A recent commentary in the Journal of Clinical Pharmacy and Therapeutics by Moore et al. entitled “A doggy tale: Risk of zoonotic infection with Bordetella bronchiseptica for cystic fibrosis (CF) patients from live licenced bacterial veterinary vaccines for cats and dogs” discusses concerns about using the most common “kennel cough” vaccines in animals owned by people with CF.

Bordetella bronchiseptica is just one of the causes of “kennel cough” (or using our current terminology, canine infectious respiratory disease complex (CIRDC)). It’s a highly transmissible bacterium that can cause disease in dogs and cats, but is also found in some healthy animals.

We have two main types of vaccine for this B. bronchiseptica: injectable vaccines with a killed form of  the bacterium, and oral / intranasal vaccines with a modified live form of the bacterium.

• Modified live vaccines use bacteria or viruses that have been attenuated to cause no or very mild disease, but still maintain the ability to cause a low grade, transient infection.
• These vaccines can be highly effective, because they induce an immune response that’s the same as when the “wild type” circulating strain of the pathogen that causes diseases is encountered, but without the same risk of illness.  They are much more protective than the injectable, killed vaccines.  If I want a pet to be protected against B. bronchiseptica, I absolutely want to use a modified live vaccine.
• However, while the vaccine strains are attenuated, they’re not completely harmless.  The concern is that they could cause disease in individuals (human or animal) who have compromised immune systems. That’s why we typically avoid giving this type of vaccine to individuals with significant immunodeficiencies.

The published commentary focuses on the use of modified live vaccines in pets whose owners have CF, who are at particularly high risk for certain respiratory infections. Infections with Bordetella bronchiseptica have been reported in people with CF, so there’s good reason to consider the risks. However, like most things, we need to think about the risks and the benefits, and it’s often not as clearcut as it might seem at first glance.

Concerns about vaccinating pets

The issue is exposure of the owner to an attenuated form of a bacterium that can (uncommonly) cause disease in people.

Is this really a significant concern?

• That’s debatable. Attenuated B. bronchiseptica in the vaccine is a much less virulent form of a bacterium that even in its normal, unattenuated state rarely infects people. Add “less virulent” to an already minimal-risk bacterium and you get a very low risk situation, but it’s not no risk, which is why we’re talking about it.

What evidence of risk do we have?

• Not much. There’s one report that suggests a B. bronchispetica vaccine caused disease in a boy, but he was squirted in the face with the vaccine, so it’s different than exposure to a vaccinated animal. Furthermore, they never tested to see if the boy was actually infected with the vaccine strain (or even Bordetella bronchiseptica). They simply attributed his respiratory disease that occurred shortly after the exposure to the vaccine (reasonable but presumptive).
• Another report described B. bronchiseptica infection in a transplant patient. Their dog had been vaccinated with a modified live vaccine, but they didn’t do any testing to see if the person was infected with the vaccine strain or not. So it’s suggestive, at best.

That’s not meant to dismiss the risk. We want to avoid infections, but we need to keep things in perspective. Millions of dogs are vaccinated with these modified live vaccines every year. Huge numbers of high risk people have contact with these dogs. Large numbers of high-risk veterinary personnel also get exposed (often at high levels) routinely. Yet, there’s very little evidence of anyone getting infected. Little isn’t zero, but we rarely have zero risk situations when dealing with zoonotic diseases of any kind.

Concerns about NOT vaccinating pets

This part doesn’t often get discussed, but we have to consider the downsides of not vaccinating, and there are a few:

• The obvious concern is that the animal will be more likely to get infected withe the “wild type” disease-causing bacterium. Human infections with wild type strains are rare but presumably much (much!) more likely than with the attenuated vaccine strain. I’d be much more worried about a person with CF being exposed to a sick dog with wild type B. bronchiseptica, than to a dog that was just vaccinated with the attenuated strain.
• A sick pet may increase exposure of the owner to various other bacteria from coughing, sneezing and nasal discharge. The mouth and nose harbour myriad bacteria, many of which can cause disease (and do so much more commonly than B. bronchiseptica). An animal that is coughing, sneezing or depositing nasal discharge everywhere presumably greatly increases the risk of exposure of people to this array of bacteria.
• A sick pet may be more likely to be treated with antibiotics, which are often used (often unnecessarily, but that’s a different issue) to treat kennel cough. Antibiotic use is a known risk factor for dogs and cats acquiring and shedding antibiotic-resistant bacteria, some of which cause disease in people, and can be very difficult to treat.

Overall, whether or not to vaccinate a pet is a cost-benefit decision, but we have limited data on the true costs and benefits when it comes to modified live B. bronchiseptica vaccines. My main considerations when deciding whether to recommend a kennel cough vaccine are focused on the dog/cat: what’s their risk of exposure, and what are the potential implications of infection for the animal. A high-risk owner doesn’t mean I shy away from modified live kennel cough vaccines. In some ways, it pushes me towards wanting to use them, because my concerns with the pet getting sick because it’s inadequately vaccinated and then exposing the owner to something are greater.  We can take some basic measures to reduce the risks associated with vaccination procedure itself. The commentary rightly talks about the potential importance of Bordetella transmission from pets to CF patients, but if anything, I take that as more of a reason to vaccinate, to reduce the risk of this happening.

Risk reduction measures are pretty straightforward. The main concern is avoiding human exposure to large numbers of live bacteria from the vaccine.

• Keep the owner out of the room when vaccinating to prevent inadvertent exposure to the vaccine itself, including when the animals (often) sneezes right after it’s given.
• Consider wiping residual vaccine off the pet’s face after vaccination.
• Tell the owner to avoid close contact with the pet’s face, and to avoid allowing the pet to lick the owner for at least a day (but really I recommend both these measured most of the time for high risk owners regardless).

It’s great to see the human medical folks putting some thought put into pet related disease issues. Too often, these issues are not on the radar at all. However, zoonotic disease issues related to pets in high risk households are often complex and nuanced, and that is often not recognized or considered.

The concluding statement of the commentary is great, and to me is the most important recommendation they make:

“Patients should make their veterinarian aware of their CF status, so that a safe and efficacious vaccine strategy is formulated, both mitigating the potential infection risks from live components of the vaccine, but simultaneously offering maximum immunological protection to the animal.”

That’s the key. People need to engage their veterinarian about risks in their households, and veterinarians need to be part of the team that manages risk to all members of the household.

Photo credit: Dr. Kate Armstrong (from Weese & Evason, Infectious Diseases of the Dog and Cat, A Color Handbook)