B. burgdorferi have different outer surface proteins (Osps) that act as antigens and are expressed at various phases in the tick feeding cycle. When a Borrelia-infected tick attaches to a dog, high levels of OspA are expressed by the Borrelia, which are residing in the tick’s midgut.

Soon after the tick takes its bloodmeal, OspA expression decreases and OspC expression increases, becoming the dominant outer surface protein. Infection occurs when the Borrelia, which are now expressing only OspC, pass from the tick’s midgut to its salivary glands and then into the dog.¹ 

Reference:
1. Schwan TG, Piesman J. Vector interactions and molecular adaptations of Lyme disease and relapsing fever spirochetes associated with transmission by ticks. Emerg Infect Dis. 2002;8(2):115–121.