Ivermectin is a commonly used anti-parasitic in animals, and it’s also used in people to treat some parasitic diseases. Additionally, it’s still widely discussed in some internet circles for treatment or prevention of COVID-19. That’s based on mainly anecdotes, some in vitro study, and very poor quality “clinical trials”.

It’s led to stories of people using or even hoarding veterinary ivermectin products. I’ve had lots of questions from the general public and veterinarians as a result (including stories of people buying more heartworm meds for their dog  than they’d ever need).

Bad idea. Here are some reasons why:

  1. There no evidence that it works. An in vitro study showed ivermectin inhibited SARS-CoV-2, but only at massive doses, well beyond what would be safe for use in people.
  2. Anecdotes aren’t data.
    • I don’t dismiss clinical observation, as it is the starting point of many important discoveries. However, a lot of things I have thought I observed or initially appeared to “work” didn’t pan out to be true over time.
    • Observations should lead us to take specific steps to figure out if something is real.  They help raise questions, not answer them.
  3. Other factors may be involved that make the drug appear effective in some siutations, but those factors don’t apply broadly.
    • One potential reason ivermectin may seem to work in some areas is because it’s an anti-parasitic. Strongyloides stercoralis is a human respiratory parasite that is common in some countries. People get infected and the parasite can lay dormant in the body, but when they are immunosuppressed, it can be re-activated.
    • Dexamethasone is a common and effective treatment in people with moderate to severe signs of COVID-19.  It reduces inflammation, but also impacts the function of the immune system (since that’s what triggers inflammation).
    • If someone is infected with dormant Strongyloides stercoralis, dexamethasone treatment for COVID-19 might lead to re-activation of the parasite, and that would complicate respiratory disease.
    • I don’t dismiss that potential, but it would mean that ivermectin might be effective in people with dormant Strongyloides stercoralis infection that are also receiving dexamethasone.
    • That’s a lot different than “ivermectin works against SARS-CoV-2 and everyone, everywhere, who is infected should be on it.”  They should not.
  4. Ivermectin isn’t a very commonly used drug in people, but it is important for treatment of certain parasitic infections.
    • Diversion of the relatively small amount of human ivermectin products towards unnecessary use compromises the care of people that really need it. (I realize that isn’t associated with “don’t steal Fido’s stash” but it’s still an important point.)
  5. Your dog’s heartworm preventative is a low dose treatment.
    • To get the levels used in the aforementioned in vitro study, a person would require a dose of  about 3500 ug/kg. Heartworm prevention in dogs is dosed at about 6 ug/kg.
    • So, my dog Merlin gets one 272 ug chewable a month. To get 3500 ug/kg, I’d need 965 tablets per dose. If I had a small dog, I’d need even more of his supply.
    • If I wanted to self-treat for Strongyloides stercoralis (not sure why I would, but let’s pretend), I’d need 52 of my dog’s chewables – per day.
  6. Ivermectin also comes in more concentrated oral, injectable (don’t even think about it) and pour-on (topical) forms. While I have good confidence in mainstream veterinary pharmaceutical companies, I still don’t want people taking a product that is only intended for (studied in and approved for) use in animals.

Ivermectin is one of many so-called “miracle cures” that we’ve seen pushed on the internet. Like most others, there’s limited substance to the stories behind it. There’s no magic bullet for COVID-19. I’d love it if ivermectin was one, as it’s a drug we know how to use and how to produce. Production could be ramped up and it’s relatively cheap. While conspiracy theories abound, no one has come up with any plausible explanation why an effective drug would be suppressed. Dexamethasone is cheap, widely used and widely produced, and it’s standard of care for some patients. That’s because it works. Ivermectin fits all of those except the “works for COVID-19” which is the most important one.

We’ve once again updated the Guide to Mitigating the Risk of Infection in Veterinary Practices During the COVID-19 Pandemic (14-Apr-2021)It can also be accessed through the Ontario Veterinary Medical Association Coronavirus FAQ webpage (member login required).

I’ll be happy when we can stop updating these guidelines. Progress is good and adding new information is useful. I just long for the day when we don’t need them.

Previous versions of the guidance and other related documents can be found on the Worms & Germs COVID-19 Veterinary Resources page.

