The contagious cat bug: how it might protect you from Alzhiemer’s

What links mind-altering parasites, rat mazes, and sartorially-challenged cat owners – and what have they all to do with treating diseases of the brain? You might of heard about toxoplasma infection in the news – but it’s not necessarily all bad. Guest contributor Kathryn Lougheed discovers more.

Have you ever had an impromptu Facebook session when you should have been working? Or have you experienced a lapse of concentration and ended up driving a little bit too fast? Your poor attention span could be due to a brain parasite egging you on to take unhelpful and risk-taking behaviours. Unsettling as it sounds, you could be home to a parasite called Toxoplasma gondii (T. gondii), an uninvited guest that may be subtly messing with your mind.

Toxoplasma infection has recently hit the news, with newly published data showing an estimated 350,000 new cases in the UK, and 1.5 million new cases in the USA every year. Most people don’t realise they’ve become infected – symptoms are usually no worse than a mild flu. More fascinatingly is the effect the infection has when it gets into your brain…

Pick of the pets

A recent poll by Hunch took a lighthearted look at the differences between ‘cat people’ and ‘dog people’. It was discovered that people with cats tend to be more introverted, liberal, and ironic than dog owners. Friends of felines also prefer the Beatle George Harrison over Paul McCartney, and are 11% more likely to describe themselves as fashion-challenged than their canine-crazy counterparts (possibly as a result of all that cat hair – it gets everywhere). Taken together, it suggests that personality traits play their part in deciding which pets we pick. Or do they? Could the opposite be true – could owning a cat change the way your brain works?


dj sugar

T. gondii (full name: ‘Toxoplasma Gondii’) is a microbe that can infect a number of species, humans included. Infection rates are as high as 70% in certain populations thanks, in part, to our love of undercooked meat. But the parasite is more than a little choosy when it comes to getting in the mood for sex: T. gondii insists on being within the intestinal system of a cat when reproducing (how romantic). In order to get there, it makes good use of one of the least appreciated species in the world, the rat.

Rats tend to eat anything they can get claws on, and frequently pick up a T. gondii infection by eating something contaminated with cat dung. When ingested, T. gondii has cleared the first hurdle: it’s now safely inside the rat. Getting from rat to cat (where T. gondii longs to be), however, takes a bit of cunning – and some mind manipulation. The parasite’s big problem is that rats are quite wily and avoid anything that they don’t much like the look of, including traps, poison and cats. (As anyone who’s had an epic battle against rodent squatters will tell you, this makes rats extremely hard to get rid of). In short, they seem to have an innate fear of anything new.

So T. gondii has evolved mechanisms to alter the behaviour of an infected rodent, turning these normally fearful creatures into emboldened risktakers with a suicidal attraction to the smell of cat urine (let’s not dwell on that). Sniffing around a cat’s toilet, the unfortunate rodent practically presents itself on a dinner plate and quickly becomes a lunch for a hungry cat. And the cycle is complete: T. gondii is back inside a feline, and ready to experience a bit of intestinal loving.

Mind games

"I'll Be Losing My Mind, Anyway."There is also evidence that T. gondii affects human behaviour too. Like rats, if we put anything in our mouth that has touched cat poo, we may become infected. It has been reported that infected people display slower reaction times, differences in attention span, and an increase in the ratio of male to female babies born. ‘Hang on,’ I hear you say, ‘what does the sex of your baby have to do with a mindaltering parasite?’ Well, it appears that mothers experience miscarriages with boys at a slightly higher rate than with girls (a male embryo can sometimes be recognised as foreign by the mother’s immune system – causing the foetus to be rejected). But T. gondii suppresses the mother’s immune system, reducing this risk of male miscarriage.

Given its effects on normal brain function, it’s not too big a leap to suppose that T. gondii infection may play a role in disorders of the brain too. And, sure enough, a link has been suggested between T. gondii infection and conditions including schizophrenia, Obsessive Compulsive Disorder, Parkinson’s disease, and Alzheimer’s. But these studies have one problem – what if they get the cause and effect the wrong way round? It may be possible that T. gondii infection causes Alzheimer’s, or, equally, patients who already have Alzheimer’s simply tend to spend more time indoors with pet cats.

Mr friendly parasite

A recent study by Dr Jung of Seoul National University and his team explored whether the increase in T. gondii infection associated with Alzheimer’s is, in fact, a red herring and whether T. gondii may perversely be a good thing for Alzheimer’s sufferers. I’ve already mentioned how T. gondii can play around with a mother-to-be’s immune system. It seems particularly adept at suppressing brain inflammation – and such inflammation is central to the progression of neurodegenerative diseases such as Alzheimer’s. So could T. gondii actually be slowing the progression of Alzheimer’s?

To test this theory, Dr Jung and his team bred rats that suffered a rodent equivalent of Alzheimer’s and forced them to swim around in pools of opaque water looking for an invisible platform hidden beneath the surface. This wasn’t some kind of rodent torture, but a way to test how quickly rats learn and remember. Amazingly, they found that the parasite-infected rats had a much better memory than their uninfected cousins – and their brains showed less inflammation and degeneration.

It’s too early to say whether T. gondii can have the same effect in humans as in rats, but it opens the door to a better understanding of brain disorders and the development of new treatments for diseases such as Alzheimer’s. Perhaps mind-altering parasites aren’t all bad after all, or maybe that’s my T. gondii speaking…


Jung BK, Pyo KH, Shin KY, Hwang YS, Lim H, Lee SJ, Moon JH, Lee SH, Suh YH, Chai JY, & Shin EH (2012). Toxoplasma gondii infection in the brain inhibits neuronal degeneration and learning and memory impairments in a murine model of Alzheimer’s disease. PloS one, 7 (3) PMID: 22470449

Article by Kat Lougheed

August 10, 2012

Kathryn Lougheed is a research scientist at Imperial College London, working on the lung disease tuberculosis. She has an unhealthy interest in bacteria, blogging about research of the single-celled variety at in addition to running a popular science website for kids at, answering such important questions as ‘Why do papercuts hurt so much?’.

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One thought on “The contagious cat bug: how it might protect you from Alzhiemer’s”

  1. AT age 8, back in 1944, during a routine health check by a Health Department doctor’s visit to my primary school.. in Hastings New Zealand, it was found that I had very poor vision in my right eye.
    From age eight to thirteen I wore glasses, and after each visit to the eye doctor in nearby Napier,he would tell my mother my sight was improving, which of course it wasn’t
    A new eye specialist took over and he told my mother the glasses were useless…..that I had somehow damaged my eye, probably with a stick or some sharp object.
    He was on to it.
    But it wasn’t until we went to live in Epsom, Surrey, in the mid 1960’s when I went through a Health Screening clinic, that the eye specialist, I was referred to…..Dr Dorrington Ward, told me that the damage to my sight, was the result of TOXOPLASMOSIS.
    There was nothing that could be done about it.
    He did tell me that my good left eye had very good sight, obviously compensating for the very poor peripheral sight in the eight eye.
    Doctors back in NZ have told me it is the most extensive damage to an eye, that they have seen.

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