Michael R. Gallagher, Jesse K. Kreye, Erika T. Machtinger, Alexis Everland, Nathaniel Schmidt, Nicholas S. Skowronski, October 2022
RESEARCH BRIEF #42
Restoration of degraded eastern US forests through prescribed fire may have public health benefits by reducing tick populations and disease transmissions.
Fire intensity, severity, frequency, seasonality, and spatial variation have differential effects on tick species and life stages; more work is needed to incorporate tick population control into forest restoration objectives.
Tick control by prescribed fire will require recurring fire and should not be considered as a single treatment.
In this paper, the authors describe how fire suppression in eastern US forests has greatly expanded microclimatic conditions favoring tick abundance and pathogen transmission. They propose an ecologically-based tick population control strategy as an alternative to current tick management focused on wildlife host control, spraying aracacides or fungal tick pathogens, and individual practices of tick avoidance. They provide evidence of the direct and indirect effects of prescribed fire on tick populations, habitat, and hosts, and propose that tick control can be an additional outcome of recurring prescribed fire for ecological restoration.
More than a century of fire suppression in the eastern US forests has resulted in the replacement of fire-adapted species by shade-tolerant species at higher densities. This process (forest mesophication) has been widely documented throughout the region and is considered to be a degradation of once widespread, more open ecosystems. These structural and compositional changes have affected forest microclimates and greatly expanded conditions that favor tick survival and interactions with their wildlife hosts. For example, short periods of dry conditions can induce tick population mortality and restrict host-searching behaviors (questing), but litter accumulation increases forest floor humidity, and increased shrub density stabilizes understory temperatures at optimal ranges for tick survival and questing.
Currently, more than 75% of US vector-borne human disease cases are tick-borne. The diseases are primarily carried by three tick species: black-legged tick (Ixodes scapularis), lone star tick (Amblyomma ameicanum), and dog tick (Dermacentor variabilis). Black-legged ticks carry the pathogens for Lyme disease, babesiosis, anaplasmosis, ehrlichiosis, and other diseases. Lone star ticks transmit pathogens that cause tularemia and other diseases, and are the likely source of alpha-gal syndrome. Dog ticks transmit Rocky Mountain spotted fever and tularemia. Ticks require a blood meal from a vertebrate host at each life stage in a 2-year cycle. Though developmental phenologies vary among tick species, in general, the larval stage is most vulnerable to temperature and moisture extremes. Pathogens are acquired when ticks feed upon a reservoir species (a host infected with the pathogen); most commonly, white-footed mice, eastern chipmunks, and shrews. Other vertebrates, including white-tailed deer, wild turkey, and racoon are not reservoir species, but serve to spread and amplify tick populations.
The authors argue that the absence of wildland fire has been overlooked as a contributor to tick population spread and cite numerous studies of direct and indirect effects of prescribed fire on ticks, their hosts, and predators. Their conceptual model illustrates these interactions and feedbacks among ticks, other organisms, and their environment. Direct effects of flaming and smoldering combustion include mortality from increased temperatures or oxygen deprivation in vegetation, litter, and soil. Indirect effects of fire include reduction or removal of sheltering and questing habitats and the subsequent increased frequency of lethal temperature and moisture fluctuations. Indirect effects can also include increased abundance of tick predators, including fire ants and bobwhite quail, or decreases in host populations such as shrews and cotton rats.
Conditions achieved by prescribed fire for forest restoration will require maintenance with a suitable fire regime. However, fire intensity, severity, frequency, seasonality, and spatial variation can have differential effects on tick species, their life stages, and their host populations. The authors suggest suitable fire return intervals likely range between 1 – 20 years, depending on location, based on studies supporting this range for eastern US forests. Dormant season burns may need to be more frequent or intense to affect ticks in the litter, while growing-season fires can target questing ticks. The authors note that adjusting prescribed fire applications in wildland-urban interface areas where human contact with ticks is elevated could help guide development of prescribed fire treatments for tick control.
To build mechanistic models to guide development of tick population control objectives for fire prescriptions, more research is needed to better understand tick and host ecology, the direct and indirect effects of fire on ticks and their interactions with hosts, and the long-term effects of prescribed burning on tick populations and disease transmission. Specific examples of needed research include how the timing of fire alters direct heating effects on ticks, how fire behavior effects differ among forest types or restoration stages, the magnitude of change needed to produce desired microclimate or vegetation structure, and where geographically various approaches are most effective. In particular, understanding the interacting effects of fire regime components and how fires influence future fires will maximize the utility of decision-support tools.
Public concern about the spread of existing and novel tick-borne diseases, and wider public acceptance of prescribed fire as a management tool presents an opportunity to include tick population control into prescribed fire and restoration objectives. The authors recognize the limits of prescribed fire for tick control, particularly in residential areas where fire is not a common management tool, and in severely degraded forests where fire exclusion has led to major community and environmental shifts, requiring mechanical or herbicide interventions. Tick reduction through application of prescribed fire depends on a long-term process and should not be considered as a single treatment or a limited number of treatments.