I’ve taken a look back at some posts from the start of the COVID-19 pandemic, to see how my thoughts have evolved, what I got right and what I screwed up. We have a lot of COVID-19 posts (starting from when we called it “novel coronavirus” or “Wuhan coronavirus” before the SARS-CoV-2 terminology existed). Here are some highlights and “grading” of my comments from a selection of posts

January 20, 2020 “New coronavirus: Companion animal concerns?”: Very early on I said While this virus still seems to be less transmissible and less virulent than its relative, the SARS coronavirus, it’s pretty early to have a lot of confidence in that.

  • Grade B: It turns out SARS-CoV-2 is not less transmissible than the first SARS virus, but I guess I covered myself by saying it was early to have much confidence.

For containment measures for SARS, this new coronavirus or any other new disease, we need to assume that multiple species can be affected until proven otherwise, and we need to act accordingly. That doesn’t necessarily need to be complex. It might just be making sure animal contact questions are asked along with human contact questions, that quarantine protocols consider what to do with exposed animals, and that quarantined individuals are kept away from animals. 

  • Grade A: This turned out to be a fairly accurate statement, but fortunately risks from animals seem to be pretty limited (let’s hope this statement holds up). This is also around the time I sent some (essentially ignored) emails to try to get some consideration of animal aspects in any human exposure response planning here in Ontario.

January 23, 2020 “Novel coronavirus in China… Hold off on blaming snakes”:  In a post talking about how snakes might be the source of SARS-CoV-2, I wasn’t convinced and said I’m sure we’ll see much more genetic analysis of this virus by many different groups, but I suspect it will keep coming back to bats as the source. 

  • Grade A:  A final answer is still needed but this seems correct.

February 5, 2020 “Novel coronavirus and animals”: In talking about whether there’s actually a problem with companion animals, I said we need to focus on ensuring exposed animals are quarantined, using good old fashioned infection control like hand washing, keeping fear at bay and applying logic to the problem.

  • Grade B+: I’ll dock myself some marks on this one since it was focused on dogs. To be fair, the post was about concerns regarding over-reaction and euthanasia or abandonment of dogs, but it didn’t get into the bigger risks we now know are present with other species.

February 14, 2020 “More on COVID-19 (novel coronavirus) in animals”: I said We still have no evidence that this virus affects domestic animals, but since we also still have no real evidence that it doesn’t, it’s best to continue to take reasonable precautions to reduce the risk of exposure of animals to infected people, and to properly manage pets of people who are infected. “

  • Grade A+: Especially since it was at the same time groups like the US CDC were actively pushing back against there being any risk to/from animals.

February 18, 2020 “COVID-19 and potential animal hosts”: This post talked about a study that predicts species susceptibility based on ACE2 receptors. The potential susceptibility of cats is obviously a concern given their commonness as pets and the close interaction many people have with their cats. Pigs could be an even worse issue. If pigs could be infected and shed the virus, and it got into the commercial pig population, it would potentially be an even worse issue.  As with SARS, mice and rats are likely resistant to infection – that’s good from the standpoint of them not being reservoirs in the wild, but it also means they can’t be used for experimental study (as these are the most common lab animal species).

  • Grade C+: I think I over-estimated how useful these studies would be. They were interesting, but some species predicted to be high risk (e.g. pigs) aren’t, and some that were predicted to be lower risk (e.g. cat) most certainly as susceptible to SARS-CoV-2.  I also missed a couple of points. It seems like infection of pigs isn’t a concern but I think I missed some of the main potential concerns, as I wasn’t thinking about them as sources of virus mutants or wildlife exposure.

February 28, 2020 “COVID-19 in a dog”: A post about the first SARS-CoV-2 positive dog: Overall, my concerns are still more about cats. Dogs will probably get investigated more because there are more pet dogs than cats in most regions, and people tend to seek healthcare for the dogs (or alternative caretakers if the owner is indisposed due to illness) quicker than for their cats. 

  • Grade A: That turned out to be true.

