In the most severe cases, dogs can develop anaphylactic shock. In canines, the shock organ is the gastrointestinal (GI) tract (in contrast to cats and humans, in which it is the lungs). Dogs in anaphylactic shock do not necessarily develop difficulty breathing. They are much more likely to develop sudden onset of vomiting, diarrhea, and collapse. The diarrhea and vomit can both be extremely bloody, in some cases.