“Gastro” means stomach, “Entero” means intestine, so “gastroenteritis is an infection of the stomach &/or intestines (bowels). If there’s vomiting, the stomach is involved; if there’s diarrhea, it’s the intestines; if both, then both are. See Staphylococcal Food Poisoning for a discussion of the condition that causes vomiting alone.
Acute Gastroenteritis causing significant diarrhea is caused by one of the following germs in the bowels (see Diagram — Gastrointestinal System). We can try to guess whether it’s in the Small Intestine or Large Intestine, even though there are often exceptions:
- Small Intestine: stools are large volume and completely watery; non-bloody
- Large Intestine: frequent small-volume stools (squirts), with blood or mucus
Basically, viruses last a short time & go away on their own. Bacteria are potentially more serious, but can also go away on their own. Sometimes they can be treated, sometimes not. Protozoa come & go, come & go; there’s often treatment available. Scroll below for brief summaries of the main germs (may see also Differences Among Germs).
Germs that Cause Acute Diarrhea (Gastroenteritis)
|xxxxxxSmall Intestine Germs |
• Other viruses
• Clostridium perfringens
• E. coli (Traveler’s diarrhea, other strains)
• Cryptosporidium *
• Microsporidium *
• Isospora *
|xxxxxxLarge Intestine Germs |
• Cytomegalovirus *
• E. coli (hemorrhagic)
• C. difficile
• Entamoeba histolytica (Amebic dysentery)
* = AIDS-related germs (Cryptosporidium also affects people with normal immune systems)
Germs that Infect the Small Intestine
Norovirus — Very common; the most likely cause of acute diarrhea. Vomiting & diarrhea for 2-3 days, sometimes with fever. Goes away on its own. Most dangerous in babies. Spread person-to-person & contaminated food; outbreaks have occurred in restaurants, catered events, cruises, schools, camps, nursing homes, prisons, & more. Incubation period (time from transmission to beginning of illness) is 1-3 days.
Rotavirus — Vomiting, diarrhea, & fever, lasts about 4 days & goes away. Severe cases last longer. Most severe in children, can even cause breathing problems, seizures, other complications. Lots milder in adults. All babies these days get vaccinated, so disease is now much less common.
Salmonella — Related to typhoid, but the species that cause diarrhea are different. Humans get this from undercooked eggs, chicken, raw milk, sometimes meat or veggies, & from touching chickens or reptiles (pet turtles, snakes, etc.). Incubation period from transmission to onset of illness is 8 hours to 3 days.
There’s diarrhea with nausea, fever, &/or abdominal cramps, the fever eases in a few days, everything’s better in 7-10 days. However, many people with Salmonella don’t get sick at all, so finding the germ doesn’t prove it’s the cause. Also, antibiotics may make the person a carrier (the germ stays in the bowel without causing illness, but can be spread to others). So we usually only treat patients who are very sick, and also people at risk of severe illness (babies, people over 70, those over 50 with heart disease or risk factors, and anyone with weak immune systems from other diseases).
E. coli (Traveler’s diarrhea, other strains) — The Escherichia coli (“E. coli“) bacteria that causes diarrhea in the small intestine pulls fluid out of the bowel wall. There may be some nausea, the main danger is dehydration. Diarrhea goes away on its own in a few days, although 10% of patients may have it for over a week, and rare cases last a month. For travelers, instead of giving preventive antibiotics, it’s best to take a supply & begin treatment if you have 4 episodes of diarrhea in a day (usually occurring at the beginning of a trip). Note that a different E. coli strain causes large intestine diarrhea, which can be serious.
Listeriosis — A rare cause of diarrhea, which comes with fever, vomiting, maybe achy joints. It gets better in 2-3 days and isn’t serious. But Listeria makes the news, because it occurs in outbreaks from contaminated cold food, and can be dangerous in pregnancy. It causes lots of miscarriages in early pregnancy, and if a newborn gets it during delivery, the baby can be very sick.
Pregnant women can get Listeria in the blood or brain, but that’s very uncommon. The danger is greater for the elderly and people with weak immune systems from diseases like cancer, organ transplants, maybe HIV, etc.
Cholera — Wiped out in wealthier countries, Cholera causes such massive watery diarrhea that victims can die of dehydration very quickly. Certain antibiotics help, but the main treatment is lots & lots of fluids. In poor countries where IVs aren’t feasible for large numbers, patients lie on “cholera cots” with holes in them, drinking special rehydration fluid (not just water) while the diarrhea goes into a bucket underneath.
Giardia — A common parasite in poor countries, Giardia also occurs in the US. It can contaminate well water, is all over mountain streams, and is spread sexually among men who have sex with men. It causes large volume watery stools and gas, may ease on its own, but come back again if not treated. The main risks are dehydration and weight loss.
Standard tests for parasites may need to be repeated up to six times, and may still miss the diagnosis. The best test to order is a stool specimen for Giardia antigen (searching for just a fragment of the germ). Medication is available for easy treatment.
Cyclospora — Mostly a disease among travelers or immigrants from poor countries. But outbreaks have occurred from contaminated imported food. Symptoms are typical gassy watery diarrhea like Giardia; also like Giardia, it can ease up & recur, and the main risks are dehydration and weight loss. Standard parasite exams will detect it, although it may be necessary to do several stool tests. There are several medications for treatment.
