This is one of those blogs that I feel should be written and shared with everyone who vaccinates their pets but is also possibly going to upset some of my colleagues. So, I’ll start this one off with another reminder that this blog projects the opinions of a single veterinarian (me) and while my opinion is shaped by my understanding of the evidence available, my opinion is not necessarily shared by every other veterinarian. Nor am I certain that my interpretation is necessarily the correct one, it is the one that makes the most sense to me with what I understand. Should my understanding change, so will my opinion.

Nearly every pet is brought to the vet’s office annually for “shots”. This is done so routinely that many times people might not think about what is in the “shots” we are giving and what the consequences might be. Today I will try to address that. I will do my best to keep this species neutral but vaccine reactions do tend to be more of a dog problem.

A vaccine is meant to stimulate the immune system to produce antibodies to a specific infectious disease. As a result, vaccines contain either killed or attenuated infectious organisms. These organisms are the same organisms that cause diseases like Parvovirus, Distemper and Rabies. This stimulation of the immune system is naturally an inflammatory process. The muscle aches and soreness associated with illnesses like influenza are a good example of this inflammation. An allergic reaction is an individual inflammatory reaction to specific proteins. In the case of vaccines one of two reactions is possible. A type I reaction is immediate and usually occurs before the patient has even left the building. A type IV reaction, on the other hand is also called a delayed hypersensitivity reaction and can occur anytime in the 48 hours following vaccination. Vaccine reactions can include nausea, hives or facial swelling. In extreme cases they can include anaphylaxis and possibly death.

A study performed by the American Veterinary Medical Association in 2005 showed that vaccine reactions occur in 1 out of every 250 pets receiving vaccines. It turns out that small breed neutered male dogs were more likely than others to have reactions and that smaller dogs have more reactions than larger dogs. This may make a person think that giving smaller doses of vaccines to smaller dogs might be the way to go with this issue. Vaccines are not dose dependent, there are a particular number of antigen or disease particles that have to be present in order to stimulate a proper immune response and protect your pet regardless of size. If only half of these particles are given, for example, then it is possible and likely that your pet is not protected against the disease we are trying to prevent. In the case of Rabies this is a very serious matter.

It also turns out that giving more vaccines in a single visit increases the chances of causing a vaccine reaction.

After a vaccine visit some soreness, lethargy and even mild swelling at the injection sites is to be expected. This is the body’s natural reaction to being exposed to diseases and as unpleasant as it is, this is what we are trying to accomplish. If we can get your pet’s body to “believe” that it has been exposed to Rabies, for example, then it will make antibodies against the disease. With these antibodies in place the body can then fight off the Rabies virus should it ever be encountered. This saves not just animal but also human lives.

If your pet is having a reaction you can expect to see some or all of the following. Vomiting, hives, swelling of the face and complete collapse/unconsciousness. Notify your veterinarian even if you think that the vomiting might have been due to car sickness from the ride home. We will want to make a record of it and may consider taking preventative measures in the future. If your pet is showing signs of facial swelling or loses consciousness do not call your vet, bring your pet to them quickly!

Once your pet has had a vaccine reaction you will most likely not take any comfort in knowing that your pet is in less than 1/2 of 1 percent of the pet population. If they have had a severe or life threatening reaction they are even more rare. In the future you will want to take some preventative measures to be sure that this doesn’t happen again. There are 4 major things we can do to help to prevent vaccine reactions from repeating themselves in the future. The first thing we can plan on is adding in a preventative dose of an antihistamine before vaccination from now on. This will have the added benefit of helping to prevent motion/car sickness of the ride home. We might also put off that car ride home and keep your pet for observation for the following few hours. This will mean making vaccine appointments in the morning and picking Fluffy up in the evening. This means more expense and time from your part but it is well worth the money and effort. The second thing we can do to help to prevent vaccine reactions is to not give more than one vaccine at a time. Because giving vaccines within two weeks of each other can interfere with each other and the immune system we will never split up vaccines and give them less than two weeks apart. This would run the risk of your pet not being properly protected against the diseases we are trying to prevent. The third line in prevention would be to remember to tell us if your pet has reacted to vaccines in the past. This is especially important when changing veterinarians and in pets who travel and may see more than one vet. The fourth and final preventative measure is more a word of caution. If your pet has ever had a reaction to a vaccine, never have them vaccinated at a vaccine clinic. These busy and high paced events are typically not properly equipped to handle a vaccine reaction.

As an aside, a vaccine clinic is not the same as a low cost clinic. Vaccine clinics are those high number events that do not typically occur at a veterinary clinic and are aimed at increasing the total number of vaccinated pets. Low cost spay/neuter and vaccine clinics are typically veterinary practices that offer the basic preventative care for people who may not be able to afford typical veterinary care. The aim in these clinics is to increase access of veterinary care to pets that may not otherwise receive it.

Another aside, a quick word about cats. If your cat received an injectable vaccine and has a lump at the injection site that is either greater than one inch in diameter or has persisted for more than 3 months then it should be surgically biopsied and submitted to a laboratory for analysis. Some cats have a genetic mutation that causes them to overreact to any type of injection with unregulated inflammation. This inflammation lowers their natural safeguards against cancer and injection site tumors can occur.

If your pet has had a severe vaccine reaction and we know the culprit, we will typically cut that vaccine out of your pet’s schedule. If we do not know the culprit splitting up the vaccines and observing your pet post vaccination is the best way to determine which vaccine was the offender. If your dog is an adult that has finished a puppy series and started reacting to vaccines later in life, we may opt out of all vaccines except for Rabies (State law). In these cases your pet will still require an annual check up. In fact all pets should be examined by their veterinarian yearly regardless of health status with the exception of older patients who should be seen twice a year. While vaccine reactions are a scary and unfortunate complication to preventative care the rarity of adverse events combined with the prevalence and severity of the diseases they prevent keep vaccines a staple in responsible pet ownership.

Thanks for reading.


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