Amblyomma americanum Is Likely Not a Tick Vector for Lyme Disease

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

disease transmission.

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.

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