Why is leptospirosis controversial and DHP not?
Three dog vaccines as advised by the WSAVA (distemper, hepatitis, and parvovirus) are considered core vaccines; that is, they are recommended for every dog who is able to have them and the benefits are proven beyond reasonable doubt to far outweigh any risk. These diseases are highly contagious and frequently lethal, and ensuring most dogs are vaccinated protects the dogs who can't receive the vaccination or who fail to respond through herd immunity. It is also known that it is possible to ascertain reasonably well (more in-depth analysis by Skeptvet) if a dog is immune to these diseases by performing a titre test. We know from previous research that a positive titre is a very good indicator that a dog is immune. This is why most manufacturers of these vaccines advise that the duration of immunity (DOI) is three years minimum. This is because they studied dogs in a laboratory for three years and found the titre of the dogs reliably indicated immunity. However, this does not mean immunity ends after three years, as the laboratories didn't continue the experiment for any longer, and vets and many people who titre test their dogs have found the DOI to be much longer, and probably it is safe to assume that these vaccinations will last a lifetime, and if there is any question, we can do a simple titre test to check.
However, leptospirosis is different in a number of ways. Firstly, it's impossible to titre test for leptospirosis immunity, and the only way to find out how long the DOI from these vaccinations lasts is to do a laboratory study in which researchers try to infect dogs with leptospirosis at various intervals after being vaccinated, e.g. Paper. As this kind of research requires a control group and a group for each serovar (see later) covered in the vaccine for each interval of the study, it's easy to see why this is expensive both in terms of money and in terms of the welfare of the dogs being infected with diseases and euthanised to examine what effect this has had. This is why the earlier lepto vaccines only had a published DOI of 6 months -- this was as long as the study lasted. Most modern leptospirosis vaccines have been tested in this way for a year or slightly longer. This doesn't mean the immunity disappears after this time, and it could well last a lifetime just as DHP immunity appears to. But we don't know and we can't show that, and because there is no titre, we have no information from vets or dogs out there in the real world.
Secondly, leptospirosis is a kind of bacteria termed a spirochete, that has many species and many serovars and serogroups (I'm not going to go into detail explaining these terms, but serovars and serogroups are to do with immunity and are not the same as species). The vaccinations do not protect against all serovars that can cause the diseases they are associated with. The older bivalent vaccines are effective against the Canicola and Icterohaemorrhagiae serogroups whereas the newer tetravalent vaccines also include Grippotyphosa and Australis. There is a dearth of information on leptospirosis in the UK, and the sources I found useful when trying to look this up was this PhD thesis by Christopher Ball and this paper about leptospirosis in people. Canicola apparently used to be found in dogs, but is now uncommon (presumably either because of the vaccination or because dogs are less likely to live outside in unsanitary conditions today). Grippotyphosa I can find very little evidence about in the UK, other than some sources say it occurs in 'Europe' (which could mean probably anything). Australis and Icterohaemorrhagiae there is good evidence of existing in the UK, based on studies of wild animals and human patient information. However, there are many serogroups and it is unlikely either of these vaccines provide much if any protection against them.
Thirdly, leptospirosis, although it's infectious and can be fatal, simply isn't as contagious or dangerous to dogs as DHP. Leptospires are carried by wild animals, where they colonise the kidneys and are excreted in urine and semen. The leptospires need damp, mild conditions to survive any length of time without a host. They enter the body through cuts on the skin or through the mucous membranes. That leptospirosis can be zoonotic and spread to humans is probably the reason the BSAVA goes against the WSAVA's guidelines and argues that leptospirosis should be considered a core vaccination, despite admitting serious lack of evidence as to the risk of the disease and the DOI of the vaccines, as a legal backside-covering exercise rather than anything to do with dogs. In reality, it's unlikely a person would catch lepto from their dog or dogs to transmit it to other dogs. Unless your dog is incontinent and you are in the habit of wandering around the house barefoot and standing in its wee, or you take your dog out and let strangers' dogs urinate on it and have sex with it. There is little or no evidence of people being infected with leptospirosis from dogs. Additionally, as leptospirosis infection is acquired from the environment, vaccinating dogs does nothing for herd immunity and preventing the spread of disease.
It's worth emphasising that just because a vaccination is non-core, that doesn't mean the vaccination is useless or should not be used. Other examples of non-core vaccinations given by the WSAVA in the link above include canine herpesvirus, which is given to a bitch just before mating and just before whelping to protect her unborn pups from a viral disease, and kennel cough, which provides some protection against a contagious flu-like illness that isn't dangerous to most dogs, but might be recommended to a dog in regular contact with another dog who might be vulnerable to infection, in the same way the influenza vaccination is given to carers of elderly or immunocompromised human individuals. When speaking about non-core vaccinations, we should talk about the dogs' individual situations and the risk and benefit analysis for each individual dog.
It's worth noting that there are also vaccinations for leptospirosis available for humans. These certainly aren't considered core vaccines (probably for all the reasons above as well as low incidence) and are only really considered in serious risk situations, e.g. aid workers attending a flood-stricken area in a warm climate. Instead, people are advised to use sensible precautions, such as not to stand in swamps, sewers, and stagnant water and not handle dead wildlife, and to wear suitable protective clothing if they have to do these activities. If your situation makes it impossible to keep your dogs away from these sorts of dangers, the vaccination might reduce (but not eliminate) the risk.
"Once upon a time, there was a rat. It was dead and it stank. The end."
-Pandora's favourite bedtime story.
There are three informed decisions you can make:
1. You could use the vaccination according to the manufacturer's guidelines, that is, to do the initial course of two vaccinations and from then on to do one vaccination annually, in the knowledge that the vaccination does not provide full immunity and infection with other serovars of lepto is still possible, and also aware that the DOI is not properly established and administering vaccinations to which an animal is already immune may put the animal at higher risk of a reaction.
2. You could do the initial course of two vaccinations a fortnight apart, and then not repeat the vaccination, in the knowledge that the vaccination does not provide full immunity and infection with other serovars of lepto is still possible, and also aware that the DOI is not properly established and that immunity may (or may not) be lost over time.
3. You could eschew the vaccination entirely, and argue that more research needs to be done into the DOI and to invent a vaccine that covers all strains of leptospirosis that can be pathological to dogs in the UK.
Any of these choices, when made based on proper understanding and risk assessment, are valid.
Take-Home Messages and Advice:
If you have a terrier whom you use for ratting or a dog who regularly swims in ponds etc., the benefits of either 1 or 2 almost certainly outweigh the risks.
If you decide vaccination is right for your dog, you might as well use the tetravalent version (lepto 4) and not the bivalent version, as there is good evidence that one of the additional serovars (Australis) in the tetravalent vaccine is endemic to the UK.
Whether you vaccinate or not, remember your dog is still at risk of lepto and you should bear this in mind should it be ill when there is not a more obvious explanation.
Always take measures to keep rats away from your house and the area your dogs use as a toilet. Use fencing that rats cannot walk through and repair any holes or gaps underneath that rodents could get through. Do not use rat poison as the rats can die anywhere and there is a risk the dogs will find and play with/eat the dead rats, potentially exposing themselves to both the risk of lepto and secondary poisoning. Where rats are a problem, use breakback traps (probably a faster and more humane death for the rats as well) in locations where dogs and wildlife cannot access them, and dispose of the dead rats responsibly while wearing gloves.