Superspreaders are not limited to humans. When researchers began to deconstruct the outbreaks of West Nile virus that hit the United States in the first decade of the 21st century, they found that certain areas, like the western suburbs of Chicago, were hit especially hard. Tony Goldberg, an epidemiologist at the University of Wisconsin-Madison, began to study the mosquitoes that transmitted West Nile virus in an attempt to figure out why some places had so many more cases than others.
Using traps and a device that operates like a giant vacuum cleaner, Goldberg and colleagues collected mosquitoes from around Chicago. A few of these mosquitoes had blood in their abdomen from a recent meal, which allowed researchers to identify which animals the mosquitoes had been biting. Overwhelmingly, the mosquitoes had been feeding on Turdus migratorius, the American robin.
West Nile virus is traditionally associated with crows, blue jays, and other corvids, Goldberg said. These are the birds that get sick and die in large numbers during West Nile outbreaks. Public health officials knew to brace themselves for a West Nile outbreak in humans this summer when they began to see large numbers of dead crows. Although researchers knew that other birds could become infected, they were thought to be fairly minor contributors to West Nile spread. The fact that robins don’t die or become noticeably ill made them both invisible to public health authorities and remarkably good spreaders of West Nile. When crows and related birds die, they are unable to transmit the virus to other birds. They become literal dead ends. Robins, on the other hand, continue the everyday activities that bring them in contact with new mosquitoes, which drink the virus from the birds’ blood and continue the spread of the disease.
Whether they are individuals or an entire species, superspreaders have certain traits that make them efficient engines of infection. According to disease ecologist Sara Paull at the University of Colorado, Boulder, superspreaders share three major qualities. They shed large quantities of the pathogen. They transmit it to a large number of people. And they do so for a long period of time. A combination of an individual’s physiology and behavior determines whether he or she will become a superspreader.
Take Typhoid Mary. If she hadn’t been a cook and hadn’t harbored an asymptomatic infection of Salmonella typhi in her gall bladder, she wouldn’t have infected many other people. In the case of SARS, many local outbreaks centered on hospitals. A health care worker who hacked up large amounts of virus had a much greater potential of spreading SARS to a wide range of people, many of whom had impaired immune systems, compared to someone who rarely had contact with the outside world. Scientists still don’t know why SARS superspreaders were so effective at transmitting the disease, although a major factor appears to be invasive respiratory procedures like intubation and nebulizer treatments that aerosolized large amounts of virus.
Identifying superspreaders seems like it would be a logical place for public health officials to start fighting disease outbreaks. “If you just apply infectious disease control measures randomly, you may not be able to actually eliminate an infectious disease. But if you target the most relevant 20 percent, then you can vaccinate or treat enough of these individuals to eliminate the disease much more quickly,” said Virginia Tech disease ecologist Dana Hawley.
In order to figure out who these superspreaders really are, scientists need to define superspreading in a concrete way that can be applied to multiple different diseases. Thus far, noted Lloyd-Smith, this hasn’t been done. “This term is being bandied about, and everyone is using their own personal definition,” Lloyd-Smith said. The basic definition Lloyd-Smith developed was a person who spreads an infection to a significantly higher number of contacts than average. But no one knows exactly how much higher that number needs to be before someone crosses the line from unlucky to superspreader. Nor do researchers understand why some diseases are more prone to superspreading than others.
In the meantime, scientists have begun to look at superspreaders’ mirror image—those people who don’t transmit infections to anyone else—as a way to halt disease outbreaks. Are their immune systems fundamentally different? What about their behavior? Perhaps through these individuals, researchers will crack the mystery of superspreading and ultimately stop local infections from becoming pandemics.