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Most of us think of parasites as either bitey little insects. Perhaps even worse, wriggly worms wiggling about inside our pets’ intestines. Serious infestations of these parasites are happily a thing of the past. Modern, risk-assessed treatments that we now give to our pets are so effective that these days, we rarely come across a serious parasite infestation. But what lies beneath? The answer to that is a microscopic world that’s a hidden universe of interconnected life. It is only revealed by the microscope and even then, with difficulty.
One member of this microverse is Toxoplasma. (This is not to be confused with Toxocara, the dog roundworm.) A microscopic parasite that not only has a worldwide distribution but also has a wide range of host mammals, including humans. The most important host, however, is the cat, both wild and domestic.
Rats and rodents are also essential to the spread of Toxoplasma as they are predated by cats. This creates a life-cycle. They also love to live near livestock and humans, where food is plentiful. So a cat might eat a rat, become infected with Toxoplasma. In turn, it will pass millions of parasite eggs in faeces for up to two weeks. These eggs, in turn, infect more rodents, humans, dogs, cows and sheep, etc. with an infection that lasts for life. Luckily, only rodents and cats can excrete the eggs in their faeces (in slightly different forms), and the rest of us just harbour the parasites in a dormant and harmless state in our tissues. So if so many humans and animals are carrying this tiny pest, why don’t we hear more about it? The answer lies in the classic device of a successful parasite: don’t kill your host. In fact, don’t even let them know you’re there!
Development of clinical disease from Toxoplasma is rare in all species. That way, everything goes on as usual, and Toxoplasma is free to spread without hindrance. It’s a clever little devil. There are, however, important and serious exceptions to this which affect pregnant women and immunosuppressed individuals. Advice regarding exposure to cats’ faeces and pregnancy hasn’t changed much over the years:
But the fact is, humans are more likely to contract toxoplasma from undercooked meat, unwashed fruit and vegetables and raw milk. The danger here is to an unborn child. Should the mother become infected during pregnancy and not been exposed to Toxoplasma before, no immunity is passed on. The consequences of this can be serious to the baby later on in its life.
Let’s get back to cats. You cannot catch Toxoplasma through normal contact with your pet cat. If your cat becomes infected for the first time, it will excrete eggs for 1-2 weeks. The eggs need to ‘mature’ for 1-5 days howeber, before becoming infectious to other animals. So a twice-daily cleaned litter tray represents minimal risk (but expectant mothers should still take precautions). Will the infection harm your cat? Very rarely, but it does have the capacity to cause eye, brain, liver and lung disease. In humans and dogs, neurological symptoms as a result of infection are similarly rare but can still be serious.
Hope I haven’t scared you all too much…I’ll lighten things up by mentioning some recent research that shows just how clever Toxoplasma might be. Rats infected with this parasite change their behaviour in quite the most remarkable way. They become more risk-averse and even find cat urine (usually repugnant to all of us) attractive. Why? Because it increases the chance of becoming a cat’s snack, infecting the cat that then excretes millions of eggs, and the life-cycle is complete.
The researchers found that this devil-may-care attitude is in part produced by increased dopamine levels in the infected rat brains, changing their behaviour. What’s even more interesting is the possibility of a similar effect in humans! There is some evidence that people with Toxoplasma exposure are more likely to be involved in traffic accidents and find the odour of cat urine more enticing. How about that? Whether this has a bearing on the number of cats an owner has is perhaps another question.
Clearly, more work needs to be done. In the meantime, don’t feed your cat raw meat, and if you start finding tom-cat smell attractive, drive even more carefully!
Written by Mark Newton-Clarke
