Search leptospirosis

Leptospirosis is a bacterial infection that’s been described as a re-emerging problem in dogs in North America. (It’s been described as that for many years now so maybe we should drop the "re-emerging" and just say it’s a problem). The causative agent, Leptospira interogans, is a widespread bug that’s carried by a variety of wildlife species, and it can cause disease in many different animals, including dogs and people.

In dogs, lepto is an important cause of kidney disease in some regions, and infected dogs pose some degree of risk to people who come in contact with their urine. While it used to be mainly associated with rural dogs here in Ontario, it’s increasingly being found in urban dogs because of the proliferation of raccoons (that can shed the bacterium in their urine) in cities.

A recent study from the University of California Davis (Hennebelle et al, Risk factors associated with leptospirosis in dogs from northern California: 2001-2010, Vector Borne and Zoonotic Diseases, 2014) looked at 67 dogs with lepto and 271 non-lepto controls. You can’t extrapolate all the results to other regions, because there are different animal reservoirs and other factors to consider, but the study provides some good information.

Here are the highlights:

  • Vomiting, lethargy, increased white blood cell count and increased kidney values (azotemia) were the most common presenting problems. That’s not surprising but it’s a constant problem. Dogs don’t come in screaming “I have lepto!!!” They often have vague signs and it may be a little while (and a lot of handling) before lepto is considered or diagnosed. That in-between period poses a risk to handlers if good practices to avoid urine contact aren’t used, so practicing good general infection control and keeping lepto under consideration in any of these cases are important to reduce human risks.
  • Dogs with lepto can be pretty sick and treatment can be pretty expensive. On average, affected dogs were hospitalized for 11 days at a cost of $5459 (USD). This doesn’t mean it’s always this expensive. This is a referral hospital that probably sees a caseload that’s sicker than average, but regardless, it’s a serious and often very expensive disease.
  • 13% of affected dogs died. Again, that’s based on a biased caseload, but still shows it’s not to be taken lightly.
  • The main serovar was Pomona. That’s different than we see here in Ontario, where Grippotyphosa (mainly from raccoons) predominates.
  • There were regional differences even in California, with more cases from the central or south coast, Sierra Nevada foothills, San Francisco bay area or north coast compared to the distribution of control dogs.
  • Owners of dogs with lepto were more likely to report that their dog had contact with water or wildlife, or visited a ranch. These are risk factors for lepto that have been found in other studies as well, and make sense biologically.
  • Other risk factors included being 5-10 years of age or over 10 years of age, or being hound breeds.

Lepto’s a big problem in many regions, including around here. That’s why my dog Merlin’s vaccinated against the disease. Lepto vaccines have gotten a bad rap because the older ones were relatively ineffective and associated with increased risk of adverse reactions. However, today’s vaccines protect against the important strains (for most regions) and are quite safe. Discussing the risk of lepto and whether vaccination is indicated is something every dog owner should do with their veterinarian. Knowing regional trends in lepto help make that determination.

Some information about lepto distribution in dogs is available at http://www.wormsandgermsmap.com We don’t have a lot of cases entered yet, so more data would help.  If you are a veterinarian or veterinary technician and would like to know how you can help contribute data, click here.

Leptospirosis is disease caused by several types of Leptospira bacteria. It’s often called a re-emerging disease in dogs, because the incidence has been increasing over the past couple of decades in many areas. While the overall number of infections is limited, when it does occur lepto can cause serious disease, including kidney failure. It’s also a zoonotic disease: lepto can be transmitted from sick dogs to the people who handle them. Infected animals shed the bacteria in their urine, and when the urine comes in contact with broken skin or mucous membranes (mouth, eyes, nose), transmission of infection can result.

Canine vaccines against lepto are available. In the past, lepto vaccination hasn’t been all that popular because the vaccines tended to have a higher rate of side effects compared to most vaccines, and the types of lepto that the vaccines targeted were often not the most important types causing disease. This has changed more recently as newer vaccines have become available. These vaccines seem to have low rates of adverse effects and provide protection against the four types of lepto that tend to be the most important. These vaccines have also been shown in research studies to protect against disease and to reduce shedding of the lepto bacterium in urine (thereby decreasing the risk of transmission to other animals or people).

