1 00:00:06,857 --> 00:00:10,397 Is there a disease that makes us love cats, 2 00:00:10,397 --> 00:00:12,450 and do you have it? 3 00:00:12,450 --> 00:00:13,570 Maybe, 4 00:00:13,570 --> 00:00:16,011 and it's more likely than you'd think. 5 00:00:16,011 --> 00:00:18,486 We're talking about toxoplasmosis, 6 00:00:18,486 --> 00:00:22,608 a disease caused by toxoplasma gondii. 7 00:00:22,608 --> 00:00:27,450 Like all parasites, toxoplasma lives at the expense of its host, 8 00:00:27,450 --> 00:00:30,451 and needs its host to produce offspring. 9 00:00:30,451 --> 00:00:35,042 To do that, toxo orchestrates a brain manipulation scheme 10 00:00:35,042 --> 00:00:36,354 involving cats, 11 00:00:36,354 --> 00:00:37,666 their rodent prey, 12 00:00:37,666 --> 00:00:40,978 and virtually all other birds and mammals, 13 00:00:40,978 --> 00:00:43,083 including humans. 14 00:00:43,083 --> 00:00:47,856 Documented human infections go as far back as ancient Egypt. 15 00:00:47,856 --> 00:00:50,605 We found samples in mummies. 16 00:00:50,605 --> 00:00:54,478 Today, about a third of the world's population is infected, 17 00:00:54,478 --> 00:00:57,260 and most of them never even know it. 18 00:00:57,260 --> 00:01:00,760 In healthy people, symptoms often don't show up at all. 19 00:01:00,760 --> 00:01:04,197 When they do, they're mild and flu-like. 20 00:01:04,197 --> 00:01:06,833 But those are just the physical symptoms. 21 00:01:06,833 --> 00:01:10,477 Toxoplasma also nestles into our brains 22 00:01:10,477 --> 00:01:14,149 and meddles with our behavior behind the scenes. 23 00:01:14,149 --> 00:01:18,928 To understand why, let's take a look at the parasite's life cycle. 24 00:01:18,928 --> 00:01:22,677 While the parasite can multiply in practically any host, 25 00:01:22,677 --> 00:01:27,298 it can only reproduce sexually in the intestines of cats. 26 00:01:27,298 --> 00:01:31,678 The offspring, called oocysts, are shed in the cat's feces. 27 00:01:31,678 --> 00:01:36,270 A single cat can shed up to a hundred million oocysts. 28 00:01:36,270 --> 00:01:40,728 If another animal, like a mouse, accidentally ingests them, 29 00:01:40,728 --> 00:01:45,486 they'll invade the mouse's tissues and mature to form tissue cysts. 30 00:01:45,486 --> 00:01:47,792 If the mouse gets eaten by a cat, 31 00:01:47,792 --> 00:01:51,198 the tissue cysts become active and release offspring 32 00:01:51,198 --> 00:01:54,076 that mate to form new oocysts, 33 00:01:54,076 --> 00:01:55,698 completing the cycle. 34 00:01:55,698 --> 00:01:56,865 But there's a problem. 35 00:01:56,865 --> 00:02:01,476 A mouse's natural desire to avoid a cat makes it tough to close this loop. 36 00:02:01,476 --> 00:02:04,762 Toxoplasma has a solution for that. 37 00:02:04,762 --> 00:02:10,230 The parasites invade white blood cells to hitch a ride to the brain 38 00:02:10,230 --> 00:02:14,260 where they seem to override the innate fear of predators. 39 00:02:14,260 --> 00:02:19,178 Infected rodents are more reckless and have slower reaction times. 40 00:02:19,178 --> 00:02:24,479 Strangest of all, they're actually attracted to feline urine, 41 00:02:24,479 --> 00:02:28,263 which probably makes them more likely to cross paths with a cat 42 00:02:28,263 --> 00:02:31,363 and help the parasite complete its life cycle. 43 00:02:31,363 --> 00:02:33,791 How does the parasite pull this off? 44 00:02:33,791 --> 00:02:38,837 Although the exact mechanism isn't known, toxo appears to increase dopamine, 45 00:02:38,837 --> 00:02:44,013 a brain neurotransmitter that is involved in novelty-seeking behavior. 46 00:02:44,013 --> 00:02:48,859 Thus, one idea is that toxo tinkers with neurotransmitters, 47 00:02:48,859 --> 00:02:52,226 the chemical signals that modulate emotions. 48 00:02:52,226 --> 00:02:53,332 The result? 49 00:02:53,332 --> 00:02:55,041 Fatal attraction. 50 00:02:55,041 --> 00:02:58,704 But mice aren't the only animals that end up with these parasites, 51 00:02:58,704 --> 00:03:03,791 and that's where humans, and all of toxo's other hosts, come in. 52 00:03:03,791 --> 00:03:08,984 We can accidentally ingest oocysts in contaminated water, 53 00:03:08,984 --> 00:03:10,855 or unwashed produce, 54 00:03:10,855 --> 00:03:12,892 or from playing in sandboxes, 55 00:03:12,892 --> 00:03:15,362 or cleaning out litter boxes. 56 00:03:15,362 --> 00:03:20,825 This is behind the common recommendation that pregnant women not change cat litter. 57 00:03:20,825 --> 00:03:23,709 Toxo can cause serious birth defects. 58 00:03:23,709 --> 00:03:26,679 We can also get toxo from eating undercooked meat 59 00:03:26,679 --> 00:03:30,512 from other animals that picked up some oocysts. 60 00:03:30,512 --> 00:03:34,060 And it turns out that toxo can mess with our brains, too. 61 00:03:34,060 --> 00:03:38,292 Studies have found connections between toxo and schizophrenia, 62 00:03:38,292 --> 00:03:39,768 biopolar disorder, 63 00:03:39,768 --> 00:03:41,643 obsessive compulsive disorder, 64 00:03:41,643 --> 00:03:43,638 and aggression. 65 00:03:43,638 --> 00:03:47,326 It also slows reactions and decreases concentration, 66 00:03:47,326 --> 00:03:51,727 which may be why one study found that people involved in traffic accidents 67 00:03:51,727 --> 00:03:56,846 were almost three times more likely to have toxoplasma. 68 00:03:56,846 --> 00:04:01,919 So is toxo manipulating our brains as an evolutionary strategy 69 00:04:01,919 --> 00:04:05,792 to get predatory cats to eat us? 70 00:04:05,792 --> 00:04:09,367 Or are our brains just similar enough to a rodent's 71 00:04:09,367 --> 00:04:15,140 that the same neurological tricks that lure them in catch us in the net, too? 72 00:04:15,140 --> 00:04:21,196 And is toxo the reason so many people love cats and keep them as pets? 73 00:04:21,196 --> 00:04:23,888 Well, the jury's still out on that one. 74 00:04:23,888 --> 00:04:26,697 Some recent studies even contradict the idea. 75 00:04:26,697 --> 00:04:31,358 Regardless, toxoplasma has definitely benefited from humans 76 00:04:31,358 --> 00:04:34,422 to become one of the world's most successful parasites. 77 00:04:34,422 --> 00:04:38,355 It's not just our willingness to let cats on our dining room tables 78 00:04:38,355 --> 00:04:39,793 or in our beds. 79 00:04:39,793 --> 00:04:42,865 Raising livestock and building cities which attract rodents 80 00:04:42,865 --> 00:04:45,452 has provided billions of new hosts, 81 00:04:45,452 --> 00:04:49,636 and you and your cat may be two of them.