Mind-bending parasites

I suspect that when most of us think about parasites, if we think about them at all, we conjure up a notion of something like a tapeworm or a tick, say ‘ICK’, and quickly go on to think about other things. For many decades, parasitologists have conducted intensive research on the fascinatingly complex life cycles of many parasites, often involving a sequence of several different body forms for the parasites and two or three kinds of hosts. Parasites have long been known to cause an assortment of infirmities in their hosts (blindness, liver malfunction, various diseases…the list is very long). However, research in the past thirty years or so has revealed that parasites need to be viewed in a much broader and deeper way: Parasites can interact with their hosts in insidious ways, often causing changes in behavior that are beneficial to the parasite but often not beneficial to the host.

There are now so many intriguing examples of mind-bending and behavior-changing by parasites that here I will feature some examples from non-human organisms. Humans are certainly subject to the same kinds of host-manipulations by parasites, and perhaps I’ll write a second essay presenting some cases that affect humans in serious ways. In many cases, perhaps most, the precise physiology by which the minds of the hosts are bent is still a subject of intense research, but the examples themselves should open some of our mental doors…

There is a wasp that parasitizes certain kinds of spiders, laying an egg in a selected host. The wasp larva somehow induces the host spider to stop weaving a web and start spinning a cocoon. When the cocoon is finished, the larva kills the spider and enters the cocoon, using it as housing while the larva transforms into an adult wasp.

Another wasp parasitizes woolly bear caterpillars by laying eggs inside. Parasitized woolly bears choose to eat a diet richer in carbohydrates than unparasitized woolly bears, which normally choose a more protein-rich diet. When the wasp larvae emerge from the carbo-loaded caterpillars (now dead), they are bigger than those from protein-fed caterpillars; the carbs are turned into high-energy fats, and the wasp larvae prosper from their enriched diet.

Yet another wasp uses cockroaches as a host. An adult wasp first stings a cockroach in the thorax, causing temporary paralysis. It then stings the helpless roach in particular portions of the brain, feeling around inside the roach’s head until its stinger finds the right places for egg-laying. After an egg is laid, the roach grooms obsessively and does not run away, even though it is then physically capable of doing so. Meanwhile the wasp goes off to find a burrow to store the parasitized prey. The wasp returns to the roach, breaks an antenna and drinks some blood, and hauls off the roach to the burrow. Wasp venom slows the roach metabolism so it stays alive long enough to be eaten (fresh!) by the larva.

And don’t forget Sacculina, the remarkable barnacles that parasitize crabs (and have appeared in these essays before). An infected male crab is unable to molt and grow, and is effectively castrated, becoming female-like in both shape and behavior—reportedly even courting and egg-tending like a female.

Still other parasites make their hosts behave in ways that increase the probability that the parasite gets transmitted to the next host or a better habitat. A spiny-headed worm infects some amphipods (small crustaceans), causing the hosts to move toward light (the water surface), where they are more likely to be eaten by ducks (or accidentally by aquatic mammals), which serve as the subsequent host. The larvae of a horsehair worm infect crickets and grasshoppers, causing them to jump into water, where the adult worm is at home. A parasitic fungus gets into certain ants, causing them to leave their usual habitat in the tree canopy and go someplace lower in the tree, where the fungus then erupts from the ants’ heads, showering its spores into salubrious habitat below. A certain fluke in an ant makes the ant crawl up to the top of a grass stem where it is likely to be consumed by an herbivore, which is the ultimate host.

A very famous example comes from a group of bacteria known as Wolbachia. These bacteria live inside the cells of many insects and other arthropods and even some nematodes (round worms). Because they live in the cytoplasm of the cells (not the nucleus), they are typically transmitted from mother to offspring, but sometimes may be carried by vectors (mites, parasitoid wasps?) that visit more than one host. There are many Wolbachias, infecting many different hosts, and their effects on the hosts are remarkably varied. They can affect reproduction by inducing parthenogenesis (females reproduce asexually, without males) or feminizing males (or sometimes killing off males altogether)—all of those effects would benefit transmission of Wolbachia and are probably not advantageous to the hosts. They can change egg-laying behavior and mating preferences, which might not benefit the hosts at all. Or they can change locomotor activity of the hosts such that the hosts do more foraging, which could benefit both Wolbachia and the host.

Viruses are intracellular parasites and many stories could be told about them. Here it is pertinent to note that recent research has found that when immune systems of both vertebrates and arthropods respond to a pathogen invasion of the host, the brains of the infected animals may be influenced in ways that affect their social behavior. This opens a door to the possibility of myriad fascinating links between parasites, immune systems, and behavior; the story will continue to unfold as research follows the links.

