Tick-Borne Zoonoses in the United States: Persistent and Emerging Threats to Human Health

Abstract

In the United States, ticks transmit the greatest diversity of arthropod-borne pathogens and are responsible for the most cases of all vector-borne diseases. In recent decades, the number of reported cases of notifiable tick-borne diseases has steadily increased, geographic distributions of many ticks and tick-borne diseases have expanded, and new tick-borne disease agents have been recognized. In this review, we (1) describe the known disease agents associated with the most commonly human-biting ixodid ticks, (2) review the natural histories of these ticks and their associated pathogens, (3) highlight spatial and temporal changes in vector tick distributions and tick-borne disease occurrence in recent decades, and (4) identify knowledge gaps and barriers to more effective prevention of tick-borne diseases. We describe 12 major tick-borne diseases caused by 15 distinct disease agents that are transmitted by the 8 most commonly human-biting ixodid ticks in the United States. Notably, 40% of these pathogens were described within the last two decades. Our assessment highlights the importance of animal studies to elucidate how tick-borne pathogens are maintained in nature, as well as advances in molecular detection of pathogens which has led to the discovery of several new tick-borne disease agents.

Figure 1

Generalized distributions of key human biting ticks in the United States These maps provide general insight into the expected distribution of common human-biting ticks in the contiguous United States. Populations of these ticks may be found outside noted areas. The maps are not meant to represent risk for tick-borne diseases. The brown dog tick, Rhipicephalus sanguineus, is found throughout the continental United States and in Hawaii. The geographic distribution of the Pacific Coast tick, Dermacentor occidentalis, has not been assessed in recent years and is not shown.

Figure 2

Reported human cases in the United States of Lyme disease, 1992–2014 (A); tularemia, 1944–2014 (B); and anaplasmosis (diagonal lines), ehrlichiosis (stippled), and spotted fever group rickettsioses (black) 2000–2014 (C).

Figure 3

Reported distribution of key tick-borne diseases in the United States, 2014 Each dot represents one case. Cases are reported from the infected person’s county of residence, not necessarily the county of exposure. During 2014, babesiosis was reportable in Alabama, California, Connecticut, Delaware, Illinois, Indiana, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Montana, Nebraska, New Hampshire, New Jersey, New York, North Dakota, Ohio, Oregon, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Washington, West Virginia, Wisconsin, and Wyoming. CDC was not notified through the national surveillance system of cases in other states. In 2014, no cases of tick-borne illness were reported from Hawaii. In 2014, Alaska reported eight travel-related cases of Lyme disease.