Allergic reactions are extremely common in dogs and less common in cats. Two of the most common cause of allergic reactions are bites or stings from flying insects (bees, wasps, yellow jackets, hornets) and vaccinations reactions. The bite/sting of one insect usually can not be found on the animal, even though the skin can be very red, swollen and covered with hives. However, dogs that have been attacked by a swarm can certainly show hundreds of stings all over the body.
Allergic reactions caused by insect stings usually result in clinical signs associated with the skin. The signs usually occur minutes after the sting, though a delay of several hours is also possible. Hives (urticaria) and/or a swollen nose or face are extremely common signs. Many animals will also be very anxious, pant, appear to itch all over (rubbing face or body on the ground), and possibly vomit. Rarely, the inside of the mouth and throat can swell enough to cause breathing problems. The vast majority of this type of allergic reaction requires injections to decrease the animal's discomfort. Oral medication is seldom effective enough to treat the reaction.
Vaccination reactions can also produce the same skin abnormalities mentioned above several hours after the vaccination was administered. However, some vaccination reactions can occur within an hour of the vaccination and produce very serious signs such as collapse and shock. Many of the serious vaccination reactions can occur while owners are still at the veterinarian's office! Immediate and aggressive therapy to stop the reaction and treat shock is warranted when reactions as a result of vaccinations. This type of reaction can be prevented by your veterinarian administering medications just prior to the vaccination.
Contrary to popular belief, spider bites do not cause allergic reactions. Most spider bites that can cause problems produce other signs such as a wound that does not heal and seems to spread (brown recluse) and neurologic signs (black widow spider). Common spiders found in houses and in yards (wolf spider) are not known to cause allergic reactions or serious wounds.