Risk of Tick-Borne Disease May Be "Drastically Underestimated" in Western US

With around 300,000 cases reported every year, Lyme disease, caused by the bacterium Borrelia burgdorferi, is one of the most commonly reported vector-borne disease in the United States. Because of this high incidence, and the difficulty that often comes with diagnosing and treating the disease, researchers are channeling their efforts into learning more about Lyme disease and the ticks that transmit it.

At the American Society for Microbiology (ASM) Microbe 2017 meeting in New Orleans, Louisiana, Nathan C. Nieto, PhD from Northern Arizona University, delivered a presentation on how citizen science—a research collaboration between scientists and volunteers to answer “real-world” questions—has helped identify the distribution of ticks and tick-borne disease.

In their research, the researchers mention that although cases frequently occur in the northwest and upper mid-western regions of the United States, the disease is hyperendemic in a specific state: California. In this state, the primary vector responsible for transmitting the disease is the western-blacklegged tick, or Ixodes pacificus.

The researchers report that in Russia in 2011, Borrelia miyamotoi, a species of “spiral-shaped bacteria,” had been confirmed as a human pathogen. The United States was soon to follow, also confirming that B. miyamotoi was a human pathogen in the northeastern region of the country “resulting in a viral-like illness following the bite of an infected tick.” B. miyamotoi is more distantly related to the spirochetes that result in Lyme disease and are more closely related to spirochetes that cause tick-borne relapsing fever (TBRF).

Interestingly enough, however, rather than being transmitted by Argasid soft ticks—ticks that are responsible for the majority of TBRF cases—B. miyamotoi is transmitted by Ixodid hard ticks. Because B. burgdorferi and B. miyamotoi are both transmitted by the same tick, and that they are “sympatric species,” meaning they are found in the same geographical area, western United States, the potential risk of being infected with a tick-borne disease has drastically increased for this region.

For their study, the researchers focused specifically on California because Lyme is hyperendemic there. After directed surveillance techniques and collecting ticks from citizen scientists throughout the state, the researchers compared the prevalence of B. burgdorferi with that of B. miyamotoi.

According to the researchers, “Transects were established in popular recreation preserves where we flagged for I. pacificus and citizen-collected ticks were mailed to us directly.” The researchers identified the life-stage and sex of each tick and then extracted DNA “using a kit.” Then, “each extract…was subjected to quantitative polymerase chain reaction (qPCR) for Borrelia spp.” The samples that tested positive were then “subjected to an additional nested-PCR followed by traditional Sanger sequencing.”

Their findings? In the state of California, both B. burgdorferi and B. miyamotoi were found to co-exist in I. pacificus, “from identical locations.” In fact, in some cases they noted “comparable prevalence estimates.” The researchers note that a particularly interesting finding was that the ticks that had been collected by the public “did not peak in abundance until one month following peak tick activity.” This finding led the researchers to postulate that there was “a disconnect between the biology of the tick and actual human exposure.”

The researchers note that B. miyamotoi infections in humans “may not be accurately diagnosed”—blood tests that are often used to diagnose Lyme disease “are not helpful” when it comes to detecting B. miyamotoi—and “there is a highly diverse B. burgdorferi sensu lato complex in the region.”

Given this limitation, the researchers concluded that the dynamics of tick-borne diseases in the state of California are “much more complex” than other regions in North America. In fact, the researchers suggest that risk of tick-borne disease might be “drastically underestimated” in the western region of the United States.

Knowing the distribution of ticks and tick-borne disease promotes awareness for those who are living in the western region of the country, who may be at increased risk of infection, and given that knowledge, perhaps more infections can be prevented by taking the proper precautions against tick bites.