Lyme Disease is a bacteria spread by ticks. If left untreated it can cause arthritis, kidney disease, and in some cases cause a heart condition called mycocarditis. Usually the first sign of Lyme Disease in dogs is limping, or lameness in a limb. Although there are very effective topical treatments available to help prevent ticks, the risk is still there.
If the prevention was not properly applied, or if you lapse a few days in applying the prevention, a tick can bite and transmit the disease. There are several different variables that may put your pet at risk. Topical prevention along with annual vaccines are highly recommended.