We tend to think of a parasite as:
a) a disgusting wormy-squiggly thing that inhabits our bodies and gives nothing in return, or
b) our 24 year-old nephew who is still couch surfing in our living room and continually raiding our refrigerator.
For (a) at least, the relationship might be more complicated than we think. I'd like to take a running start at this explanation, from about 500 million years ago. Paleoparasitologists have found fossil evidence of parasites dating back to the lower Cambrian era. This means that both the host and the parasite have spent a long time climbing the evolutionary ladder together and have developed a remarkable balance. Twenty-five percent of the world's human population have parasites. In more primitive settings the percentage approaches one hundred. And, it turns out, even parasites have parasites.
If a parasite is clever, it will develop a means to stay on or in its host without killing it. And that's where some of the latest research is showing that some parasites do give something back in return. But before I press on I want to say that many of the parasites out there are truly hideous (have a look through a parasitology text book) and do not follow the guidelines for parasitic civility: they can and do kill, especially in a host who has a weakened immune system, is malnourished, or for some reason has a very heavy parasite burden.
A good example of clever is the hookworm. Each worm that attaches to the intestine sucks out about 0.1 ml of blood per day. Hey, what's a dab of hemoglobin shared between friends? Our immune system mounts a strong attack on the worm. And now here's the clever part: the worm has developed the ability to modulate and tamp down our immune response so we don't kill it. In rat experiments, scientists have shown that this kind of down-regulation of the immune system actually helps in conditions that are autoimmune-based, such as allergies, asthma, lupus, rheumatoid arthritis, and type 1 diabetes. In 2010 NPR aired a radio segment on Jasper Lawrence, whose life was being ruined by his severe asthma and allergies. After all the usual medical treatments failed, he read about this immune phenomenon, took a trip to Africa, and purposely infected himself with hookworm. He reported marvelous improvement. So it seems that the host-parasite relationship isn't just one way. The current hygiene hypothesis of illness suggests that we could all use a bit more dirt and scum in our lives.
However, before you jump online and order larvae from the Burpee Parasite Catalog (or start asking questions like, "are three enough....are six too many?") please remember that much more research needs to be done before we can be sure what is beneficial vs. harmful, here. For now this development in our understanding of parasites in our lives is food for thought (so to speak). Whether human, horse, hookworm, or hantavirus....the tapestry of life is woven with infinite complexity.
(Originally published November 2013)
