I hope that my last post persuaded you that the appendix is not the pathetic remains of our forbearers’ large cecum, but is in fact a nifty piece of anatomy that maintains a safe house for the normal micro flora of our gut (If you’re interested in gut micro flora, Melissa wrote a great post here). While this little organ seems to work well in developing countries where there are frequent outbreaks of enteric pathogens and minimal hygiene, something seems to have gone awry in the developing world. While appendicitis is exceedingly rare in developing countries, it has been reported that up to 6% of the population in industrialized countries develop appendicitis necessitating appendectomy [1]. Why has our bacterial safe house turned into a ticking time bomb?
As early as 1505, Leonardo da Vinci identified the appendix and recognized that it sometimes became inflamed and burst. Much of his medical knowledge was lost, and it wasn’t recognized again until 1705 when the (then very young) father of clinical case reports, Giovanni Battista Morgagni, dissected a man who had died of appendicitis and subsequent peritonitis. That case actually revolutionized the understanding of medicine, with Morgagni and his mentor Valsalva recognizing that a specific disease was caused by a specific condition in a specific part of the body. This showed that illness was not caused by an imbalance of humors or a generalized malaise, but rather a specific cause. This one case led Morgagni and Valsalva to perform autopsies on all their deceased patients, and their detailed notes of over 700 cases were analyzed and published in the book On the Seats and Causes of Disease as Indicated by Anatomy. This book, and the idea that disease is caused by specific disorders, revolutionized medicine.
While appendicitis was one of the first diseases for which the anatomical source was recognized, we still don’t clearly understand why the condition occurs. It is generally believed that appendicitis occurs when the appendix is obstructed (by obstruction of the opening into the cecum by feces or swelling of the appendix due to proliferation of the tissue of the appendix itself), and the mucinous products of the appendix build up, leading to increased pressure and eventually tissue death. This dead tissue encourages bacterial proliferation (and we’re no longer talking about the friendly house-keeping type). Acute appendicitis is a medical emergency, and one that must be diagnosed and handled quickly. The removal of an inflamed, but intact, appendix is a much easier and neater procedure than trying to manage the aftermath of a ruptured appendix and subsequent peritonitis. If you think you might have appendicitis- get thee to the emergency department!
But why has appendicitis become so common? Appendectomy is sometimes referred to as ‘bread and butter’ for a general surgeon, but in developing countries this condition is almost unheard of. The rate of appendicitis is reported to be about 35-fold higher in the United States than in areas of African unaffected by modern health care and sanitation. Additionally, as communities adopt Western sanitation and hygiene practices, the rate of appendicitis increases [2]. Could appendicitis be another result of the “hygiene hypothesis”- the idea that modern medicine and sanitation can lead to an under-stimulated and over-active immune system?
As discussed in my first post, the appendix is associated with a large amount of gut-associated lymphoid tissue (GALT). While I pointed out that the appendix does secrete some substances that actively encourage the formation of biofilms for friendly bacteria, GALT also plays a role in the more typically recognized ‘keep the bad guys out’ aspect of the immune system. It’s that part of the system that tends to go awry with our modern hygienic world. Our immune system evolved to handle and control a number of different pathogens, including unfriendly bacteria and parasites. In the absence of pathogens, however, the system can go amiss The immune system is primed and looking for a fight, and if nothing appropriate comes along to take a beating, the immune system can start getting self-destructive, going after the body in which it is housed. It’s a classic case of ‘idle hands’ (or an active teenager with no good way to get the energy out!). This may well play a role in the prevalence of appendicitis in the developed world: overactive GALT tissue causes the appendix to swell, plugging the appendix, stopping the secretions from exiting into the cecum, and leading to increased pressure and subsequent necrosis and disease. (This is the condition that tends to occur in young people. In older people, appendicitis tends to be caused by the physical blockage of the appendix by a coprolith).
So is that it? In the past, and in the developing world, the appendix operated as a safe house for commensal bacteria. In the modern/hygienic world the appendix isn’t really needed, and can in fact get a bit out of whack because it doesn’t have anything to direct it’s immune-related functions towards. It definitely seems as though this might be the case, and unfortunately the problem appears to extend beyond the appendix. It turns out that an overactive appendix may also play a role in ulcerative colitis- an inflammatory condition of the large intestine. In some people with ulcerative colitis, an appendectomy improves the symptoms of ulcerative colitis, and in others it can completely cure the condition. The intended purpose of the appendix may shed light on why this pathology occurs. First- in a hyper-immune state, the appendix may house bacteria that the immune system aberrantly attacks. Alternatively (or additionally), the GALT tissue may drive the gut into a hyper-immune state. In either case- understanding the evolutionary purpose of the appendix can help understand and treat the conditions that occur in our modern hygienic world. Furthermore, it offers evidence that we should think about the impact of our uber-hygienic world, and consider how we might best handle the mismatch between our immune system that evolved to keep us safe in a dirty world and our modern clean environment.
(If you’re looking for a scholarly discussion of this topic, I highly recommend The cecal appendix: one more immune component with a function disturbed by post-industrial culture [2].)
1. Bollinger, R.R., A.S. Barbas, E.L. Bush, S.S. Lin, and W. Parker, Biofilms in the normal human large bowel: fact rather than fiction. Gut, 2007. 56(10): p. 1481-2.
2. Laurin, M., M.L. Everett, and W. Parker, The cecal appendix: one more immune component with a function disturbed by post-industrial culture. Anat Rec (Hoboken), 2011. 294(4): p. 567-79.
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