kw: book reviews, nonfiction, animals, parasites
'Big fleas have little fleas
upon their backs to bite 'em.
Little fleas have lesser fleas,
and so ad infinitum.'
This paraphrase of a classic ditty by Jonathan Swift expresses the fact that much more than half of all species are parasitic. Just considering the 1.8 million known species of eukaryotic life, most are parasites. And as the ditty says, parasites have parasites. I thought of putting a couple of photos with this article, but I am squeamish, and couldn't bear to spend any time sorting through the images that are publicly available.
Parasites: Tales of Humanity's Most Unwelcome Guests, by Rosemary Drisdelle, a Canadian clinical parasitologist, deals nearly exclusively with parasites that cause human disease. This keeps the book within bounds; stories of parasites that afflict our livestock (from bees to bovines), plus wild animals and plants, could easily fill a five-foot shelf.
There are parasites—technically commensals—that cause us no harm, such as the tiny Demodex mites that live in the hair follicles of all human faces. These get a brief mention in the book, but the focus is on the ones that do damage or kill us, from tiny protozoans such as Malaria organisms to yard-long Guinea worms and even longer tapeworms.
I once read that if you went outside, away from the paved existence we suburbanites live, and then all matter except nematode worms was removed, the scene would look more ghostly, but almost exactly the same. The ground, the plants, the animals, and possibly you, would be outlined, and usually permeated, with tiny worms. The common lab animal C. elegans is no longer than a millimeter and is nearly invisible, but your back yard contains billions of them. They don't parasitize humans, so any we carry are incidental to the fact that even the air contains a few floating around with the dust that we breathe. They die inside of us.
Many, many species don't. In fact, for most of the worm-type parasites, being breathed in or eaten by someone is their preferred way to invade. Their eggs are too small to be damaged by chewing, they are unfazed by our stomach acids, the waterborne ones brush off the Chlorine in our drinking water unharmed, so that getting into us is the easy part. Fortunately, particularly in the "first world", getting to us in the first place is much harder, which is why many of us are nearly parasite-free (except for those face mites).
Dr. Drisdelle begins her book with a fresh look at Jericho. The town was a fortified enclave surrounding a spring. Springs are good places for snails, and snails carry flukes and schistosomes and other bad actors. People with intestinal infestations shed the worms' eggs with every visit to the toilet, and in Biblical times, the toilets drained into the spring. Rahab and the rest of the townspeople were probably sick, and easy prey for the invading Hebrews, wall or no wall. Rahab saw the invaders as her ticket out of town, a place of sickness and early death.
At least half the human race lives in similar conditions today (at least 2 billion have no toilets whatever, using the ground "out back"). The amazing thing is that parasitic diseases afflict anything less than 100% of the population in the "developing" countries. For the more fortunate half, the good news is that filtered water is comparatively safe to drink, and most municipalities have at least minimal filtering of the water supply. The bad news is that breakdowns of the system are getting more frequent. Growing populations and aging infrastructure tend to collide in ways that lead to widespread diarrhea, at the very least, and coma or death for some.
Then there's your friendly neighborhood produce market. What is in the water that the store uses to spray the lettuce to keep it looking fresh? Where was it grown? What animals lived uphill from the field, and how healthy are they? And fresh meat: Do you dare to eat it in any way other than cooked well done?
My wife is paranoid about meat. She cooks hamburgers to leather, and prefers to stir-fry ground meats quite thoroughly. And with good reason. She had to be de-wormed once. Luckily her invader was one of those that a medicine can cure. Some invaders die only of old age, which can be 10-20 years unless the host dies first. Then, water: we filter all our drinking water, no longer trusting the municipality to keep it save 100% of the time.
Their long lives make elimination of parasitic diseases a very long-term proposition. The book's closing chapter explores efforts to eliminate several of the most damaging parasitic diseases. While rich countries have eliminated Malaria and a great many other fellow travelers, poverty and disease are still nearly inseparable. Sometimes it seems new parasites are arising faster than we can exterminate the older ones.
One minor caveat about the book: I am used to zoological terminology, but not all readers will know that the abbreviation "spp." means "species", plural, nor that when you see something like B. procyonis, it is shorthand for Baylisascaris procyonis, which will have been mentioned earlier in the section, and is a binomial species name. The book is a fascinating survey of the crawlies that we hate to think about. My fondest wish for you all is that "think about" is as close as we ever come to them!