March 27, 2020 “Human-to-cat COVID-19 transmission: Belgium”: I said If you’re worried about getting COVID-19, worry about your human contacts, not your pets. Keep pets away from high risk people, but otherwise, your risk is from exposure to people, not your pet

  • Grade A: I haven’t really changed that line in the past year.

March 29, 2020 “Social distancing WITHIN veterinary clinics”: I said Masks can reduce the risk of transmission if someone is unknowingly shedding the virus . Masks aren’t perfect but there can be some benefit. Whether it’s a good use of masks is questionable. Putting on masks for occasional close contact procedures (and ideally reusing that mask for the whole shift) isn’t unreasonable, but whether it’s really worth the mask use is hard to say.

  • Grade D: My line on masking was influenced by influenza data and concerns about mask availability, and I underestimated the usefulness of routine masking. I also focused on it being needed for known close contact situations, not more broadly. Those were the standard recommendations at that time but I still get a “D” for this one.

March 30, 2020 “COVID-19 and ferrets”: Here, I discussed concerns about ferrets.

  • Grade C:  We haven’t seen much SARS-CoV-2 activity in ferrets. Maybe that’s just because they aren’t common pets and don’t get taken to a veterinarian as often. Where I missed the boat was thinking about related species. Mink were not on my radar at all… (see below.)

April 24, 2020 “Pets and COVID-19 fears”: A commentary sent in partnership with the Ontario Veterinary Medical Association to veterinarians to balance awareness and paranoia when talking to pet owners about SARS-CoV-2.

  • Grade A: I’m not sure I’d change anything a year later.

April 26, 2020 “COVID-19 in mink: The Netherlands”: I said We’ll put this in the “interesting but not really surprising” file.

  • Grade C-: Fair statement, but while I talked about the concerns, I don’t think I really appreciated how susceptible mink would be and the issues that would develop from that.

April 28, 2020 “COVID-19 modelling and the impact of releasing cats”: This was a commentary about a crap-tastic paper about releasing infected cats and the potential impact on virus tranmission. Little did I realize this was one of many future “studies” based on little substance, little common sense and a complete abandonment of any principles of peer review. Academic opportunism has abounded in the past year and a bit, with some people putting out utterly useless papers, reviews and commentaries, and sometimes causing harm.

  • No grade… just a rant.

May 28, 2020 “Veterinary clinic staff cohorting”: I said One concept that we’ve recommended for COVID-19 control in veterinary clinics is staff cohorting.

  • Grade D: This turned out to be too impractical in most situations. It might have been useful if this was a short lived problem, but obviously that hasn’t been the case. I dropped this idea pretty quickly.

In the same post Routine cloth mask use whenever a 6-foot gap can’t be maintained between people is emerging as a key infection control tool. Cloth masks are far from perfect, but they can do a good job containing most infectious droplets, which are probably the main source of exposure.

  • Grade C: We now know the focus on the 6-foot distancing as a risk/no risk cutoff isn’t valid. Emphasizing mask use was good, but focusing on known close contact situations wasn’t.

I’d give a higher grade for the rest of the post. Most of it was actually pretty good, but these are some glaring issues.

I’ll pause here to avoid this post getting even longer. Overall, I think we had some pretty good thoughts about animals from the start. I missed the ball on a couple of key things:

  • Importance of masking: I mentioned masking above, but I didn’t realize how much of a core lifestyle component this would be.
  • Duration of the pandemic: I was expecting the pandemic to hit hard in the spring of 2020 based on modelling, so I wasn’t surprised at what we encountered then. However, I didn’t expect the pandemic to drag on this long. I was truly thinking we just needed to hit it hard in the spring of 2020 to return to normalcy in the fall or winter.

As spring approaches, a pressing question has come to the minds of many kids: “Can the Easter bunny get COVID-19?” or “Can Easter bunny eggs spread COVID-19?

Fortunately, the answer is no. Easter bunnies are safe from this virus and kids don’t have to worry about whatever the Easter bunny leaves behind.

Based on what we know to date, “regular” rabbits aren’t very susceptible to the SARS-CoV-2 virus.  Some types of rabbits can be infected with SARS-CoV-2 experimentally, but they don’t seem to get sick and they only shed low levels of virus. So, even a regular rabbit that was infected probably poses little to no risk.  (Don’t worry kids, no one’s going to try that with the Easter bunny.)

