Amblyomma americanum Is Likely Not a Tick Vector for Lyme Disease
Dr. Natalie Stilwell provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting. In addition to her DVM obtained from Auburn University, she holds a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida.
Evidence from 54 studies suggests that the lone star tick cannot transmit Borrelia burgdorferi.
This photomicrograph of a heart tissue sample processed using Warthin-Starry stain reveals the presence of Borrelia burgdorferi spirochetes.
In the United States, the black-legged tick (Ixodes scapularis) and western black-legged tick (Ixodes pacificus) are the most common vectors for Borrelia burgdorferi, the bacterial spirochete that causes Lyme disease. Several other ticks, including the lone star tick (Amblyomma americanum), historically were suspected to be vectors because of their feeding behavior on known wildlife reservoirs of B burgdorferi, including the white-tailed deer.
However, the authors of a recent review article compiled evidence from studies conducted by more than 35 research groups, all demonstrating that A americanum is an unlikely vector for B burgdorferi.1
Early Diagnosis of Lyme Disease
Lyme disease was first described in 1975,2 and vector competence of I scapularis for the spirochete B burgdorferi was demonstrated experimentally soon afterward.3 Several other Ixodes species worldwide are now known to maintain sylvatic transmission of B burgdorferi in wildlife reservoirs.4
Over the past 30 years, at least 35 research groups have published 54 studies screening more than 50,000 A americanum collected in the United States for the presence of B burgdorferi. Ticks were gathered from a variety of sources, including vegetation (35 studies), animals (10 studies), and humans (10 studies).
Screening methods used until the mid-1990s relied predominantly on identification of B burgdorferi using dark field microscopy (9 studies), direct fluorescent antibody (DFA) testing (10 studies), indirect fluorescent antibody (IFA) testing (11 studies), and bacterial culture (5 studies). With these methods, several research groups identified spirochetal bacteria suspected to be B burgdorferi in A americanum tick samples.4-20
It is now understood that early detections were often complicated by false-positive results. Spirochetes closely related to B burgdorferi may appear morphologically similar on dark field microscopy, resulting in incorrect species identification. Also, DFA or IFA tests using polyclonal antibodies can produce a false-positive result by cross-reacting with multiple Borrelia species.21 Furthermore, fluorescent antibody tests are incapable of distinguishing between viable and nonviable B burgdorferi spirochetes. Results from several studies indicated that the B burgdorferi found in A americanum samples were nonmotile, dead, or degraded.22 It is now known that in such cases, the spirochetes were likely ingested in a blood meal from an infected host but were unable to replicate in the tick vector and transmit to a new host.23
IFA studies using a monoclonal antibody specific for B burgdorferi detected a low prevalence of B burgdorferi; however, efforts to culture positive samples were unsuccessful, providing further evidence that the isolated spirochetes were nonviable.22,24
Around the same time, several experimental infection studies failed to demonstrate vector competence of A americanum for B burgdorferi. Spirochetes collected from A americanum did not culture in standard media used for B burgdorferi (Barbour-Stoenner-Kelly media) in any of the studies.4,7,11,16,25 Also, B burgdorferi collected from A americanum and inoculated into mice failed to establish in the rodent host, again suggesting the spirochetes were nonviable.11
To date, no published study has successfully transmitted B burgdorferi from an infected host to a naïve host using A americanum as a vector. Furthermore, results from 3 studies specifically demonstrated that the tick’s salivary mechanism prevented B burgdorferi from colonizing A americanum.26-28
Identification of a Novel Borrelia Species
Beginning in the mid-1990s, molecular diagnostic tools (including polymerase chain reaction [PCR] and phylogenetic analysis) became the screening tools of choice in Lyme disease studies. These species-
specific methods revealed that the spirochete most commonly infecting A americanum was genetically distinct from B burgdorferi and likely represented a novel Borrelia species.21,29
Through screening of about 33,000 A americanum for Borrelia species, 25 research groups established a 1.7% prevalence of what is now known as Borrelia lonestari, once hypothesized as the causative agent of the human disease Southern tick-associated rash illness. In contrast, only 3 PCR-positive samples from A americanum were characterized as B burgdorferi, with abundant evidence pointing to detection of blood meal DNA rather than replicative infection in the tick vector.30
Ultimately, the switch from early diagnostic methods to molecular tools provided clarity to decades of confounding diagnostic results. In all, nearly 3 dozen studies concluded that A americanum serves as a vector for B lonestari rather than B burgdorferi and does not play a role in Lyme
Dr. Stilwell is a medical writer and aquatic animal veterinarian in Athens, Georgia. After receiving her DVM from Auburn University, she completed an MS degree in Fisheries and Aquatic Sciences, followed by a PhD degree in Veterinary Medical Sciences, at the University of Florida.
