Tick-borne disease

Tick-borne diseases, which afflict humans and other animals, are caused by infectious agents transmitted by tick bites.^[1] They are caused by infection with a variety of pathogens, including rickettsia and other types of bacteria, viruses, and protozoa.^[2] The economic impact of tick-borne diseases is considered to be substantial in humans,^[3] and tick-borne diseases are estimated to affect ~80 % of cattle worldwide.^[4] Most of these pathogens require passage through vertebrate hosts as part of their life cycle. Tick-borne infections in humans, farm animals, and companion animals are primarily associated with wildlife animal reservoirs.^[5] Many tick-borne infections in humans involve a complex cycle between wildlife animal reservoirs and tick vectors.^[5] The survival and transmission of these tick-borne viruses are closely linked to their interactions with tick vectors and host cells. These viruses are classified into different families, including Asfarviridae, Reoviridae, Rhabdoviridae, Orthomyxoviridae, Bunyaviridae, and Flaviviridae.^[6]

The occurrence of ticks and tick-borne illnesses in humans is increasing.^[7] Tick populations are spreading into new areas, in part due to climate change.^[8][9] Tick populations are also affected by changes in the populations of their hosts (e.g. deer, cattle, mice, lizards) and those hosts' predators (e.g. foxes). Diversity and availability of hosts and predators can be affected by deforestation and habitat fragmentation.^[8]

Because individual ticks can harbor more than one disease-causing agent, patients can be infected with more than one pathogen at the same time, compounding the difficulty in diagnosis and treatment.^[2] As the incidence of tick-borne illnesses increases and the geographic areas in which they are found expand, health workers increasingly must be able to distinguish the diverse, and often overlapping, clinical presentations of these diseases.^[10]

As of 2020^[update] 18 tick-borne pathogens have been identified in the United States according to the Centers for Disease Control^[10] and at least 27 are known globally.^[8][11][12] New tick-borne diseases have been discovered in the 21st century, due in part to the use of molecular assays and next-generation sequencing.^[13]