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The cause or etiology of the fever and gastroenteritis
has to be confirmed first along with the severity of dehydration.
If the child is accepting fluids and has mild to moderate dehydration then WHO oral rehydration solution is preferred
as much as he can take or 75 ml/kg over four hours.If severe dehydration
suspected then it's best to give intravenous fluids either normal saline or ringer lactate would be the preferred ones .Isolyte P contains potassium and you should know the renal function and electrolytes values before you start that.
The rate would be calculated by the treating pediatrician
depending on the weight , grade of dehydration and ongoing losses.
The treatment for fever would be acetaminophen
15 mg /kg per oral , the vomiting may subside once hydration is corrected or may require antiemetic like domeperidone or metoclopramide
or onadansetron as per prescription.
Diarrhoea if viral should decrease on it's own or with supportive probiotics and zinc therapy .
However I would advise you to get stool routine and culture done since problem has persisted for three days .
Your pediatrician may want to start antibiotics like ofloxacin
or colistin or cefixime in prescription doses if bacterial infection is suspected.
The most important thing is to prevent dehydration by giving frequent sips of fluids or iv fluids.
All the best .
Wish your child a speedy recovery.