Lepto is not among the "core" vaccines that groups say all dogs should have. That’s because the risk of lepto exposure varies greatly between regions and dogs. The key thing to consider when deciding whether or not to vaccinate is what each individual dog’s risk of exposure is. The main reservoir of lepto is wildlife such as rats and raccoons. If infected wildlife are in the area, they may be peeing out lepto and contaminating the environment, particularly any type of standing water (lepto loves to live in warm, wet environments).

In the past, much of the concern has been focused on dogs that go out and swim in streams or ponds.  The "poster child" for lepto has typically been the Labrador Retriever that goes out gallivanting in the bush, but that may be changing too.  When you consider where wildlife such as rats and raccoons live and how high the wildlife infection rates can be, remember that these pests can be present in urban areas in incredibly large numbers.  Living in a city does not make a dog safe from exposure to lepto. In fact some urban areas, with large numbers of wildlife crowded into high-traffic areas like parks, are probably higher risk than rural areas.

Deciding on vaccinating requires an understanding of how common lepto is in the area and whether there is a risk of exposure for the individual animal. This is a disease for which a veterinarian’s understanding of disease patterns in an area (including any specific areas that be particularly high risk) and the types of possible exposure of the dog play a big role. There’s no "one size fits all" recommendation for lepto vaccination.

British champion rower Andy Holmes has died of leptospirosis, which was suspected to have been acquired from the water during the annual Boston Rowing Marathon on the River Witham (UK) in September. The 51-year-old Holmes, an accomplished Olympic rower from the 1980s, started to feel unwell in the days after the race, and developed a fever. He was subsequently diagnosed with Weil’s disease, a serious form of leptospirosis that can cause liver failure.

In some ways, this is being written off as a very rare and unfortunate event. It’s always hard to determine how aggressive to be when making recommendations about avoiding infections that can be acquired from common recreational and occupational activities.

The race’s welfare officer stated "Part of any rower’s training is being warned about water safety. If you fall into water you must wash thoroughly and if you think you have ingested any water seek medical advice." The problem is, exposure to water during rowing and similar events is basically unavoidable. Splashes of small amounts of water into the eyes, nose, mouth or cuts/scrapes could be enough to inoculate potentially harmful microorganisms into the tissues. People aren’t going to run to the physician after every potential exposure. Knowing whether or not the water source has previously been implicated in leptospirosis infections may be useful, but it doesn’t tell you anything for certain.

General recommendations for people working around water include:

  • Covering cuts and sores with waterproof bandages.
  • Washing hands, particularly before eating.
  • Avoiding contact of water with the eyes, mouth and nose, whenever possible.
  • Avoiding ingestion of any amount of water.
  • Ensuring their physician knows about the potential for water exposure should they become sick.

Obviously, complete avoidance of water exposure is impossible for many people, and the overall risk is very low. Weil’s disease is a rare condition but it does occur, both as sporadic cases and large outbreaks. It’s usually treatable but can be fatal, so it shouldn’t be dismissed.

Issues with pets and leptospirosis are similar. Pets, mainly dogs, become exposed from contact with water that has been infected by Leptospira bacteria from the urine of infected wildlife.  Infection can cause a broad range of disease in dogs as well, from subclinical to acutely fatal.  Vaccines for certain strains are available for dogs who are at higher risk of exposure.  Talk to your veterinarian about whether your dog should be vaccinated against leptospirosis.  More information about leptospirosis in dogs and cats is available on the Worms & Germs Resources page.

Image: Andy Holmes sits behind Steve Redgrave after winning a gold medal for Britain at the 1988 Seoul Olympics.

Veterinarians are reporting a potential cluster of leptospirosis cases in dogs near Lake Aquitaine in Mississauga (Winston Churchill Blvd. & Derry Rd. area). Leptospira gryppotyphosa has been confirmed in one dog, with other cases being suspected but not confirmed because owners declined testing.

