Ticks that transmit B. burgorferi to humans can also carry and transmit several other parasites such as Theileria microti and Anaplasma phagocytophilum, which cause the diseases babesiosis and human granulocytic anaplasmosis (HGA), respectively. Among early Lyme disease patients, depending on their location, 2-12% will also have HGA and 2-40% will have babesiosis. Cat scratch fever is another common co-infection, although there is debate among experts on this topic on tick-to-human transmission.
Co-infections complicate Lyme symptoms, especially diagnosis and treatment. It is possible for a tick to carry and transmit one of the co-infections and not Borrelia, making diagnosis difficult and often elusive. The Centers for Disease Control (CDC)’s emerging infections diseases department did a study in rural New Jersey of 100 ticks and found that 55% of the ticks were infected with at least one of the pathogens.