Spillover: Animal Infections and the Next Human Pandemic

The narrative opens with a description of Hendra virus, which was discovered in Western Australia. Though the disease infected very few people, and mostly harmed horses, the outbreak occurred suddenly and is a useful illustration of how diseases emerge, especially a phenomenon known as zoonosis: “an animal infection transmissible to humans” (14). Many well-known diseases, like AIDS and flu, belong in this category, and the phenomenon reminds us that human societies are closely connected to those of the animals around them.

The Hendra virus outbreak began in 1994, when racehorses in a Melbourne suburb of the same name became ill in large numbers. The first was a mayor named Drama Series, but many others became sick afterward, as did the horse’s human owner, who died. A stable hand survived and was never hospitalized. Necropsies (animal autopsies) revealed a new virus, unrelated to any known diseases. The researchers next had to determine where the virus typically resided—outside of horses and people—and how it was transmitted. When Quammen visited Melbourne, his guide showed him a fig tree in the horse’s pasture, where bats liked to roost. The bats were significant because diseases, like plants or people, have typical or preferred habitats.

The moment when a virus enters a new host is called “spillover.” Viruses, protists, prions, worms, and bacteria are all capable of zoonosis. About 60% of human diseases result from zoonosis; others, such as polio or smallpox, only infect humans and are therefore much easier to eradicate. Zoonosis often happens when animal habitats are disturbed by human activity. Most zoonotic diseases develop long-term evolutionary relationships with an animal, their “reservoir hosts,” and these animals often show no symptoms or ill effects from the relationship (23). This allows the pathogen to survive and successfully reproduce, largely unknown to humans. Viruses evolve quickly and do not respond to antibiotics, and they can easily become global phenomena as they “ride” around the world in their host.

Tracking the origin of a zoonotic disease requires specialized scientific training. Many of these “field biologists” are veterinarians or have pursued graduate degrees in ecology or public health (26). The “virus hunter” who found Hendra, Hume Field, worked as a veterinarian before studying ecology. Hume’s initial searches involved trapping many kinds of animals, including rodents and reptiles. The local veterinarian suggested checking bats, but no one notified field of this, so further progress was not made until another outbreak nearly a year later.

In January 1996, a scientific “brainstorming session” resulted in Field testing bats. Field eventually tested Australian bats, known as “flying foxes,” and determined that many bats carried antibodies. A later necropsy on a pregnant bat revealed live virus. Field’s final data revealed that Hendra virus was present in close to half of the population. This had some negative effects on popular perception of bats, and some people in the horse trade suggested all bats be exterminated.

Hendra virus needed an “amplifier” host to infect humans—specifically, the horse (34). Quammen explains this further with reference to Australia’s particular history and further comparison to other diseases. Hand Foot and Mouth disease can only infect humans via infected pigs. In the case of Hendra, Bats are a native species to the continent, while horses were imported by one British colonist in 1788. There is no record of any Hendra-like outbreak prior to this. At the same time, there is no clear record of Hendra in people prior to 1994. Quammen posits that infection likely occurred, but the disease was mistaken for something else.

Returning to zoonotic disease in general, Quammen notes that it is largely a late 20th-century phenomenon. A likely origin case was the Machupo virus in Bolivia, which killed many people until its reservoir was identified. Quammen calls them the “results of things we are doing” (39). Specifically, humans “disrupt” animal habitats and ecosystems through activities like hunting, mining, or agriculture, in turn disrupting traditional animal behaviors and provide viruses with new hosts. The rapidity and scale of human population growth only exacerbates these trends.

The continuance of zoonosis is of huge concern to public health professionals, as they see it as likely to contribute to future pandemics—the event they call “The Next Big One,” or NBO, which could result in as many as 40 million deaths (42). Zoonosis gives rise to what ecologists call “emerging disease,” defined as “an infectious disease whose incidence is increasing following its first introduction into a new host population” (42). Diseases can also “re-emerge” as they evolve or ecosystems change (43). Another concept closely related to emergence is known as “spillover”—a single event in which a disease that formerly had one host finds another.

This happened in Hendra in 1994. Quammen introduces data that demonstrates why zoonosis and spillover matter: Most emerging diseases result from wildlife spillovers—well over 60%. However, the matter is rarely as simple as monitoring wild animals and studying their diseases, as the Hendra case illustrates. We now know what Hendra is, but it is not clear why and how the horse passed on the infection, and why only some humans who had close contact with the horses became ill. Which diseases will emerge when, and how, are pressing public health questions that remains difficult to answer.

Quammen concludes his examination of Hendra by interviewing a human survivor of the virus, a veterinarian in Cairns. The young vet was called out to see an “off color” family horse in the middle of the night. The horse had froth” around his nose, some of which got on the vet’s arms. In the rush and at the late hour, she didn’t wear a mask or gloves. She later did a brief necropsy that resulted in more fluid on her legs, but the owners declined to pursue further testing after it was obvious the horse died of heart failure. About 10 days later, the vet herself became sick. She later tested positive for Hendra antibodies and underwent continuous monitoring of her health in case of a recurrence. She remained anxious and uncertain, since so little was known about the disease and its long-term effects on humans.