Leptospirosis Vaccine: A Silent Killer?

A blizzard of misinformation about leptospirosis is putting animals and human health at risk

Leptospirosis is an infectious disease caused by bacteria of the genus Leptospira. It is recognised as an important disease in dogs due to its potential to cause severe illness and its ability to spread to humans.

In the United Kingdom, leptospirosis vaccination is classified by the British Small Animal Veterinary Association (BSAVA) as a core vaccine. Core vaccines are those recommended for all dogs, regardless of lifestyle, circumstances, or geographical location, because they protect against serious and potentially life-threatening diseases that are widely distributed.

The World Small Animal Veterinary Association (WSAVA) Vaccination Guidelines define core vaccines as those that protect animals from severe diseases with global distribution and which all dogs and cats should receive, irrespective of their individual risk factors.

In the UK, the BSAVA recommends that dogs are vaccinated against the following core diseases:

• Canine Distemper Virus (D)

• Canine Adenovirus / Infectious Canine Hepatitis (H)

• Canine Parvovirus (P)

• Leptospirosis (L)

It is important to note that vaccines are multivalent, meaning they can contain protection against several strains of Leptospira, depending on the formulation used.

The primary aim of vaccination is to protect both individual animals and wider animal populations from serious infectious disease. Some of these infections are zoonotic, meaning they can be transmitted from animals to humans. Leptospirosis is one such disease, and controlling infection in the animal population plays an important role in reducing the risk of human exposure.

Veterinary professionals frequently encounter misinformation about leptospirosis vaccination, particularly online. This can understandably cause concern for pet owners, especially given the potential health implications for both dogs and people.

To help address this, we have created a series of posts designed to provide clear, evidence-based information about leptospirosis and vaccination, supporting owners who may have come across conflicting information and would like to better understand the science behind current veterinary recommendations.

Myth One: The Leptospirosis vaccination, particularly the quadrivalent version (L4), has a high rate of serious side effects

There have been concerns raised about reactions to quadrivalent vaccines (L4) and about reactions to leptospirosis vaccination; these are unsubstantiated.

The overall incidence of suspected adverse reactions for both L2 and L4 vaccine products is considered to be rare. Reports of adverse events following leptospirosis vaccination are very low and have continued to decline over time.

Historically, data from the Veterinary Medicines Directorate (VMD) showed fewer than 2 adverse reactions per 10,000 doses for L2 vaccines and fewer than 7 per 10,000 doses for L4 vaccines. In 2023, the VMD issued an update confirming that the rate of reported adverse events had been decreasing over time with the incidence of adverse events being 0.016% for L2 vaccines and 0.045% for L4 vaccines, equivalent to fewer than 2 reports per 10,000 doses for L2 and fewer than 5 per 10,000 doses for L4.

The latest analysis conducted in December 2025 shows rates have fallen even further. The incidence of adverse events remains 0.016% for L2 vaccines and has reduced to 0.040% for L4 vaccines, meaning the VMD has received fewer than 2 reports per 10,000 animals treated with L2 vaccines and fewer than 4 per 10,000 animals treated with L4 vaccines.

VMD reaffirms safety of Leptospira vaccines in dogs - GOV.UK

Overall, this data demonstrates that adverse reactions to leptospirosis vaccines are rare and continue to decrease over time.

An informal poll on Veterinary Voices UK found:

• 92% Had never seen a serious adverse effects of the vaccination

• 6% Had seen a case that happened shortly afterwards but was never attributed to the vaccine. Remember correlation doesn't always mean causation.

• 2% Had seen an adverse effect

Correlation doesn't equal causation

These statistics includes every suspected adverse event reported, even cases that were considered unclassifiable or were later found to be unrelated to the vaccine.

One member of Veterinary Voices stated:

'We had a 14 week old puppy found dead in its crate less than 24hrs after second L4 vaccine. The vaccine company paid for a Post Mortem (PM) which found it choked on some kibble. Without the PM though it’s one of those stories that would have been all over social media.'

