Prep for USMLEPrep for USMLE
         Forum      |     Resources New Posts   |   Register   |   Login

 Bloody vs Watery diarrhea  

Post Reply  
  • 0/5
  • 1
  • 2
  • 3
  • 4
  • 5

Author18 Posts

Which Organisms cause bloody diarrhea?
Which Organisms cause watery diarrhea?
Which Organisms cause BOTH?

Is there any way to remember this?
Is it safe to assume that invasive bacteria can cause bloody diarrhea, and toxigenic bacteria cause watery diarrhea?

*EHEC is toxigenic, and causes watery diarrhea but can also cause bloody diarrhea.


vibrio cholera, ETEC, Rotavirus, cryptosporidum, and Girardia

salmonella, shigella, campylobacter, EHEC, EIEC, E.coli O157 H7, Yersinia enterocolitica, and Entamoeba histolytica.

If anyone knows more, please add to the list.


bloody diarrhoea - if there is necrosis of epithelium
1. invasive organisms - like Salmonella and Shigella which kill enterocytes
2. toxins which lead to necrosis

watery diarrhoea - without necrosis,
if 1. bacteria cover absorptive surface or 2. decrease intestinal villi (due to toxins) so less can be absorbed from the intestines or
3. increase water and electrolytes secretion (due to toxins)


Thanks mildus.
Can you please give some examples for each of your points (which organisms work by each mechanism?)
Thank you. smiling face


bloody diarrhoea
1. invasive organisms
Salmonella, Shigella and EIEC (they enter enterocytes, multiply in their cytoplasm, and then spread to the next enterocytes while the previous die by necrosis) (necrosis --- so bleeding --- and reactive secretion of water and mucus)
2. toxins which lead to necrosis
Shigella dysenteriae with shiga toxin which inhibits protein synthesis thus leading to the cell death (necrosis), also EHEC which has shiga-like toxin

watery diarrhoea - without invasion and necrosis
1. bacteria cover absorptive surface so less can be absorbed from the intestines e.g. EAEC and DAEC and Giardia
2. decreased number of intestinal villi
e.g. EPEC
3. increase water and electrolytes secretion (due to toxins)
e.g. Vibrio cholerae with cholerae toxin,
ETEC with LT-I toxin which is like cholerae-toxin and acts via cAMP,
ETEC with St-a toxin which acts via cGMP


Good job! Thanks mildus.


smiling face


just to diverge a little: osmotic diarrhea caused by malabsoption(eg lactose intolerance) of osmotically active substanes causing a net transudate of water and electrolytes from the blood to the gut lumen


Here's something i found:

Organisms that cause bloody diarrhea:

Edited by DrVirgo on Feb 15, 2006 - 6:16 AM


And for Watery diarrhea:

We wreck the GI: V, R, E, C, the Gi.
These organisms wreck the GI and cause watery diarrhea.



nice mnemo


Great work DrVirgo!


clostridium difficile ;in FA microbiology section it is mentioned as one of causes of bloody diarrhea ,but in one of FA questions a case presented by watery diarrhea and the answer was c.difficile so now i am confused .

anyone can help ?


Having personally experienced both Shigella flexneri in 1982 and Salmonella Group B a couple of years ago, I can attest to the fact that Shigella produces bloody mucus (and high fever of 104 in my case) about 8 hours after onset. I was exposed via a culture I worked on about 36 hours previous to onset.

Salmonella can cause a self limiting diarrhea or it can become systemic. I contracted Salmonella B from another culture I worked on. I did not have blood, but I did have a general malaise and loose stool, with kind of a low grade temperature. I was suspicious I had gotten it from work as I had worked on a Salmonella, and the one I worked on and my serotype (sent to the state Dept. of Health lab) were identical.

I worked on a patient's specimen(s) a couple of weeks ago. An elderly woman had Salmonella in her stool, and I checked her other cultures that were set up, to find out that she had a gram negative rod in a blood culture. Luckily it was growing enough to type it, and she also had sepsis (invasive) Salmonella. I called the floor and the only information I got from them is that she came from Florida. I am in Ohio so that's quite a distance. Did not find out what caused her infection, but I did notice that the susceptibility pattern was rather resistant for a Salmonella (they are usually fairly sensitive to most drugs) although we only report out a few for Salmonella (and I don't remember them off the top of my head - sorry).

Bottom line - you can memorize poop characteristics all you want, but it's always best to take a history and get a culture. It absolutely amazes me how many doctors will order an Ova and Parasite examination on a diarrheal specimen for a patient who has never left the country but don't order a stool culture. I cannot tell you how many bloody mucousy stools I've seen over 35 years that have never had a culture run on them. I've also seen specimens come in with the diagnosis "diarrhea" and have the specimen be perfectly formed or hard as a rock :-(.

We run a lot of C. diff toxins on stool specimens and reject those on formed specimens. Many times the nurses can tell the patients have C. diff because of the smell of their stool - same with Rotavirus. Trust your nurses on this point - they see it all. C. diff poops usually are foul looking with mucus. Rotavirus poops tend to be yellowish and loose.

And PLEASE take a history from the patient before you order O&P testing. We now only do Giardia Antigen/Cryptosporidium Ag testing on stools ordered for O&P's unless the doc can tell us the patient has a travel history to an endemic area or long-standing (>one MONTH) diarrhea. I have heard that Giardia infections (have heard this from people who have had it) produce lots of gas and foul smelling and copious stools. If they have well water, the antigen testing is far more sensitive than an O&P exam.

I'll get off my soapbox now nod


Giardia causes malodouress steatorrheanod


Also Norwalk virus causes watery diarrhea
In general, parasites & viruses cause watery diarrhea
Exception for parasite is Entamoeba histolytica which causes bloody diarrhea




You can get off your soapbox now.

Bookmark and Share

This thread is closed, so you cannot post a reply.

Login or Register to post messages

show Similar forum topics

bloody or watery diarrhea
Goljan p. 79 mentions bloody diarrhea
bloody diarrhea
show Related resources

Advertise | Support | Premium | Contact