Emerging Tick-Borne Diseases

Abstract

Increases in tick-borne disease prevalence and transmission are important public health issues. Efforts to control these emerging diseases are frustrated by the struggle to control tick populations and to detect and treat infections caused by the pathogens that they transmit. This review covers tick-borne infectious diseases of nonrickettsial bacterial, parasitic, and viral origins. While tick surveillance and tracking inform our understanding of the importance of the spread and ecology of ticks and help identify areas of risk for disease transmission, the vectors are not the focus of this document. Here, we emphasize the most significant pathogens that infect humans as well as the epidemiology, clinical features, diagnosis, and treatment of diseases that they cause. Although detection via molecular or immunological methods has improved, tick-borne diseases continue to remain underdiagnosed, making the scope of the problem difficult to assess. Our current understanding of the incidence of tick-borne diseases is discussed in this review. An awareness of the diseases that can be transmitted by ticks in specific locations is key to detection and selection of appropriate treatment. As tick-transmitted pathogens are discovered and emerge in new geographic regions, our ability to detect, describe, and understand the growing public health threat must also grow to meet the challenge.

Keywords: Amblyomma; Anaplasma phagocytophilum; Babesia; Babesia microti; Borrelia; Borrelia burgdorferi; Borrelia mayonii; Borrelia miyamotoi; Crimean-Congo hemorrhagic fever virus (CCHFV); Ehrlichia chaffeensis; Ehrlichia ewingii; Ixodes; Lyme; Powassan virus (POWV); babesiosis; blood-borne parasites; deer tick virus (DTV); emerging disease; tick borne; tick-borne encephalitis virus (TBEV).

FIG 1

Areas of the United States where I. scapularis, A. americanum, and I. pacificus are endemic. Pathogens that can be transmitted by I. scapularis include A. phagocytophilum, B. burgdorferi, B. miyamotoi, Babesia microti, Babesia divergens, and POWV. Pathogens that can be transmitted by I. pacificus include A. phagocytophilum, B. burgdorferi, B. miyamotoi, Babesia microti, and Babesia duncani. Pathogens that are transmitted by A. americanum include E. chaffeensis, E. ewingii, Heartland virus, and Bourbon virus. Where tick species ranges overlap, all pathogens need to be considered in making a diagnosis.

FIG 2

Areas where Ixodes species are endemic are at risk for transmission of Lyme borreliosis, A. phagocytophilum, and Babesia sp. Data presented in this map were collected through the VectorNet project. Countries and regions are displayed at different scales to facilitate visualization. Antic., anticipated; Obs., observed. (Adapted from European Centre for Disease Prevention and Control and European Food Safety Authority maps at https://ecdc.europa.eu/en/disease-vectors/surveillance-and-disease-data/tick-maps.)

FIG 3

Incidence of bacterial tick-borne diseases in the United States in 2017. (A) Lyme disease. Note that the scale is different for Lyme disease due to the greater incidence. (B and C) Anaplasmosis (B) and ehrlichiosis (E. chaffeensis) (C). NN, not notifiable. (Based on data from the Centers for Disease Control and Prevention at https://www.cdc.gov/lyme/stats/tables.html [panel A], https://www.cdc.gov/anaplasmosis/stats/index.html [panel B], and https://www.cdc.gov/ehrlichiosis/stats/index.html [panel C].)

FIG 4

Increase in the number of reported cases of babesiosis in the Unites States since the disease became nationally notifiable in 2011. The inset shows the number of cases in New York (where babesiosis has been a reportable disease since 1986) and the number of specimens that were positive for Babesia microti by RT-PCR at the Wadsworth Center, New York State Department of Health.

FIG 5

Geographical distribution of Crimean-Congo hemorrhagic fever (CCHF). Source data for the map are from the World Health Organization. The boundaries and names shown and designations used on this map do not imply the expression of any opinion whatsoever on the part of the World Health Organization concerning the legal status of any county, territory, city, or area or of its authorities or concerning the delimitation of its frontiers or boundaries. (Modified from the World Health Organization map at http://www.who.int/emergencies/diseases/crimean-congo-haemorrhagic-fever/en/.)