Leptospirosis is a bacterial infection caused by different types Leptospira spp. These bacteria like to live in water and in moist conditions, and infections in dogs most often result in kidney disease. The bacterium is shed in the urine of infected animals. Leptospira gryppotyphosa is mainly found in wild voles, raccoons, skunks and opossums, and these animals can infect various environmental areas. Any animals exposed to outdoor environments in endemic areas can become infected from ingesting infected water or from contact of infected water with the mouth, eyes or nose, or cuts or other broken skin.

People in the area where these cases have been found should take particular care and probably avoid letting their animals wander into the water or wet areas. A vaccine is available to reduce the risk of leptospirosis, including disease caused by this Leptospira type. Vaccination of pets that are exposed to water or wet habitats in areas where lepto is present is a good idea. People in the Lake Aquitaine area should be particularly vigilant and vaccination of pets would be a good idea.

(click image for source)

Veterinarians are reporting an apparent spike in cases of leptospirosis in dogs in southern Michigan. Leptospirosis is considered a re-emerging disease in many areas of North America. This disease, caused by various types of the Leptospira bacterium, can affect many different species, including dogs and people. A wide range of illnesses can result, including fatal infections. In dogs, kidney failure is a common problem. 

Classically, leptospirosis is diagnosed in dogs that spend time in the woods and similar areas, where they may be exposed to the bacterium from contact with the urine of infected wildlife. Different types of Leptospira have different animal hosts, and infected hosts can shed large numbers of bacteria in urine. These bacteria can survive in wet conditions for long periods of time, and other animals can be infected through ingestion of urine-contaminated water or contact of urine-contaminated water with broken skin (e.g. tiny cuts or open sores on their feet) or mucous membranes (eyes, mouth, nose).

Michigan vets have suggested that the recent spike in cases is the result of local highway construction, which may have driven rats out of their normal habitats and into areas that people and dogs frequent. That’s possible, but it could also be increasing natural re-emergence of the disease, or increasing recognition of the disease, as more attention is being paid to it. Regardless, an understanding that this disease is a problem in the area is important to allow for prompt diagnosis (and proper treatment), as well as preventive measures.

A vaccine is available, but it is not 100% protective and only protects against certain strains of Leptospira. Nonetheless, it’s still a good idea in areas where disease is caused by the strains present in the vaccine and when dogs have a reasonable chance of being exposed.

People can also get leptospirosis. Most often, they are exposed just like dogs: from the outdoor environment. However, pet-to-human transmission has been reported, mainly involving pet rats (since rats are an important reservoir host). People who have contact with an infected dog must take precautions to reduce the risk of transmission. This includes avoiding contact with urine, good attention to personal hygiene (especially hand washing), and proper cleaning and disinfection of any areas potentially contaminated with urine. Prompt diagnosis of canine lepto is very important because treatment rapidly stops the animal from shedding the bacterium. The earlier it’s diagnosed, the quicker it can be treated, and the less contamination can occur.

More information about leptospirosis and Leptospira is available on the Worms & Germs Resources page, and in our archives.

(photo by costi)

When we have a -30C windchill and snow on the ground, my first thoughts usually aren’t about survival of bacteria in the outdoor environment. However, some microorganisms are well adapted for survival in various adverse conditions and we shouldn’t assume that cold=dead for every bug of concern. Along that line, we received a question recently about survival of Leptospira and I passed it along to our lepto expert, Dr. John Prescott. Here’s his guest post:

A reader in Ohio owns a dog that had leptospirosis, and had some questions about leptospirosis that may be of general interest.

Q1. Since the yard is likely contaminated with leptospires, she asked “How cold does the temperature have to get before the Lepto organisms are killed?”

A1. Once it’s frozen, as it is now in January, they’re dead. Leptospires are fragile bacteria that are killed by dry heat and by freezing. They survive well in moist or wet environments, with moderate temperatures. In some countries leptospirosis is called “mud fever” or “fall fever” since this description captures so well the environmental conditions under which they thrive.

Although leptospirosis in dogs can occur at any time in the year, it mainly causes disease in the fall, late September to December, peaking in November. The increasingly mild and prolonged falls that we have experienced in the last decade are thought to be an important reason that leptospirosis has resurged in dogs. Interestingly, there is often a “blip” of leptospirosis in dogs in March in Ontario (and likely Ohio), since this is when the snow melts and conditions are wet, even though we can still get freezing at that time. I suspect that this is also the time when the raccoons that are thought to be the main source of leptospirosis for dogs are again active after the winter, and are foraging for food for themselves and their babies.

