We tend to think of the appendix as something we can easily manage without, but recently science has started to take a more favourable view of this unassuming little organ. Since it persists in several evolutionary lineages, surely it must have some useful purpose? The problem up to now has been trying to work out what that might be, but a new study has found a fascinating link to a disease that’s one of the biggest killers of children around the globe.
Diarrhoea is the cause of over 500,000 deaths in children under 5 every year, according to the WHO, making it the second leading cause of death in this age group. The majority of cases are caused by infections with bacteria, viruses, or parasites, such as E. coli and rotavirus, and it’s often spread through contaminated food and water.
In the past, it’s been noticed that patients who have had their appendix removed are at an increased risk of developing certain forms of infectious diarrhea, or are more likely to get severe symptoms. Although this association has been observed, the reasons behind it are less clear.
Wishing to investigate this further, a team from the French National Institute of Health and Medical Research (Inserm) turned to some of our closest animal relatives, which are also disproportionately affected by infectious diarrhea.
The researchers examined the veterinary records of 1,251 primates housed at La Vallée des Singes zoological park in Romagne, France. The individuals were of 45 different species, of which 13 – including chimpanzees, ring-tailed lemurs, and western gorillas – naturally have an appendix, and 32 – like mandrills, golden lion tamarins, and common woolly monkeys – do not.
The team was able to collect data on the frequency and severity of diarrhea episodes in these animals between 1998 and 2018.
Over the 20-year period, almost 3,000 incidences of diarrhea were recorded, and 13 percent of these were classed as severe. Although about half the individuals experienced diarrhea at least once during that time, it became clear that the burden was not equally shared between the different species.
“We identified a lower risk of severe diarrhea among primate species with an appendix, especially in the early part of life when the risk of diarrhea is maximal,” the authors wrote in their paper. “Moreover, we observed a delayed onset of diarrhea and of severe diarrhea in species possessing an appendix.”
Couple these findings with the observation that humans sans appendix are more susceptible to diarrheal disease, and it does appear that what was once considered a vestigial organ may be playing a much more useful role than many suspected. It’s particularly notable that this protective effect seems to be strongest during early life, as co-lead author Éric Ogier-Denis explained in a statement:
“The observation of a particularly high protective effect in the first part of life, the period most vulnerable to severe diarrhea, but also the most optimal in terms of reproductive capacity, argues in favor of a selective advantage role in evolution.”
Another important finding was that none of the primates with an appendix included in the study had suffered acute appendicitis during the 20-year follow-up period. Co-first author Maxime Collard explained that this potentially life-threatening complication, which is the usual reason for surgery to remove the appendix, is more common in humans, but it’s still important that more research is done to build on these results, and hopefully quantify exactly how beneficial an intact appendix can be.
“[I]f the protection associated with the presence of the appendix in humans is of the same level as that observed in primates, it would very much counterbalance the risk related to fatal appendicitis,” Collard concluded.