Remember correlation doesn't equal causation. One member stated:

'One of my dogs with a history of severe immune mediated disease was due a DHP/L4 booster on a Friday. The following Tuesday she had a severe relapse and ended up at a referral centre ...Only thing was, I was so busy on the Friday that she never got her booster…

If I had given it, it would be extremely likely I would have thought the vaccination was the cause when this simply wasn't the case.'

Myth Two: After the primary course, other core vaccinations don’t always need to be given yearly. Why are we giving Leptospirosis yearly? You are 'over vaccinating'!

Some vaccines are highly effective at inducing long-lasting protection by stimulating a durable immune memory response. Others, due to the nature of the pathogen or the type of vaccine used, provide only shorter-term immunity or primarily reduce the severity of clinical disease rather than completely preventing infection. Canine leptospirosis vaccination falls into this latter category.

Immunity induced by vaccination with current Leptospira bacterins is serogroup‐specific, knowledge of serogroups that commonly cause disease within a particular geographic region remains important for vaccine design.

Vaccines against leptospirosis are inactivated (killed) vaccines, meaning they do not contain live organisms capable of replicating. As a result, they stimulate a protective immune response that is shorter in duration compared with many other core vaccines. For this reason, the initial vaccination course requires two doses, followed by annual booster vaccinations to maintain protection.

This differs from other core canine vaccines, such as those protecting against distemper, adenovirus, and parvovirus, which typically provide a longer duration of immunity and therefore require boosters less frequently. Because immunity to leptospirosis is relatively short-lived, maintaining annual vaccination is important to ensure continued protection.

Leptospirosis is also a zoonotic disease, meaning it can be transmitted from animals to humans. For this reason, maintaining vaccination is not only important for protecting dogs, but also contributes to reducing potential public health risks. A systematic review and meta‐analysis of experimental trials of commercially available vaccines found >80% protection against clinical disease and kidney carrier status in dogs.

Annual vaccination appointments also provide an opportunity for a comprehensive veterinary health check, allowing owners to discuss any concerns and enabling veterinary surgeons to identify potential health issues that may otherwise go unnoticed.

Why can’t we simply use titer testing?

For some diseases, blood titer testing can be used to measure antibody levels and assess whether an animal still has protective immunity. However, leptospirosis is different.

In leptospirosis, antibody levels in the blood do not reliably correlate with protection.

Research has shown that some animals with high antibody titers may still lack effective protection, while others with low titers may remain protected. This means that standard antibody testing cannot reliably determine whether a dog is immune to leptospirosis.

Assessing immunity to leptospirosis therefore requires more complex laboratory methods used in research settings, rather than routine clinical testing.

Studies demonstrate that immunity following both vaccination and natural infection is relatively short-lived <12-15 months), which is why regular booster vaccination remains the recommended approach.

In a recent poll conducted by Veterinary Voices UK, 99.6% of veterinary professionals supported annual leptospirosis vaccination. The remaining 0.4% suggested that, depending on individual risk factors and geographical location, some dogs may even benefit from more frequent vaccination in high-risk situations.

Myth Three: Leptospirosis is not a dangerous life threatening disease

• Canine leptospirosis is a bacterial zoonotic disease with worldwide distribution.

• The term zoonotic (also known as zoonoses) are caused by germs/ viruses/ bacteria that spread between animals and people.

As mentioned above Leptospirosis is Zoonotic meaning it puts ourselves, our children, our relatives and veterinary professionals at serious risk, subclinical shedding has been documented in shelter dogs.

One member of Veterinary Voices UK stated:

'An acquaintance of mine is currently in an induced coma in ICU with Leptospirosis. I think the zoonotic potential is often underestimated.'

It can cause numerous serious issues such as renal and hepatic disease, coagulopathies and other abnormalities. Case mortality rate ranges in various studies from 10–43.3+%

A UK study noted a 29% mortality rate with clinical signs of the disease including inappetence, vomiting, lethargy, polydipsia and polyuria, abdominal pain with hepatic (liver) and renal (kidney) involvement.

When polled 100% of Veterinary professionals from Veterinary Voices UK believed Leptospirosis to be serious and life threatening. These are professionals who have been the front line in dealing with these cases in practice and have diagnosed, intensively nursed, treated, medicated and fought to keep these Leptospirosis cases alive. Sadly many have to be euthanased due to multiple organ failure and poor prognosis.

