0:00:06.857,0:00:10.397 Is there a disease [br]that makes us love cats, 0:00:10.397,0:00:12.450 and do you have it? 0:00:12.450,0:00:13.570 Maybe, 0:00:13.570,0:00:16.011 and it's more likely than you'd think. 0:00:16.011,0:00:18.486 We're talking about toxoplasmosis, 0:00:18.486,0:00:22.608 a disease caused by toxoplasma gondii. 0:00:22.608,0:00:27.450 Like all parasites, toxoplasma lives[br]at the expense of its host, 0:00:27.450,0:00:30.451 and needs its host to produce offspring. 0:00:30.451,0:00:35.042 To do that, toxo orchestrates a brain[br]manipulation scheme 0:00:35.042,0:00:36.354 involving cats, 0:00:36.354,0:00:37.666 their rodent prey, 0:00:37.666,0:00:40.978 and virtually all other birds and mammals, 0:00:40.978,0:00:43.083 including humans. 0:00:43.083,0:00:47.856 Documented human infections go as far back[br]as ancient Egypt. 0:00:47.856,0:00:50.605 We found samples in mummies. 0:00:50.605,0:00:54.478 Today, about a third of the world's[br]population is infected, 0:00:54.478,0:00:57.260 and most of them never even know it. 0:00:57.260,0:01:00.760 In healthy people, symptoms often[br]don't show up at all. 0:01:00.760,0:01:04.197 When they do, they're mild and flu-like. 0:01:04.197,0:01:06.833 But those are just the physical symptoms. 0:01:06.833,0:01:10.477 Toxoplasma also nestles into our brains 0:01:10.477,0:01:14.149 and meddles with our behavior[br]behind the scenes. 0:01:14.149,0:01:18.928 To understand why, let's take a look[br]at the parasite's life cycle. 0:01:18.928,0:01:22.677 While the parasite can multiply[br]in practically any host, 0:01:22.677,0:01:27.298 it can only reproduce sexually[br]in the intestines of cats. 0:01:27.298,0:01:31.678 The offspring, called oocysts,[br]are shed in the cat's feces. 0:01:31.678,0:01:36.270 A single cat can shed up to [br]a hundred million oocysts. 0:01:36.270,0:01:40.728 If another animal, like a mouse,[br]accidentally ingests them, 0:01:40.728,0:01:45.486 they'll invade the mouse's tissues[br]and mature to form tissue cysts. 0:01:45.486,0:01:47.792 If the mouse gets eaten by a cat, 0:01:47.792,0:01:51.198 the tissue cysts become active[br]and release offspring 0:01:51.198,0:01:54.076 that mate to form new oocysts, 0:01:54.076,0:01:55.698 completing the cycle. 0:01:55.698,0:01:56.865 But there's a problem. 0:01:56.865,0:02:01.476 A mouse's natural desire to avoid[br]a cat makes it tough to close this loop. 0:02:01.476,0:02:04.762 Toxoplasma has a solution for that. 0:02:04.762,0:02:10.230 The parasites invade white blood cells[br]to hitch a ride to the brain 0:02:10.230,0:02:14.260 where they seem to override the innate[br]fear of predators. 0:02:14.260,0:02:19.178 Infected rodents are more reckless[br]and have slower reaction times. 0:02:19.178,0:02:24.479 Strangest of all, they're actually[br]attracted to feline urine, 0:02:24.479,0:02:28.263 which probably makes them more likely[br]to cross paths with a cat 0:02:28.263,0:02:31.363 and help the parasite [br]complete its life cycle. 0:02:31.363,0:02:33.791 How does the parasite pull this off? 0:02:33.791,0:02:38.837 Although the exact mechanism isn't known,[br]toxo appears to increase dopamine, 0:02:38.837,0:02:44.013 a brain neurotransmitter that is involved[br]in novelty-seeking behavior. 0:02:44.013,0:02:48.859 Thus, one idea is that toxo tinkers[br]with neurotransmitters, 0:02:48.859,0:02:52.226 the chemical signals [br]that modulate emotions. 0:02:52.226,0:02:53.332 The result? 0:02:53.332,0:02:55.041 Fatal attraction. 0:02:55.041,0:02:58.704 But mice aren't the only animals[br]that end up with these parasites, 0:02:58.704,0:03:03.791 and that's where humans,[br]and all of toxo's other hosts, come in. 0:03:03.791,0:03:08.984 We can accidentally ingest oocysts[br]in contaminated water, 0:03:08.984,0:03:10.855 or unwashed produce, 0:03:10.855,0:03:12.892 or from playing in sandboxes, 0:03:12.892,0:03:15.362 or cleaning out litter boxes. 0:03:15.362,0:03:20.825 This is behind the common recommendation[br]that pregnant women not change cat litter. 0:03:20.825,0:03:23.709 Toxo can cause serious birth defects. 0:03:23.709,0:03:26.679 We can also get toxo [br]from eating undercooked meat 0:03:26.679,0:03:30.512 from other animals that picked up[br]some oocysts. 0:03:30.512,0:03:34.060 And it turns out that toxo can mess with[br]our brains, too. 0:03:34.060,0:03:38.292 Studies have found connections between[br]toxo and schizophrenia, 0:03:38.292,0:03:39.768 biopolar disorder, 0:03:39.768,0:03:41.643 obsessive compulsive disorder, 0:03:41.643,0:03:43.638 and aggression. 0:03:43.638,0:03:47.326 It also slows reactions [br]and decreases concentration, 0:03:47.326,0:03:51.727 which may be why one study found[br]that people involved in traffic accidents 0:03:51.727,0:03:56.846 were almost three times more likely[br]to have toxoplasma. 0:03:56.846,0:04:01.919 So is toxo manipulating our brains[br]as an evolutionary strategy 0:04:01.919,0:04:05.792 to get predatory cats to eat us? 0:04:05.792,0:04:09.367 Or are our brains just similar enough[br]to a rodent's 0:04:09.367,0:04:15.140 that the same neurological tricks that[br]lure them in catch us in the net, too? 0:04:15.140,0:04:21.196 And is toxo the reason so many people[br]love cats and keep them as pets? 0:04:21.196,0:04:23.888 Well, the jury's still out on that one. 0:04:23.888,0:04:26.697 Some recent studies [br]even contradict the idea. 0:04:26.697,0:04:31.358 Regardless, toxoplasma has definitely[br]benefited from humans 0:04:31.358,0:04:34.422 to become one of the world's [br]most successful parasites. 0:04:34.422,0:04:38.355 It's not just our willingness to let[br]cats on our dining room tables 0:04:38.355,0:04:39.793 or in our beds. 0:04:39.793,0:04:42.865 Raising livestock [br]and building cities which attract rodents 0:04:42.865,0:04:45.452 has provided billions of new hosts, 0:04:45.452,0:04:49.636 and you and your cat may be two of them.