I’ve finally read a book by Carl Zimmer, after seeing his (and his brother’s) byline on numerous pieces online. For some reason I thought he had also written “Survival of the Sickest”, but that’s apparently by somebody whose name rings no bell. Much of the book is sort of a complaint that people haven’t found parasites very interesting even though they really really are (did you know that most species of animal are parasites? I didn’t), but I’m going to write about that things that stuck in my head rather than what Zimmer may have wanted to emphasize.

Bit one is that the term “parasite” is not used to apply to everything occupying what we might call the parasitic ecological niche. Only eukaryotes & multicellular organisms are given the name, for reasons of historical accident rather than because it carves reality at the joints. Zimmer says this lead to parasites being neglected by scientists, because europe is more afflicted by bacteria & virii while tropical reasons have “parasites” proper. This made me wonder why that was the case. The classic explanation for the prevalence of tropical disease is that many species have to live in water (it is useful for carrying in nutrients and carrying out waste) and warmth is also better for most life than cold. But I would think that would apply especially to single-celled organisms (and eukaryotes admittedly qualify), while colder weather is associated with larger size. I mentioned earlier William McNeill’s claim that microparasites result in less selection for IQ relative to macropredation, and it occurs to me that a number of the macroparasites in the tropics actually should be visible without a microscope. But perhaps the overall fitness cost from all entities in the “parasite” ecological niche is still lower in northern europe.

Continuing on with the eukaryotic distinction, Zimmer argues that such parasites are more likely to manipulate their host than bacteria/virii. The reasoning seems to be that many “parasites” proper need multiple hosts for the different generational stages of their life cycle (the different forms generations can take made it difficult to tell at first different organisms were one species and satisfy Koch’s postulates, I was surprised to learn that Plasmodium have a sexually reproducing generation inside mosquitoes since they are single-celled). Could it also have to do with the complexity of the organism allowing for more complicated effects on a host, or am I placing too much weight on a fuzzy notion of “complexity”? At an rate, I was under the impression that a number of venereal diseases like syphilis (bacterial) affected their host’s behavior to spread more easily, as discussed in one of the chapters from “The Man Who Mistook His Wife For a Hat” by Oliver Sacks. I assume most readers are familiar with the Red Queen theory of infections giving rise to sexual reproduction, but Zimmer adds that William Hamilton & Marlene Zuk also argued that showy displays for sexual selection would be particularly common in response to “parasites” proper because bacteria/virii tend to just kill their hosts or get promptly eliminated themselves. I don’t know why one would expect that to be the case. Helicobacter pylorii and other bacteria can persist in hosts for a long time, and H.I.V is a well known persistent virus. Furthermore, one of the most well known examples of showy mate display is the peacock’s tail, which I had originally heard was supposed to be a credible signal that the peacock was not afflicted with diarrhea, which can often result from bacteria such as cholera (though I don’t know if cholera specifically is a common affliction of peacocks).

That was the most interesting bit, the rest are just some assorted observations. The non-cell like nature of red blood “cells” (a corpuscle) has been discussed at this blog before, and it comes up again in the case of the Plasmodium protozoan parasite which causes malaria. It’s common to infect a regular cell, which has lots of machinery and activity that can be hijacked by an intruder. But red blood cells have hardly any of that, which is why Zimmer finds it so impressive that plasmodium are able to get such use out of it (the upside is that no major histocompatibility complex is produced to tip off the immune system). Zimmer’s description (mere “crates” pushed around by bloodflow) led me to be uncertain as to whether RBCs even had a metabolism, but apparently they do.

A major part of Ewald & Cochran’s “New Germ Theory” is disputing a common view that parasites & hosts are inevitably reconciled over ti me to a symbiotic form. The existence of organelles like mitochondria inside eukaryotes is itself thought to be the result of a parasitic-turned-symbiotic relationship in the distant past. I won’t go into that, but I felt like highlighting a couple of examples given of one species making use of another to prey on a third species preying on it. One surprising bit is that owls sometimes grab snakes and bring them to their nests, not to eat the snake but so that the snake can eat parasites living inside the nest. I guess eating the nest parasites is not the comparative advantage of the owl. I assume many readers are familiar with the disturbing infestation of caterpillars by wasp larva which consume it from the inside. What you may not have known is that some plants emit a chemical in response to caterpillars precisely in order to attract the wasps. That actually didn’t make that much sense to me, since infected caterpillars keep eating a lot of plan in order to feed the parasites inside them. Perhaps it keeps down the total population (and in fact wasps have been “carpet-bombed” across fields to do just that to cassava bugs in Africa), but the particular plant with a caterpillar emitting the signal doesn’t stand to benefit much.

Finally, in the off-chance that Zimmer ever reads this, I have a question for him. It is stated that birds and whales probably didn’t get tetrabothriid tapeworms from fish, because the tapeworm’s closest relatives infest reptiles on land. But then right afterward it is stated that Eric Hoberg thinks that birds got the parasite by eating fish which served as intermediate hosts between reptiles. That sounds like a contradiction, how can those statements be consistent?