The field of disease ecology faces the challenge of linking ecological and epidemiological approaches to better inform and understand human disease risk from vector-borne diseases [1]. While vector-borne diseases are attributable to bacteria, viruses, and parasites transmitted through the bites of ticks, mosquitoes, and fleas [2], the majority of vector-borne diseases in the United States (U.S.) are spread by ticks; these Tick-Borne Diseases (TBDs) accounted for 77% of the 642,602 cases reported to the U.S. Centers for Disease Control and Prevention (CDC) between 2004 and 2016, consisting of 16 diseases including Lyme Disease (LD), Ehrlichiosis (EHR), and Spotted Fever Rickettsioses (SFR) [2]. The CDC notes that these numbers reflect significant under-reporting, such that the true extent of LD and other TBDs is largely undetermined and potentially underestimated ^[3][4][3,4]. Ixodes scapularis (I. scapularis) ticks are known to be widespread in the eastern U.S. However, limited surveillance may account for underreporting outside of the Northeast ^[5][6][7][5,6,7]; yet the geographic range of I. scapularis is spreading rapidly, contributing to the increase in LD cases [6].

Passive tick surveillance is the primary framework for current public health system and practice in the U.S., and relies on reporting of ticks found on people, livestock, or pets, and identification of concomitant disease. Active tick surveillance entails direct collection of ticks from host animals, such as deer and other mammals. Both approaches are limited by time involved in tick collection and testing, and could be improved with more detailed documentation, identification of relevant tick habitats, and incorporation of proxy data associated with TBD risk.

Vector-borne pathogen transmission is affected by a variety of factors from tick ecology, the number of pathogens carried, their reservoirs, and hosts. White-tailed deer (Odocoileus virginianus) serve as primary reservoir hosts for Ehrlichia chaffeensis (E. chaffeensis), which is the pathogen that causes human monocytic ehrlichiosis (HME) [8]. The cycle of tick transmission involves feeding on infected reservoir hosts and then transmitting Borrelia burgdorferi (Bb) to the host [9]. White-tailed deer (also referred to in this article simply as “deer”) present differing roles as non-competent hosts for LD, but primary hosts for the adult ticks that carry Lyme disease [10]. Unlike small mammals, deer are not reservoirs for LD in particular, but rather are inadvertent hosts. Deer are also noted as important hosts for E. ewingii, another pathogen found in humans and canines, transmitted by the Lonestar tick, as well as the Blacklegged Tick Ixodes scapularis [11]. The role of wildlife in the circulation of Anaplasma phagocytophilum has yet to be clearly determined, but several species of wild ruminants, including deer, have been considered by some researchers as possibly important reservoirs ^[12][13][12,13]. A natural opportunity for assessing deer density and disease risk occurred on a small island in Denmark, where deer density plummeted following an epidemic; human Lyme neuroborreliosis cases declined in tandem with the host and tick reduction [14]. Despite their resilience to contracting LD, deer nonetheless serve as large “mobile hosts” for pathogen-carrying ticks such as I. scapularis infected with bb or other bacterial agents [15].

Ixodes scapularis ticks responsible for transmitting LD have a broad range of potential hosts—including lizards, rodents, and deer—that may cross paths and transmit infected ticks to humans and/or their pets. The complex feeding cycle of ticks often involves multiple feeding and egg-laying rounds on their deer or rodent hosts before transmission of pathogenic bacteria to a susceptible host; these cycles are impacted by factors such as vegetation, ecological suitability, and a variety of other environmental considerations.