Cryptosporidium — A parasite spread mostly from water in swimming pools, lakes, occasionally drinking water, and sometimes farm animals. Many infected people have mild or even no symptoms, but some get 1-2 weeks of watery diarrhea, maybe with nausea or abdominal cramps.
The parasite isn’t seen on routine stool tests, so clinicians have to tell the lab ahead of time to look for it. Some labs can test for Cryptosporidium antigen (a fragment of the parasite), which is easier. There’s no good treatment, which isn’t important for healthy people (it goes away on its own), but Cryptosporidium is quite dangerous for AIDS patients and others with weak immune systems.
Germs that Infect the Large Intestine
Shigella — Patients have frequent small squirts of diarrhea, often with blood or mucus. Most have abdominal pain (more steady than just cramps), and many have fever and/or vomiting. The species Shigella dysenteriae and flexneri are worst, while Shigella sonnei causes milder watery diarrhea. All species are pretty contagious.
In healthy people the illness goes away on its own in a week. Complications such as bowel obstruction or perforation are rare, but more common in babies, the elderly, diabetics, and others with weak immune systems (like HIV). Antibiotic treatment is available, but the germ has become resistant to more and more drugs. The stool culture that makes the diagnosis will also let us know which antibiotics work on that specific strain, and which won’t.
Campylobacter — A bacteria that infects adults as much as children, it causes frequent squirts of diarrhea (sometimes bloody), with vomiting, fever, and abdominal pain. Occasionally the pain is the main symptom, making us think of appendicitis. Most cases are milder, and get better on their own in a week.
The germ is transmitted mainly by undercooked chicken. Even if the chicken is cooked well, sometimes other foods get contaminated if prepared on the same cutting board as the meat. The germ is also contagious among men who have sex with men.
Diagnosis is made by a routine stool culture, which also identifies which antibiotics will work best. Unfortunately, Campylobacter has become resistant to many common medications, because antibiotics are often given to chickens to promote growth. Unfortunately, efforts to remove antibiotics from animal feed get stopped by lobbyists in Congress.
Campylobacter can cause some unusual late complications, by triggering an immune reaction. Our body’s own immune system can then attack joints or nerves, causing arthritis or nerve paralysis (Guillain-Barré Syndrome).
E. coli (hemorrhagic) — Escherichia coli, or E. coli for short, is the main bacteria we all have in our bowels. But the strain O157:H7 lives in cattle & other animals, and causes disease if beef isn’t cooked well. It became famous in 1993 when people across the country were infected at Jack-in-the-Box restaurants, but outbreaks have also occurred from unpasteurized juice, bagged greens, petting zoos, swimming in lakes, county fairs, etc.
Bloody diarrhea without fever is the main clue. The belly is usually tender when pressed on. Patients may need to be hospitalized. The disease goes away on its own in a week, if there are no complications.
But the most feared complication is Hemolytic-Uremic Syndrome (HUS): “hemolytic” means red blood cells fall apart, “uremic” means kidney failure. Half of patients need temporary dialysis, some wind up with permanent kidney damage, seizures occasionally occur, one out of 25 patients die. HUS occurs in about 10% of persons with O157:H7, but 15% in children.
All specimens of bloody diarrhea should be tested specifically for O157:H7. Some laboratories do this automatically, but others won’t unless the test is specifically requested. Another problem is that in Europe and elsewhere, other strains cause HUS (e.g. O104:H4 in Germany), and may or may not show up depending on the test that’s done. Ideally, tests should also be done for Shiga Toxin, which is what the bacteria produce to cause HUS.
Even though antibiotics can kill E. coli, they also make it more likely for HUS to develop. Therefore, antibiotics should not be given to patients with bloody diarrhea and no fever. Bloody diarrhea with fever could be Shigella, and antibiotics may help.
Clostridium difficile (“C. Diff”) — This nasty bacteria used to be acquired mainly in hospitals. Now, you can pick it up anywhere. The elderly are at risk, and especially people who’ve recently taken antibiotics for anything else (they kill off harmless bacteria in our bowels, so C. Diff can establish a foothold and multiply).
Many people who have C. Diff don’t get sick (but can be carriers who spread it around). Others have a chronic diarrhea. Some get severe diarrhea. Some keep getting it over & over, even after treatment. And in rare cases, the bowel can swell up enormously (“toxic megacolon”), which can be fatal.
We diagnose C. difficile by a stool test for its toxins. The test is important to do on anyone with diarrhea who’s been in a hospital recently (including employees, even long-term visitors), on those who’ve taken antibiotics in the last 2 weeks, and on people with diarrhea that goes away and keeps returning. Treatment is available. We don’t test people without diarrhea, because treating carriers doesn’t work.
Entamoeba histolytica — A parasite that’s the cause of Amebic dysentery. Bloody diarrhea is the main symptom; it may be mild, or severe. We think about Ameba in people who’ve traveled to poor countries, and also men who have sex with men.
We usually diagnose Entamoeba histolytica by stool tests for parasites, looking for them under the microscope. The problem is, there’s a related species Entamoeba dispar that’s much more common, and is harmless. It’s especially found among gay men. So we should never diagnose “ameba” without doing an extra “antigen detection test” to prove that the species is E. histolytica (the bad one). In fact, the antigen test is more accurate than traditional parasite exams, and should be done to begin with.