Acute diarrhea is majorly infectious, particularly viral, yet, bacterial causes make up most cases of severe diarrhea. 

In adults with acute diarrhea who present to medical care, the initial evaluation should contain an assessment for extracellular volume depletion (e.g., dark yellow or scanty urine, reduced skin turgor, orthostatic hypotension) and determine the symptom duration, frequency, and stool characteristics, and associated symptoms (e.g., fever and peritoneal signs).

The presence of inflammatory features (e.g., fever or bloody or mucoid stool) suggests an infection of the large bowel, which encloses pathogens distinct from small bowel infections. Potential exposures, like eating history, residence, occupational exposure, recent and remote travel, pets, and hobbies, render further diagnostic clues about the potential microbiologic etiology.

Most patients are suitable for receiving expectant management for several days without undergoing microbiologic stool tests. However, standard stool cultures (or multiplex molecular testing) are needed for patients with acute community-acquired diarrhea and those presenting-

• Severe illness (profuse watery diarrhea, signs of hypovolemia, the passage of ≥6 unformed stools/24 hours, severe abdominal pain, need for hospitalization).
• Signs of inflammatory diarrhea (bloody diarrhea, small volume mucous stools, fever).
• High-risk host features (e.g., age ≥70 years, cardiac, inflammatory bowel disease, immunocompromising condition, pregnancy).
• Symptoms continuing for more than a week.
• Public health concerns (e.g., diarrheal illness in food handlers, health care professionals, and individuals in daycare centers)

Further diagnostic testing relies on the presenting features. Grossly bloody diarrhea needs testing for Shiga toxin (to identify Shiga toxin-producing Escherichia coli [STEC]) and fecal leukocytes or lactoferrin, if available. Testing for Clostridioides difficile is needed in cases of recent antibiotic use or healthcare exposure. Parasite testing is not needed in the majority of patients with acute diarrhea, yet it is useful, in patients with persistent diarrhea, in men admitting sexual intercourse with men, in immunocompromised patients, during a community waterborne outbreak (associated with Giardia and Cryptosporidium), or with bloody diarrhea with few or no fecal leukocytes (associated with intestinal amebiasis). 

The essential therapy in diarrheal illness is volume repletion, preferably by the oral route, with solutions containing water, salt, and sugar. Adults with severe hypovolemia should be given intravenous fluid repletion followed by switching to oral rehydration solutions. 

In most patients with community-acquired, non-travel-associated diarrhea, routinely administering empiric antibiotic therapy is not required. Antibiotic therapy can reduce the diarrheal duration and other symptoms by several days but may be associated with side effects, bacterial resistance, normal flora alterations (and increased susceptibility to C. difficile infection), and cost. 

However, empiric antibiotic treatment may have a greater relative benefit in selected patients, such as in patients with severe disease, features indicating invasive bacterial infection (bloody or mucoid stools), or host factors associated with an increased risk for complications.

Empiric antibiotic therapy should be withheld pending stool testing to rule out E. coli O157:H7 or Shiga toxin production in stable patients where there is an increased possibility of STEC (e.g., bloody diarrhea in the setting of an outbreak or in an afebrile patient). 

For patients needing empiric antibiotic therapy, azithromycin or fluoroquinolone is preferable. Azithromycin is a choice for patients with fever or dysentery (bloody or mucoid diarrhea) and in patients at risk for a fluoroquinolone-resistant pathogen (e.g., in patients with diarrhea after travel to Southeast Asia or during outbreaks of resistant pathogens).

Despite identifying a bacterial pathogen, not all patients warrant antimicrobial therapy, and STEC particularly should not be managed with antibiotics. 

For those who desire symptomatic therapy, the antimotility agent loperamide can be used cautiously in afebrile or low-grade fever patients and with non-bloody stools. For patients with clinical features suggestive of dysentery (fever, bloody or mucoid stools), antimotility agents should be avoided unless antibiotics are given because of concerns of prolonging disease in such infections. In such patients, bismuth salicylate can be used as an alternative. Racecadotril is another effective antisecretory agent; however, not universally available. 

UpToDate[Internet]. LaRocque R, Harris JB. Approach to the adult with acute diarrhea in resource-rich settings. 2022. Available from: https://www.uptodate.com/contents/approach-to-the-adult-with-acute-diarrhea-in-resource-rich-settings/print.