This is a really interesting question, and one that a lot of animal owners have been asking. There are also different views by vets – on a quick “straw poll” in our office, we have had three different answers so far! Ultimately, it boils down to questions of effectiveness, safety, cost and convenience – but in this blog we are going to try and use evidence to determine what the best solution is in the real world.

What is the lepto vaccine for?

As the name suggests, the vaccine provides limited, but fairly effective, protection against Leptospira infection. This is caused by a group of bacteria called leptospires that are spread in the urine of infected animals, primarily rats. The bacteria can infect many animals, including dogs, cattle and humans, and may lead to liver damage, kidney failure, and are potentially fatal, with UK vets reporting a 60% mortality rate in infected dogs.

That said, clinical infection is relatively uncommon, possibly as a result of relatively widespread vaccination – while the vaccine does not prevent infection with leptospires, it is pretty good at preventing that infection from causing clinical disease, and excellent at preventing the patient from becoming a carrier, infecting other dogs, animals or people.

What’s the difference between the two vaccines?

The main difference is that the L2 contains only 2 strains of leptospire – Icterohaemorrhagiaeand 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 – firstly, they increased the amount of antigenic compound (i.e. how many dead bacteria were present), and secondly they 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). Contrary to popular belief, there has been no change in the adjuvant component of the vaccine between the two versions.

Why are some puppies being started on L2 anyway?

Originally, the manufacturers of the L4 series of vaccines (Intervet, a subsidiary of Merck’s MSD Animal Health division) had intended to phase out the L2 vaccine, and entirely replace it with L4.

However, there has been a widely publicised campaign reporting large numbers of deaths and other harmful effects from the vaccine, and a lot of breeders refuse to use it; as a result, many vets offer either the L4 or the L2.

Meet Ella: The Dog Who Saved My Life: The Sunday Times Bestseller
Meet Ella: The Dog Who Saved My Life: The Sunday Times Bestseller
Buy Now
We earn a commission if you make a purchase, at no additional cost to you.

Is the L4 vaccine really dangerous?

It depends how you define your terms! Ultimately, any medication is potentially dangerous – a drug that never has any side effects at all is, sadly, either a myth or a product that doesn’t do anything at all. As soon as you start altering or tweaking the physiology of an organism as complex as a mammal, there is the potential for unexpected and undesirable results. In actual fact, vaccines as a whole are much safer than most medicines, because they are actually far more natural than any pharmaceutical or herbal medicine.

We need to remember what a vaccine is – essentially, it’s not intended to alter the animal’s physiology or biochemistry. Instead, we can think of it as being like a flight simulator – it allows the immune system to “practice” how to fight an infection, with a lower (or in the case of a killed vaccine like Lepto, zero) risk of real infection. However, there is always the chance of the immune system “overreacting”. As a result, both the manufacturers and the regulators are required to keep data as to how many suspected adverse reactions occur.

For L2, the risk is 0.015% of a suspected adverse reaction – so of 10,000 dogs given the vaccine, we would expect 2 to become ill enough to require treatment, and therefore get a report of a significant side effect. For L4, the risk ishigher, at 0.069% (so 7 in 10,000). This is a similar risk to you developing anaphylactic shock, an asthma attack, or a bleed on the brain after taking an aspirin tablet– it can happen, but it probably won’t.

What about all the dogs who are supposed to have died?

Hundreds of thousands of doses of L4 are given every year – sadly, sooner or later statistically some dogs will develop more serious problems, some of which may be fatal. The trouble is that a “suspected adverse reaction” may or may not actually be due to the vaccine – it could be that the patient developed some disease at a similar time, that it was due to some other component in the combination vaccine given, or (most commonly of all) that the dog was examined by a vet who noticed the problem at the time of vaccination.

That said, there is a very real possibility that any vaccine can trigger an autoimmune disease. Is this really a vaccine related event? The consensus of opinion is that these dogs would sooner or later experience autoimmune disease, as they were genetically predisposed to do so – either triggered by a vaccine, or by a natural infection, it wouldn’t really matter.

Overall, though, dogs having L4 are roughly 4 ½ times more likely to become unwell than dogs having L2.

So, is it worth it?

Now here’s the rub! Unfortunately, there is no central database for how many Leptospirosis cases there are in the UK that are due to each serotype of the bacteria. We are pretty sure that Grippotyphosais not natively found in the UK (although this would be relevant to any dogs visiting Europe). However, 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.

Assuming a 5% risk of infection at any one time, this means that 1 in 20 (500 in 10,000) of the L2 but not L4 vaccinated dogs poses a risk to others of spreading Leptospira bacteria; assuming that 98% of cases are subclinical, then 0.1% (10 in 10,000) will get ill – a rather higher number than the risk of suspected adverse reactions.

So overall, based on the evidence we have now, yes, you are better off vaccinating against L4 than just limiting it to L2.

How do we move forward?

There are three options –

  • Get them upgraded now. This will, however, cost more and be inconvenient, for a relatively small benefit to your dog.
  • Don’t upgrade, just keep them on L2. This is cheapest and most convenient, but does mean that there’s a substantial risk that they will be shedding the bacteria, and there is still a markedly increased risk of harm.
  • Upgrade when your dog is next due their vaccination. This is probably the best “compromise” position.

Overall, it’s really important that you and your vet agree a vaccination plan for your dog as an individual. Also, don’t trust online scaremongering – there are lots of very misleading websites out there (mainly anti-vaccination, but some over-enthusiastically pro-vaccine as well) whose output is based entirely on emotion and illogic, rather than evidence and clinical reality.

You may also be interested in;

For any health concerns, please also use our free Dog Symptom Checker.

Thoughts of Dog - 2025 Day-to-Day Calendar
Thoughts of Dog - 2025 Day-to-Day Calendar
Shop Here
We earn a commission if you make a purchase, at no additional cost to you.
Chuckit Indoor Fetch Ball Dog Toy - Orange and Blue
Chuckit Indoor Fetch Ball Dog Toy - Orange and Blue
See More Dog Toys Here
We earn a commission if you make a purchase, at no additional cost to you.