A Reputation With A History

 Leptospirosis vaccines have been used in dogs for decades, with vaccines against two important groups of Leptospira available in North America since the 1960s. These vaccines are bacterins (vaccines made using killed bacteria) and earlier generations developed a reputation for causing more post-vaccination reactions than veterinarians were comfortable with That raised an obvious question: what was causing those reactions? Researchers began looking not only at the immune response to Leptospira itself, but at what else remained in vaccine formulations after manufacturing. Work from roughly two decades ago found that some canine vaccines containing Leptospira had relatively high concentrations of residual proteins, including bovine serum albumin, which is a protein found in cow blood. These proteins were investigated as one possible contributor to allergic reactions. They weren’t the only possible explanation, but the findings helped researchers understand how vaccine formulation and manufacturing could influence reaction risk. As manufacturing methods improved, newer Lepto vaccines were refined with the goal of reducing adverse reactions while still producing a protective immune response. But the vaccine wasn’t the only thing changing. As larger populations of vaccinated dogs became available to study, researchers could begin looking for patterns across thousands of doses and comparing Lepto directly with the other vaccines dogs routinely receive. For the first time, there was enough information to take an old concern and put it to a much bigger test.

What Modern Research Found

So, with better data and modern vaccines to study, did Lepto still stand out?

Vaccines are designed to get the immune system’s attention, so mild reactions can happen with any vaccine. A dog may be sleepy, sore, eat less than usual, or run a mild fever for a short time. Serious allergic reactions are possible, but much less common. The interesting part is what researchers didn’t find: modern Lepto vaccines don’t appear to cause reactions unusually often compared with other routinely administered canine vaccines. AAHA reports fewer than 53 adverse events for every 10,000 doses, with most reported reactions being mild.  In the most recent large safety study, the odds of an adverse event following Lepto vaccination were actually lower than after either DA2PP or rabies vaccination. Serious anaphylactic reactions have also not been reported more frequently with Lepto antigens than with other vaccine antigens. It is also important to note that “adverse event” is a broad term for a health problem reported after vaccination and doesn’t necessarily mean the vaccine was proven to have caused it.

BUT WHAT ABOUT SMALL DOGS? Here’s where the picture gets a little more interesting because there really is a connection between a dog’s size and the chance of a vaccine reaction, but it isn’t specific to Lepto. Smaller and younger dogs are more likely to experience vaccine reactions in general, and receiving multiple vaccines during the same visit can increase that risk. That’s why, for a dog with a history of reactions or other medical concerns, a veterinarian may choose to give fewer vaccines at one appointment and spread them across separate visits instead. What we don’t do is simply give a smaller dose. Unlike many medications, vaccines aren’t dosed according to body weight. The manufacturer’s recommended dose is designed and tested to provide the amount needed to produce a reliable immune response, whether it’s being given to a Chihuahua or a Great Dane. Giving a smaller dose may not provide adequate protection, so for a small dog (or any dog with a history of vaccine reactions) the goal is to adjust the vaccination plan, not the dose. Age, health, previous reactions, vaccine history, and how many vaccines are being given at once can all help your veterinarian decide the safest way to vaccinate an individual patient.

The Many Paths to Exposure

While researchers were taking a closer look at the vaccine, the disease itself was becoming harder to put in a box. For years, Leptospirosis had a fairly recognizable risk profile: the outdoor dog. Think hunting dogs, farm dogs, hikers, swimmers, the ones wading through creeks, drinking from ponds, or regularly crossing paths with wildlife. But cases began turning up outside that picture, including in small dogs living in urban and suburban environments. Today, dogs of any age, breed, or sex can be susceptible, with Lepto documented everywhere from rural properties to cities and even in dry regions where it might seem unlikely. The reason has a lot to do with how Leptospira travel. Infected mammals can shed the bacteria in their urine, but the bacteria don’t have to stay with the animal that carried them. Once urine reaches wet soil or water, the bacteria can survive there for weeks to months. Rodents are important reservoirs, but dogs, livestock, wildlife, and other mammals can carry the bacteria too. Some infected animals may show few (or no) obvious signs while still shedding bacteria into the environment. That turns a surprisingly ordinary puddle, patch of wet soil, backyard, or shared outdoor space into a possible meeting point between two animals that never actually meet.

None of this means every puddle or trip to the dog park is going to be an emergency. Exposure is still a matter of circumstance, and not every encounter leads to infection. What it does show is why the old picture of the “Lepto dog” has gotten so much harder to draw. Risk isn’t confined to one particular landscape or lifestyle anymore.

And Then There's Us

Dogs aren’t the only ones in this picture. Leptospirosis is zoonotic, meaning the same bacteria moving between animals and their environments can infect us too.  Human infection can cause fever, headache, muscle aches, and vomiting, while severe cases can damage the kidneys and liver, cause meningitis or breathing problems, and occasionally be fatal. Worldwide, Leptospirosis causes an estimated one million human illnesses and nearly 60,000 deaths each year. And when it comes to dogs, proximity matters. They share our homes and yards, climb onto our furniture, ride in our cars, lick our hands, and depend on us to care for them. But that also means if a dog becomes infected and leptospira is being shed in their urine, they might be bringing the bacteria into an environment we share. Fortunately, documented dog-to-human transmission is uncommon. But our relationship with dogs gives us far more frequent and intimate contact with them than with most of the wildlife that also carry the bacteria. It’s one reason cautious handling of urine, good hygiene, and prompt diagnosis and treatment are important when Leptospirosis is suspected. It also adds another dimension to prevention. Modern Lepto vaccines reduce the risk of disease while also dramatically reducing or preventing Leptospira from colonizing the kidneys and being shed in urine after exposure. For a zoonotic disease carried by an animal that lives right alongside us, reducing both disease and shedding matters.

The 2024 Turning Point

That brings us to the biggest change. The vaccine had improved, the safety data had grown, and the dog considered “at risk” was no longer so easy to define. Add in a potentially serious disease that can affect both animals and people, and the old lifestyle-based recommendation was becoming harder to justify. In 2024, AAHA updated its canine vaccination guidelines and moved Leptospirosis from non-core to core. That means Lepto is no longer considered a vaccine reserved mainly for dogs with a particular lifestyle.  For dogs beginning Lepto vaccination, AAHA recommends a 4-serovar vaccine. The initial series starts at 12 weeks of age or older with two doses given 2 - 4 weeks apart, followed by revaccination within one year and annual boosters thereafter. Four serovars matter because immunity to one group of Leptospira may offer little or no protection against another. And “core” still doesn’t mean one-size-fits-all. Your dog’s age, health, vaccine history, previous reactions, and individual medical needs are still part of the conversation with your veterinarian.

Where the Science Stands Today

Medicine changes when the evidence changes. That’s a good thing. The Lepto vaccine has carried its reputation for a long time, and there are real reasons it developed. But we have considerably more information now than we did when many of those concerns began. That doesn’t erase its history, but it gives us the context to understand it. Our goal with this article isn’t to tell you what to think about the Lepto vaccine. It’s to make sure you have the information to understand it. We hope knowing where the concerns came from, what researchers have learned since, and why recommendations have changed helps you make an informed decision about your dog’s care – and feel confident in the decision you make. And if you still have questions, you can always call us or talk with your veterinarian at your next visit!