In the summer of 2017, specimens of an “unusual-looking” tick were reported to public health officials in New Jersey, and they were later determined to be a species, Haemaphysalis longicornis, from the other side of the world. No established population of the species, native to Asia, had ever been previously documented in the United States. Pictured above are engorged (left) and partially engorged (middle) adult females and an engorged larva (right). Scale at top is in millimeters. (Photo credit: James L. Occi, Rutgers University)

You’d be forgiven if you thought we already had enough ticks to worry about here in North America. The blacklegged tick, the lone star tick, the dog tick, the Rocky Mountain wood tick, et cetera, et cetera.

Unfortunately, these masterful ancient parasites and the whims of global trade and travel have little sympathy for our concerns. Case in point: The discovery last year of an infestation of the tick species Haemaphysalis longicornis on a rural property in western New Jersey, USA.

As reported in a short paper published Monday in the Journal of Medical Entomology, public health officials in and around New Jersey are on alert for the presence of Haemaphysalis longicornis, a tick species native to Asia, after a significant infestation was discovered in Hunterdon County, New Jersey, last summer. A resident submitted samples of unusual-looking ticks from a sheep, and the ticks were later identified as an exotic species.

A 12-year-old Icelandic sheep living on a property in Hunterdon County, New Jersey, was found in 2017 to be infested with numerous ticks of the species Haemaphysalis longicornis (shown above in a closeup of the sheep’s ear). Further investigation found a large infestation of H. longicornis, in multiple life stages, on the property. The tick species is native to Asia, and the appearance in New Jersey marks the first potentially established population of the species ever documented in the United States. (Image originally published in Rainey et al 2018, Journal of Medical Entomology)

“At this point, we really don’t know how it got here,” says Andrea M. Egizi, Ph.D., research scientist at the Tick-Borne Disease Laboratory at Rutgers University and senior author on the report. “We know it didn’t come here on the sheep where it was found, since the animal had no travel history. … It’s possible it hitchhiked in on a different animal, but there were no other domestic animals on the property where it was found, so it would still have had to make its way from somewhere else without being detected in the original location. It’s a puzzle, and I’m not sure we’ll ever know.” Egizi co-authored the report with Tadhgh Rainey at the Hunterdon County Division of Health, James L. Occi at Rutgers University, and Richard G. Robbins, Ph.D., at the Smithsonian Institution’s Walter Reed Biosystematics Unit.

Haemaphysalis longicornis is cause for concern primarily as a livestock pest, Egizi says, though it has been reported to parasitize humans, as well. It is native to East Asia but is an invasive cattle pest in New Zealand, Australia, and several Pacific islands. In the United States, it has been intercepted on a few occasions at quarantine stations, in small numbers, but a potentially established population of multiple life stages has never before been documented in the U.S.

Nearly unique among its peers, however, is H. longicornis‘ capacity for parthenogenesis—i.e., reproduction without fertilization, in which females produce offspring that are (roughly) clones of herself. And that’s what appears to have enabled the infestation in New Jersey to arise; among 27 adults, 41 nymphs, and 1,058 larvae collected on the site, just a single specimen was male.

“There are only a couple species in the world that are known to exhibit this trait, and H. longicornis is one of them,” Egizi says. “While both types of reproductive methods are found in populations within its native range, invasive populations are exclusively parthenogenetic. This makes sense because the ability to reproduce without males makes it easier for them to establish in a new location: Only one tick is needed to start a population, and they can grow to high numbers quickly. They are not limited by the need to find mates, which can be difficult in a small population.”

When the researchers visited the New Jersey property, “the ticks in the paddock were so numerous that they crawled on investigators’ pants soon after setting foot inside” and “the sheep was supporting hundreds of ticks, including all three active life stages,” according to their report. The paddock itself was about one acre in size and surrounded by manicured lawn, and no other domestic animals live on the property. The infested sheep was treated with permethrin and later found to be free of ticks, and the researchers found no ticks on the property upon inspection in mid-November—though they note that preceding nights of freezing temperatures could have driven any remaining ticks down to the soil. This spring, when the ticks would begin to re-emerge, researchers from the New Jersey Division of Fish and Wildlife will return to the property and set traps for potential wild host animals nearby to check for H. longicornis. Egizi says New Jersey residents on the lookout for H. longicornis should report suspected sightings to the Rutgers Cooperative Extension Service.

“Even if this tick turns out to not be established, I think we should view this discovery as a wake-up call that this kind of thing could happen in the future,” says Egizi.