My leisure reading tonight was about Toxoplasma gondii, a unicellular parasite which infects mammals. It can replicate asexually in any mammal, but can only mate and reproduce sexually inside a cat.
Cats usually become infected by eating a prey animal in which T. gondii has spread into muscle tissue and formed dormant tissue cysts. The parasite then reproduces in the cat and is shed in its feces in the form of oocysts. If another mammal ingests the oocysts, the cycle starts again.
Alternately, non-feline carnivores can become infected directly by eating the tissue cysts.
Humans can acquire the parasite from exposure to cat feces. (Oocytes are shed mainly when the cat has first become infected, for about the first week or two, so kitten poop is more dangerous.) As omnivores, humans can also ingest tissue cysts in the muscle of another infected animal. Pork and lamb have the worst safety record.
But it was the story of an experiment which astounded me. In a Paris orphanage, 10% of the children had detectable T. gondii. After being fed rare steak and horsemeat for a year, the infection rate went up to 50%. Then rare lamb was added to the diet, which brought the infection rate up to 100%. The old, dark days of experimenting on orphans. It was 1965.
In most cases, there are no obvious symptoms of an infection, but sometimes people can have a severe reaction, called toxoplasmosis, which can be like a bad dose of the flu lasting four months.
Heating to 65 celsius will fairly reliably kill the parasites in their tissue cyst state, but cooking a steak “medium rare” or less usually means that the centre stays below 60 celsius. You might as well rub your steak with cat shit.
It has been found that rats with the parasite behave differently to non-infected rats. They have less fear and less aversion to the smell of cat urine. T. gondii achieves this by manipulating the epigenetics of their neurons, obviously an evolutionary adaption which makes the rat more likely to be eaten and the parasite passed on. It hasn’t been shown definitively yet, but some studies indicate that there are psychological effects in humans as well.