Re: Why do people have Appendicitis?

Is it a sign of the evoluation from animal to human? 
	
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That’s a pretty big question!  First of all, let’s review what we know
about appendicitis. Appendicitis is the inflammation of the appendix.  From
a quick search of medical literature about appendicitis, we can learn about
the epidemiology of acute appendicitis and its causes. 

“The peak incidence of acute appendicitis is in the second and third
decades of life; it is relatively rare at the extremes of age. Males and
females are equally affected, except between puberty and age 25, when males
predominate in a 3:2 ratio. Perforation is more common in infancy and in
the aged, during which periods mortality rates are highest. The mortality
rate has decreased steadily in Europe and the United States from 8.1 per
100,000 of the population in 1941 to <1 per 100,000 in 1970 and
subsequently. The absolute incidence of the disease also decreased by about
40% between 1940 and 1960 but since then has remained unchanged. Although
various factors such as changing dietary habits, altered intestinal flora,
and better nutrition and intake of vitamins have been suggested to explain
the reduced incidence, the exact reasons have not been elucidated. The
overall incidence of appendicitis is much lower in underdeveloped
countries, especially parts of Africa, and in lower socioeconomic groups.”

The general cause of appendicitis is obstruction of the intestinal lumen by
a fecalith (a solid mass of feces) most commonly, but can also be caused by
a tumor.  However, obstruction is only seen in 30-40% of cases.  In the
majority of cases, the mucosa of the lumen is ulcerated, which can be
caused by infection.

From Harrison's Principles of Internal Medicine
Part 12: Disorders of the Gastrointestinal System
SECTION 1: Disorders of the Alimentary Tract
Copyright 2004 by The McGraw-Hill Companies, Inc
William Silen 
  
  
Let's also review the appendix itself, and what its function is.  
Here is some information from an immunology textbook:
“The peripheral lymphoid organs are specialized to trap antigen, to allow
the initiation of adaptive immune responses, and to provide signals that
sustain recirculating lymphocytes.”
“The appendix is a gut-associated lymphoid tissue located at the beginning
of the colon.”
“The gut-associated lymphoid tissues (GALT), which include the tonsils,
adenoids, and appendix, and specialized structures called Peyer's patches
in the small intestine, collect antigen from the epithelial surfaces of the
gastrointestinal tract. In Peyer's patches, which are the most important
and highly organized of these tissues, the antigen is collected by
specialized epithelial cells called multi-fenestrated or M cells. The
lymphocytes form a follicle consisting of a large central dome of B
lymphocytes surrounded by smaller numbers of T lymphocytes. Similar but
more diffuse aggregates of lymphocytes protect the respiratory epithelium,
where they are known as bronchial-associated lymphoid tissue (BALT), and
other mucosa, where they are known simply as mucosal-associated lymphoid
tissue (MALT). Collectively, the mucosal immune system is estimated to
contain as many lymphocytes as all the rest of the body, and they form a
specialized set of cells obeying somewhat different rules.”
Immunobiology : the immune system in health and disease / Charles A.
Janeway, Jr. et al.-- 5th ed. Garland Publishing, New York 2001

And here is some information on the normal flora (bacteria and archaea)
that inhabit the intestine of healthy animals, including humans.  

“The intestinal microflora is a complex ecosystem containing over 400
bacterial species. Anaerobes outnumber facultative anaerobes. The flora is
sparse in the stomach and upper intestine, but luxuriant in the lower
bowel. Bacteria occur both in the lumen and attached to the mucosa, but do
not normally penetrate the bowel wall.”

“Concentrations of 10^9 to 10^11 bacteria/g of contents are frequently
found in human colon and feces.”

“Although the normal flora can inhibit pathogens, many of its members can
produce disease in humans. Anaerobes in the intestinal tract are the
primary agents of intra-abdominal abscesses and peritonitis. Bowel
perforations produced by appendicitis, cancer, infarction, surgery, or
gunshot wounds almost always seed the peritoneal cavity and adjacent organs
with the normal flora.”

Medical Microbiology, Samuel Baron MD, University of Texas Medical Branch
at Galveston. 1996 Fourth Edition.

The second part of your question is about differences between humans and
other animals, and the evolution of the appendix and appendicitis.  
There is an entire webpage in http://www.talkorigins.org/
dedicated to this
question here. http://www.talkorigins.org/faqs/vestiges/appendix.html
There is also an interesting New York Times article, reprinted on this
blog, which addresses a similar question.  http://johnhawks.net/weblog/topics/senescence

So we can see from the medical literature that the appendix has a function
in immunity, but we know that people without an appendix can still survive.
 So its presence is not necessary for life, like the heart is, but it still
serves a function.  As you will read in the previous link to the
talkorigins website, “vestiges are very often complex or specialized
structures, in fact overly complex for their functions, and prime examples
are the wing of the ostrich and the eyes of blind cavefish. A vestige can
be a complex structure, in an absolute sense, while simultaneously being
rudimentary or degenerate relative to the same homologous structure in
other organisms.”

As for the specific question about appendicitis being a sign of evolution:
Some non-human primates have appendices, as do some non-primate animals.  I
don’t see any reason why those appendices could not also succumb to
inflammation and infection.  I wasn’t able to find any examples of
appendicitis in my search, but perhaps a veterinarian or someone who takes
care of research primates might be able to answer that question.  

I hope this helps!