The bacteria that cause Lyme disease are transmitted to humans primarily by the black-legged tick (Ixodes scapularis). Often presumed guilty by association is the lone star tick (Amblyomma americanum). However, a new review of three decades' worth of research concludes the latter should be exonerated: While lone star ticks are guilty of transmitting bacteria that cause several human illnesses, the scientific evidence says Lyme disease is not one of them.

In an article published in the Journal of Medical Entomology, a group of tick researchers led by Ellen Stromdahl, BCE, argue that the lone star tick plays no role in the spread of Lyme disease, based on their comprehensive review of more than 60 published articles over 30 years. 

Lone star tick bites can indeed cause a range of human illnesses, including ehrlichiosis, Southern Tick-Associated Rash Illness (STARI), and red meat allergy. STARI produces rashes that mimic those seen with Lyme disease; however, Lyme disease, properly termed "Lyme borreliosis," is caused by a bacterium named Borrelia burgdorferi.

In the eastern U.S., researchers discovered 30 years ago that infected black-legged ticks spread B. burgdorferi bacteria to humans. Lone star ticks were also tested for B. burgdorferi at that time. Most studies concluded that they played no role in the spread of Lyme disease, but the notion that lone star ticks were capable of transmitting Borrelia emerged, and it persists today.

In testing lone star ticks for B. lonestari, these studies also included testing the ticks for the actual Lyme bacteria, B. burgdorferi. Thus, the evidence was "hiding in plain sight," Stromdahl says, and a thorough literature search revealed papers describing the analysis of tens of thousands of ticks using definitive, modern molecular methods.

A chemical found in lone star tick saliva destroys Lyme bacteria — preventing lone stars from transmitting Borrelia burgdorferi. "Lone star tick saliva is a very effective barrier against B. burgdorferi — it literally explodes them," says Graham Hickling, who contributed to the review.

"Lone star ticks are constantly being exposed to B. burgdorferi as they feed on infected animals, but the bacteria species has never been cultured from a lone star tick in a lab. However, it has been cultured from rodents and black-legged ticks in the Southeast."

Hickling adds that the lone star saliva's protective effect is evident in the greater than 99.5% of lone star ticks found in the research team's review that tested negative for B. burgdorferi — opposite the consistent findings in black-legged ticks, in which up to half of the adult ticks can be infected withB. burgdorferi.

The review article notes that nine different studies have tested for transmission of B. burgdorferi by lone star ticks, and transmission was never observed. A 10th study determined that the saliva kills the bacteria.

Debunking the perception that lone star ticks can transmit Lyme disease is important because imprecise inferences about the geographical distribution of infected ticks can have negative public health consequences, says Robyn Nadolny, co-author of the review.

"The media coverage of Lyme disease might make many Americans think that a tick-bite means only one thing: Lyme disease," Nadolny says. "We aim to raise awareness of ehrlichiosis and the other problems caused by lone star ticks so that the real threat from these ticks does not go unrecognized."