Interagency Cooperation Drives Discovery of Lyme Disease Spirochete in Exotic Tick
By Brooke Coder
Nearly everyone in the United States has heard about Lyme disease, knows someone who has contracted it, or has their own personal Lyme disease story. Lyme disease is the most commonly reported vector-borne disease in North America, and Pennsylvania ranks first in the nation with reported Lyme disease cases.
In 2018, the Pennsylvania Department of Environmental Protection received State funding that enabled the creation of a statewide active tick and tick-borne pathogen surveillance program. This program began monitoring all 67 counties within the Commonwealth for tick distribution, density, and associated pathogens, helping to contribute to a publicly available, nationwide tick surveillance data set. Today, Pennsylvania is one of just a few states in the nation with such a comprehensive surveillance and testing program. To fully extend this surveillance benefit to Commonwealth citizens, we are now coordinating with 38 local government agencies to help collect ticks and enhance program capabilities.
Pennsylvania’s tick program monitors pathogen prevalence in collected ticks through molecular assays adopted from published methodologies developed by partners at the U.S. Centers for Disease Control and Prevention (CDC). Initially, our program tested for the “big three” human pathogens commonly found within the nymphal and adult stages of the blacklegged tick (Ixodes scapularis), including Borrelia burgdorferi, Anaplasma phagocytophilum, and Babesia microti. However, as new pathogens continue to emerge in nature and reports of human infections from our state’s Department of Health are shared, our lab has developed additional tests. Pathogen testing now includes Borrelia miyamotoi (causative agent of hard tick-borne relapsing fever [Borrelia miyamotoi disease]), Powassan lineage II (Deer Tick Virus), and another recently described Lyme disease-causing agent, Borrelia mayonii.
Expanding testing efforts will not only gauge prevalence of these other pathogens in Pennsylvania but also benefit development of epidemiological models for predicting where human infection could occur. Through collaborative discussions and information sharing with the Pennsylvania Department of Health, we have commenced a “One Health” approach to tick-borne diseases, realizing that the intersection of our distinct work provides the greatest benefit.
Since inception, the primary goal of the tick program has been to collect and test blacklegged ticks because of the public health risk they pose. However, over time and through targeted collections throughout the Commonwealth, established populations of additional tick species have been discovered. One of those species is the newly invasive Asian longhorned tick (Haemaphysalis longicornis). Since its first discovery in New Jersey in 2017, it has rapidly expanded its range across the mid-Atlantic to now inhabit 15 states. The Asian longhorned tick is a known carrier of human pathogens throughout its native range in southeast Asia, but none had been discovered in the U.S. In 2019, our statewide sampling of blacklegged tick nymphs led to the first collections in Pennsylvania of Asian longhorned ticks. A total of 668 Asian longhorned ticks were collected from four southeastern Pennsylvania counties via dragging understory vegetation and leaf litter with a felt cloth.
Given the uniqueness of our finding and strong interest in monitoring pathogens the U.S. tick populations may harbor, we sought the advice and expertise of collaborators at the CDC familiar with molecular testing of this exotic tick. After initial discussions, a subset of 263 ticks was sent to the CDC in Fort Collins, Colorado, for testing for presence of B. burgdorferi, B. mayonii, B. miyamotoi, and B. microti. As my colleagues and I reported in February in the journal Emerging Infectious Diseases, one adult female collected from a county park in Bucks County, Pennsylvania, was found to contain genetic material of the Lyme disease spirochete, B. burgdorferi. Just one out of 263 gives an overall infection rate among Asian longhorned ticks tested that is quite low (0.4 percent). Comparatively, blacklegged ticks collected over the same surveillance period in the same counties tested positive for B. burgdorferi ranging from 16.7 percent to 57.1 percent, depending on the county. This is consistent with recent laboratory findings that the Asian longhorned tick is averse to feeding on the primary reservoir of the Lyme disease spirochete, the white-footed mouse. Additionally, research has shown that Asian longhorned ticks are unable to carry the infection from one life stage to the next (e.g., from nymph to adult), making them unlikely to play a significant role in the disease transmission.
None the less, detecting B. burgdorferi DNA in an Asian longhorned tick in Pennsylvania emphasizes how crucial tick surveillance and pathogen testing remains on a local, state, and national level. While the Asian longhorned tick is not an avid human biter, there are documented cases of it biting humans in the U.S., which supports continued use of personal protective measures for anyone entering habitats where ticks may be present. This tick has only just arrived in the U.S., so there is still uncertainty regarding its role in ecology and human health, and because of this, continued surveillance has never been more important.
Pennsylvania’s Tick Surveillance and Testing Program is off to a great start, and collaborations like this have helped progress it in the right direction. The discovery of the Lyme disease spirochete in the Asian longhorned tick not only highlights the importance of tick surveillance and pathogen testing but demonstrates the value of building collaborative networks to recognize ecological processes potentially contributing to greater public health awareness.
Emerging Infectious Diseases
Brooke Coder is an aquatic biologist in the Tick Program at the Pennsylvania Department of Environmental Protection’s Division of Vector Management in Harrisburg, Pennsylvania. Email: firstname.lastname@example.org.