This book by notable
popular science author
Carl Zimmer details the
primarily unseen world of
parasites. Beginning with a discussion
of why
parasitism inspires such universal fascination and revulsion, and
concluding with a slightly reaching analogy describing human beings as
pests leeching off the Earth, Zimmer covers a wide range of topics
within
parasitology,
biology, and
evolution.
Zimmer has obviously performed extensive research on his subject, which the reader can duplicate with the aid of
his helpful selected
bibliography. He relates
case studies of many
specific parasite species, from the well known
Plasmodium which
causes
malaria by infecting human red blood cells, to the parasitic
barnacle of
genus Sacculina,
which invades the body of a
crab, sterilizes it, and alters the host's
behavior such that the infected crab actually nurtures the barnacle's eggs
instead of its own -- even if the crab is male. These tales are often gruesome, and always fascinating.
In addition to gleefully expounding upon the
gross-out factor inherent within
parasitology, Zimmer also explains important scientific concepts introduced to the fields of biology and
ecology by researchers studying parasites. He describes the scientists who struggled to understand parasite behavior -- how does a parasite who enters the host through its digestive tract find its way to other
organs or
systems where it will take up permanent residence? The original idea was that they follow
concentration gradients, moving toward higher concentrations of some key
chemical that is secreted by their target organ. However, the
environment within a
host differs from that of, say, an open ocean -- the host organism's job is to maintain uniformity,
homeostasis, within its organ sysems, and to make sure that various chemical
secretions stay where they belong -- not to spread them throughout the body leaving faint
scent trails for parasitic
worms to follow. Instead, it was found that
internal parasites perform specific
behaviors when exposed to location-specific chemical cues -- perhaps reacting to a
liver secretion, even if they do not ultimately infect the liver. As they drift through the body, they await exposure to these cues, at which point they begin sudden swimming, thrashing, or burrowing behaviors. Performed in the right stepwise sequence, these behaviors ultimately lead the parasite to its final destination, through a process that had been unknown in conventional behavioral
zoology.
Zimmer's personal fascination with
evolution shines through in the book, as well. He mentions theories that attempt to explain
sexual reproduction -- that much-beloved source of variability that drives
evolutionary processes in you, me, the birds, and the bees -- as a result of organisms attempting to boost their
resistance to parasite infection. This "
arms race" between host and parasite has been described as the
Red Queen effect, in which both parties evolve as fast as they can just to stay in the same place. Parasites evolve toward greater infectious capacity just as quickly as hosts evolve stronger resistance. (For more on this, try reading
The Red Queen by
Matt Ridley.) The author also details some of the more
creative mechanisms that parasites have evolved in their pursuit of infectiousness. Some of these include such profound alterations of behavior as those seen by crabs infected with
Sacculina, taken to an even more chilling level: there are seemingly parasites which
infect an animal, then subsequently alter its behavior so that it is more likely to be eaten by the parasite's final host. The parasite
Toxoplasma, for example, can infect
mice, but its primary host is a
cat. (It can also infect the human brain, producing
toxoplasmosis -- a condition not without its own behavioral effects on the host.) Scientific studies have revealed that, among other things, mice infected with the parasite exhibit less aversion to the smell of cat
urine -- typically a powerful deterrent to all
rodents. It follows that these mice would be slower to flee from a cat, and thus more likely to fall prey to one, which is just what
Toxoplasma "
wants." This particular type of behavioral alteration has come to be known as the theory of
Parasite Increased Trophic Transmission (PITT), and is believed to have strongly influenced the evolution of parasites whose
life cycle depends upon more than one host species.
Overall, I think this
book is a great read. I found Zimmer's writing to be entertaining and approachable. As a
student of biology, I came in with a bit more background knowledge than the typical
layperson might have, but I still learned a lot of new things. The book isn't all hard science -- Zimmer references
pop culture phenomena like the series of
Alien movies and
The X-Files to provide evidence of widespread
morbid fascination with
parasitism, for example. However, most of the text involves descriptions of various scientists and their research in parasitology, which opened a window into a world I'd never seen before. If you've any interest in reading a work of popular
science, I'd heartily recommend this book. Thanks to Carl Zimmer's fascinating portrayal of parasitology, I may just apply to a few
graduate schools in the field.