When we think of animals in the African Sahara, we picture herds of zebra, families of elephants, and flocks of birds. We see animals travelling in groups because we assume that there is a safety to large numbers or a mating and breeding advantage. But underlying this assumption is the notion that these animals are acting of their own accord. We assume that like us, animals are in control of their own behavior.
But are they really?
Artemia, a brine shrimp, more commonly known as the sea monkey, typically lives a solitary life in the depths of the ocean. However, these shrimp are occasionally found in large red swarms that sometimes span for meters. Biologists have been puzzled by this phenomenon, and after years of research, have found that their change in social behavior is due to a manipulative parasite. A parasitic tapeworm invades the shrimp’s system and not only drains nutrients, but also changes its body color from transparent to bright red. Additionally, it causes the shrimp to swim in large groups. The reason for this is that ultimately, the shrimp are just a temporary stepping stone in the tapeworm’s ultimate voyage toward reproduction, which it can only do in the intestine of a flamingo. Thus, to get there, it manipulates its shrimp-host into forming conspicuous swarms that are easy for flamingo to spot. Instead of the shrimp acting socially of its own accord, it is being controlled by an insidious parasite.
Another example of a parasitic manipulation is the suicidal cricket. When a cricket swallows the larvae of a certain worm, the worm hatches and grows within the cricket’s body. The worm, however, can only mate and reproduce in water, so it releases proteins that addle the cricket’s brain, causing it to behave unpredictably and irrationally. Thus, when the cricket nears a body of water, it jumps in and drowns itself, and the worm, wriggling out of its host, mates in its ideal habitat.
Examples of parasitic hijacking are numerous, which leads to the terrifying question of just how prevalent these occurrences are, and just how much influence they have over living organisms. After reading about these phenomena, I would like to find solace in the fact that these high tales of mind-control are mere outliers of the natural world that oppose the norm.
However, Kevin Lafferty’s research group found that parasites exist in extreme abundance. Just a single species of parasites called trematodes are microscopic on an individual level, but collectively, weigh as much as all the fish in the coastal body of water he was studying. In addition, Japanese scientist Takuya Sato found that just one stream he was studying contained so many corpses of the above mentioned suicidal cricket that the weight of the drowned insects made up 60% of the diet of local trout. Therefore, manipulation is not the oddity. As disquieting as it sounds, it is critical and common in the natural world around us.
What makes these stories about parasites so sinister – and yet so compelling – is that they expose our most primal fears of being stripped of the independence and free-will we pride ourselves on. While we, humans, are no strangers to manipulation – taking drugs to shift the chemistries of our brains to change our moods, and succumbing to advertisements that are designed to influence our thinking – we have yet to experience the fine-tuned artistry of parasitic control. The premium we place on our own freedom is manifest when the thought of losing it is written in literature as the stuff of the darkest fiction. We simply cannot fathom the possibility of being controlled by anything other than our own minds. And so we wonder – with tentative and halting breath – whether there are, in fact, incidences of sinister parasites affecting human behavior.
One parasite, Toxoplasma gondii, or toxo, for short, infects a wide variety of mammals, but can only reproduce inside a cat. Scientist Joanne Webster has found that when toxo gets into a rat or a mouse, it chemically alters its brain to deliberately seek out the smell of cat urine, rather than to run away from it. Thus the cat preys on the mouse, and toxo, now at its final destination, reproduces. The eerie thing about this example is that cats, like people, are mammals. We are made of the same basic structure, the same type of cells, and we have the same chemicals running through our brains. It’s not surprising, therefore, to learn that toxo exists in humans as well.
According to the CDC, more than sixty million people in the United States may be infected with the toxo parasite. Similarly, serological studies estimate that up to one third of the world’s population has been exposed to and may be chronically infected with toxo, although infection rates vary by country. Though it is not connected to any overt and deathly illness, it can manifest itself in those with compromised immune systems, such as infants and pregnant women. The most common ways that toxo is transmitted include: consuming raw or undercooked meat containing T. gondii tissue cysts, by ingesting water, soil, or vegetables contaminated with cysts shed in the feces of an infected animal, from a blood transfusion or organ transplant, or trans-placental transmission from mother to fetus , especially in a case where T. gondii is contracted during pregnancy.
Further research has shown that toxo infection can cause changes in personality and some researchers even say that those with schizophrenia are more likely to be infected with toxo. Therefore, while evidence and data are still highly inconclusive, it would be implausible for us to think that humans are resistant to parasitic manipulation. We too, are affected, and it is worth recognizing the beauty of their prevalence, to explore these invisible critters that crank the wheels of Mother Nature, and shape the world as we know it.