Leptospirosis is disease caused by several types of Leptospira bacteria. It’s often called a re-emerging disease in dogs, because the incidence has been increasing over the past couple of decades in many areas. While the overall number of infections is limited, when it does occur lepto can cause serious disease, including kidney failure. It’s also a zoonotic disease: lepto can be transmitted from sick dogs to the people who handle them. Infected animals shed the bacteria in their urine, and when the urine comes in contact with broken skin or mucous membranes (mouth, eyes, nose), transmission of infection can result.
Canine vaccines against lepto are available. In the past, lepto vaccination hasn’t been all that popular because the vaccines tended to have a higher rate of side effects compared to most vaccines, and the types of lepto that the vaccines targeted were often not the most important types causing disease. This has changed more recently as newer vaccines have become available. These vaccines seem to have low rates of adverse effects and provide protection against the four types of lepto that tend to be the most important. These vaccines have also been shown in research studies to protect against disease and to reduce shedding of the lepto bacterium in urine (thereby decreasing the risk of transmission to other animals or people).
Lepto is not among the "core" vaccines that groups say all dogs should have. That’s because the risk of lepto exposure varies greatly between regions and dogs. The key thing to consider when deciding whether or not to vaccinate is what each individual dog’s risk of exposure is. The main reservoir of lepto is wildlife such as rats and raccoons. If infected wildlife are in the area, they may be peeing out lepto and contaminating the environment, particularly any type of standing water (lepto loves to live in warm, wet environments).
In the past, much of the concern has been focused on dogs that go out and swim in streams or ponds. The "poster child" for lepto has typically been the Labrador Retriever that goes out gallivanting in the bush, but that may be changing too. When you consider where wildlife such as rats and raccoons live and how high the wildlife infection rates can be, remember that these pests can be present in urban areas in incredibly large numbers. Living in a city does not make a dog safe from exposure to lepto. In fact some urban areas, with large numbers of wildlife crowded into high-traffic areas like parks, are probably higher risk than rural areas.
Deciding on vaccinating requires an understanding of how common lepto is in the area and whether there is a risk of exposure for the individual animal. This is a disease for which a veterinarian’s understanding of disease patterns in an area (including any specific areas that be particularly high risk) and the types of possible exposure of the dog play a big role. There’s no "one size fits all" recommendation for lepto vaccination.