March 27, 2002
                 Written by: Lisa C. Beagan, DVM

                 Pets can have adverse reactions to their annual booster shots.Although generally speaking, vaccines are wonderful things that protect our furry friends from fatal diseases, when you consider how a vaccine works, it’s easy to understand why some animals will respond to a vaccine with signs ranging from a mild swelling at the site of injection to sudden death.

                 The vaccines contain antigens (foreign substances) which initiate your pet’s immune system to mount a normal response to something foreign. This immune stimulation protects against disease through the formation of antibodies. Your pet develops antibodies to the antigen in the vaccine, which will fight off the disease if the animal is exposed to it.

                    The signs of an allergic reaction vary widely but generally may include hives, facial swelling that can progress to more severe swelling of the neck, vomiting or nausea, anaphylaxis/shock, or even sudden death.

                 This immune stimulation can also cause a normal inflammatory response (swelling) at the site of the injection. Another common reaction is a host of flu-like symptoms that may last 24 to 48 hours and may include lethargy, soreness, and even a mild fever. In these cases, the pet often will eat normally and will return to       normal within 24-48 hours without treatment.

                 However, the same response that causes the desired immunity against disease can also, in very rare situations, cause more severe allergic reactions that may be worse than the disease itself. These allergic reactions are termed hypersensitivity reactions and  can occur right away (type I hypersensitivity reactions) or manifest up to 48 hours after the vaccine is administered (type IV or delayed hypersensitivity).

                 It is not uncommon for pets to have an allergic reaction to vaccines after the second dose is administered and not after the first, although some will react after the first as well. The reason for the reaction to the second dose is that because the body has already developed antibodies with the first vaccination, the pet experiences a heightened reaction to the next vaccination. It is possible that animals react to different components in the vaccine including the adjuvant (substance that the antigen is mixed with) or the antigen. Because different manufacturers use different adjuvants, pets may react to one brand of vaccine and not another.

                 The signs of an allergic reaction vary widely but generally may include hives, facial swelling that can progress to more severe swelling of the neck, vomiting or nausea, anaphylaxis/shock, or even sudden death. Most of these reactions occur within a few hours of vaccination and if your pet exhibits any of these symptoms, it is important to call your veterinarian immediately.

                 Your veterinarian will likely recommend that you bring your pet back to the hospital (or in the case of a horse that the veterinarian will return) as soon as possible. Depending upon the severity of the reaction, the doctor will likely administer medications to reduce the body’s response, including antihistamines and           anti-inflammatories like steroids. If the pet has had a severe anaphylactic reaction, the vet will treat it with epinephrine and fluids.

                 If your pet has had a reaction to a vaccine in the past, there are several things that your veterinarian may recommend to ensure that the next round of booster shots are worry-free. By giving different vaccines at different times (usually separated by two-week intervals), the doctor may be able to determine which vaccine your pet reacts to; by splitting vaccines in the future, your pet will be exposed to fewer antigens at one time. The veterinarian also may recommend performing vaccines in the morning to allow for the opportunity to monitor your pet throughout the day. In some cases, pets are pretreated with antihistamines or anti-inflammatories before receiving the vaccine. If your pet had a reaction to a vaccine in the past, it is important that you have a veterinarian administer and monitor all future vaccines.

                 In cases of very severe reactions, it may even be recommended that your pet not receive specific vaccines in the future. In these cases, the veterinarian often will also recommend performing antibody titers (a blood test that looks for antibodies to a specific antigen). If the titers are not sufficient and the risk of exposure is high enough, the veterinarian may opt to vaccinate the animal while taking all the necessary precautions. If the titers are high enough, however, the veterinarian will likely not want to take the risk of vaccination. These titers should not be used as a replacement for vaccination in pets that do not have reactions        since it is unclear what levels are considered protective. Because of this, performing titers is not considered the best way to ensure protection against disease but it is the best option when the risk of a severe reaction is high.

                 The chance of any type of vaccine reaction is very low, but it is important that owners know the symptoms of an allergic response and what to do should one occur.

                The chance of any type of vaccine reaction is very low, but it is important that owners know the symptoms of an allergic response and what to do should one occur. Just because your pet reacted to one specific type of vaccine does not mean that he will react to a different one; in fact, pets often will only react to one antigen within a vaccine and not to others. Also, it may be recommended that a vaccine from another manufacturer be used since the reaction may have been to the specific adjuvant in the vaccine.