Furthermore, as we all know, the Easter bunnies have inherent magical properties that protect them from various problems (e.g. nosy household dogs). Their elusive nature (have you actually ever seen an Easter bunny depositing its wares?) and strategic placement of gifts in empty rooms mean the Easter bunny is following our “3 Cs approach to COVID-19 control,” that is avoiding closed spaces, close contact and crowded settings. And maybe the Easter bunny wears a mask too? We’ll probably never know for sure…

Don’t be afraid of the Easter bunny.

Don’t worry about what the Easter bunny leaves behind

No, you don’t need to disinfect Easter eggs.

The biggest risk from the Easter bunny is when the family dog finds all that chocolate first (yes, I am speaking from personal experience). That has nothing to do with COVID-19.

Ontario Animal Health Network Veterinary PodcastsThe companion animal Ontario Animal Health Network has produced a series of mini-podcasts on COVID-19 precautions in veterinary clinics, featuring none other than Dr. Scott Weese.  Each mini-podcast features a quick 3-5 minute “lighting round” on common questions and topics – bite-sized bits for busy practitioners and clinic staff who may only have a few minutes to spare these days.  Current topics include:

  • Avoid the 3 Cs: Crowding, close contact, confined spaces
  • Rethinking clinic spaces: The end of the waiting room and more
  • Masks and the trouble with bubbles
  • Patients and procedures that warrant extra precautions
  • Don’t panic! Talking to clients about SARS-CoV-2 risk to and from pets
  • Staying safe with the swiss cheese approach
  • What’s on your face? The why, when and what of masks and face shields

If you have a COVID-19 related question about which you’d like to hear a podcast or mini-podcast, you can email OAHN at oahn@uoguelph.ca.  Also check out the OAHN COVID-19 resources page for veterinarians. Stay safe!

Lyme disease vaccine is a non-core vaccine, meaning it’s not needed for all dogs in all areas. It’s an effective vaccine, and I’d consider it a reasonable vaccine to give to dogs in (or visiting) higher risk areas, especially when there might be owner compliance issues with tick preventive medication. Available tick preventatives are very good, but sometimes people forget to give them on time, so vaccination is a good backup plan for those situations and in areas where the risk of exposure is particularly high.

Lyme disease vaccines are a bit unusual, in that they are primarily aimed at vaccinating the tick, not the dog (strange as that sounds). They usually target two proteins on Borrelia burgdorferi , the bacterium that causes Lyme disease. One of those is outer surface protein A (OspA), which is “expressed” on the  surface of the bacterium when it’s inside the tick. After the tick has attached to a host (like a dog) for a while, the bacterium changes to make itself more adept at infecting animals. That results in a change in the outer surface protein from OspA to OspC.

Lyme disease vaccines contain OspA, which induces the dog’s immune system to produce antibodies against that protein. When a tick starts to feed, it ingests the antibodies in the dog’s blood, which attack the bacterium before it’s ready to migrate to the dog.

Lyme disease vaccines can also contain OspC, to target the bacterium in the tick as it starts to produce that protein, and provide backup protection if the bacterium happens to evade the OspA antibodies and makes it into the body.

With typical vaccines, if an individual is exposed to the bacterium/virus for which they’ve been “primed” by the vaccine, they then get an immune response boost to generate even more antibodies. However, that doesn’t really apply to Lyme disease.  Since the dog’s antibodies flow into the tick, there’s no extra immune boost because the bacterium isn’t yet in the dog’s body (so the dog’s immune system doesn’t get exposed directly). Decreased antibody levels in the dog therefore more directly correspond with decreased protection. There will be some booster effect with exposure to OspC if the bacterium makes its way into the dog, but ideally we’d like to stop the process before it gets that far.

Lyme disease vaccines are given as an initial series of 2 doses, 2-4 weeks apart, and then an annual booster. But,  there’s concern that immunity from Lyme disease vaccines doesn’t last as long as others, so there’s less leeway for overdue dogs. It’s been recommended to re-start the 2 dose series from scratch if the dog is overdue for it’s yearly booster by more than 1 month. That’s pretty conservative, but it’s fair to assume that this vaccine’s long-term protective effect could be less predictable and solid than leptospirosis vaccines (where we accept up to 3 month delay).

