November 2024

Viral evolution makes the news again (in its usual, cruel way) as mpox cases surge in Africa. The outbreak, centered in the Democratic Republic of the Congo (DRC), threatens to spill over to other continents, constituting a global health emergency according to the World Health Organization. In 2022, mpox began to spread widely for the first time outside of Africa, where the virus naturally infects animals and sometimes makes the jump to human hosts. Most people infected with mpox fully recover without treatment. Although the 2022 outbreak eventually spread to 116 countries, it killed only around 220 people. The viral strain behind the current outbreak, however, appears to be deadlier. The DRC has already reported more than 1000 deaths. The difference seems to come down to the evolutionary origins of the two viral strains.

Where's the evolution?

Several headlines about the current outbreak have used a word normally found only in evolutionary biology textbooks: clade. “Germany reports first case of clade Ib mpox variant.” “India reports first case of mpox from fast-spreading clade Ib variety.” From context, you’d probably guess that a clade is a variety, type, or group – and you’d be largely correct. But it’s more specific than that.

A clade is a group composed an ancestor, all the descendants of that ancestor, and no others. You can think of a clade as a branch of an evolutionary tree – a branch that includes all of its original twigs and leaves, and no other twigs or leaves.  If a group includes an organism that is not a descendent of the ancestor shared by all the others in the group, the group is not a clade.  If a group excludes even one of the descendants of the most recent common ancestor of the organisms in the group, the group is not a clade.

As shown below, clades are nested within one another; they form a nested hierarchy. Furthermore, clades can be any size. A clade can be a single species – or millions.  It just depends on how many descendants the ancestor you are interested in has. For example, the animals form a clade known as Animalia, which includes all the living and extinct animals. Within the animals, the vertebrates (again, living and extinct) form a clade. Within the vertebrates, the primates form a clade…and so on.

In evolutionary biology, the concept of a clade is useful because it gives us an indication of evolutionary relatedness. All members of a clade are more closely related to each other than they are to organisms not in the clade. All primates are more closely related to each other than they are to non-primates.  All vertebrates are more closely related to each other than they are to invertebrates, and so on. Furthermore, because traits are passed on through lines of descent, all the organisms in a particular clade are likely to share certain similar traits that they inherited from their common ancestor.

The mpox clade behind the current outbreak is called Ib, shown in light peach in the diagram below. It is nested within clade I, shown in pink below. On the other hand, the viral strain that caused the 2022/23 outbreak, which is associated with only a few hundred deaths, was clade IIb, shown in green below.  Studies in animals suggest that clade I viruses have inherited a higher level of virulence from their shared ancestor compared to clade II viruses. Around 5% of people who have contracted mpox in the current outbreak in the DRC have died, while clade II viruses have a fatality rate between 0.1 and 3.6%.

So far, clade Ib mpox has not gotten far. The United States has not had any cases, and only a few other non-African countries (Germany, India, Thailand, and Sweden) have reported isolated cases related to recent travel. But of course, the situation is very different in and around the DRC, where the virus is spreading person to person and health challenges that make mpox more deadly (such as malnutrition and HIV infection) are more common. Furthermore, an unusually large number of children have been infected in this outbreak. Seventy-eight percent of deaths have been among children under 15. While surveillance and testing are important in avoiding a global health crisis, so is preventing and treating the disease where it arises. More than 5 million doses of the mpox vaccine have been pledged by countries like Japan and the U.S., but only a few hundred thousand have been delivered thus far.