Researchers may have found a way to use ticks to test for persistent Lyme disease in humans, a switch on the pest’s usual role in spreading the disease.
Xenodiagnosis is being considered to detect Lyme disease when there are persistent or relapsing nonobjective symptoms such as fatigue, musculoskeletal pain and cognitive complaints, which are loosely summarized by the phrase posttreatment Lyme disease syndrome, or PTLDS.
In the study, 36 people had 25 to 30 pathogen-free Ixodes scapularis larval ticks placed over a 7-cm2 area under a modified retention dressing placed close to an area of erythema migrans or close to an affected joint. Ticks fed to repletion for 3 to 7 days. Tissue samples were collected by 2 skin punch biopsies of 2-cm. These were tested for Borrelia burgdorferi by polymerase chain reaction (PCR), culture, amplification followed by mass spectroscopy. In addition, researchers tried to infect immunodeficient mice.
Results appeared in Clinical Infectious Diseases.
The procedure was well tolerated with no severe adverse events. The most common adverse event was mild itching at the tick attachment site. Xenodiagnosis was positive for B. burgdorferi DNA in 1 patient with erythema migrans early during therapy and in 1 patient with PTLDS. Authors noted that there wasn’t enough evidence to conclude that either patient had viable spirochetes.
Researchers wrote, “Our initial results show that the majority of the patients with Lyme disease treated with antibiotic therapy are negative by xenodiagnosis. An important caveat is that the number of tested xenodiagnostic ticks per participant in general was small, particularly in the early subjects. The number of engorged ticks tested for each individual is likely to be an important variable. The more engorged ticks tested, the larger the probability of detecting a positive. However, our testing of the ticks included multiple modalities and very sensitive assays targeting B. burgdorferi.”
Not so fast, an editorial commented.
“The authors concluded that xenodiagnosis can be performed safely and is generally well tolerated,” the editorial stated. “We agree, with the caveat, based on their own experience, that this is far from a user-friendly technique.”
Also, detection of DNA alone is not enough. Highly sensitive PCR tests may enhance detection of rare DNA fragments, while recovery of live spirochetes is the only reliable criterion for a positive xenodiagnoses, the editorial stated.
Also, xenodiagnoses works in mice because spirochetes spread from the feeding site to skin, a kind of natural reservoir. This doesn’t happen in humans.
“Ironically, a method employed to seek evidence for the persistence of spirochetes in PTLDS may actually provide evidence against the biologic plausibility of a hypothesis that has fueled controversy for nearly 30 years,” the editorial concluded.