From the worm to the mite, from the single cell of sinister purpose to the complex biological machines that dig, crawl, swim and fly, parasitic organisms have been around more or less since there were other living things to mooch off of. That they have evolved so widely, yet with such singular adaptations to the hosts they invade, is testament to the boundless specialization that can be achieved by creatures often so simple in their own design it almost defies rational analysis. Almost, but not quite, as parasitologist Rosemary Drisdelle makes abundantly clear in her new book Parasites: Tales of humanity’s most unwelcome guests(University of California Press). Her research spans examination of Biblical pestilence, wars, the environmental impact of human migration and industrialization, and detailed analysis of modern food and water supplies that are will make you (briefly) never want to eat or drink again. More importantly, however, she digs deep (sorry about that) into the complex and sometimes even beneficial relationships between parasites and their hosts, which have induced numerous positive developments in agriculture, infrastructure, and even law enforcement (CSI, eat your heart out—oops). So for all those who skip over John Hurt’s big dinner scene in Alien, sink your teeth into this book, and some well-cured beef jerky, if you dare.
1. You make the case that parasites acclimated to humans quite quickly. Given that humans have been around only a fraction of the time that the invertebrate orders containing parasitic organisms have, how do you account for the speed of this familiarity?
That’s true for some parasites, but not for others.
We can divide the organisms that parasitize humans into two groups, sometimes referred to as heirlooms and souvenirs. The heirlooms evolved with us (and with our nonhuman ancestors) and thus had millions of years to perfect a relationship with us. We co-evolved. The souvenirs have been acquired more recently, often through close contact with other species. When a newly acquired parasite successfully infects humans, it is probably because we are similar enough to the original host to make that possible. Unfortunately, infection with a parasite that hasn’t evolved with us often results in severe disease.
To use an example, there are two species of hookworm that routinely infest human intestines. Necator americanus is thought to be an heirloom parasite that evolved with us in Africa, while Ancylostoma duodenale is thought to be a souvenir, acquired in East or Southeast Asia. Predictably, A. duodenale causes much more severe disease, and at much lower numbers of worms than N. americanus does. Thus, you might say A. duodenale has acclimated to humans, but the relationship is still not a happy one for us.
2. How old is parasitism? Put another way: when autotrophic and heterotrophic organisms first diverged, which came first—the predators or the parasites?
Fascinating question! Parasitism is virtually as old as life itself. When the first single celled organism engulfed another single celled organism and the engulfed one didn’t die, that may have been the first parasite. In that scenario, the predator came first, but just barely. It might also be the case that that original host was invaded by another organism, in which case, the parasite was first. No one knows for certain of course.
Even more fascinating, at least to me, is the theory that eukaryotic cells—the cells that make up our bodies—are the result of the scenario described above. Organelles such as mitochondria, vital to the functioning of our cells, are probably the descendents of organisms that were engulfed, or that invaded, and then took up permanent residence inside the cell.
3. What is the distinction between disease-inducing parasites and extant viruses or bacteria?
This question leads us into a tangle of definitions. A basic definition of a parasite is: an organism that lives on, or inside, another species, and obtains everything it needs from that host species. Normally, there is some damage to the host. Taken literally, this would include quite a few bacteria, fungi, and viruses. (I’m reminded of the debate as to whether viruses are actually alive, but let’s not go there now.)
For convenience, in medicine and often in science, parasitology includes the protozoa, helminthes (worms), and ectoparasites such as lice, fleas, and mites. It doesn’t include bacteria, fungi or viruses. Within all these groups, there are some that cause disease and many that don’t.
4. What are the epidemiological criteria for tracing an outbreak of parasite-induced disease back to its source (as opposed to a viral or bacterial one)?
I’m not an epidemiologist, but I can say that these two processes would have many things in common. Remember that a parasite in a human could be a protozoan, a worm, an insect, an arachnid etc. Some require an intermediate host or a particular environment outside the host, while some can pass directly from one host to another. Thus, any epidemiological approach would begin with identification of the parasite involved and work from there.
