Q. I am wondering if you wouldn’t mind clarifying something for me about which bacteria are actually the ones that will cause BLOODY diarrhea vs watery diarrhea. Do they necessarily have to be invasive? And with that, does invasive necessarily mean fever and systemic signs? I know this is a large question, but I feel like we have heard a few conflicting categorizations of these in addition to what it says in FirstAID.

A. Regarding classifying bugs into watery vs. bloody diarrhea: the short answer is, it depends on which source you’re looking at. Different authors classify the bugs into different categories. Some simply talk about watery vs. bloody diarrhea, and others include other descriptive categories, like “dysentery” (small volume, bloody and pus-filled stools), or “bloody watery.” Here’s a summary:

Bloody diarrhea: Shigella and certain strains of E. coli, especially EHEC. Shigella is invasive; EHEC is technically not invasive – it remains confined to the surface of the intestine where it exerts its effects through toxins (but the toxins can “invade”, getting into the bloodstream and finding their way to the kidneys). Shigella is sometimes put into a separate “dysentery” category, because the stools are not just bloody, but pus filled, and they are frequent and small in volume.

Bloody and watery diarrhea: Salmonella, Campylobacter, Yersinia. These bugs are (or can be) invasive.

Watery diarrhea: Certain strains of E. coli (like ETEC), and Vibrio cholerae. Campylobacter may also cause watery diarrhea, especially in infants. These bugs do not invade (campylobacter is capable of either invading, and causing bloody/watery diarrhea, or not invading, and causing just watery diarrhea).

The bugs listed above are the most important ones for you to remember – either because they are the most common, or because they are the most dangerous. Obviously, there are many other bugs that can cause diarrhea – and I think it may be best to think of these as simply causing diarrhea which is not particularly watery or bloody. These bugs are usually described as simply causing “enterocolitis” – which is a general term for inflammation of the intestine.

Finally, you asked about whether invasion means the patient will have systemic signs, and the answer to that is yes. Any time you have an invasive infection – meaning the bug gets into the tissue in question and causes inflammation, the patient will likely get a fever. All those inflammatory cells (particularly the neutrophils) secrete cytokines that act on the fever centers in the brain. EHEC infection, as nasty and potentially fatal as it is, does not usually present with fever, because the bugs are not technically “invasive”.

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4 Responses to Which bugs cause bloody diarrhea?

  1. Mukarram says:

    I searched Google for “bacteria that cause bloody diarrhea” and got pretty excited when pathologystudent.com was the first result. Your site is awesome, and your answers are always helpful, concise, and easy to understand. Thank you.

  2. Kristine says:

    Thanks so much, Mukarram! So glad you like the site 🙂

  3. madeeha says:

    V nicely answered.

  4. Ivy says:

    Thanks so much for this! Quick question: what is the difference between osmotic and secretory diarrhea? Are ETEC and vibrio considered secretory diarrhea? And is there a way to know if WBCs will be in the stool? I know some questions usually ask that..I guess it depends if the bug is causing the problem or if it’s due to a toxin. Sorry…I guess this wasn’t a quick question haha. But thank you for your help!

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