An outbreak of gastroenteritis occurred in a youth group camp. Water at the camp, which was not chlorinated or filtered, was obtained from a spring on the premises. The farmland near the camp was grazed by cattle and sheep. Run-off from the pasture entered the camp spring. The isolated microorganism required an atmosphere containing reduced oxygen and increased carbon dioxide for its growth. In most cases, the gastroenteritis was self-limiting. Those requiring antibiotic treatment responded to erythromycin. Which is the most likely causative agent?

Campylobacter jejuni may be part of the normal flora of different domesticated animals including poultry, sheep, and cattle. Surface waters and soil where animal graze may be contaminated with C. jejuni. Therefore, C. jejuni outbreaks, which are zoonotic, may result from the ingestion of contaminated water that has not been treated. The main symptoms of C. jejuni infection are gastroenteritis, abdominal pain, fever, and malaise. The symptoms often peak at 24 to 48 hours after onset or may last for 7 to 10 days. The disease is generally self-limited. Due to the susceptibility of Campylobacter to different antibiotics including macrolides, erythromycin is the drug of choice to treat severe enteritis caused by C. jejuni. Optimum growth of Campylobacter occurs under reduced oxygen and increased carbon dioxide (5%-10%). Infection with V. cholerae results in a massive watery diarrhea that progresses to severe dehydration and hypovolemic shock. Infection with V. parahaemolyticus is associated with explosive watery diarrhea, abdominal pain, and fever. The gastroenteritis is self-limited. V. parahaemolyticus is usually found in an estuarine and marine environment and is associated with ingestion of contaminated shellfish. Enteropathogenic and entero-invasive E. coli strains are major causes of diarrhea in underdeveloped countries. Both cause watery diarrhea and vomiting. E. coli grows rapidly under aerobic conditions.