Tick-borne diseases are increasingly important public health problems as case numbers and the list of identified pathogens continue to grow. A recently published retrospective study on the medical costs associated with Lyme disease suggests that $0.7 -1.3 billion is spent annually in the US to pay for direct medical costs related to the illness. The scientific understanding of these pathogens, their vectors, and the illnesses they produce continue to evolve. Despite the importance of these diseases, clinician training regarding these illnesses is limited and the Entomological Society of America states there is “a severe ‘tick literacy deficit’ allowing TBDs to explode.”
The US hosts a wide variety of hard and soft tick species. There is considerable overlap in the ticks’ ranges and, as this map illustrates, several areas of the country are home to 3 or more species.
Soft ticks transmit members of the relapsing fever group of Borrelia, including B. hermsii, B. parkeri, and B. duttoni. However, B. miyamotoi, which is also in the relapsing fever group, is transmitted by Ixodes ticks. Combined, species of Ixodes, Amblyomma, Dermacentor, and Rhipicephalusticks transmit 18 illnesses and counting.
Many tick species are capable of transmitting multiple pathogens. Thus, a single bite could lead to multiple simultaneous infections. The various tick-borne diseases seen in the US often produce similar nonspecific symptoms and because the available diagnostic testing is often insensitive when patients first present, arriving at the correct diagnosis (or diagnoses) can be challenging.
In addition to the diagnostic challenges, managing patients with tick-borne illnesses can also be vexing. There is little high-quality trial evidence to guide treatment decisions for patients with a single infection and no trial evidence regarding multiply infected patients. Additionally, some infections appear to be difficult to cure and may produce chronic disease
Incidentally, this website uses the term “tick-borne disease” as a general descriptor that can be applied to any of the illnesses that are transmitted to humans via a tick bite. “Co-infection” is more specific, referring only to the other non-Lyme illnesses that are also transmitted by black-legged ticks. Thus, while every co-infection is also a tick-borne disease, not all tick-borne diseases are co-infections.