Infantile gastroenteritis is a common illness worldwide, causing significant morbidity and mortality, especially in developing countries. Its main characteristic is the presence of diarrhea, defined as stools more than 15g/kg for infants younger than 2 years and greater than 200 g for children and beyond that age and adults.
Infantile gastroenteritis presentation widely varies depending on its etiology and severity. In viral gastroenteritis, most often caused by rotavirus, there is low fever, vomiting, and water-like stools. Most rotavirus patients are younger than 2 years . Blood is absent in the fecal matter.
The existence of dehydration should be thoroughly assessed. Important clinical indicators are low blood pressure, tachycardia, thready pulse, significant weight loss, dry mucous membranes, and oliguria. The consciousness state may be decreased in severely affected infants.
The physician should inquire about the types of food the patient has ingested, which may point to a specific pathogen: dairy products may contain Staphylococcus, Campylobacter, Listeria or Salmonella species. Meat can be infected by Clostridium perfringens, Aeromonas, Staphylococcus and Salmonella or Campylobacter species, while seafood intake may lead to astrovirus, Vibrio, Aeromonas or Plesiomonas species infection. If symptoms occur sooner than 6 hours after ingestion, a preformed toxin, like those produced by Bacillus or Staphylococcus should be suspected. Nosocomial infection of various types has also been documented .
Aeromonas induces acute watery diarrhea or a more severe, cholera-like illness, with blood present in the stool . Bacillus cereus causes precocious emetic syndrome that usually resolves within 24 hours  and watery diarrhea accompanied by severe cramps . Campylobacter infection is characterized by a pre-diarrhea period, with fever, myalgia and abdominal pain . Clostridium difficile may be complicated by pseudomembranous colitis . Escherichia coli leads to enteritis that may progress to hemorrhagic colitis and hemolytic uremic syndrome . Diarrhea usually lasts for 1 to 3 days and is accompanied by headache, dizziness, lymphadenopathy, rash and myalgia . Nontyphoidal Salmonella infection is characterized by diarrhea that usually lasts less than a week, but may lead to extraintestinal complications, such as urinary tract infections, osteomyelitis or arthritis. Salmonella typhi is the etiological agent of typhoid fever, a condition with diarrhea, vomiting, anorexia, fever, headaches and rose spots. Bloody stools may be present both in Salmonella  and Shigella patients . Cholera is described as afebrile, watery, painless diarrhea. A more severe form, cholera gravis may give rise to an immense liquid loss which rapidly progresses to severe dehydration and death . Yersinia enterocolitica can be the causative agent of terminal ileitis and mesenteric lymphadenitis and may mimic appendicitis . Complaints may persist up to one year in Yersinia infection . The physician should inquire about associated symptoms of parasitic infections, like anal pruritus, that may be accompanied by diarrhea.
In cases where physical examination suggests a bacterial, protozoal or parasitic infection, laboratory tests are required in order to elucidate the etiology. In all situations where dehydration signs are observed, the physician should evaluate the gravity of the condition by ordering complete blood cell count, serum electrolytes, urea, and creatinine.
Clinical judgment indicates what tests may be necessary for a specific patient. Giardia lamblia is identified by enzyme immunoassay. Rapid antigen stool testing may highlight the presence of rotavirus, while polymerase chain reaction is used in calicivirus infection. Human astrovirus genotyping is possible in selected cases . The stool should be examined for parasite ova and larvae , as well as leukocytes, that signify enteroinvasive infection. Bacterial cultures are extremely valuable  and should always be performed if the patient is febrile. Cultures for Campylobacter, Salmonella, and Shigella should be obtained if white or red blood cells are identified in the stool. Escherichia coli is identified if the stool is cultured on a specific environment such as a chromogenic media . This is method valuable in Yersinia , Salmonella  and Vibrio  species, as well. A blood agar plate is useful for the detection of Aeromonas spp., Vibrio spp. and Plesiomonas spp. Campylobacter should be cultured on blood-free charcoal-cefoperazone-deoxycholate agar or Skirrow medium  or it can be identified using its characteristic Gram stain morphology. Antibiotic susceptibility testing is indicated in infants younger than 6 months or immunocompromised children, as well as those with prolonged evolution.
If the diarrhea is considered to be part of an ulcerative colitis or Crohn's disease, a colonoscopy may be indicated. This procedure sometimes visualizes pseudomembranes in Clostridium difficile infection. If this microorganism is identified, the physician should keep in mind and monitor the risk of developing toxic megacolon , intestinal perforation , renal failure or septic shock.