Parasite that controls cat minds may infect billions of people. The clue is in their eyes.

Toxoplasma gondii is currently the world's most successful parasite. This microscopic creature may infect any mammal or bird, and it has affected people on every continent. A person infected with Toxoplasma is infected for life. We don't have a medicine that can completely eliminate the parasite from the body yet. Furthermore, no human vaccination has been approved.

Toxoplasma infections are expected to affect 30–50 percent of people worldwide, and infections may be on the rise in Australia. In the 1970s, a review of studies conducted at blood banks and prenatal clinics across the country estimated that 30% of people were infected. In a recent community-based study in Western Australia, however, 66 percent of persons were infected.

This parasite's sickness might leave a scar on the back of the eye. Our current study searched for symptoms of disease in otherwise healthy patients and discovered that a large number had the Toxoplasma infection.

We don't just get it from cats

Illustration of how humans can be infected by the parasite.
The cat is the primary host for Toxoplasma.

When cats eat infected prey, they pick up the parasite. They then pass vast numbers of parasites in their feces for a few of weeks in a form that can live in the environment for long periods of time, even in adverse weather.

Parasites lodge in the muscle of grazing livestock and survive after they are butchered for meat. Humans can contract the disease by eating this meat, as well as fresh produce and water contaminated by cats. It is also conceivable for a pregnant woman who becomes infected for the first time to transfer the infection on to her unborn child.

While Toxoplasma infection is exceedingly prevalent, the rate of toxoplasmosis, the disease produced by the infection, is the most important health statistic.

How it affects the eye

The retina, the multi-layered nerve tissue that lines the eye and creates vision, is a favorite of Toxoplasma. Infection can result in recurrent retinal inflammation and irreversible damage. Ocular toxoplasmosis is the medical term for this condition.

Contrary to popular belief, ocular toxoplasmosis mostly affects healthy individuals, according to medical study. However, it can be more severe in the elderly or those with a compromised immune system, particularly when infected during pregnancy.

"Floaters" and hazy vision are common symptoms of active inflammation. If the inflammation advances to scarring, visual loss may become irreversible.

We assessed lowered vision to below driving level in more than 50% of eyes with ocular toxoplasmosis examined at a large ophthalmology clinic, and 25% of eyes were irreversibly blind.

How many eyes?

Ophthalmologists and optometrists are well-versed in ocular toxoplasmosis treatment. However, even the medical community is unaware of the scope of the problem. Until date, the number of Australians with ocular toxoplasmosis has never been counted.

We wanted to find out how common ocular toxoplasmosis is in Australia, but we knew it would be difficult to secure financing for a large-scale study of this neglected disease. So, as part of the Busselton Healthy Ageing Study, retinal pictures were acquired from over 5,000 baby boomers (born 1946–64) in Busselton, Western Australia. Other eye illnesses such as macular degeneration and glaucoma were investigated using the photos.

We calculated the prevalence of ocular toxoplasmosis at one in 150 Australians by screening these retinal pictures. This may appear unusually common, yet it is consistent with how people contract Toxoplasma.

Australia has a large number of wild cats in addition to pet cats. And Australia has a lot of farmland, accounting for more than half of the global organic growing area.

Above all, many Australians want their red meat rare, placing them at serious risk.

How the condition is treated

A retina examination is required to identify ocular toxoplasmosis, ideally with the pupils dilated.

Because of the way Toxoplasma triggers retinal cells to create certain proteins, the retinal lesion is easy to identify, and an ophthalmologist or optometrist can recognize it right once. A blood test is often used to confirm the diagnosis.

If the illness is modest, the doctor may decide to let the body's immune system handle it, which can take a few months. Anti-inflammatory and anti-parasitic medications are commonly prescribed together.

Stopping the spread

Although toxoplasma infection is incurable, it can be avoided. Toxoplasma may be present in meat sold in Australian shops. The parasite can be killed by boiling meat to a temperature of 66°C or freezing it before cooking.

Drinking untreated water (such as straight from rivers or creeks) should be avoided, and fresh fruit and vegetables should be rinsed before eating. When changing cat litter, gloves should be worn and hands should be cleansed afterwards.

For diseases that affect humans, animals, and their ecosystems, the World Health Organization and other international and state health agencies are supporting a One Health approach. Different sectors collaborate to promote excellent health. Now that we know how frequent ocular toxoplasmosis is in Australia, there is a strong case to be made for using One Health to battle Toxoplasma infections.