group of 4 puppies

We’re getting reports of potential canine parvovirus infection in vaccinated dogs in some parts of northern Ontario. Parvo is a nasty but vaccine-preventable disease, and available vaccines are very effective. True vaccine breakthrough (i.e. infection in a fully vaccinated individual) seems to be rare with canine parvo, but no vaccine is 100% effective, so it is possible.

Whenever I hear reports of canine parvovirus vaccine failure, the first thing that comes to mind is whether the dogs are actually vaccinated, and vaccinated properly. Most of the time, these “breakthrough” infections are not actually due to vaccine failure, but failure to vaccinate the dog properly, or infection that occurs before vaccination has a chance to take effect.

Most young animals some acquire short-term immunity to certain diseases from their mothers. That’s critically important to prevent infections early in life, but it also impacts the effectiveness of early vaccination, because the antibodies that youngsters have from their dam (maternal antibodies) can interfere with vaccine response.

When we vaccinate puppies against parvovirus, we typically start at 8 weeks of age. At that age, vaccination probably won’t work well because of maternal antibodies, but if it does, great. If the pup doesn’t have a lot of maternal antibodies (left) at that stage, we don’t want to risk leaving it unprotected.

We do the same thing at 12 weeks of age, with the same rationale.

By 16 weeks of age, we’re getting confident that maternal antibodies have dropped enough for the pup to respond to a vaccine. A single dose of core vaccines (usually given as a combination vaccine for parvo, distemper and hepatitis) at this age should provide solid immunity. We’ll sometimes give an extra dose at 20 weeks of age if it’s a higher risk area, the dog has a higher risk lifestyle, or we’re not convinced about the dog’s age. That’s a bit of an insurance policy.

We then revaccinate 6-12 months later, and usually go on an every 3 year cycle after that.

Most of the time, when I see parvo “vaccine failures,” it’s when vaccination stopped at 12 weeks of age. That’s not a vaccine failure to me. That’s incomplete vaccination.

True vaccine failure can happen, but it seems to be very rare. We need to watch out for it in case there’s a problem with the vaccine (very unlikely) or a new strain of the virus that current vaccines don’t cover (also very unlikely, but something we’d want to catch as early as possible).

When we hear of large numbers of possible breakthrough parvo cases, it gets more concerning, but it’s still most often from incomplete vaccination. We’ll often start to hear about more cases that were already occurring in the background simply because people start talking about it more. So, an increase in reported cases itself doesn’t necessarily mean something new happening.

For me, the first step in sorting out issues like this is clarifying vaccination status of the affected dogs. If all the dogs were incompletely vaccinated, we have our answer (and an opportunity to better educate owners). On the other end of the spectrum, if we have large numbers of dogs with reliable full vaccination histories (particularly when done by a veterinarian, as we have more confidence in proper administration, as well as vaccine cold chain and handling), I get more concerned, and would want to get samples from cases for virus sequencing.

To start at step 1, we’re trying to collect information about parvo cases occurring in dogs that are reportedly vaccinated (to some extent). People can voluntarily report cases here:

We do this type of on-the-fly surveillance periodically, as needed. Some flop because there’s not much reporting or the problem burns out. Sometimes, we find interesting and actionable information. We’ll see what happens here.