Parasites The Inside Story

For many people, parasites top the list of nature's most unwanted creatures. For biologists, however, they offer a window into the ecology and evolution of their hosts, while a full understanding of an ecosystem needs to take its parasite fauna into account. Parasites offers an engrossing and well-illustrated introduction to some of these processes. However, the brevity of the chapters and the input from three authors does turn this into a bit of a medley that might leave readers wanting more.

This book has resulted from a collaboration between three authors. Curator of parasites Scott L. Gardner and parasitologist Gabor Rácz both work at the H. W. Manter Laboratory of Parasitology at the University of Nebraska State Museum where Judy Diamond is a professor and curator. Between them, they have written thirteen short chapters divided over three parts and serve up a parasitical potpourri of interesting studies and findings. Fortunately, they avoid going for the low-hanging fruit of presenting stories to gross you out.

In the first part, they discuss the human burden of various nasty diseases, including a chapter on the discovery and treatment of river blindness. But this book is not a compendium of unpleasant human infections. Equally interesting is what parasites can reveal about, for example, past human migrations, such as the peopling of the Americas during the last ice age. It is widely accepted that humans migrated across the Bering Land Bridge from Asia into North America. But could some people separately have crossed the Pacific by boat and landed in South America? The authors here mention that genetic analysis of parasites recovered from coprolites shows different parasite communities in different places. Some of these include cold-sensitive hookworms, which would support the idea that not all settlers in the Americas arrived via the frozen northern route.

Humans are not the only victims; parasites are found all over the tree of life and some of the most fascinating and well-written chapters follow in the second part. I never thought I would say this, but I have a renewed respect for tapeworms. Generally, biologists think that parasites and hosts evolve in lockstep in a process of co-evolution, but studies on the tapeworms of seabirds reveal a far deeper evolutionary history, stretching back in time some 300 million years. "Like old wine in new bottles, the parasites were vastly older than their seabird hosts" (p. 50). These parasite ancestors might have lived in ancient fish, marine reptiles, or pterosaurs. Meanwhile, when whales returned to the sea, it seems their parasites did not come along during this evolutionary transition. None of their current parasites has a recent common ancestor found in land mammals. I simply have never given much thought to the intersection of parasitology and phylogenetics, but the authors have piqued my interest. The other fun side of parasites is the direct manipulation of host behaviour, frequently in an attempt to be passed on to the next host in their lifecycle. From slugs whose eyestalks puff up and pulsate—something birds find irresistible—after infection with the fluke Leucochloridium variae to ants who turn "into a nightly automaton" (p. 67) and climb up grass stalks at night waiting to be eaten by deer after infection with the fluke Dicrocoelium dendriticum—these stories are guaranteed to be a hit at your next dinner party.

The last part of the book, which focuses on research done by parasitologists today, often in remote locations, continues the theme of avoiding simple war stories and instead presenting fascinating science. Next to parasites influencing animal behaviour, the reverse is also true, and animal ecology and behaviour intersect with parasitology. For example, pocket gophers and grasshopper mice in the Nebraska Sandhills ecosystem do not share parasites despite sharing the same burrows. However, in South America, the burrow-sharing tuco-tucos (a rodent) and pink fairy armadillos do. Understanding why requires detailed knowledge of the ecology of the hosts and parasites involved, but it seems that the former example resembles one of "next door neighbours with different job schedules" (p. 101). Of course, given their close relationship, hosts and parasites co-evolve, engaged in an endless arms race in which each gain achieved by the parasite invites a countermove from the host. I appreciated, though, how the authors earlier on point out that "host-parasite relationships are less a victory between contestants than a compromise in which both parties meet their survival needs to some degree" (p. 43). Consider, also, that parasites face double the evolutionary challenge as their success depends not just on their own adaptations but also those of the host. "Choose a host on its way to extinction, and that could be the end of the parasite. The ability to switch hosts is a great insurance policy against ending up with a dud for a host" (p. 11). Admittedly, host switching is risky and challenging, but some of the abovementioned examples reveal that certain parasites have succeeded.

These three parts are generously padded out with extra material, setting an example for the kind of attention I wish more publishers would lavish on popular academic titles. Most chapters contain a full-page illustration of a parasite's lifecycle, drawn by graphic artist Brenda Lee. Add to this 16 unpaginated plates containing 29 colour photos, an 18-page illustrated appendix to the parasites mentioned in the text (consider it a parasite bestiary), a 13-page glossary, and a 32-page bibliography. Put together these make up just under half of the book. And herein lies perhaps the book's greatest weakness: it really is quite short. With three authors writing thirteen chapters in only 118 pages, and chapters being 6–11 pages each, they are spreading their material thin. There are really interesting ideas covered here but the book goes off in so many directions at once that it risks losing coherence a bit and I would have loved to see certain topics developed more. The matter of authorship is also unclear. There are noticeable stylistic differences between some chapters, but who wrote what chapter, or whether they were all co-written by everyone is not mentioned. Finally, the text does not refer you to the plates, nor does it insert references to the generous bibliography. You will have to do some searching if you want to follow up on the studies described in the text.

Parasites is thus a very readable, interesting, and well-presented introduction to the often overlooked evolutionary and ecological aspects of host-parasite interactions. As an introduction to parasitology, this will likely whet your appetite, though be warned that it might leave you hungry for more.