Q2. Do dogs still shed leptospires after they’ve been treated?

A2. No. Leptospires are quickly killed by the antibiotics used in treatment, amoxicillin or doxycycline. There is no danger that dogs treated for a week with these drugs are a risk to people or other animals. You may read in otherwise very reputable textbooks that these antibiotics “do not eliminate the carrier state” but I have no idea where this misunderstanding comes from.

Q3. Where can I find out more about leptospirosis in dogs?

A3. I like the web site http://www.leptoinfo.com, which is maintained by a vaccine company. I was surprised how many web sites devoted to leptospirosis that there are, but like much on the internet some contain highly misleading information. The “Worms & Germs” site has good past blogs about canine leptospirosis and is usually (just kidding, Scott) a reliable source of information.

One very common entrenched misconception, which is very hard to kill, is that vaccination does not stop animals shedding the organism. This is quite wrong. I suspect this misconception came from an experimental study half a century ago when dogs with pre-existing kidney infection with a leptospiral serovar called Canicola were vaccinated. It would not be expected by anyone that these animals would stop shedding since antibodies don’t penetrate into the place in the kidney where the leptospires live and from which they are shed in the urine. What vaccination does incredibly effectively is to prevent leptospires from actually reaching the kidney and setting up home there. The leptospires are removed by antibodies in the blood, so they never reach the kidney.

Back in the 1980s, Vietnamese Pot-Bellied Pigs were a popular fad pet.  These stout little oinkers are still out there, though they’re not quite as popular as they once were.  Potbellied pigs are cute (at least to some people… to each their own!), supposedly quite smart, and can even be house trained/litter trained.  As with any new pet though, it’s very important to do your research before going hog-wild and getting yourself a pot-bellied pig.  Talk to your veterinarian about what your pig will need in terms of medical care – vaccines, deworming, spay/neuter, hoof trimming, tusk trimming… Because they are uncommon pets, some veterinarians may not be comfortable treating a pig.  Make sure you ask ahead of time so you know to which veterinarian(s) in your area you can (and will!) take your pig.

We recently received a question about vaccination of pot-bellied pigs.  Just like dogs, in some areas pigs need to be licensed by the city, and certain vaccines are required in order to obtain a license.  In this particular case, pigs are required to be vaccinated against rabies, swine erysipelas and leptospirosis.  Regular visitors to this site are no doubt familiar with the issues around rabies and why it’s important to vaccinate for this deadly disease.  (More information about rabies is available on the Worms & Germs Resources page and in our archives.)  Swine erysipelas is a systemic bacterial infection caused by Erysipelothrix rhusiopathia, which can rarely cause a skin infection known as erysipeloid in humans.  This is not to be confused with human erysipelas, which is a skin infection caused by various species of Streptococcus (particularly Streptococcus pyogenes). 

But the question was about leptospirosis vaccination in pot-bellied pigs.  Pigs are susceptible to infection by Leptospira interrogans, just like dogs and people, and if infected a pet pig would be equally capable of shedding the bacterium in its urine and potentially transmitting the disease.  The issues around requiring vaccination of pigs for leptospirosis are very similar to those around making leptospirosis a "core" vaccine in dogs.  More information about this is available in the Worms & Germs post entitled "Should all dogs in Ontario be vaccinated for leptospirosis?"  A pet pig would likely be exposed to the same serovars of Leptospira as a dog kept in the same area, typically by coming in contact with urine from infected wild animals such as raccoons and skunks when they go outside.  However, the risk of exposure for a pig that rarely or never leaves the house would be extremely low compared to a pig that has outdoor access.  Another important consideration is whether or not the pig vaccine is against the same serovars that a pet pig, instead of a commercial pig, might encounter.  This will also vary depending on in what area the pig lives.  The Leptospira servoars pomona and bratislava are actually host-adapted to pigs.

