Researchers at the U.S. Centers for Disease Control and Prevention have found that the invasive Asian longhorned tick (Haemaphysalis longicornis) can, at least under lab conditions, acquire and transmit the bacteria that cause Rocky Mountain spotted fever. (Photo by KIM, Hyun-tae via iNaturalist, CC BY 4.0)

By Andrew Porterfield

Andrew Porterfield

Rocky Mountain spotted fever, a disease usually spread by native ticks in the United States, may have a new potential spreader—the Asian longhorned tick.

The tick species Haemaphysalis longicornis was detected in 2017 in a handful of U.S. states. Native to eastern Asia, H. longicornis, dubbed the Asian longhorned tick, traveled to New Zealand, Australia, and Pacific islands before its U.S. arrival. Researchers discovered that the tick’s new territory also overlaps with areas of the country where the rickettsial disease Rocky Mountain spotted fever is found.

Could the new arrival take up and spread the disease? Researchers at the U.S. Centers for Disease Control and Prevention, led by Michael Levin, Ph.D., medical entomology director at CDC’s Rickettsial Zoonoses Branch, tested the ticks on guinea pigs to find out. Their results showed that H. longicornis can acquire and transmit Rickettsia rickettsii (the parasite behind Rocky Mountain spotted fever) very efficiently, at least in laboratory conditions. The team’s work is reported in a study published in April in the Journal of Medical Entomology.

Michael Levin, Ph.D.

R. rickettsii is a native North American bacterium, long associated with Rocky Mountain spotted fever. The disease saw an increase of 46 percent in new cases from 2016 to 2017. Spotted fever afflicts about 20,000 people in the U.S. ever year. It proceeds rapidly and is sometimes fatal, but antibiotics are generally successful at treating it.

At the same time H. longicornis appeared in 12 states, nine of those states reported higher incidences of spotted fever. The invasive tick feeds on blood from opossums, raccoons, and dogs (among other animals), which are known to be susceptible to R. rickettsia infection. Finally, H. longicornis reproduces parthenogenetically (solely by females), allowing it to reproduce without mating and accelerate its population growth. Thus, the CDC researchers wanted to know if, and how well, the new arrival could also spread spotted fever.

Levin and his team took a parthenogenetic colony of female ticks originally from Westchester County, New York, and grown in a lab for two generations. Pathogen-free and tick-free guinea pigs were infested with ticks and then examined for signs of rickettsial infection. The scientists inoculated 13 guinea pigs with R. rickettsia to see if H. longicornis could catch the pathogen from an infected animal. Then, eight of the guinea pigs were infested with about 500 tick larvae, and three were exposed to 75 nymphs, so the researchers could look at the ability of H. longicornis to transmit R. rickettsii to susceptible hosts.

Both larvae and nymphs of the tick could catch R. rickettsia by feeding on the blood of infected guinea pigs and carry the pathogen through molting from larva to nymph stage at rates exceeding 80 percent, a highly efficient process. The pathogen also could pass from mother to offspring in enough quantities to cause infection and illness in the host animal. This rate was not as efficient as the larva-to-nymph rate but still enough to result in illness.

The team was not surprised by H. longicornis’ ability to acquire R. rickettsii from infected guinea pigs and transmit it through successive life cycle stages, but “we were surprised how efficiently the ticks acquired the bacteria and how many bacteria they were able to transmit into naïve animals following feeding,” Levin says.

The longhorned tick is not known for biting smaller animals, so successful feeding on guinea pigs lagged behind feeding rates of native vectors of R. rickettsii—the American dog tick (Dermacentor variabilis) and the lone star tick (Amblyomma americanum). This study, however, showed that H. longicornis transmission rates to animals were similar to its native tick relatives.

The researchers caution that this study was conducted under laboratory conditions and may not be directly replicated in the field. “We used laboratory animals which are easily infected with R. rickettsii and that allow R. rickettsii to multiply readily in their bodies. We also used relatively large numbers of ticks,” Levin says. “In nature, H. longicornis ticks feed on various wild and domestic animals—many of which are less susceptible or not at all susceptible to R. rickettsii infection.”

While Levin and colleagues’ research continues, the study stands as a warning that H. longicornis could intensify transmission of R. rickettsii, and thereby increase cases of spotted fever in natural environments in the United States.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.