Based on that, when it comes to Lyme disease vaccines, if a dog gets its:

First dose, but is late for the 2-4 week booster

  • Restart the whole series (i.e. 2 doses 2-4 weeks apart, then yearly)
  • We don’t have good guidance on what constitutes “late” or “overdue” here.  The American Animal Hospital Association vaccination guidance says within 6 weeks of the first dose is still okay, and that’s reasonable. It’s quite possible that the dog would respond well to a later booster, but we don’t have confidence in that.

First dose, 2-4 week booster, and then a yearly booster not more than 1 month late

  • Continue with the single annual boosters

First dose, 2-4 week booster, but is more than 1 month late for the yearly booster

  • Restart the whole series (i.e. 2 doses 2-4 weeks apart, then yearly)

As I mentioned above, tick prevention is still key, particularly for unvaccinated or inadequately vaccinated dogs. If vaccination has lapsed, it’s even more important to avoid ticks, do tick checks and use a good tick preventive medication (on schedule).

This is probably the vaccine about which I get the most questions when it comes to delays. Leptospirosis (aka lepto) is a regionally important and potentially life-threatening infection of dogs (and people) caused by serovars of the Leptospira bacterium. It’s generally considered a non-core vaccine, meaning it’s not needed for all dogs in all areas. However, it’s probably best considered a regionally core vaccine. If leptospirosis occurs where a dog lives or anywhere it to which that dog might travel, I consider this an essential vaccine. This disease is pretty widespread internationally, so that includes a lot of dogs.

Lepto vaccines are killed vaccines, meaning they contain bits of the dead bacterium. Killed vaccines can be effective, but typically require multiple doses given within a specific interval to maximize immunity. For lepto, we typically start with two doses of vaccine 2-4 weeks apart, and then yearly boosters. We need that initial 2-4 week booster to make sure there’s a good immune response (unlike the modified live virus (MLV) core vaccines I mentioned in Part 1). If that 2-4 week booster is missed, we can’t assume there’s much protection or ability to respond to a future single booster shot. Also, yearly booster shots are needed for lepto to maintain immunity, unlike the core vaccines that can often be given every 3 years after the initial series.

Considering all that, if a dog is late for it’s first booster or annual shot, the default is to restart the entire series. The World Small Animal Veterinary Association vaccination guidance gives some leeway for the annual booster, indicating that re-dosing within 15 months (not the labelled 12) is likely okay. Personally, I suspect we can go longer, since the vaccines are good and immune systems usually aren’t dumb. However, the farther we go off-label, the less confidence we have. While many (or maybe most) dogs are probably protected well after a year and will still respond to a single booster, as a veterinarian, I can’t tell an owner with much confidence that that’s the case. So, the default is to go back to the start so we can be sure.

Based on that, when it comes to lepto vaccines, if a dog gets its:

First dose, but is late for the 2-4 week booster

  • Restart the whole series (i.e. 2 doses 2-4 weeks apart, then yearly)

First dose, 2-4 week booster, and then a yearly booster not more than 3 months late

  • Continue with the single annual boosters

First dose, 2-4 week booster, but is more than 3 months late for the yearly booster

  • Restart the whole series (i.e. 2 doses 2-4 weeks apart, then yearly)

What if someone doesn’t want to restart the series (due to cost, logistics, not keen on vaccines, etc.)?

Odds are reasonable the dog will still respond to a single late booster, but confidence in the immunity this will generate drops the later the booster gets. There’s no way to say exactly what the risk is, so not going back to repeat that 2-4 week primary series means the owner has to accept some degree of uncertainly regarding how well protected their pet will be. Additionally, manufacturers tend to stand by their products, but they’re not likely to offer any support if a dog gets lepto when the recommended vaccine schedule isn’t followed. Those are all things that need to be discussed and considered.

I’d prioritize dogs for lepto vaccination in this order:

1a) Starting initial lepto series

1b) 2-4 week booster for dogs that received the first dose

2) Yearly booster for dogs approaching the end of the 3 month extension window

3) Yearly booster for dogs not yet near the 3 month extension window

More information about leptospirosis can be found on the Worms & Germs Resources – Pets page.