It is also important to vaccinate an animal with vaccines that are labeled for use in its own species.  Vaccinating a pig with a vaccine meant for dogs could have unpredictable results – it may increase the risk of an adverse reaction, or it may not adequately stimulate an immune response, thereby leaving the pig essentially unvaccinated.  Your veterinarian can discuss the pros and cons of vaccination in your pet with the available vaccine products.

More information about leptospirosis is also available on the Worms & Germs Resources page.

From Guest Author Dr. John Prescott, Professor, Department of Pathobiology, University of Guelph.  More information about Leptospira and leptospirosis can be found on our Resources page.

The last decade has witnessed a surge in leptospirosis in dogs throughout much of North America. Ontario and Québec have been part of the surge, which is associated with two serovars of Leptospira, grippotyphosa and pomona.

The reasons for the dramatic increase relate to: 1. The apparent spread of infection in raccoons and to a lesser extent skunks; 2. A changing climate that favours prolonged survival of these bacteria outdoors in the milder fall temperatures; 3. Perhaps to a minor extent increased awareness of the disease by veterinarians. Gillian Alton, a Masters student at the University of Guelph, has shown that the increased infection rate observed in recent years appears to have leveled off, which may be the result of widespread vaccination.

Leptospirosis should be suspected whenever there is kidney or liver inflammation of unknown origin, particularly in the fall of the year. In 2007, there were about 80 positive and 170 suspicious cases in Ontario based on blood tests submitted to the Animal Health Laboratory (AHL), University of Guelph. Since not all such blood tests go through the AHL, it is likely that there would have been about 160 positive and 350 suspicious cases based on this testing throughout Ontario, and an unknown number of cases diagnosed by PCR (a DNA-based test). If one includes cases diagnosed based on clinical signs but without laboratory testing, and about half the suspicious cases as positive cases, then there may be about 400 clinical cases (i.e. cases where the animal actually gets sick) of leptospirosis in dogs occurring annually in Ontario. Clinical leptospirosis in dogs is a serious disease and this number, if the assumptions are correct, represents a high burden of infection.

Arguments for recommending the new 4-way leptospiral vaccines as a “core” vaccine (i.e. all dogs should be vaccinated) in Ontario are:

  1. The suggested size of the problem;
  2. The often serious nature of the disease;
  3. The zoonotic potential of the infection (a small number of human infections acquired from dogs have been recognized in Ontario and Québec in recent years);
  4. The ongoing widespread presence and sometimes large numbers of raccoons in suburban and urban Ontario;
  5. The diagnosis of canine leptospirosis throughout the province;
  6. The diagnosis of the disease in dogs of all types, not just the “male hunting dog” which is sometimes conventionally regarded as “high risk”.

Arguments against recommending the new 4-way vaccines as a “core” vaccine are:

  1. The sporadic nature of the infection, including the lack of exposure of some dogs to raccoons and other wildlife sources;
  2. The number of vaccine reactions associated with leptospiral vaccines (this is not a significant problem with at least one of the vaccines);
  3. The considerable confusion caused by the (almost certainly totally unfounded) suspicion that serovar autumnalis causes canine leptospirosis, but is not in the new vaccines;
  4. The lack of inclusion of serovar bratislava in the vaccine (although this serovar seems to cause only mild disease in dogs);
  5. The annual cost of revaccination.

The vaccine manufacturers have the responsibility to provide the supporting data on which a “core vaccine” recommendation should be based, by testing dogs in Ontario for exposure to the different serovars. In the absence of such data, but knowing the possible extent of the problem, veterinarians should always discuss the pros and cons of leptospiral vaccination with dog owners. My opinion is that, barring problems of vaccine reactions and hypersensitivity in individual dogs, annual leptospiral vaccination with a 4-way vaccine should be recommended.

A large and eagerly-awaited follow up study on adverse post-vaccination events in dogs was recently published recently, and it provides a lot of solid – but unsurprising – data.

The study (Moore et al. 2023), published in the Journal of the American Veterinary Medical Association and led by Dr. George Moore from Purdue, used medical records from a large corporate practice network in the US (Banfield) to study adverse events that occurred within 3 days of vaccination of dogs. In total, they had data from 4,654,187 dogs (quite impressive) from 1119 veterinary clinics.