Advertisement

You’re not entirely what you think you are!

Usually I write about things that I like to imagine that I know at least a bit about. Today is an exception. I have little background in this subject, but some recent readings got me so interested that I’m writing about microbes—bacteria and viruses, which are too tiny to see without fancy equipment but which could be said to rule the world. The implications for medical matters of some relatively new information are enormous.

There are way more kinds of microbes, with lots more genetic diversity, than all other organisms on earth. The numbers are so big that most of us can’t really comprehend them. It turns out that microbes are involved with almost everything we are and do! Mind-boggling!

For starters, each of us harbors a couple of pounds of bacteria (as well as trillions of the even-smaller viruses, of which more, presently). We are all familiar with the idea that bacteria can cause unpleasant diseases, but many bacteria are actually beneficial. Some live on our skin, where they can help keep harmful bacteria at bay. Many live in our digestive tracts, where they perform major feats of breaking down food into usable particles. Bacteria produce helpful enzymes too, such as one that allows a baby to extract nutrition from milk—reportedly, no human gene encodes an enzyme that can do this.

Having the right assortment of resident bacteria can, at least sometimes, help determine if one is obese, diabetic, has high blood pressure or multiple sclerosis, for example, or if one escapes these conditions. Bacteria that affect metabolism may even be involved with the condition of autism, or the avoidance of same. It may become possible to combat a nasty sort of bacteria with the proper application of another one, which outcompetes or destroys the first one. The previously unsuspected roles of bacteria in many human ailments—or in our avoidance of them—are just starting to be explored, and the medical implications for treatments are fascinating.

Now consider viruses for a moment. Viruses consist of a small protein shell and usually just a few genes. They make their living, so to speak, by invading the cells of other organisms and taking control of the metabolic machinery in those cells, forcing the cell to make more viruses. However, the process of replication is sloppy, with lots of mutations, and if two viruses occur together in one cell, they may exchange genetic material. So new versions of viruses are emerging all the time. Sometimes the new versions find an opportunity to invade entirely different species, jumping from bird to human, or pig to primate.

Inside the cell, reproduction of the invading virus may be extremely rapid, eventually exploding the cell and releasing viral offpring. Flu viruses and the common-cold virus work this way. Other viruses don’t kill the host cell, but they speed up cell division and cause the host cell to make more cells. That’s the makings of a possible cancer. Whether or not a cancer actually develops, however, depends on many factors, including the specific kinds of cells involved and if a previous exposure has already alerted the immune system.

Surprisingly, viruses may offer a way to treat some bacterial infections. Many, perhaps most, viruses attack bacteria, and they are commonly host-specific, attacking only one kind of bacterium. These viruses are called bacteriophages, or phages, for short. So there is hope that medical treatments with phages might help control some bacterial infections. Of course, the bacteria can mutate, and some would probably become phage-resistant (just as happens with antibiotics). But phages can evolve too and might acquire a mutation that overcomes resistance by bacteria.

What really ‘blew me away’ was learning a little about retroviruses, which insert their genes into the DNA of the host cell. These viruses can cause host DNA near the insertion to make proteins, in effect turning on host genes that had been turned off, and cancer can sometimes result. But not inevitably!

After a retrovirus inserts itself into the host DNA, the host’s immune system may inactivate it or mutations may cripple it. The inserted but inactive retrovirus can spread harmlessly through the host as the host’s body makes new cells (which it does frequently). If the retrovirus gets into an egg or sperm cell, it can be passed on to offspring and to future generations.

We now know that this has happened many, many times in the long history of life on earth. Viruses have been around for billions of years. Retroviruses are found in all kinds of vertebrates, from fishes to humans. In fact, about eight percent of DNA in humans comes from retroviruses that long ago inserted themselves into our ancestors’ DNA!

Even crippled retroviruses can be dangerous, if a mutation re-activates them, or if they insert new copies of themselves into the host genome. So, not surprisingly in this co-evolutionary arms race, humans (and probably other animals too) have evolved at least a few means of defense, by producing certain proteins that disable the retrovirus’ process of replication.

It turns out that retroviruses are essential to human (and perhaps most mammalian) reproduction. Most species of mammals, including humans, feed a growing fetus by means of a placenta. A particular retrovirus plays a critical role in the development of a placenta, without which a fetus would quickly starve to death.

So, in effect, you are not entirely what you probably thought you are. In addition to truly human DNA from your parents, you are partly an assortment of good, neutral, and bad bacteria, some resident and some transient, and good, neutral, and bad viruses, some of them buried in your DNA. In short, each of us is an ecosystem (living in an environment that itself is a gigantic ecosystem). Just think: ecosystem management applied to medicine!