In some ways, the approach to rabies vaccine is easy. In other ways, it’s complicated. To some degree, the medicine is easy, but other considerations (like regulatory requirements) cloud the picture.

The medicine:

Rabies vaccines are highly effective. A single initial dose provides at least 1 year of protection. The first dose is supposed to be given at 12 or 16 weeks (depending on the jurisdiction – rabies vaccines in Canada are labelled for use in dogs and cats 12 weeks of age and older), with a booster one year later. Getting that first shot is critical, so the animal is protected as soon as possible against this deadly – and zoonotic – disease.

After that, we can use 1 yr or 3 yr vaccines.

  • For a 1-year vaccine product, if the animal is overdue, we’d just give another dose as soon as possible, but the yearly cycle does not change.
  • For a 3-year vaccine product, after the initial dose and the first 1-year booster, the animal can safely go 3 years between boosters.  However, if the 1-year booster is missed, then we’d have to restart that primary series to stick to the manufacturer’s instructions, meaning the late booster would only be considered good for a year. Then we’d go to every 3 years after the next dose a year later. If the animal misses any of its 3-year boosters, the same would apply, i.e it would need a booster a year later to get back on the 3-year schedule.
  • Note: As of March 2019, there is only one 1-year rabies vaccine product for cats still available in Canada, and none for dogs; all other rabies vaccines for dogs and cats in Canada are 3-year products.

That’s all pretty straightforward and by-the-book (or label, in this case).

The challenge is what constitutes “late.” Strict interpretation of the label would be that even a single day overdue would require the primary series to be repeated. Many would consider that overkill since rabies vaccines are so highly effective. However, there’s not much appetite for guessing when it comes to rabies. Once an animal is more than a couple of months overdue, it’s harder to say that the booster should count as a 3-year dose, since we’re deviating ever more from the label recommendations. So, prudence would dictate we go back to the start. If there’s concern that the provincial/state/regional authorities (especially border authorities) would use a strict interpretation and consider any lapse an indication that the vaccination series had to be restarted, then it’s safer to be more conservative and do that in your practice as well. More on that below).

From a rabies protection standpoint, I don’t worry about late vaccines (within reason) since rabies vaccines are so good, especially in an animal that has received multiple doses over the course of its life.

The “other considerations”:

What makes rabies vaccination delays complicated is most often regulatory/public health rules and interpretations thereof. In many areas, rabies vaccination is required by law; for example, in Ontario, all dogs, cats and ferrets over 12 weeks of age are legally required to be kept up-to-date on rabies vaccination, regardless of whether they have indoor or outdoor lifestyle.  I doubt there would be a fine applied in most cases for reasonable delays in vaccinating a pet due to COVID-19 restrictions over the last year (but each jurisdiction is its own…). However, the bigger issue is the response to a potential rabies exposure in such an overdue animal. When an dog or cat is exposed to a rabid (tested) or potentially rabid (untested suspicious) animal, the response depends on the pet’s rabies vaccination status. That response varies a bit by region; some default directly to the guidelines in the NASPHV Rabies Compendium, some adapt those guidelines, and others are… well, a bit unpredictable.

In Ontario, the response could range from simple observation with no strict confinement (e.g. a fully vaccinated pet that gets a booster within 7 days of the encounter) to a strict 6 month confinement (e.g. an unvaccinated pet that does not get a rabies vaccine within 7 days of the encounter). Animals with a history of lapsed vaccination are dealt with on a case-by-case basis, with things such as the time since the last dose and the number of lifetime doses being considered (among other factors). The pet could ultimately be treated as fully vaccinated or unvaccinated, depending on the details, and that can be the difference between life-and-death, as euthanasia is sometimes elected by owners in lieu of having to strictly quarantine the pet.

Understanding how your region addresses overdue vaccination is useful, to determine how strict you need to be about getting animals in for their boosters. If they’re not flexible and consider an animal’s vaccination status to be lapsed the minute they pass the 1- or 3-year mark, then we need to make sure we’re prioritizing rabies vaccinations so there are no gaps.