Thankfully, generally although the potential exists for zoonotic transmission of pathogenic leptospires from sick dogs with leptospirosis to humans, the 'Updated ACVIM consensus statement on leptospirosis in dogs 2023' states that the risk of such transmission appears to be low, especially when basic precautions are taken. However because leptospiruria usually does not commence until 7 to 10 days after infection, dogs in the first few days of illness (before veterinary care is sought) may not represent a clinically relevant source of zoonotic infection. Treated dogs returned to the home environment also represent a low risk to household members. Nevertheless, until 48 hours of treatment with doxycycline has been completed, owners should avoid contact with their dog's urine and wear gloves and eye protection when cleaning up urine.

Myth Four: Leptospirosis is easily treated, you don't have to worry about it.

It is often cited that a 'quick course of antibiotics' is enough to treat leptospirosis in dogs.

Although it is true that dogs with leptospirosis are recommended a minimum 14 day course of antibiotic, this is rarely the only treatment these cases receive given the hepatic and renal involvement and severity of the disease.

One consensus paper states 'Based on the marked morbidity and potential mortality of the disease, and the risk of zoonotic transmission, as recommended for humans... dogs with suspected or probable leptospirosis should be treated with appropriate antimicrobials'

In a paper, reporting severe clinical symptoms associated with L. Australis infection in a dog vaccinated with a commercial bivalent vaccine against leptospirosis, the paper noted Leptospirosis in dogs can present with a wide range of clinical signs, including vasculitis, acute kidney injury (AKI), and hepatic injury, with the exact manifestations varying depending on the infecting strain and the host’s immune response. Additional reported clinical features include fever, pulmonary haemorrhage, uveitis, myositis, and reproductive failure. Pulmonary complications of this nature are increasingly recognised as a frequent and life-threatening manifestation of canine leptospirosis in some regions of Europe. The paper stated 'Leptospira infections are a common cause of acute renal disease, and, during the acute phase of infection, the predominant renal lesions are those of acute interstitial nephritis, with tubular cell necrosis, apoptosis, and regeneration. As previously reported, the most common clinicopathologic findings in dogs with L. Australis infection identified multiple organ damage with consistent renal involvement, as well as muscle, heart, and liver impact. In the majority of symptomatic dogs, clinicopathologic evidence of nephropathy mirrored histologically apparent interstitial damage and was characterised by low urine-specific gravity, glycosuria, proteinuria, and cast formation. Similar findings were reported in previous studies in which infections were caused by serogroups other than Australis.'

Dogs commonly require hospitalisation, intravenous fluid therapy, analgesia, intensive nursing care, anti-emetics, anti-hypertensives, and nutritional support. In severe cases with pulmonary injury they may even require oxygen therapy and mechanical ventilation.

Devastatingly dogs are often extremely sick, with multiple organ failure and have to be humanly euthanised. This is extremely upsetting for both the owners and veterinary professionals.

On member on Veterinary Voices reflects on a case:

One case I particularly remember survived but required a feeding tube, a weeks hospitalisation and very intense barrier nursing. After she went home, she had weekly blood tests to monitor kidney and liver parameters for around 8 weeks. Very expensive for the owner, compared to vaccinating. Also, not covered by insurance, as she was not vaccinated and it’s a preventable disease.

Dogs presenting with kidney injury associated with leptospirosis can take months to recover and in approximately 50 per cent of dogs impairment persists for more than one year. This will incur veterinary costs for ongoing treatment and monitoring of the animals renal health.

When polled 100% of veterinary professionals from Veterinary Voices UK agreed that Leptospirosis was not easy to treat and often requires intensive care or euthanasia.

Myth Five: We don’t need to vaccinate against L4 in UK!

Key points summary:

• There are over 300 pathogenic serovars of Leptospira, and classification can be complex.

• Vaccines are serovar-specific, so understanding which strains circulate in a region is important.

• Serovars are grouped into serogroups based on similarities in their outer lipopolysaccharide antigens.

