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November 30, 2023 — Peanut, a beloved 5-pound chihuahua adored by all Morris Animal Foundation staff, faced an unexpected challenge after a routine Leptospirosis booster. This vaccination reaction prompted her veterinarian-owner to reconsider Peanut’s vaccine schedule, leading to a positive turnaround. Yet, incidents like Peanut’s raise concerns for pet owners and veterinarians, fueling discussions about the necessity of vaccinations.  

Peanut’s owner changed the timing of her vaccinations, and Peanut’s doing fantastic. Still, experiences like these can be problematic for pet owners and veterinarians alike, and many pet owners might question whether their pet needs to be vaccinated.  

Clarity is crucial as vaccine hesitancy grows among pet owners, compounded by recent disease outbreaks. In this article, we delve into understanding vaccination mechanisms and scrutinize old and new data on adverse events. By examining success stories and shortcomings, we aim to empower pet owners to make informed decisions about their furry friends' vaccinations.  

The Basics of Vaccination: Getting the Immune System Ready to React  
Our pets encounter potential disease-causing agents daily, relying on their immune system's prowess for defense. Vaccination capitalizes on this system, aiming to train it to efficiently recognize and combat foreign or abnormal agents. This strategy, designed to prompt a quicker immune response, aids in lessening disease severity or preventing symptoms altogether.  

Vaccines vary in composition, using live attenuated viruses, killed agents, or novel technologies like mRNA vaccines. The formulation also includes mediums that aid immune response, known as adjuvants. Vaccine duration, influenced by factors like virus mutability, dictates the need for boosters.  

The medium in which vaccines are delivered represents another component found in all vaccines. While it can be an inert substance, in some instances, these other substances can also stimulate the immune system, generating a more robust immune response. These substances are commonly referred to as adjuvants - more about these later. 

Vaccines do not similarly stimulate the immune system. An additional polio vaccination is seldom required for adults vaccinated as children, indicating that some vaccines have a long-lasting effect. Some vaccines have a short duration. An example is the Bordetella vaccine given to dogs every six months to prevent kennel cough. The duration of immunity often dictates when vaccines and boosters are recommended, with most vaccines falling somewhere in between. 

There are a few known factors and many unknown reasons why there is a difference in how long a pet is protected. One factor that influences the duration of immunity is how fast a virus mutates. The more a virus mutates quickly, the more tweaking is required to keep the vaccine effective. Influenza is a classic example of a rapidly changing virus requiring yearly vaccine updates — unfortunately, a lot of the why isn’t known yet.   

Examining Adverse Reactions  
Several studies have confirmed that adverse reactions to vaccination are low in cats and dogs. A recent article published in the Journal of the American Veterinary Medical Association describes results from a recent study that looked at adverse events associated with dog vaccinations and provides some of the most current data available.  

Studies affirm that adverse reactions to vaccinations in cats and dogs are infrequent. Recent research published in the Journal of the American Veterinary Medical Association includes data from millions of dogs, showing a low incidence of adverse events post-vaccination. Younger and smaller dogs exhibit slightly higher risks, with certain breeds marginally more susceptible.  

This new study was a repeat of a study performed by the same group in 2003. The team wanted to learn if anything had changed since the publication of their first study when it came to adverse events.    

The team reviewed the data from about 4.6 million dogs. They looked at adverse reactions reported within three days of vaccination. They found a rate of 19.4 adverse reactions per 10,000 vaccinations. Younger dogs (2-18 months) were more likely to react than older dogs (but the numbers were still low, roughly 25 per 10,000 vaccinations).  

The smaller the dog by body weight, the higher the risk for an adverse reaction. The team also found that some breeds were more likely to have a vaccine reaction than others. The top three breeds with higher adverse events include:  

  • French bulldogs   
  • Dachshunds  
  • Boston terriers

However, the rates of adverse events were still low for these breeds (roughly 45-55 events/10,000 vaccinations).  

The team also confirmed previous data that suggested that the greater the number of vaccinations given during a visit, the higher the risk of an adverse reaction. Owners need to understand this refers to the actual number of vaccines, not the number of antigens in a vaccine (for example, the combination distemper/parvovirus/adenovirus/Parainfluenza virus vaccine would count as one vaccine).  

The same team published a study in 2007 looking at adverse events in 500,000 cats within 30 days of vaccination (note this time frame was quite a bit larger than the two dog studies). The team reported that the rate of adverse events was 51.6/10,000 cats vaccinated. Like dogs, the more vaccines given at one time, the higher the likelihood of a reaction. Neutered cats also had a higher risk of a vaccination reaction, and the chance was most significant for cats that were approximately 1 year old. Lethargy was the most commonly reported clinical sign, with swelling, pain or soreness at the sight of the injection being the second most commonly reported adverse event.  

Unlike in the dog study, the team followed cats for one to two years. The long follow-up time was related to injection-site sarcoma formation, a severe cancer that forms at the site of previous injections. In this study, the team did not find that cats who had localized vaccine reactions developed ISS during the study follow-up period.  

The bottom line for the dog and the cat study is that the rate of adverse reactions is still meager.   

Success Stories and Ongoing Research: Impactful Vaccination Endeavors   
In 1986, two kittens showed up at the home of a pair of animal lovers who were reeling from the loss of their last cat to cancer. One was pure white with startling blue eyes, coined Sinatra, and the other was a red tabby that reminded the owners of a beloved cat from their childhood – Little Red.   

