Venom the movie...REAL??? not clickbait almost died

The products of evolution are fascinating and complex. Life intersects in countless interlocking processes, and we find many of these to be beautiful. There are some ways of life, however, that are less photogenic, and those are the kind I want to talk in this post. Specifically, one of the reproductive strategies we find most disturbing: behavior-altering parasitism, in which the behavior of the host is altered to maximize the success of its parasite.

(like this guy)

So what does that mean? In this case, we’re not talking behavior like the usefulness of a cough for spreading germs. We’re talking a perceptive shift that changes the way that that host interacts with the world. For most of us, the most familiar example is rabies. The basic physical symptoms of the rabies virus are fever, headache, and vomiting, with partial paralysis at later stages, but behavior is also affected, causing anxiety, confused agitation, and hyperactivity. Weirdly, the infected develop a fear of water. Nocturnal animals become active during the day. Behavior becomes increasingly reckless. 

Saliva collecting in the mouth is probably the most well known symptom of rabies.

This all happens for a reason. Digging a little deeper, the direct correlation between these symptoms and transmission of the parasite is obvious. The deranged nervous behavior makes it much more likely that the infected animal will pass on the rabies to another host, while the aversion to water and drink in general keeps saliva (through which rabies is passed) collected, pooling, in the mouth. All the better to infect you with! 

The Baculoviridae family of viruses cause even grosser effects. Acting on caterpillars, they cause their hosts to gorge themselves providing nutrients for the parasite. When the virions (virus bits) are ready to leave the host, the caterpillar’s cells begin producing enzymes that “dissolve the animal into goo.” This scatters clumps of infected material that can be ingested by future hosts! (Yay recycling!) 

Another great example of parasitically altered behavior is toxoplasmosis. Rodents that consume fecal matter from infected animals become more reckless and less fearful of predators-- even, dramatically, attracted to the scent of cat urine. 

(Interestingly, mice that have been cured of toxoplasmosis remain unafraid of cats, suggesting a lasting structural change in the brain.) 

This all, of course, makes those infected mice much more likely to be eaten by a cat: the definitive host. Definitive hosts are what you might call final or “preferred” hosts, a stable environment in which the parasite can kick back, reach maturity and reproduce. 

Not only mice and cats are affected by toxoplasmosis. Considered a neglected parasitic infection in the US, it causes "mild flu-like symptoms" at worst in those humans whose immune systems are able to battle the parasite. Most immune systems are; according to the CDC, over 40 million people in the States carry the parasite, with only a tiny fraction showing symptoms. But it can cause death if your immune system is compromised by something like AIDS, and if you become infected while pregnant (or just before you get pregnant), the results for your kid could be equally serious. The effect could be as obvious as losing the baby, or symptoms (seizures, blindness, or mental disability) might only surface in that child years after birth. Some scientists think that the effect of toxoplasmosis on the general (human) population is even more staggering. One fairly prolific proponent of this is Jaroslav Flegr, a parasitologist and evolutionary biologist who has written about toxoplasmosis' influence on everything from traffic accidents to sex ratios, mental illness and human personality. Much of this writing, to summarize very simply, discusses the increased recklessness also seen in infected mice. 

This effect on the mice is called parasite-increased trophic transmission, in which the host behavior is altered to make that host more likely to be eaten by an animal at a higher trophic level (in layman’s terms, a predator), thus passing on the parasite through ingestion (in layman’s terms, eatin it). 

Behavior-altering parasitism is a wide-ranging strategy, not only exhibited by viruses like rabies and protozoa like Toxoplasma gondii, but crustaceans, insects, and parasitic worms (like leeches and tapeworms). 

(above) A horsehair worm with cricket host.
(below) A flatworm (green dude) with a snail host 

(both images from wikipedia)

It can also lead to more complex behaviors than increased recklessness. For example, the larvae of the wasp Hymenoepimecis argyraphaga are grown in Leucauge argyra spiders. Before they finish killing the host, those larvae inject the spider with a chemical that alters weaving behavior. The spider, before dying, weaves a different sort of web for the larvae to pupate in, one specifically to keep their cocoons supported and dry. These more complicated changes in behavior are what are most spectacularly cool to me. 

Another very dramatic change in behavior is caused by the Cordyceps genus of fungi. Acting on insects, it is also known as the zombie ant fungus, due to its effects on its host. Spores attach to the ant and "dig" down into its exoskeleton, where they spread out along the muscles. David Hughes (an entomologist who's studied zombie ants) describes how the fungus replaces the ant from the inside out: "We found that a high percentage of the cells in a host were fungal cells. In essence, these manipulated animals were a fungus in ants’ clothing." The results of Hughes' team's research suggest that the fungus directly manipulates the legs and mandibles of the ant, like a puppeteer, and that a "large proportion" of the fungal cells seem connected to each other in a 3D network, working collectively to control the different behaviors required from the different parts of the host's body. Infected ants are moved to ideal places for the fungus to reproduce, over time (perhaps even years) creating mass graves of strange, overgrown little corpses. Cordyceps out of control can destroy an entire colony of ants, robbing itself of its own legs, so to speak, and so there is a balance struck by the ecosystem that continues to be researched, where just enough ants are infected to help the fungus grow without stranding itself. (This is really cool- other fungi that don't harm the ants help protect their colonies from cordyceps.)

We don’t understand the mechanisms behind every kind of behavioral change yet, but one that isn't too difficult to wrap our brains around is altered neurochemical communication. So what does that mean? The brain is a physical thing. Thought and action both start as energy between neurons. If you alter the brain’s ability to communicate to the body or itself by messing with the chemicals that allow that communication, you alter the behavior. For example, the emerald cockroach wasp injects venom directly into the brain of its host, messing with the chemical balance and therefore with the brain’s responses to threatening stimuli (the escape! responses triggered by something frightening, like a wasp trying to inject stuff into your head). 

(image from wikipedia)

The host becomes usefully paralyzed without any interference on the part of the wasp to the parts of the brain responsible for movement control. Put simply: the physical ability of the host to move has not been touched. What has been changed is that the host is no longer stimulated to move-- it has no motivation, there is no incentive to react. In its way, this solution is as satisfyingly elegant as anything else evolution has produced. Energy is more efficiently spent changing the host’s innate reaction to stimuli--cutting off behavior at the source-- than simply handling the behavior once it happens. 

We often like to consider our rational thinking as something divorced from our physical selves. These parasites remind us that even the mind is just a collection of physical components-- hormones, proteins, impulses, chemicals-- that can be attacked by an outside force, just the same as our muscles and organs. Isn’t it cool?