Next to mosquitoes, ticks are the most common parasitic disease vector in the world and, in the U.S., ticks are the most prevalent vector for infectious disease, responsible for LD, Rocky Mountain spotted fever and Spotted Fever Rickettsiosis (SFR), Anaplasmosis (ANA), southern tick associated rash illness, and a number of other serious diseases. Active TBD diagnosis, monitoring, and surveillance remain limited by data gaps and time, geography, equipment, funding, and testing opportunities. Carbon dioxide (CO₂) trapping, removal of ticks from hosts such as deer, birds, lizards, or small mammals, and dragging surveys to collect ticks from their natural environments are some of the most common collection, monitoring, and active surveillance strategies. Although repeated visits and collection provide greater precision in estimating tick density and identifying disease, the limitations are many, including personal risk, securing permits, or identifying suitable habitats. Moreover, these techniques are designed to acquire in situ ticks, and so may be less representative of the tick populations and habitats where most humans are infected with TBDs. Active tick surveillance outcomes also are affected by the sample techniques, such as CO₂ trapping, rodent collection, or dragging ^[16][19]. In a study of ticks in Southern Indiana, tick presence and species were affected by sampling methods as well as climate ^[16][19]. To date, and with over 3100 counties and municipalities in the U.S., the information sourced from both active and passive surveillance does not fully inform public health disease prevention, estimation, or intervention.

Passive surveillance that extends beyond simple reporting to public health agencies has the potential to provide further evidence-based, timely, and geographically salient information to medical practitioners, the public, patients, and public health officials. Specifically, the promising use of data proxies, based on, for example, canine, livestock, or other wildlife data can help inform human disease risk at the local level ^[17][18][19][16,17,18].

Deer have been evaluated for their role in spreading tick-borne pathogens, including LD. Deer hosts are not susceptible to Bb infection and, therefore, may dilute the prevalence of these related pathogens and thus reduce infectivity of ticks that feed on them [20]. Conversely, high deer density has the capacity to contribute to disease risk by sustaining a high nymphal tick population [20]. Deer are known to serve as a solid host for blood feeding, even if not suitable reservoirs for Bb ^[21][22][21,22].

Higher densities of I. scapularis are found in more forested areas [23], which is where higher levels of deer population density occur and, thus, have a higher likelihood of serving as hosts. Deer populations increase the spatial distribution of ticks given that deer habitats serve as primary breeding grounds for wintering tick eggs, even if the deer hosts do not directly spread disease [24]. Adult ticks tend to be more common on deer overall; moreover, adults ticks are less-successful vectors for transmitting LD, as they are more easily detected and removed by their human hosts [24]. Although deer may play a role in the tick life cycle, studies point to a limited association of deer populations with human TBD risk. Some research shows a an increase in LD rates that occurred decades after increases in deer populations, or indicate an association between deer density and increased ticks ^{[25][26][27][28]}[25,26,27,28]. Nymphal tick populations were also found to be unaffected by increases in white-tailed deer populations ^[29][30][31][29,30,31]. Recent research expands active surveillance by assessing deer population movement by sex, differing times of day, and by seasons, finding that deer have expansive reach into residential areas and backyards [32]. Overabundance of deer is also found in areas of low LD risk ^[33][34][33,34].

Related findings are compared to tick presence, including nymphal ticks, and deer density proxies, by county in Indiana. Hunters, as reporters of “deer kills,” have served as observers and checks on the deer population, assisting in building One Health multimodal datasets for determining human disease risk.

Medically important ticks are found throughout Indiana. For example, the American Dog tick is present in all 92 counties, and is known to feed on a wide range of mammals, including deer ^[35][36]. The Lonestar tick is found in moist woodlands throughout the state, but primarily in the southern counties. I. scapularis is found throughout the state, with adults feeding on deer and other mammals, and nymphal ticks acting as the primary vector of LD transmission to human and canine hosts. The Brown Dog tick, although not native to Indiana, is present in the state and is a known vector of canine EHR ^[35][36]. I. scapularis is an increasing public health concern across the United States, as it is associated with multiple pathogens such as Lyme disease, anaplasmosis, babesiosis, Borrelia miyamotoi disease, Powassan virus disease, and ehrlichiosis, with LD accounting for more than 70 percent of TBD cases [26]. Changes in Indiana’s ecosystems reveal expanding tick migration patterns and disease risks due in large part to climate shifts and environmental changes. 

Ecoregions are defined by spatial characteristics ranging from hydrology, geology, wildlife, vegetation, and climate (Figure 1). Indiana has eight identified ecoregions containing diverse mixtures of prairies, marshes, dunes, forests, and streams. Within Indiana’s ecosystems, smaller climate and vegetation divergences are notable. For example, within the Central Cornbelt Plains, four separate smaller divisions provide a changing landscape from prairie to marshes and swamp forests ^[36][37].