Brainerd diarrhea is a syndrome of acute onset of watery diarrhea (3 or more loose stools per day) lasting 4 weeks or longer, which can occur in outbreaks or as sporadic cases. It is named after Brainerd, Minnesota, the town where the first outbreak occurred in 1983. Patients typically experience 10–20 episodes per day of explosive, watery diarrhea, characterized by urgency and often by fecal incontinence. Accompanying symptoms include gas, mild abdominal cramping, and fatigue. Nausea, vomiting, and systemic symptoms such as fever are rare, although many patients experience slight weight loss.

Despite extensive clinical and laboratory investigations, the cause of Brainerd diarrhea has not yet been identified. Although it is thought to be an infectious agent, intensive searches for bacterial, parasitic, and viral pathogens have been unsuccessful so far. The possibility remains that Brainerd diarrhea is caused by a chemical toxin, but no such toxin has yet been found.

Because the etiologic agent is unknown, there is no laboratory test that can confirm the diagnosis. Brainerd diarrhea should be suspected in any patient who presents with the acute onset of nonbloody diarrhea lasting for more than 4 weeks, and for whom stool cultures and examinations for ova and parasites have been negative. Care should be taken to exclude other causes of chronic diarrhea, both infectious and noninfectious (e.g., lymphocytic colitis, collagenous colitis, tumors, drug reactions). Brainerd diarrhea is not characterized by any specific laboratory abnormalities. On colonoscopy, petechiae, aphthous ulcers, and erythema may be observed. Microscopic examination of colonic tissue biopsy specimens often reveals mild inflammation, with an increased number of lymphocytes, particularly in the ascending and transverse colon. The stomach and small intestine generally appear normal.

There is no known curative treatment for Brainerd diarrhea. A variety of antimicrobial agents have been tried without success, including trimethoprim-sulfamethoxazole, ciprofloxacin, doxycycline, ampicillin, metronidazole, and paromomycin. Neither has there been any response to steroids or antiinflammatory agents. Approximately 50% of patients report some relief in symptoms with high doses of opioid antimotility drugs, such as loperamide, diphenoxylate, and paregoric.

Brainerd diarrhea usually resolves on its own or without specific medical treatment. Symptoms may last a year or more, and typically come and go. Long-term follow-up studies have shown that almost all patients get better by the end of 3 years. There have been no known cases of a person developing a chronic condition after the acute onset of Brainerd diarrhea or of a person relapsing once the illness has resolved completely.

Seven outbreaks of Brainerd diarrhea have been reported since 1983. Six occurred in the United States, five of which were in rural settings. One outbreak occurred on a South American cruise ship based in the Galapagos Islands. The original Brainerd outbreak, which involved 122 persons, was the largest outbreak. An outbreak in Henderson County, Illinois, involved 72 persons; the Galapagos Islands outbreak involved 58. A survey of gastroenterologists suggested that many patients who are not associated with a recognized outbreak seek treatment for illness compatible with Brainerd diarrhea. Further work needs to be done to establish the incidence of sporadic cases of Brainerd diarrhea.

In the original Brainerd outbreak, raw (unpasteurized) milk was implicated as the vehicle for disease transmission. Contaminated and inadequately chlorinated or unboiled water has been identified as a source of Brainerd diarrhea in several other outbreaks. For example, illness was strongly associated with drinking untreated well water in the Henderson County outbreak. Persons who drank the same water after it was boiled did not get sick. Contaminated water was also implicated in the Galapagos Island outbreak. The diarrheal illness does not spread contagiously from one person to the next.

Avoiding drinking raw (unpasteurized) milk and water that have not been properly chlorinated or boiled will help reduce the risk for Brainerd diarrhea and many other diseases. Once the cause of Brainerd diarrhea is identified, more specific prevention measures can be formulated.