Read This Before You Vaccinate For Lepto
As an example, there are five strains of Parvovirus. Interestingly the first strain, CPV-1 that wreaked havoc with dogs in the 1980s no longer infects dogs. They have become immune to this strain through naturally acquired or herd immunity. This strain is still in the environment and there is no vaccine for it - Parvo vaccines cover the more recent CPV-2 strains only. Where did the CDV-2 strains come from? Mutation, likely from parvo vaccination. While scientists were busy creating Parvo vaccines, dogs acquired herd immunity to the original dangerous strain all on their own. Now dogs are only susceptible to the strain of Parvo for which we have a vaccine - is this coincidence?
The same may be happening now with Lepto: as we vaccinate for it, the vaccines cause a shift or mutation in the pathogen and, as it adapts, a new strain develops.