Breuner NE, Ford SL, Hojgaard A, Osikowicz LM, Parise CM, Rosales Rizzo MF, Bai Y, Levin ML, Eisen RJ, Eisen L. Failure of the Asian longhorned tick, Haemaphysalis longicornis, to serve as an experimental vector of the Lyme disease spirochete, Borrelia burgdorferi sensu stricto. Ticks and Tick-borne Diseases. 11(1)
Post image by Kim, Hyun-tae
Ticks don’t care about social distancing
Spring, the time of year we lace up our hiking boots, perhaps dust off the grill, and head outside to enjoy the warmer weather. For some parts of the United States, however, spring is the time people start worrying about ticks again, and this year there might be more to worry about than usual.
In August of 2017, another invasive species found its way to the United States. The Asian longhorned tick (Haemaphysalis longicornis) was first documented in New Jersey, and in a little over a year it had spread into nine states, mostly along the east coast, with potential to spread further into the interior United States. This range expansion has been aided by the tick’s ability to hitch a ride on the back of birds and large mammals while it feeds.
Ticks survive by finding vertebrate hosts and feeding on their blood. This meal is required for the tick to molt from one stage to the next (larvae-to-nymph, nymph-to-adult), and for reproduction. Some ticks, such as the Asian longhorned tick, will occasionally use humans as hosts. In fact, human bites from this invasive tick have already been reported in the United States.
Due to their blood feeding nature, ticks are often vectors of disease, as they can ingest pathogens during their meal. In its natural habitat, the Asian longhorned tick has been associated with several infectious diseases caused by viruses and bacteria, and in the US they have been found to be infected with the human pathogenic bacteria passed to humans through the bite of the native black-legged tick (Ixodes scapularis). The reason that the Asian longhorned tick is a public health concern in the United States is because of its demonstrated ability to bite humans and vector disease. Furthermore, its new habitat (the Northeast US) is already home to the nation’s most prolific tick-borne disease. In essence, the concern is that as the Asian longhorned tick spreads, it picks up B. burgdorferi and spreads more Lyme disease.
The objective of this research conducted Nicole Breuner and colleagues at the Centers for Disease Control and Prevention Division of Vector-Borne Diseases, was to experimentally test if Asian longhorned ticks originating from the United States can be vectors for the bacteria responsible for Lyme disease.
Uninfected ticks pick up B. burgdorferi when they feed on non-human hosts. Therefore, to test if Asian longhorned ticks are competent vectors for B. burgdorferi, uninfected larval ticks were first allowed to feed on laboratory mice that were infected with B. burgdorferi. Asian longhorned ticks were placed either freely on the fur of mice, or on the shaved back of mice in feeding tubes. Black-legged ticks were placed on two mice as a control to compare the infection prevalence with B. burgdorferi. After the ticks fed to completion, larvae were placed in chambers mimicking natural conditions to allow them to transition into nymphs. This step is critical, as it will show if the infection with B. burgdorferi can withstand the molting process. If it can, then nymphal ticks can pass B. burgdorferi to humans or uninfected hosts when they take their next meal.
Following the feeding, 80% of Black-legged larvae and 56% of Asian longhorned larvae tested positive for B. burgdorferi. In the newly molted nymphs, 73% of black-legged nymphs were infected, whereas 0% of the Asian longhorned nymphs were infected.
Not for Lyme, at least
These results show that while Asian longhorned ticks are capable of acquiring the pathogen that causes Lyme disease when they feed, they are not capable of maintaining the infection during the molting process. Furthermore, the experiments showed that Asian longhorned ticks were unwilling to attach to mice when placed freely upon them, which suggests they prefer other hosts. The most competent host for B. burgdorferi are white-footed mice, which means that, while Asian longhorned ticks currently occupy the same geographic areas as black-legged ticks, white-footed mice are not effective feeding hosts for this new invasive tick and thus less likely to transmit Lyme disease.
Therefore, the potential public health concern posed by Asian longhorned ticks with respect to Lyme disease may be small, as they are unlikely to be proficient vectors of B. burgdorferi and unlikely to feed on the most competent reservoir host, white-footed mice. We’re not out of the woods yet, however, as more studies are needed to determine if Asian longhorned ticks can transmit other pathogens in the United States.