What it is
The lining of the stomach and gut is thin but hard-working: it digests food, absorbs water and protects the body from the bacteria that live in the gut. When a bad mouthful or an infection irritates it, the lining turns red and swells, food is no longer digested, and fluid that should be absorbed leaves the body in the stools or vomit instead.
Gastroenteritis is a sudden, short-lived condition, which sets it apart from long-standing gut inflammation that lasts for weeks or keeps coming back. The biggest danger is the loss of fluid. Puppies and small and elderly dogs weaken faster than strong adult dogs, and a dog that is vomiting and has stopped drinking cannot replace what it loses.
Stop your dog eating what it finds on walks, keep the bin closed, and change foods gradually, over several days. If your dog is vomiting or has diarrhoea, watch whether it drinks and note how many times it has happened.
Signs you may notice
The dog vomits, has diarrhoea, refuses food, turns lethargic, its stomach gurgles and its belly may hurt when touched. The stools may be watery, with mucus or streaks of blood.
When to see a vet
Book a visit if the vomiting or diarrhoea lasts more than a day. A puppy, a dog that vomits again and again and has stopped drinking, a very lethargic dog, or blood in the stool or vomit needs a vet as soon as possible.
This is general information, not veterinary advice. For your own dog's health, tests and treatment, see a vet.