A 3 year old boy presents with a 1 day history of loose stools, fever, abdominal cramping, headache, and myalgia. He has no blood in the stool. A careful history reveals that he has several pet tules. Which of the following is most likely the causative agent of his diarrhea?

Salmonella spp., including S. enteritidis and S. typhimurium, produce a gastroenteritis or enterocolitis. Patients with decreased gastric acidity, sickle cell disease, defects in immunity, or children younger than 4 years have a more severe course of disease. Salmonella spp. are carried in nature by animal reservoirs such as poultry, tules, cattle, pigs, and sheep. The incubation period is 8-48 hours after ingestion of contaminated food or water. Chlamydia psittaci produces an interstitial pneumonitis accompanied by headache, backache, and a dry, hacking cough. A pale, macular rash is also found on the trunk (Horder's spots). Patients at risk include pet shop workers, pigeon handlers, and poultry workers. Entamoeba histolytica produces a diarrhea (frequently bloody or heme-positive), right lower quadrant crampy abdominal pain, and fever. Patients frequently have weight loss and anorexia. There is usually a history of travel outside the U.S. Most cases are chronic. Complications include liver abscesses. Staphylococcus aureus produces a self-limited gastroenteritis due to the production of preformed, heat-stable enterotoxins. The incubation period is 16 hours. The toxins enhance intestinal peristalsis and induce vomiting by a direct effect on the CNS. Ref: Ray C.G., Ryan K.J. (2010). Chapter 33. Enterobacteriaceae. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.