• Different Leptospira species can share the same serovar name (e.g., Grippotyphosa occurs in both L. interrogans and L. kirschneri).

• Pathogenic leptospires are also classified as P1 and P2 pathogens, with P1 (especially L. interrogans and L. kirschneri) responsible for most disease in humans and animals.

• Modern classification can also use genomic sequence typing.

• Serogroups were once thought to be linked to specific reservoir hosts, but research shows multiple species can carry the same serogroup.

• As a result, identifying a serogroup does not reliably indicate the reservoir host.

Immunity induced by vaccination with current Leptospira bacterins is serogroup‐specific, however, a systematic review and meta‐analysis of experimental trials of commercially available vaccines found >80% protection against clinical disease and kidney carrier status in dogs. Although bacterins elicit serogroup‐specific immunity, partial immunity to heterologous serogroups has been documented in some studies.

Typically, Bivalent vaccines contains the serogroups

-L icterohaemorrhagiae -L canicola

Historically these were the most common types seen – however, in recent years others have been increasingly being seen and in the L4, the manufacturers made two changes.

Changes include:

The manufacturer increased the amount of antigenic compound (i.e. how many dead bacteria were present)

They also added two additional strains, Australis(the group containing the specific strain Bratislava) and Grippotyphosa (reported to be the most common strain in much of Europe).

So now the current L4 vaccine contains four serogroups

-L canicola -L icterohaemorrhagiae -L grippotyphosa -L Bratislava

In 1991 a major study was conducted in Edinburgh and Glasgow and found that just over 6% of dogs had been exposed to Bratislava (part of the Australis group) – and of those dogs testing positive for exposure to any Lepto strain, 5% were actively excreting Bratislava bacteria.

This study found the following:

• 150 dogs were tested for antibodies to 11 leptospiral serovars.

• 29 dogs (19.8%) were seropositive.

• Main serovars detected:

  • L. icterohaemorrhagiae: 18 dogs (48.6%)

  • L. bratislava: 9 dogs (24.3%)

  • L. canicola: 8 dogs (21.6%)

  • L. hardjo: 1 dog

• Vaccination groups:

  • Vaccinated dogs can still become infected, though risk is lower.

  • Unvaccinated or inadequately vaccinated dogs have higher risk of active infection.

Now we may also have an increase in the European serogroup risk because imported dogs are on the increase and a number of dogs travel to Europe for holidays with their owners.

The number of dogs being commercially imported has surged by more than 50% due to lockdown - with imports from Romania leaping by 67% as the demand for puppies soared in the UK. While the number of imports just from Romania increased by 67% from 19,489 to 32,525 and now represent more than half (54%) of all EU imports.

One veterinary surgeon from Veterinary Voices UK stated:

We had 2 confirmed cases in one household earlier this year, all dogs were vaccinated with lepto 2 but the strain affecting them was typed and was in lepto4 not lepto2. Sadly both dogs who were confirmed with Leptospirosis died despite intensive treatment.

NB: The Nobivac L2 vaccine has been discontinued, with manufacturers transitioning to quadrivalent vaccines such as Nobivac L4 in line with updated international guidance.

This change followed a review of the World Small Animal Veterinary Association (WSAVA) vaccination guidelines, which recommend the use of quadrivalent leptospirosis vaccines as they provide broader protection against multiple circulating strains of Leptospira.

In 2026, a new brand was released to fill this gap which is a inactivated bacterial vaccine suspension specifically formulated to stimulate active immunity in dogs against two critical serogroups of Leptospira interrogans: Canicola and Icterohaemorrhagiae.

Information is changing and this article is based on what is known as of January 2026. If any new information has been presented, please let us know and we can review and update.

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Robyn Lowe BSc Hons, Dip AVN (Small Animal), Dip HE CVN is a small animal Registered Veterinary Nurse (RVN) who regularly writes articles for academic journals and publications for animal owners. Robyn has a passion for evidence-based medicine, volunteers for Canine Arthritis Management, runs the Veterinary Voices Public Page, and campaigns on mental health and animal welfare issues.

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