The appearance of emerging infections
I’m currently reading Spillover: Animal Infections and the Next Human Pandemic, by David Quammen (available here, via Amazon.) Mr. Quammen is a science and fiction writer, whose pieces have appeared in National Geographic, Rolling Stone, as well a many other periodicals. The premise of the book is stated in the subtitle: the human species is at risk due to infectious agents acquired from animals. These are zoonotic diseases, which may cause relatively innocuous disease in various animal reservoirs, however may have much more severe symptoms when they appear in humans. The underlying assumption is that these diseases are not new to Life on Planet Earth, but are however new to us.
Quammen sets up the book in a series of chapters, with each chapter detailing the emergence of a single (or a set of highly related) from an animal population into the human population. Several of these diseases are likely familiar (malaria, Lyme Disease, AIDS), but several are essentially novel, or else best known to an infectious disease aficionado (Ebola, equine morbillavirus, Q fever.) Quammen’s central argument is that none of the pathogens responsible are new; the circumstances for their movement into humans however is new.
Let’s look at Lyme Disease, for example. In BIO230, we discuss the signs and symptoms of the disease, which has been observed to be an emerging infection in the Eastern United States. Since the first diagnosed cases in the mid-1970’s, the number of cases has steadily increased to about 30,000 cases annually. The class discussion focuses on the pathogen (a spiral bacterium called Borrelia burgdorferi) and the route of transmission via the deer tick, which acquires the bacterium from blood meals from amplifying hosts (traditionally, a deer.) Quammen points out that this discussion is very simplistic, and furthermore is not well borne out by epidemiological data for disease incidence, case location, and deer population. In fact, areas which have instituted high levels of deer control have not experienced a predicted decrease in the number of Lyme disease cases.
Blood meals on large mammals such as deer are important in the late stages of the tick life cycle, but the presence of small mammals (primarily white-footed mice) play an essential role in the early stages of the tick life cycle. The species of small mammal is important; mice flourish in environments in close proximity to humans, particularly when many of their natural predators such as foxes, weasels, raptors are declining due to habitat fragmentation. Mice, it turns out, are also particularly poor self groomers, and consequently can become infested with the ticks that transmit Borrelia. Quammen points out that the number of deer in a region necessary for supplying blood meals for Borrelia infected adult ticks doesn’t need to be particularly large at all; amplification of the bacterium occurs in the mouse, whereas the deer is only necessary for the final reproductive step. At that point, transmission to humans has likely already occurred. Control measures for preventing Lyme disease then would be what we are already doing (awareness of the disease, avoidance of ticks, prompt diagnosis and treatment), unless more aggressive measures are instituted to reverse the trends to disrupting various habitats.
Spillover is an outstanding read; the science in it is very accessible for the lay person, and any student of microbiology will have no problem following Quammen’s arguments. Further, his stories about each of the infectious diseases are supplemented with his personal field trips with scientists conducting work with each of the agents. In the chapter on Lyme disease, he described tagging tick-infested mice in New York. In the chapter on Ebola fever, he traveled to count gorillas in the Congo. No gorilla signs were found; they had apparently died of Ebola. The book also includes research notes and a bibliography for further information on many of the topics. I recommend Spillover for anyone interested in in modern infectious disease, and the potential futility of medical science in preventing a major calamity on the scale portrayed in “Contagion.” I suspect that Quammen would argue that our only defense against these diseases is being aware that the potential is constant and significant, and for our medical first responders to be able to recognize novel diseases for what they are.