But against any of the Children of Israel, a dog shall not whet its tongue (Ex. 11:7)
Animals, and particularly dogs, are the first to be aware of the approach of a natural catastrophe; their frenzied barking is often the first indication that anything is wrong. Thus, when the dogs in Egypt remained silent, it demonstrated that the slaying of the firstborn was a supernatural plague rather than an outbreak of illness or natural epidemic. (Kol Omeir Kera)
The dog is the most faithful and empathetic of all domestic animals. If someone in the household should die or be injured, a dog will make the most heart-rending noises to express its grief. Thus, after describing the terrible confusion that the slaying of the firstborn would cause - "there shall be a great cry throughout all the land of Egypt, such as there was none like it, nor shall be like it any more" - the Torah reassures us that the dogs in the Jewish sector would have no reason to bark. (Rabbi Yeshayahu Horowitz of Vienna)