Young animals are especially prone to infestation with intestinal worms and microscopic parasites called coccidia. These parasites are common causes of diarrhea in kittens and puppies. Severe infestations may cause vomiting as well. Some intestinal worms are potentially contagious to humans.
Adverse reactions to rapid diet changes also may cause the sort of gastrointestinal upset that you describe. Foreign objects lodged in the intestines, infection with a number of viruses or bacteria, and inflammatory disorders of the intestines are on the list of causes but are less common.
Your cat needs to see a vet. Be sure to take a stool sample so that he or she can test for parasites. After evaluating your pet and looking at his deworming and vaccination history, the veterinarian will be able to offer an educated assessment of the situation.
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