Here are some of the study highlights:

  • A total of 31,197 adverse events were identified. That corresponds to a rate of 19.4 events per 10,000 vaccinations, or 0.19%.
  • Forty-five percent (45%) of vaccine reactions were classified as mild, while 15% were considered severe.
  • As expected, adverse events were more common in small dogs. The figure below shows the decrease in adverse events with increasing dog size.

We’ve known about this association for a while, and it still holds true. The highest rates of adverse events were in French bulldogs (55.9/10,000), dachshunds (49.4/10,000) and Boston terriers (44.9/10,000). The lowest risk breeds were mixed breed dogs (14.0/10,000), golden retrievers (12.6/10,000), Labrador retrievers (11.1/10,000… Ozzie and Merlin will be happy about that), German shepherds (9.2/10,000) and border collies (8.6/10,000).

Adverse events were also more common in younger dogs. Rates were 24.6/10,000 for 2-9 month-old dogs and 25.6/10,000 for 9-18 month-old dogs.

There was some variation in adverse event rates between different vaccine types, but nothing dramatic. Rabies vaccines had the highest incidence of adverse reactions, coming in at 24.8/10,000, just a smidge ahead of other core vaccines (i.e. distemper virus, parvovirus, adenovirus) for which the rate was 24.6/10,000 (see table below)

There are often unwarranted fears about (current) leptospirosis vaccines, likely based on historical issues since much older lepto vaccines seemed to cause more reactions. However, the incidence of adverse events with our current vaccines was lower than that for core and rabies vaccines (21.4/10,000).

When the researchers looked at moderate and severe reactions, rabies and other core vaccines had the highest risk. Lepto vaccines were associated with highest risk of mild reactions.

  • This shouldn’t be taken as indicating core and rabies vaccines are dangerous. Rather, it highlights that lepto vaccines are low risk.

As expected, adverse events increased with more vaccines administered at a single visit. Note that this refers to more vaccines, not more antigens (a core vaccine that covers parvo, distemper, adenovirus and paraflu is one vaccine). The figure below shows that. The increase for large dogs was pretty unremarkable but it was pretty clear for medium and small dogs.

Overall, none of the results are surprising. They fit with what we’ve known and observed for a while, but it’s great to have very solid data to back it up.

Vaccines save lives. There’s no denying that.

Vaccines can cause adverse effects. There’s also no denying that.

The low risk of adverse effects and the high risk (and implication) of these infectious diseases make the cost:benefit ratio very clear to me.

test

Vaccine hesitancy by pet owners has been in the news a lot recently, largely due to a recent study about the prevalence and consequences of vaccine hesitancy among dog owners in the US (Motta et al. 2023). The survey-based study reported 37% of respondents think vaccines can cause “cognitive issues, like canine/feline autism”, 22% think the risks of vaccination outweigh the benefits, and 30% think most dogs receive vaccines that are not necessary. A colleague and I wrote a commentary for the San Francisco Chronicle about how this kind of vaccine hesitancy among pet owners is a concern for both people and pet, especially if it results in reduced rabies vaccination coverage.

The survey by Motta et al. only scratched the surface of the issues. We need a lot more information about vaccine hesitancy to truly understand the problem, and to be able to properly address it, but it’s definitely a concern.

Vaccine hesitancy, in both people and animals, is a really complex area. I think we’ve tended to over-simplify the issue in the past by lumping people into really broad groups (e.g. “anti-vaxxer,” “cheap”) without properly investigating the reasons behind their actions. People who don’t want to use vaccines aren’t one homogenous group.

  • Some are hardcore, true “anti-vaxxers.” This is often driven by mistrust in the system or science, and it’s hard to address this group. You can’t convince someone to trust, and trying to throw more facts at them doesn’t help.
  • Most are “vaccine hesitant,” with concerns that are not unreasonable but may be misplaced, misunderstood or simply not adequately addressed. This is the group on which we should focus, because it’s the group with which we have the most potential to engage, address their concerns and hopefully alleviate those concerns.

