Beware of our feline friendsby Margo White
A common cat-carried parasite could be making us sicker than we realise.
There’s more evidence that cats can be dangerous. Or at least the parasite that commonly infects felines, Toxoplasma gondii, can be. When passed on to humans, T gondii can cause toxoplasmosis, which in turn can affect memory and concentration and result in fatigue, muscle soreness and headaches.
This is according to research led by Mark Thomas, associate professor in the Department of Molecular Medicine and Pathology at the University of Auckland. “The general view is that toxoplasmosis is trivial. People get over it and only in a small proportion is it anything to worry about,” says Thomas, whose findings appear in the Scandinavian Journal of Infectious Diseases. But as he discovered in a sample of people recently diagnosed with the illness, symptoms can be more severe than they are commonly understood and than usually described in the medical literature.
Thanks to cat toilet habits, T gondii can be found in domestic gardens, from where rats and mice pick it up. But as the parasite depends on a cat to complete its life cycle, it needs to get from the rodent back to the cat.
How? By modifying rodent behaviour, causing an infected rat or mouse to put itself in the cat’s way. This might sound far-fetched, but studies have shown rodents that aren’t infected with T gondii will keep away from the scent of a cat, while infected ones are attracted by the smell. It is thought this suicidal behaviour is due to the parasites in the rodent’s brain releasing dopamine, a neurotransmitter involved in the regulation of fear.
T gondii isn’t the only parasite to mess with the mind of its host. A worm that commonly infects wetas compels an infected insect, which would normally avoid water, to drown itself. The worm is then released into the water where it can mate with another worm discharged in a similar way. French scientists have identified a worm that persuades crickets to do the same; in fact, hundreds of parasites are capable of modifying host behaviour.
It’s estimated that 30-40% of the adult human population in New Zealand have been infected by T gondii, and that one in 200 people are infected each year. The parasite remains alive and is widely distributed through the tissues of infected people, but as our immune system keeps it under control, most of us wouldn’t know we have it.
Toxoplasmosis can cause an overt reaction in some people when they are first infected, although the medical literature merely describes the symptoms as “flu-like”. Thomas puts this vagueness down to lack of research into the effects of recently acquired toxoplasmosis. He has seen patients who had prolonged symptoms that were distinct from those described in the textbooks. One previously “high-functioning” patient, for instance, had symptoms akin to agoraphobia, suffering anxiety about venturing outside the house for months.
For the study, Thomas and medical student Weng Kit Wong sent questionnaires to more than 100 Aucklanders who’d been diagnosed with newly acquired toxoplasmosis in 2011. Thirty-one responded, most of whom reported significant physical and mental health effects: 90% had fatigue, nearly three-quarters suffered from headaches, more than half had difficulty concentrating and had muscle aches, and more than a third had fever. Symptoms had often lasted for weeks.
As cats aren’t likely to eat people, T gondii hasn’t evolved to target us, but there is growing evidence that it can alter our neuropsychology. Danish researchers have shown chronic toxoplasmosis is associated with an increased risk of self-harm in women who have recently given birth, and researchers in California have linked it to neuroticism. There is evidence of an association between chronic toxoplasmosis and schizophrenia, Thomas says.
He believes the illness deserves to be taken more seriously and thinks the Auckland study points to the need for further research on toxoplasmosis’s effect on the brain, which could be a result of excessive dopamine production.
It may also be worth setting up trials to see if people with acute toxoplasmosis symptoms should be treated rather than being told to go home and rest, as they usually are. “We want GPs who see patients with acute toxoplasmosis to be aware that it can be a relatively severe illness, which can persist for several weeks.”
Researchers from Quebec working with GlaxoSmithKline have identified a way to stimulate the brain’s defence mechanism against Alzheimer’s disease – at least in mice. Alzheimer’s involves the production of amyloid beta, which forms sticky plaques when it’s unable to be eliminated. The team identified a molecule that stimulates the activity of the brain’s immune cells, known as MPL (monophosphoryl lipid A), and found that giving 12 weekly injections eliminated up to 80% of plaques in mice with Alzheimer’s. The rodents also showed improved cognitive function.
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