As awareness of canine infectious respiratory disease complex (CIRDC, formerly known as “kennel cough”) has spiked recently, there are more discussions happening about respiratory vaccines in dogs. A large number of different bacteria and viruses play a role in CIRDC. We can vaccinate against a few of them including parainfluenza virus (the most commonly diagnosed contributor to CIRDC), the bacterium Bordetella bronchiseptica (typically number 2 or 3 on the list of diagnosed contributors), canine adenovirus (pretty uncommon) and canine influenza (very sporadic).
We also have different ways to vaccinate dogs, specifically use of injectable versus mucosal (oral or intranasal) vaccines.
- Injectable vaccines tend to induce a better systemic antibody responses. Mucosal vaccines provide a better local immune response at the mucosal surface. For respiratory infections, the local immune response is probably the most effective. There’s reasonable evidence that mucosal vaccines are superior to injectable vaccines for Bordetella. We don’t have good data for parainfluenza, but I’d assume the same applies. (We only have injectable influenza vaccines for dogs.)
- Mucosal vaccines are modified live organisms – versions of Bordetella and parainfluenza that are still alive (i.e. functional) but have been attenuated so that while they can elicit an immune response, there is negligible risk of causing disease in the animal. We never say a modified live vaccine (MLV) is 100% guaranteed not to cause disease, but the risk is really low, and the protection is really good, so overall they’re beneficial for vaccination in “normal” animals. However, we tend to avoid MLVs in immunocompromised animals because low virulence organisms might be more likely to cause disease in an individual with a compromised immune system.
That’s the dog side. But, we have to remember that each dog is attached to one or more people too. When we vaccinate a dog with a mucosal vaccine, it sheds the modified bacterium/virus for a while, and might have a large load of the vaccine strain in their nose or mouth right after the initial administration.
That means people can be exposed to the vaccine strains as well. Generally, that’s not a big deal, and it’s really only a potential issue for Bordetella (because canine parainfluenza and canine adenovirus of any form don’t infect people). I get asked about this a lot, by both veterinarians and pet owners, and I write a similar post to this one every few years, but each time we have a bit more data.
Why is there concern about human exposure to Bordetella in canine vaccines?
- Bordetella bronchiseptica can cause infections in people. They are rare, but they occur. So, if the “normal” Bordetella bronchiseptica can cause disease in people, we have to think about whether the vaccine strains can cause disease too.
- The answer is “yes,” with a big “but” (actually, a series of “buts”).
Yes, there have been a couple of reports of human infections with canine vaccine-strain Bordetella, some of which are more convincing that others.
A recent report (Kraai et al. 2023) described vaccine-strain Bordetella bronchiseptica infection in a 43-year-old woman who was taking immunosuppressive medication.
- She developed bronchitis with malaise and a mild fever two weeks after her dog had received an intranasal vaccine.
- Bordetella bronchiseptica was isolated from her sputum. When it’s gene sequence was assessed, it was consistent with the vaccine strain.
- She had mild disease and responded to antimicrobial treatment.
Clearly there is some risk with human exposure, that’s certain. Some groups have said to avoid MLVs in animals living with immunocompromised people. But let’s thing about that critically for a moment. All vaccination decisions require consideration of the costs (risks) versus benefits:
- The risk to humans from canine vaccines is really low. Millions of doses of mucosal vaccines are given to dogs every year, yet human infections are still extremely rare.
- Disease that has been reported in people who do get sick is mild.
- Mucosal vaccination is superior to parenteral vaccination, and prevention of disease in dogs can also reduce the risk of exposure to the “wild type” (non-attenuated) strains of Bordetella in humans.
Broad “don’t use modified live vaccines in animals owned by high risk people” statements overlook a few big-picture issues:
- The big one is the vaccine strain is much less likely to cause disease than the circulating (non-attenuated, disease causing) strains. A person is much more likely to be infected with the Bordetella from a naturally infected dog than from a vaccinated dog, so I’d rather prevent the dog from getting infected by vaccinating with the most effective method available.
- Natural Bordetella infection (unlike vaccination) also tends to make the dog cough, which increases human exposure to any number of bugs in the dog’s respiratory tract.
- If that dog needs treatment with antimicrobials, we run the risk of the person being exposed to antimicrobial resistant bacteria, some of which can pose additional risks to people.
- Antimicrobials also increase the risk of the dog developing diarrhea, which can greatly increase human exposure to disease-causing bacteria in feces (especially if the dog poops on the floor).
Some more food for thought:
If I have a dog that was recently vaccinated with a mucosal vaccine, and I was asked to rank the top 5 zoonotic pathogens that are in the dog, vaccine-strain Bordetella wouldn’t even crack that list. There’s a mix of potentially disease-causing bacteria in every dog, all the time. Getting tunnel vision about one in particular, especially one that’s really quite low risk, is not helpful.
What about killed, injectable Bordetella vaccines?
Injectable killed Bordetella vaccines (which contain no live organisms, as the name suggested) do work, they just don’t work as well. If there’s significant concern from the owner, or some other unusual circumstance that makes use of a mucosal vaccine undesirable, then by all means, use an injectable vaccine. I’d consider that to be a rare situation.
Also bear in mind that killed “kennel cough” vaccines are just for Bordetella. They don’t include anything for parainfluenza virus, the most common cause of CIRDC. Parainfluenza is part of common combination “core” vaccines (e.g. DA2PP), but those vaccines don’t do a great job of protecting against paraflu. So, while an injectable Bordetella vaccine removes the risk of exposure to vaccine-strain Bordetella, it offers less protection against Bordetella and none against paraflu, so we have greater risk of disease in the dog overall, and the implications described above that come with it.
Let’s be clear: There’s never a zero risk situation when it comes to exposure to infectious bugs (from vaccination or pet ownership in general). We have to consider the risks and benefits in every situation.
But, almost always, for high risk households, I support vaccination whenever the dog’s lifestyle and risk factors indicate that Bordetella vaccination is warranted. I’d stick with mucosal vaccines for respiratory diseases whenever possible, since they provide much better protection and we can easily mitigate the very low risk from the vaccine. Those mitigation measures include:
- Keeping the owner outside of the exam room when the dog is vaccinated.
- Wiping the dog’s nose/mouth after vaccination to remove any major external contamination.
- Recommending that the owner avoid direct contact with the dog’s oral and nasal secretions. That’s particularly important for the first 24 hours after vaccination, but it’s something I’d recommend for a high risk owner to always avoid.
- Being diligent about routine hygiene practices (e.g. handwashing), especially after contact with the dog’s face (again, something that’s actually always important for a high risk owner).
Photo credit: Dr. Kate Armstrong (from Weese & Evason, Infectious Diseases of the Dog and Cat, A Color Handbook)