Tick-borne illnesses have been on the rise in the United States, with reported cases up sharply in the past two decades. In this literature review, we synthesize the available research on the relationship between vegetation and tick abundance for four tick species in the northeastern United States that are of potential medical importance to humans. The blacklegged tick (Ixodes scapularis) (Say; Acari: Ixodidae) is found to be positively associated with closed canopy forests and dense vegetation thickets, and negatively associated with open canopy environments, such as grasslands or old agricultural fields. The American dog tick (Dermacentor variabilis) (Say; Acari: Ixodidae) has little habitat overlap with I. scapularis, with abundance highest in grasses and open-canopy fields. The lone star tick (Amblyomma americanum) (Linnaeus; Acari: Ixodidae) is a habitat generalist without consistent associations with particular types of vegetation. The habitat associations of the recently introduced Asian longhorned tick (Haemaphysalis longicornis) (Neumann; Acari: Ixodidae) in the northeastern United States, and in other regions where it has invaded, are still unknown, although based on studies in its native range, it is likely to be found in grasslands and open-canopy habitats.

Ticks are the most important vectors of human pathogens, leading to increased public health burdens worldwide. Tick-borne pathogens include viruses (e.g. tick-borne encephalitis and Powassan); bacteria, such as the causative agents of Lyme disease, spotted fever rickettsiosis and human anaplasmosis; and malaria-like protozoan parasites causing babesiosis. Tick-borne diseases are emerging due to the geographical expansion of their tick vectors, especially in the northern hemisphere. Two examples of this phenomenon are Ixodes scapularis and Amblyomma americanum, which have expanded their ranges in the USA in recent decades and are responsible for the continuous emergence of Lyme disease and human ehrlichiosis, respectively. This phenomenon is also occurring worldwide and is reflected by the increasing number of tick-borne encephalitis and haemorrhagic fever cases in Europe and Asia. In this review, we provide a concise synopsis of the most medically important tick-borne pathogen worldwide, with a particular emphasis on emerging public health threats.