- Stromdahl EY, Nadolny RM, Hickling GJ, et al. Amblyomma americanum (Acari: Ixodidae) ticks are not vectors of the Lyme disease agent, Borrelia burgdorferi (Spirocheatales: Spirochaetaceae): a review of the evidence. J Med Entomol. 2018;55(3):501-514. doi: 10.1093/jme/tjx250.
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- Clark KL, Oliver Jr JH, James AM, Durden LA, Banks CW. Prevalence of Borrelia burgdorferi sensu lato infection among rodents and host-seeking ticks in South Carolina. J Med Entomol. 2002;39(1):198-206.
- Schulze TL, Bowen GS, Bosler EM, et al. Amblyomma americanum: a potential vector of Lyme disease in New Jersey. Science. 1984;224(4649):601-603.
- Magnarelli LA, Anderson JF, Apperson CS, Fish D, Johnson RC, Chappell WA. Spirochetes in ticks and antibodies to Borrelia burgdorferi in white-tailed deer from Connecticut, New York State, and North Carolina. J Wildl Dis. 1986;22(2):178-188.
- Schulze TL, Lakat MF, Parkin WE, Shisler JK, Charette EJ, Bosler EM. Comparison of rates of infection by the Lyme disease spirochete in selected populations of Ixodes dammini and Amblyomma americanum(Acari: Ixodidae). Zentralbl Bakteriol Mikrobiol Hyg A. 1986;263(1-2):72-78.
- Levine JF, Apperson CS, Nicholson WL. The occurrence of spirochetes in ixodid ticks in North Carolina. J Entomol Sci. 1989;24:594-602.
- Levine JF, Sonenshine DE, Nicholson WL, Turner RT. Borrelia burgdorferi in ticks (Acari: Ixodidae) from coastal Virginia. J Med Entomol. 1991;28(5):668-674.
- Luckhart S, Mullen GR, Wright JC. Etiologic agent of Lyme disease, Borrelia burgdorferi, detected in ticks (Acari: Ixodidae) collected at a focus in Alabama. J Med Entomol. 1991;28(5):652-657. doi: 10.1093/jmedent/28.5.652.
- Teltow GJ, Fournier PV, Rawlings JA. Isolation of Borrelia burgdorferi from arthropods collected in Texas. Am J Trop Med Hyg. 1991;44(5):469-474.
- Kardatzke JT, Neidhardt K, Dzuban DP, Sanchez JL, Azad AF. Cluster of tick-borne infections at Fort Chaffee, Arkansas: Rickettsiae and Borrelia burgdorferi in ixodid ticks. J Med Entomol. 1992;29(4):669-672.
- Luckhart S, Mullen GR, Durden LA, Wright JC. Borrelia sp. in ticks recovered from white-tailed deer in Alabama. J Wildl Dis. 1992;28(3):449-452. doi: 10.7589/0090-3558-28.3.449.
- Simpson KK, Hinck LW. The prevalence of Borrelia burgdorferi, the Lyme disease spirochete, in ticks and rodents in Northeast Arkansas. J Ark Acad Sci. 1993;47:110-114.
- Feir D, Santanello CR, Li BW, et al. Evidence supporting the presence of Borrelia burgdorferi in Missouri. Am J Trop Med Hyg. 1994;51(4):475-482.
- Rawlings JA, Teltow GJ. Prevalence of Borrelia (Spirochaetaceae) spirochetes in Texas ticks. J Med Entomol. 1994;31(2):297-301.
- Sonenshine DE, Ratzlaff RE, Troyer J, et al. Borrelia burgdorferi in eastern Virginia: comparison between a coastal and inland locality. Am J Trop Med Hyg. 1995;53(2):123-133.
- Ouellette J, Apperson CS, Howard P Evans TL, Levine JF. Tick-raccoon associations and the potential for Lyme disease spirochete transmission in the coastal plain of North Carolina. J Wildl Dis. 1997;33(1):28-39. doi: 10.7589/0090-3558-33.1.28.
- Kollars Jr TM, Oliver JH, Kollars Jr PG, et al. Phenotypic variation in Borrelia burgdorferi sensu lato in ticks (Acari: Ixodidae) and isolates from Missouri, USA. Int J Acarol. 2000;26(2):167-172. doi: 10.1080/01647950008684182.
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- Rich SM, Armstrong PM, Smith RD, Telford SR III. Lone star tick-infecting borrelia are most closely related to the agent of bovine borreliosis. J Clin Microbiol. 2001;39(2):494-497. doi: 10.1128/JCM.39.2.494-497.2001.
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- Rudenko N, Golovchenko M, Clark K, Oliver JH, Grubhoffer H. Detection of Borrelia burgdorferi sensu stricto in Amblyomma americanum ticks in the southeastern United States: the case of selective compatibility. Emerg Microbes Infect. 2016;5:e48. doi: 10.1038/emi.2016.45.