One interesting example of an epidemiological puzzle is the outbreak of toxoplasmosis that occurred in British Columbia, Canada, in 1995. This disease is typically acquired through contact with cat feces or from eating certain foods, but in the 1995 outbreak there was no such identifiable common source. The answer came when investigators plotted cases on a map and saw that the patients shared a common water supply.
5. Your description of the yawning gaps in food and water supply supervision through which parasites can slip is as horrifying as your summary of the 1990-91 Libyan screwworm eradication program is inspiring. How high a priority is parasite control for governments, and which ones are doing a better job?
One could answer this one briefly but accurately by saying “parasitic diseases are diseases of poverty.” To expand on that a bit, on the whole, developed nations have succeeded in making our lives parasite free. It’s also true, however, that developed nations tend to be in regions where the human parasite burden is already far lighter than in the tropics. While there are some success stories, most developing countries, typically home to many endemic parasites, can’t make much progress without a better standard of living and international aid.
One thing that has made a difference is the mass movement of people (for example the large number of people moving north from Central and South America). Because they bring their parasites with them, this has brought some formerly neglected parasitic diseases such as Chagas disease and neurocysticercosis into the spotlight in the developed world. International cooperative efforts at control are becoming more common.
6. It seems as though the fortunes of parasites have always been intertwined with the economic progress of man (Xenopsylla cheopisbeing a prime example). Given today’s darkening global economic climate and the budget-slashing austerity measures being implemented by one government after another, what is the prognosis for parasite infestation for the near future?
Parasitic diseases can surge when people lack access to health care, rely on inadequate housing, change the environment, or turn to illegal activities such as unsanctioned trade in animals. Thus, economic pressures such as budget cuts can lead directly to an increase in parasites.
I suspect that, until the economic situation improves, things will remain much the same as they are now or worsen slightly. International aid will soften the effect somewhat. I hope that the commitment remains solid for efforts already in progress, such as the Roll Back Malaria program and the Guinea worm eradication effort. So far, I think this is happening.
7. Your depiction of forensic parasitology in law enforcement is fascinating. Is this in widespread use?
Forensic parasitology typically happens by chance rather than design and cases are few and far between. If a parasite is involved somehow in a forensic case, and someone is astute enough to recognize it, the parasite can tell investigators a lot. I’ve taught some forensic parasitology at the university level and beyond providing basic knowledge of parasitology, my message is usually “expect the unexpected and if you think it’s a parasite, investigate it thoroughly.”
8. Is there such thing as a mutually beneficial relationship between parasite and host, or is mutualism by definition a wholly different concept?
Not too long ago, we would have said they’re different things, but as we discover so often, it’s not that simple. It’s becoming clearer all the time that despite the misery they cause, some parasites provide some benefit to the host. In particular, our heirloom parasites (discussed in the first question) appear to be important for the development of a healthy immune system. I think it shouldn’t surprise us that there’s a complex give and take relationship between us and organisms that have been with us for millions of years.
9. The pejorative application of the term “parasite” to a person of atavistic social, political or economic ambitions is at least as old as ancient Greek drama, and perhaps even older. How and why do you think this particular slur evolved?
The Greek word “parasite” can be translated as “beside food.” I believe that there was no negative connotation at the beginning but, over time, the term was applied to unwelcome dinner guests rather than welcome ones. The similarity between people who eat at our expense, and organisms that live at our expense was noted later, and the parasite label stuck as a scientific term. Add to that the prevalent work ethic of today—everyone should provide for him- or herself and not drain the resources of others—and parasite becomes an entirely negative label from a social point of view. These days, it’s commonly applied to anyone and anything perceived to use others, from pimps to politicians. The same brush blackens all parasitic species.
I find it interesting when every year around Christmas time, articles appear pointing out that mistletoe, a plant cherished by millions, is a parasitic plant in its natural environment. The implication is often that we should not like mistletoe because of this. That’s obviously an error in logic: mistletoe has no need of a social conscience. If parasitism was not a successful and comfortable way to live, there would not be so many parasitic species of all descriptions. They are in the majority—it’s good work if you can get it.