As an example, here’s the situation in my household:

  • My dog, Merlin, has had multiple doses of rabies vaccine and is probably effectively protected for life. Him being overdue is probably low risk, but I wouldn’t want to let it go very far. I’d booster him when feasible but not stress about it, and would keep him on the 3-year cycle. If he’s not too far overdue, I’d have a strong case to consider him completely protected if he tangles with a rabies suspect.
  • My cat, Milo, is less than a year old and has had a single dose or rabies vaccine. He’s higher priority to get boostered. If I miss the 1-year booster by much, I’d have to consider re-starting the primary series again, which means I’d give him a dose, and then another in 1 year, before switching to every three years. (That’s a picture of him… he doesn’t look very stressed about it).
  • The outdoor cats (Rumple and Alice)  have received multiple doses or rabies vaccine over their lifetimes. They’re like Merlin, but higher priority for boosters since they likely have a greater risk of exposure to rabid animals.

The take home messages on rabies vaccination delays:

  • Don’t let rabies vaccination lapse, if possible.
  • I’d prioritize pets for rabies vaccination in this order
    • Animals that may have been exposed to rabies in the last 7 days because of an encounter with a suspicious animal that couldn’t be tested for rabies (or that was tested and was confirmed to be rabid.  This is a medical urgency.
    • Animals that have never been vaccinated for rabies (or unknown if they’ve ever been vaccinated). They need a dose ASAP to make sure they’re protected and won’t have to risk a long confinement period if they’re exposed to rabies.
    • Animals that have only had one dose of rabies vaccine in their lifetime.
    • Animals that are significantly overdue for their rabies booster (more than a couple of months).
    • Other pets that are due for a rabies booster (if there’s really a need to prioritize further, focus on dogs/cats that go outside unsupervised).

I’ve been dealing with questions (and some threats) about this issue for many months:

How to handle overdue vaccines because of COVID-19 delays

It’s a complex issue that’s hard to navigate because of limited data. We know how most vaccines work when used according to the label instructions. We don’t know much about what happens when use of those vaccines differs from the label instructions.

Dr. Michelle Evason and I did a webinar for CommuniVET on this topic the other day, and it made me think a lot more about it.

The main questions are:

  • How long do vaccines actually protect an individual animal?
  • Does being overdue for a vaccine mean you have restart the primary vaccination series, or can you just give a single booster later than normal and continue with the regular vaccination schedule?

Manufacturers can’t give too much guidance because they are legally bound to the licensing (label) claims. They also don’t have much data about uses that differ from the instructions on the label, because no company is going to pay for expensive vaccine trials to test various combinations of delays and boosters once the product is licensed.

Owners want their animal protected, but they also don’t really want to come back in / pay for another vaccine, and many want to avoid giving any more vaccines than are absolutely necessary.

Veterinarians want to make sure that their patients are protected, that their owners are happy (or at least not overly unhappy), and that they can provide veterinary care during a time when that might be complicated, and when most clinics (around here at least) are completely swamped.

Our two main vaccination guidance providers (the American Animal Hospital Association and World Small Animal Veterinary Association) want to help, but are restricted by the limitations above, i.e. there’s not much hard evidence on which to base off-label recommendations.

This could become a monstrously long post, so I’ll break it down. Today’s post will cover scenarios for “core” vaccines (other than rabies). These are vaccines for diseases against which we vaccinate pretty much all dogs and cats, often using combination vaccines. Core vaccines for cats are for panleukopenia,  feline viral rhinotracheitis  (aka feline herpesvirus type 1) and calicivirus. Core vaccines for dogs are for distemper, parvovirus and canine adenovirus-2

Comments below refer to vaccines and vaccination strategies used in Canada, which are similar to many other countries, but there can be some variation in vaccines and label instructions in other jurisdictionsThe discussion will also refer primarily to modified live virus (MLV) vaccines, which are the most common type of vaccine used in dogs and cats, as they are highly effective and generally result in an excellent immune response.

1. Puppy or kitten with a delayed initial series

With MLV vaccines, a delay in the initial series is not a big deal. Core MLV vaccines are highly effective and should provide long-lasting immunity even with a single dose, but only IF the puppy/kitten didn’t have a lot of maternal (mother-derived) antibodies at the time of vaccination.

