Deadly viruses like Sars, Mers and Ebola are all quite different in nature, but they share a common tendency of cherry-picking their victims as they hunt for so-called “super spreaders,” or those capable of helping the disease make a quantum leap from just a few individuals to masses of people, according to a new study by Chinese scientists.
A person who is genetically predisposed to deliver many more viral strains than normal simply by coughing, for example, would be a strong contender to make the list.
The researchers carried out a comprehensive analysis of historical data related to the spreading of the three viruses, and they found that a number of major outbreaks were all driven by super-spreaders.
To rank as one of these, an individual must be capable of infecting a disproportionately large number of people. In such cases, a small fraction of a country’s population could be responsible for more than 50 per cent of the total number of infections.
The case of Mary Mallon (“Typhoid Mary”), a cook from Ireland who migrated to New York City and lived there in the early 1900s, sheds some light on the issue.
She is believed to have transferred typhoid to 51 people, three of whom died, and is generally held as the first person in the US to be identified as an asymptomatic carrier of the pathogen associated with typhoid fever.
As one of the first and certainly most famous super-spreaders in modern history, she was put in forced quarantined for nearly three decades until her death.
For the latest study, the Chinese team, led by Professor Gao Fu with the Chinese Academy of Sciences’ Institute of Microbiology, warned that similar mass transmissions from a single source have been observed in recent viral outbreaks.
During the outbreak of Middle East respiratory syndrome (Mers) in South Korea early this year, “approximately 75 per cent of cases can be traced back to three super-spreaders, who have each infected a disproportionately high number of contacts,” the scientists reported in their paper, which was published in the journal Cell Host & Microbe earlier this month.
Meanwhile, one patient at the Prince of Wales Hospital in Hong Kong infected 125 people during the outbreak of severe acute respiratory syndrome (Sars) in 2003, while the Ebola virus reportedly leapt from one doctor in Sierra Leone to eventually infect over 300 people this year.
“It’s not entirely the person’s fault,” said Dr Bi Yuhai, a corresponding author to the study at the institute’s Key Laboratory of Pathogenic Microbiology and Immunology.
The super spreaders often appeared after the virus underwent significant mutations, and so far no one has come up with an effective mechanism to monitor these mutations, he said.
The people capable of mass-transmitting the viruses also tend to appear in certain favourable environments. An enclosed space full of people, such as the cabin of an aeroplane, could provide an environmental trigger for a super-spreading event, scientists say.
“Our study shows the urgent need for international cooperation to proactively identify super-spreaders,” Bi added.
Pilot projects led by Gao with the support of the Chinese government have explored the possibility of raising an alert before such events, which rapidly spread the virus, occur, he said.
Some teams have been developing mathematical models in the hope of predicting the time, location and severity of a super-spreading event. Some have been experimenting using animals to identify signs of viral mutation that could help the disease in its search for super-spreaders.
Bi said it is crucial for governments to take this issue to heart and deal with appropriately with such individuals once they are identified in future outbreaks.
“Taking a soft-handed approach to a super-spreader can prefigure a disaster,” he said.
“The recent outbreak of Mers in South Korea taught us a painful lesson [in that respect].”
“Once a suspect has been identified, decisive quarantine measures must be applied immediately. It might be considered cruel … [but] it could prove a saving grace for society as a whole.”