The kittens seemed healthy and happy and promptly wormed their way into their new owners’ hearts. They received the vaccines available at the time and were spayed and neutered. Everything seemed on track for a long and healthy life.  

However, one day, the new owners noted that Sinatra was breathing harder than usual, and Little Red was beginning to lose weight. Devastation struck when they discovered that both were infected with the feline leukemia virus. Little Red succumbed to leukemia within three weeks, and Sinatra followed a month later, succumbing to lymphoma, a type of cancer, in his chest. It was a crushing blow to people who prided themselves in caring for their pets.  

In 1986, a vaccine for the feline leukemia virus was just coming onto the market. It was known to have side effects – fever, lethargy, inappetence. However, as vaccine adoption became more widespread and the vaccine improved, veterinarians noted something unique – the rate of certain cancers, especially those in young cats, plummeted. Further study revealed the reason: the use of vaccines against FeLV.  

Talk to any veterinary cancer researcher about cancer in cats. They often speak of the “pre-FeLV vaccine time” and the “after times.” FeLV infection was a significant driver of developing lymphoma and other blood cancers in cats. However, the feline leukemia vaccine's effectiveness in reducing this cancer has been astounding. Interestingly, before vaccination against feline leukemia virus, the age of cats diagnosed with lymphoma averaged 3 to 5 years. In the era following the feline leukemia vaccine, retrovirus testing is still conducted on cats, but finding them is uncommon. The type of cancer most common in cats also has trended toward GI lymphoma, and the age at diagnosis is much older, often 10 to 12 years of age.  

Of course, cats still get all types of cancer, but the remarkable success of the FeLV vaccine continues to spur research looking for other cancer-causing viruses in cats and other species, too.   

Injection-site Sarcoma and Vaccination: A Failure, A Risk Factor or Something Else?  
Dr. Rachael Thomas, a research scholar at North Carolina State University and Foundation-funded scientist, didn’t set out to study veterinary cancer. But a childhood encounter with the disease left an indelible mark on her.  

“As a child, I had a cat that developed an injection-site sarcoma,” Thomas said. “Bodkin was our family cat, and I formed a close bond with her as the youngest child. We grew up together. I remember that when I heard she had cancer, I was old enough to understand it was serious. I was anxious and paralyzed by the fact there was nothing we could do. I felt vulnerable.”  

Tumor formation can result from injections in susceptible individuals, causing injection-site sarcomas. The cruelty of ISSs is they arise from medications and vaccines to help keep animals healthy or help them recover from injury or illness. The tumor invades the surrounding tissue, making surgery difficult. Radiation and chemotherapy serve as adjunctive therapy, yet ISS is rarely curable. 

Thomas continues searching for clues explaining why some cats develop this cancer and others don’t. Her ongoing work could provide additional guidance for cat owners and veterinarians regarding this severe complication associated with vaccination.   

Rabies: An Ancient and Ever-Present Disease  
“Cujo,” “Old Yeller” and “To Kill a Mockingbird” – these movies all have one thing in common – an animal with rabies plays a prominent role. Not many of us can forget a rampaging St. Bernard attacking a Ford Pinto!  

While it’s easy for those of us in North America to take rabies for granted since it’s unlikely we, or our pets, will contract and die from the infection, not so long ago, rabies was a health threat taken very seriously in the United States. For many areas of the world, rabies remains an ever-present danger, with roughly 59,000 people (primarily children) dying each year. Estimates for how many dogs die of the disease might run into the millions, according to experts.  

Vaccination for rabies has dramatically decreased the number of cases in pets and people in the US. Rabies vaccine is one of the most effective vaccines ever developed – it must be since it must protect against a deadly virus.  

Foundation-funded researcher Dr. Darryn Knobel has been studying rabies and vaccination for a long time. As a native of South Africa, he witnessed first-hand the devastating effects of rabies in communities throughout his native country. Knobel’s work has uncovered some interesting findings.    

Knobel has conducted several studies in his native country and has a study in progress in Kenya. Rabies is a significant disease in these countries, and many communities are resource-poor when it comes to the vaccination of their community dogs. One of Dr. Knobel’s studies looked at the effect of immunization on overall mortality in puppies. He found that while rabies vaccination did not affect young male puppies, there were higher mortality rates in female puppies for unclear reasons.   

Dr. Knobel is expanding on this study in Kenya and hopes to be able to answer this perplexing question. It's worth noting that, in this case, the vaccine was administered to puppies at a younger age than is recommended. However, the vaccine manufacturer also emphasized they always recommend vaccination as vaccines have significantly reduced rabies outbreaks in the region.  

Asking Questions and Understanding Risk Can Guide Vaccine Selection  
Veterinarians often find themselves uncomfortable trying to accurately balance the risk of vaccine reaction with the risk of disease. Owners can help by discussing their pets’ lifestyle, including contact with other animals, travel history and past vaccination history.   

By working together, owners can be active partners in deciding which vaccines are best for their pets.  

The Foundation Continues to Be a Leader in Vaccine Research  
The Foundation has funded research to improve all animals' lives through improved vaccination strategies for decades. We provided crucial funding that led to the first canine parvovirus and feline leukemia virus vaccines, and we continue to fund studies focused on vaccine development.   

Our funded researchers are working on improving vaccine strategies for heartworm disease, leptospirosis, feline herpesvirus, equine strangles, and cancer vaccines.  

However, there’s still so much to do.  

Learn how you can help animals everywhere live longer, healthier lives.