@helle-devi - I still do not recommend Lepto vaccine. Too many Basenjis have had reaction to it. That said, if you are in an area with high issues with Lepto, you need to weigh averages. Lepto is typically found around standing water and usually from other dogs.... How much of an increase? Is your Basenji usually on leash? And if you decide to get the vaccine I would make sure it is separate from any other vaccines by at least two to three weeks. Then if you have a reaction you know it is from the Lepto shot. Also you should do Titer Test before giving shots (except Rabies since that is required by laws of each state). Many times doing a titer test will show that a dog is fully covered and do not need a booster. My C-Me will be 12 on Christmas Eve. She had her 1yr booster and never has needed one since. My Franie (will be 12 in January) needed a booster at age 9.