A lot of people who are vaccine-hesitant are worried about adverse effects of vaccination. We have to be honest: adverse effects occur. There’s a basic level of risk that we accept when we use vaccines because of the broader benefits. I have no doubt that the core vaccines we currently use in pets do more good than harm. But, I also have no doubt that some harm can occur. I’ve seen it.

That’s tough messaging, because a lot of people don’t really care what happens to 99.9% of the population after vaccination. They care what happens to their individual pet. While we can never tell people that the risk is zero, we can explain what the risks are and try to put them into perspective, so they can make an informed cost-benefit decision.

To do that, we need to understand what the risks really are. What data do we have on vaccine adverse effects in animals?

A landmark study about adverse events in dogs within 3 days of vaccination was published in 2005 in the Journal of the American Veterinary Medical Assocaition (Moore et al, 2005). The authors studied medical records from over 1.2 million vaccinated dogs. Here are a few of the things they found:

  • An adverse event rate of 38.2/10,000 dogs (0.38%).
  • Adverse events were more common in smaller dogs and when more vaccine doses were administered. It’s important to note that this means doses of different vaccines (i.e. number of injections), not the number of antigens. A vaccine that contains 5 different antigens is still just one vaccine dose from the standpoint of adverse event risks.
  • Each additional vaccine increased the risk of an adverse event by 27% in small (<10kg) dogs and by 12% in larger dogs.
  • The highest adverse event rates were in dachshunds, pugs, Boston terriers, miniature pinschers and chihuahuas.
  • 1.7% of the reactions were anaphylaxis (the most serious kind of reaction). That corresponds to about 0.006%.

By vaccine, the adverse event rates were:

  • Injectable Bordetella: 15.4/10,000
  • Rabies: 24.7/10,000
  • DAPP (distemper, adenovirus, parvovirus, parainfluenza): 26.2/10,000
  • Leptospirosis: 28.8/10,000
  • Borrelia (Lyme disease): 43.7/10,000

The highest overall rate was when a combination of rabies and Borrelia vaccines was given (54/10,000). The very common combination of rabies and core vaccines (DAPP) resulted in a rate of 39.3/10,000.

A subset of these records were investigated further to look in more detail at the adverse effects.

  • Facial swelling was the most commonly reported problem (31%), followed by wheals or welts (21%), general itchiness (15%) and vomiting (10%). 
  • Collapse was noted in 1% of dogs with a reaction.

A similar study in cats (Moore et al, JAVMA 2007) looked at adverse reactions in 496,189 cats within 30 days of vaccination. The overall adverse event rate was 51.6/10,000 cats. As for dogs, the risk increased with the number of vaccine doses that were administered. Lethargy was the most commonly reported problem.

The canine study has recently been repeated. I haven’t seen the results published yet, but snippets have been reported, and some data were presented by Dr. George Moore at the 2022 ACVIM Forum.

  • They evaluated the records of 4.9 million dogs that were vaccinated at Banfield Pet Hospitals in the US from 2016-2020.
  • The incidence of adverse events linked to vaccination within 3 days of vaccination was 18.45 per 10,000 dogs (or 0.18% of dogs). That’s less than half the rate of the older study.
  • Dachshunds, Boston terriers, miniature pinschers, French bulldogs and havanese were over-represented, continuing to show the increased risk in small breeds. The higher risk for certain breeds (and consistency between studies over a 20 year timespan) suggests that there are possibly genetic factors that drive the risk.
  • Increased number of vaccine doses given at the same time increased the risk in dogs les than 20kg but not in dogs over 20 kg.
  • There were no significant differences between adverse event rates for DAPP, leptospirosis, rabies or Lyme disease vaccines, with rates ranging between 19.2-21.3/10,000.

With these numbers, we can pretty confidently say that adverse event rates are low in dogs, and the most recent study suggests that they’ve actually dropped. A reason for that is unclear, but it could relate to newer, more refined, vaccines.

So, while I’d never guarantee that someone’s dog or cat won’t have a vaccine reaction, we know the rates are low and we have ways to reduce the risk even further. If we had lower vaccination rates we’d have fewer animals with vaccine reactions (that are almost always transient) but a lot more animals with severe and potentially fatal disease (which can have permanent long-term consequences, even if the animal survives.

The cost:benefit calculation is clear to me.