Lepto is, for a variety of reasons, a vaccine where the risk vs benefit analysis changes tremendously from case to case.

First, lepto vaccine causes more adverse reactions than any other canine vaccine.

Second, there are many, many strains of lepto, known as "serovars," but there is no cross-protection among lepto serovars (as there is among parvo strains, for instance). A dog can be immune to one serovar of lepto and have no protection at all from another.

To make that worse, there are only four available vaccine serovars of leptospirosis: L. Canicola, L. Gripophytosa, L. Icterohemorrhagiae, and L.Pomona. However, there are two more that are causing disease in dogs, L.Autumnalis and L. Bratislava, for which we have no vaccines.

Fourth (and this is a problem that lepto immunity shares with all bacterial immunity, natural or from vaccines), immunity to bacteria is only temporary. This is why you can get strep throat (a bacteria) over and over, but only get measles (a virus) once. So the immunity will always wear off over time, sometimes in less than a year. This means that you have to repeatedly vaccinate for leptospirosis in order to maintain immunity. Repeated vaccination of course increases the chances there will be an adverse reaction.

Fifth, while vaccination can prevent clinical disease in a vaccinated dog, it does not prevent the dog from becoming a carrier.

Which brings me to my sixth and biggest problem with lepto vaccination. Since there is no cross-protection among serovars, and immunity is temporary, you always have to assume that a dog, vaccinated or not, might have lepto if the symptoms indicate it. It takes a couple of weeks to get the results of a lepto test, so you have to treat the dog based on symptoms and can't just test first. So even if you vaccinate your dog, you may end up having to treat him or her for lepto anyway, and the dog may or may not have it. It kind of takes away the main reason that we vaccinate our dogs, ie, peace of mind and freedom from worry about a certain disease.

People need to be extremely aware of the symptoms of lepto and treat it aggressively from the very first suspicion that it's lepto, regardless of the vaccination status of the dog, and long before the diagnostic test results are known - if you wait that long, it may well be too late. Caught early, nearly all lepto can be treated with antibiotics. Caught late, many dogs will die, or require costly and mostly unavailable dialysis to survive.

I don't think that routinely vaccinating a dog for those four serovars of lepto annually or even more often makes a great deal of sense for most dogs. However, if there is a known outbreak, you know the serovar, and there's a vaccine for it, then you may want to consider it. Lepto does infect humans, so it's often a reportable disease, so your local public health department might be able to tell you if there is an outbreak. Beyond that, word of mouth among dog owners and at the vet's office is your best early warning system.

There is more information on leptospirosis on VeterinaryPartner.com.