnosis

If the condition is mild, the doctor may let the body’s own immune system control the problem, which takes a few months. Most often, however, a combination of anti-inflammatory and antiparasitic drugs is prescribed.

Stop the spread

Toxoplasma infection is not curable, but it can be prevented. Meat sold in Australian supermarkets may contain Toxoplasma. Cooking meat to an internal temperature of 66 degrees Celsius or freezing it before cooking are ways to kill the parasite.

Fresh fruit and vegetables should be washed before eating and drunk untreated water (such as straight from rivers or creeks) should be avoided. Gloves should be worn when changing cat litter and hands should be washed afterwards.

The World Health Organisation and other international and national Health agencies are promoting an approach called One Health for diseases that cross humans, animals and their environment. Various sectors are working together to: good health† Now that we know how common ocular toxoplasmosis is in Australia, there is real justification for deploying One Health to fight Toxoplasma infections in this country.

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