As I was writing last week’s piece about mind-bending parasites, I came upon many fascinating reports about microorganisms (parasitic or otherwise) in relation to humans. So I could not resist diving into some of these intricate relationships that affect us all quite directly.
We are surrounded, inside and out, by trillions upon trillions of microorganisms; some are parasitic, others are mutualistic, but the distinction between these two gets blurred in some cases. For example, humans are subject to various types of parasitic worms that create an array of afflictions, but certain of these worms are reported to have a side benefit, by increasing our resistance to asthma and perhaps diabetes. So they cannot be said to be clearly “bad” (parasitic) or “good” (mutualistic). And the more we learn, the more confusing it gets.
Viruses are arguably the quintessential parasites. They are little more than bags of genes that cannot replicate themselves except by invading everyone else, from bacteria to plants to bugs and vertebrates. They have been doing so for a very long time, probably for as long as there have been bacteria and other organisms on earth. Studies have shown that humans carry a number of permanently inserted viruses, many of which are shared with our not-so-distant relatives, the monkeys and apes, and thus have been in our lineage for at least 40 million years. And I reckon more research will be able to trace things back much farther.
Viruses insert themselves into the DNA of others and use the cellular machinery of the host to replicate themselves so they can then spread to other hosts. If the inserted viral DNA is not disabled or converted to new functions by interaction with host genes, it can still make new viruses and jump to new hosts — or reinsert itself into the host DNA in a different location on a chromosome. The much-studied human immunodeficiency virus (HIV) can insert itself into human DNA at different parts of the chromosome, and the progress of the disease depends on just where the virus is inserted.
Sometimes, instead of just making new generations of virus that jump into new hosts, the viral DNA stays permanently inserted in the host chromosome. If that host chromosome happens to be in an egg or sperm cell, the viral DNA can end up in an embryo, which will carry the viral DNA in its own eggs or sperm, and so on down the generations. Recent studies have shown that as much as 8 percent (and possibly much more!) of the human genome is made up of viral DNA that came to stay. Many viral genes appear to be inactive, at least for now. But they can be reactivated, perhaps by another pathogen such as a flu virus.
Historically, viral genes are thought to have been central to the evolution of the placenta — that critical physiological link between mother and fetus — and thence to the subsequent evolution of most of the mammals. I can’t resist presenting that intriguing thought, but the story is too complex for this space.
Viruses and other parasites trigger human immune systems to fight back. There is increasing evidence that immune systems may also have effects well beyond fighting off parasitic invasions; sometimes immune systems produce dramatic changes in behavior, including social behavior, and these effects may also be exacerbated by enzymes secreted by the parasites. Here is an example:
Toxoplasma gondii is a protozoan (single-celled organism) parasite that infects rats and mice (and sometimes birds) as intermediate hosts, invading the cells of many parts of their bodies. Cats, which often eat rodents and birds, are the usual final host where the parasite can complete its life cycle. Toxoplasma manipulates the behavior of rats and mice in ways that increase the probability of transmission to a feline host: the rodents become more exploratory, aggressive, risk-taking, and even are attracted to cat odors. Toxoplasma reproduces in the intestines of cats and is passed out in feces. Humans that handle cat litter of infected cats may accidentally come in contact with cat feces; an unknowingly contaminated hand could easily pass the parasite to the mouth and thence to the digestive tract, where it prospers; from there it invades and persists as cysts in the cells of other parts of the body, including the brain. But humans can also pick up Toxoplasma in many other ways, including eating contaminated vegetation, drinking contaminated water and eating under-cooked and raw meat (from contaminated animals); some reports suggest it can be transmitted from one human to another during unprotected sexual intercourse and by blood transfusions. I would guess the human fecal matter could also contaminate water, vegetation or other persons. Toxoplasma infects a large proportion of the global human population; one estimate puts the global average at 30 to 40 percent. Toxoplasma is carried by the great majority of people in some European countries where eating rare and raw meat is customary, as well as people in some parts of Africa and Latin America.
There appears to be little physical effect of Toxoplasma on otherwise healthy hosts, and some persons are genetically resistant to it, but the parasite can be reactivated in persons with damaged immune systems. However, the potential mental and psychological effects are legion. Humans can pass on the infection from mother to fetus, at least sometimes leading to impaired mental function of the child’s brain. Infected humans tend to have slower reaction times (and have more car accidents). Toxoplasma infection is reported to be a major risk factor in the probability of developing schizophrenia and perhaps bipolar disorders, dementia, depression and many other psychological afflictions. The effects of Toxoplasma infections tend to differ between the sexes: males are said to become more vigilant, more suspicious, more dogmatic, less ethical, have lower self-control and are more likely to disobey rules; females become more outgoing, conscientious, persistent, mellow, and ethical. Mind you: these are tendencies, not expressed to the same degree in all persons!
The many trillions of symbiotic microorganisms (hundreds of kinds of bacteria, in this case) that customarily inhabit our guts synthesize vitamins (B and K), breakdown dietary fiber into absorbable bits, maintain the gut lining, provide some protections against pathogens, and help the immune system control inflammation. But those bacteria don’t stop there. The composition of the community of bacteria in the gut normally varies with age, diet and other individual features. But disruptions of the normal composition can lead to an array of unpleasantnesses such as cancer, obesity, diabetes and bowel disfunction. Furthermore, bacterial activity can turn genes on and off, change the neural interactions among different regions of the brain, and produce compounds that permanently affect cognitive function, social behavior and stress management. In short, part of what we refer to as personality is really a result of the condition of our gut microorganism community. There are even indications that gut bacteria have effects on insomnia, depression and Parkinson’s disease, but more research is needed.
In this essay I have ventured far beyond my usual focus on observations of the natural world around us and turned the focus inward. After all, this is all a part of our natural history, too. Each of us is partly a result of the viruses stuck in our DNA, the parasites inhabiting our bodies and the bacteria in our guts. And that is also true for the organisms around us. I’ve presented only the tip of a very large iceberg, as it were; researchers are busily exploring more and more dimensions of these interactions.
• Mary F. Willson is a retired professor of ecology.
Read more Outdoors:
Pondering the infinite in Yukon Flats
