There are estimated to be up to in animals that have the potential to infect humans. But identifying them is a task of Sisyphean proportions. One of the main programmes that seeks to detect novel viruses, PREDICT, run by the US Agency for International Development, spent 10 years and more than $200 million and managed to uncover just in wild animals, livestock and humans. Some of these are deemed potential threats to humans, including novel strains of Ebola and variants of the SARS and MERS coronaviruses. However, the project didn鈥檛 detect the covid-19 virus before it spilled over into humans. Funding for PREDICT has been to support response to the current outbreak, but its primary mission ended in March.
Still, we do have some idea of what kinds of viruses to look out for. For a published by the Johns Hopkins Center for Health Security in Maryland, interviewed more than 120 global experts in infectious disease. They agreed that a global pandemic would most likely be caused by a novel virus strain that crossed over from animals (see 鈥淲e knew how to prevent a pandemic like covid-19, so what went wrong?鈥�), meaning we had no immunity to it. They also thought it would spread between humans via coughing and sneezing, and that it could be transmitted by asymptomatic people or before the onset of symptoms. Sound familiar?
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The experts concluded that highly deadly viruses like Ebola are unlikely to cause a pandemic because they tend to kill hosts before they can infect many others and are less likely to be spread by asymptomatic people, says Adalja. They also thought we shouldn鈥檛 be too worried about something like plague, which is caused by Yersinia pestis bacteria, because we have broad-spectrum antibiotics that are usually effective against new bacterial strains (while we lack broad-spectrum antivirals).
They concluded, too, that DNA viruses pose less of a risk than RNA viruses, which include coronaviruses, because they are generally more stable, so less likely to mutate into deadly strains. However, the DNA virus that caused smallpox proves there are exceptions to this rule 鈥� it after jumping from animals.
Of the 168 known virus families, the experts narrowed the field down to five that we should keep a close eye on. As well as coronaviruses, they include:
Picornaviruses
The best-known of these are rhinoviruses, which generally cause colds, and enteroviruses, which cause a range of illnesses including colds, polio and hand, foot and mouth disease.
Because they usually only cause mild symptoms, this family of viruses has largely been overlooked as a pandemic threat, says Adalja. But it is possible that they could mutate into more dangerous forms. After all, most coronaviruses only cause colds, but new deadly strains like those that cause SARS, MERS and covid-19 have spilled over from animals.
At this stage, we don鈥檛 know much about the ability of picornaviruses to jump species, but there is some .
Pneumoviruses
These include human metapneumovirus, which usually causes colds but can also result in bronchitis and pneumonia. Human metapneumovirus is thought to have originated in birds, and scientists believe that more deadly strains could spill over in the future.
Paramyxoviruses
Measles, mumps and croup are all caused by paramyxoviruses. The strain that causes measles is thought to have crossed into humans from cattle at least 800 years ago. There are also examples of paramyxoviruses jumping to people from bats, horses and pigs.
Orthomyxoviruses
These are 鈥渙n top of everyone鈥檚 pandemic list鈥� because they include the influenza virus, which has already caused several flu pandemics, says at the University of Sydney in Australia. The flu virus is 鈥渂asically everything you don鈥檛 want a virus to be鈥� as it is infectious before symptoms arise and it easily mutates and jumps between species like birds, pigs and humans.
