Spillover: Animal Infections and the Next Human Pandemic

Field is an Australian who originally trained as a veterinarian and went on to get a doctorate in ecology. He abandoned his original dissertation to study the then-new outbreak of Hendra virus, and he set to work trapping many varieties of wildlife to try to identify its reservoir. Field eventually found live virus in bats and conclusively established them as the reservoir.

Field appears briefly in the history of the 2003 SARS outbreak and again in Quammen’s account of the Nipah outbreak in Malaysia and Bangladesh. He investigated the pig farms that were known to carry vast swathes of infected animals and introduced Quammen to the idea that animals raised for food are a huge potential source of outbreaks, one virtually impossible to predict. He also conducted some initial investigations into bats in Malaysia, though other members of the team found the conclusive data. 

Leroy is one of the protagonists of Quammen’s Ebola investigations. He is a French virologist and veterinarian based at CIRMF in Gabon. He initially focused on other infections but became fascinated with Ebola, and its study shaped the rest of his career. Leroy identified bats as a possible Ebola reservoir, yet remained puzzled that bats did not seem to infect humans, only primates do. While his initial efforts in the 1990s did not find live virus in bats, Leroy’s investigation of a 2003 outbreak did trace Ebola to bat consumption. His career showcases the importance of patient fieldwork in identifying the origins of zoonosis. 

Warfield is an American virologist who trained in vaccine development while getting her PhD at Baylor University. She grew up in Maryland and was fascinated with the scientists who worked on infectious diseases at USAMRIID at Ft. Dietrich. After graduating, she got a job there and was certified to work in the BSL-4 environments that are home to Ebola research.

In 2002, Warfield was testing a possible Ebola vaccine, injecting it into Ebola infected mice, when she had a lab accident and was stuck with a needle when a mouse wriggled away. She was placed into quarantine for three weeks and subjected to routine testing. She ultimately developed no signs of illness and left quarantine. She continues to work at Ft. Dietrich, fascinated by how such a tiny virus can be so deadly to humans. Her story illustrates the dangers of close scientific work with viruses, as necessary as it may be to understanding them.

MacDonald was a Scottish malaria researcher interested in the mechanics of outbreaks, especially the application of mathematical models. He determined that a 1934 outbreak of the disease in Ceylon (now Sri Lanka) was worse than expected due to an increase in the mosquito population that year. He originated the concept known as the “basic reproduction rate,” describing what happens when “when one infected individual enters a population where all individuals are nonimmune and therefore susceptible” (147). When this threshold is higher than one, a disease will continue to spread—if it is much higher than that, outbreaks are significantly worse. MacDonald’s work demonstrates that many disciplines and approaches are necessary for a thorough grasp of emerging diseases.

The married team of infectious disease specialists discovered that Plasmodium knowlesi malaria is zoonotic. Janet Cox-Singh is Irish and met her Malaysian Sikh husband at the Liverpool School of Tropical Medicine. They established at a laboratory at the University of Malaysia Serawak and discovered an unusual cluster of malaria cases. Using a novel sample technique with filter paper, they discovered that Plasmodium knowlesi was infecting people and causing deaths.

They identified three kinds of macaque as the reservoir host. While the number of cases was relatively small, Singh and Cox-Singh were concerned the outbreak could grow larger, possibly leading to a “host switch” from macaques to humans especially as deforestation continued. Quammen notes that this would have massive regional consequences given how many countries are home to these species of macaque. 

Burnet discovered the cause of psittacosis in humans and presented a public health solution: Keeping parakeets in more sanitary conditions would keep them healthier and also keep human infections down. Burnet helped discover that Q fever is a bacterium; he also worked on the laboratory techniques to make the bacterium visible, and it bears his name.

Burnet was also an advocate for multifaceted and multidisciplinary study of infectious disease. He wrote Biological Aspects of Infectious Disease in 1940. He insisted that it was only proper to consider diseases as organisms that evolve and rely on their hosts to survive just as predators consume prey. Decades before Quammen’s other protagonists, he stressed “environmental disruption by humans as a releaser of epidemics” (237). 

Montagnier is a French scientist who had experience studying retroviruses that caused cancer. A retrovirus has a unique operating mechanism, in which:

[…] it converts its RNA into DNA within a host cell; its viral DNA then penetrates the cell nucleus and gets itself integrated into the genome of the host cell, thereby guaranteeing replication of the virus whenever the host cell reproduces itself (391).

Montagnier theorized the still-unidentified HIV might operate similarly. Montagnier tested his hypothesis with blood cells from an infected man and won a Nobel Prize for his work. 

Hahn is a medical doctor specializing in retroviruses. She and her graduate students and postdocs developed important sampling techniques for testing chimp urine and feces non-invasively, which allowed them to establish that chimp SIV was a direct ancestor to AIDS and that it originated in Cameroon before spreading to urban centers in then-Belgian Congo. She worked with Jane Goodall to study the chimps at Gombe National Park and further determined that chimp SIV is not harmless to its hosts.

Keele was a postdoc of Beatrice Hahn’s who helped invent the sampling methods that established the location of HIV’s spillover. He pioneered the preservation of stool samples for further testing. Another of Keele’s projects established that chimp SIV has significant health effects on its hosts.