Puppies and kittens get these antibodies from their dam, which provide protection while the animal is very young and its own immune system gears up to produce its own antibodies. Unfortunately, the maternal antibodies also decrease the ability of the animal to respond to a vaccine.  We usually give a series of 3-4 doses of MLV vaccine to puppies and kittens; this is not because they need an initial dose and a series of boosters, but rather, it’s because we want to make sure they’re protected as soon as those maternal antibodies run out, and that they get long-lasting immunity. So, we start the vaccination series early, and if their immune system responds that’s great, they’re protected early. If not, they get another dose in a few weeks, and then another dose a few weeks later… By 16 weeks of age, we assume they are able to fully respond to vaccination because their maternal antibodies have disappeared (please note that puppies and kittens can be vaccinated for rabies at 12 weeks – you do not (and ideally should not) wait until 16 weeks for rabies vaccination, in order to get them protected as soon as possible – more on that in Part 2 tmrw).

So, the key is we want to get a dose of MLV vaccine into a kitten or puppy at 16 weeks of age or older. If they missed one or more shots in the series, as long as they get a dose at 16+ weeks of age (even if their very first one), they should be good to go because that’s the one that gives them lasting immunity. In some high-risk situations, such as a dog that’s going to encounter a lot of dogs or be in a high risk environment like a shelter, we’d still consider giving another dose if they got their only shot at 16-20 weeks, but that’s case-by-case.

The take home for this group: Get them vaccinated when they’re young if at all possible, but once they’re 16 weeks of age or older, they may just need a single dose.   If there are delays getting a young animal vaccinated, care must be taken to reduce the risk of exposure as maternal antibodies drop, as there is a risk the animal will be unprotected for some time. That means limiting contact with other dogs/cats and high risk environments (e.g. parks, puppy classes, kennels) until vaccinated.

2. Puppy or kitten with a delayed 6 month/one year booster

Unlike killed vaccines for which the timing of the booster is important to get the full vaccine response, boosters of MLV vaccines can be given almost any time and probably results in a similar response. After the initial puppy/kitten vaccination series with a killed vaccine, these animals are usually vaccinated again 6 months or 1 year later, only because we are less confident about long-term  (multi-year) protection from the single initial vaccine.

The risk of being a bit overdue is probably low. The MLV vaccines we use are very good and immunity is probably fairly long-lasting.  We just don’t have as much confidence in it (because we don’t have the data to back it up). I’d prioritize these boosters below a younger animal needing its 16-week vaccination, but above an older dog/cat waiting for its 3 year booster.

The take home for this group: Get them boostered when you can, but don’t worry about a delay. If there is a delay, they still likely just need the single booster they were going to get.

3. Adult dog or cat with a delayed 3 year booster

After the initial vaccination series (or single dose), and the subsequent dose 6-12 months later, we now typically give dogs and cats core booster vaccinations approximately every 3 years. Being late for the 3 year booster isn’t a big deal. These vaccines are highly effective and produce an excellent response with a single dose, regardless of vaccination history. We rarely see the core vaccine diseases in vaccinated adult dogs and cats.

If there’s a need to triage who gets in for vaccines, these animals are lowest on my priority list (but remember we’re not talking about rabies vaccination – see Part 2). They’re unlikely to get these diseases as adults, and a delay will not impact how well they respond to the next vaccine. I want to get them done, though, and the need for a rabies booster may bump them up the priority list.

The take home for this group: When it comes to just the non-rabies core vaccines, get them boostered when when you can, but don’t worry about a delay. They just need the single dose they were going to get, whenever they can get it.

MLV core vaccinations are the easiest part of the decision process. It’s basically just a matter of getting them done when possible, but not changing anything else (beyond the potential for needing fewer puppy/kitten doses).

I didn’t talk about killed vaccines (yet) but I’ll mention them quickly because the story is completely different for them. With killed vaccines, we’re much more dependent on properly timed boosters. If those aren’t given, the default is to re-start the initial series to ensure there’s a robust immune response. So, if a vaccination series is started and then delayed, the whole series may need to be re-started.

More on other vaccines soon.