1770: "When a Parasite Rewires Behavior to Spread Itself"

0:00Imagine getting so sick that your body should be telling you to stop eating, stop moving around, stop looking for a mate. Now imagine the infection is interfering with that warning system. That's what researchers found in Texas field crickets, infected with a virus known as Cricket or Ritovirus. The infection was doing serious damage. Female stop producing eggs and males still produce sperm, but much of it had little or no movement. But their sexual behavior didn't shut down. Infective males actually began courting females sooner than healthy males, and that's where this whole thing becomes much more than a story about sick crickets mating. Animals have something called sickness behavior. When the immune system attacks an infection, chemical signals help coordinate the body's defenses, but they also communicate with the nervous system. An animal might become sluggish, eat less, withdraw, and reduce activities like courtship. That's not just a weakness, it's part of the body's response to infection.

1:01Biologists, Shelley Adamo and her colleagues, they wanted to know why these infected crickets, they weren't doing it. So they gave healthy and virus infected crickets, heat-killed bacteria. The bacteria couldn't reproduce or cause another infection, but the immune system could still recognize it. The healthy crickets they reacted. The infected crickets, they didn't. They just found another clue inside their immune systems, activity of phenyl-loxidase, and enzyme involved in insect defense, it was reduced. The virus was also attacking the fat body, and organistores nutrients regulate metabolism and produces important components of the insect immune response. Something in the normal connection between infection, immunity, and behavior was disrupted. And then, here it comes. The virus spread through sexual contact. The female may no longer be capable of producing eggs, but she still made it.

2:02An animal may have a badly impaired sperm, but he can still court females. And for the cricket reproduction, it's breaking down. For the virus, the root now has another path to a cricket, another cricket, and maybe another. Others weren't claiming that the virus gave crickets a new sexual urge, the evidence pointed towards something subtler. Infection appeared to interfere with the normal system that suppressed courtship when the animal became sick, but instead of pressing the accelerator, think of it as disabling the break. Nature has produced far more dramatic versions of this. The fungus, Afio-cortiseps. Infects ants and altars their behavior before killing them in locations favorable for fungal reproduction. Hair worms. They develop inside insects like crickets, and they can alter their behavior ways to bring them towards water, where the adult worm needs to reproduce. Toxoplasma Gandhi.

3:03It can alter responses to cat's odors and infected rodents, potentially increasing the chance that a rodent is eaten by a cat. After the parasite completes the sexual stage of its entire life cycle, different parasite, different host, different mechanism, but the evolutionary principle is pretty similar. A parasite lives inside a biological machine that already knows how to walk, eat, mate, hide, hunt, and react to danger. Evolution doesn't necessarily have to build new behaviors from scratch, it simply begins to exploit the machinery that's already there. In downed fear, change where the animal travels. Interrupt the signal that says you're sick, keep a host active when it normally would withdraw. None of this requires the parasite to think or plan. Utations that happen to increase transmission can leave more descendants, and over generations, natural selection can preserve those traits.

4:03It's the disturbing brilliance of host manipulation. Sometimes the most useful thing a parasite can change is not the host's body. It's what the host does next. These are interesting things with JC.