The emergence of ticks and tick-borne diseases in the USA

Ticks are the primary source of vector-borne pathogens in the USA. The native tick species of the USA are adapted for a number of distinct ecosystems and mammalian hosts. As such, several overlapping sylvatic cycles exist within the tick populations. These occur in nature without human involvement. The incursion of humans into these environments can result in sylvatic spillover of these pathogens to humans, causing clinical disease. As tick populations shift, the risk of exposure to emerging, newly introduced and re-emerging pathogens increases, posing a direct threat to human and animal health. Several genera are the source of most tick-borne pathogens in the USA. These include Dermacentor, Amblyomma and Ixodes, with a lesser role for Ornithodoros soft ticks. This expert opinion focuses on the influence of the changing landscape of ticks in the USA and its impact on the emergence of tick-borne pathogens that occurs as a result.