In September of last year, New Jersey-based entomologist Tadhgh Rainey was contacted by the owner of a tick-infested pet sheep. When he went to check on the animal, he was shocked by the vast quantity of ticks that had swarmed the sheep’s enclosure.

“A minute after we entered the paddock, even before I touched the sheep, I was covered in ticks,” Rainey, who works with the public health department of Hunterdon County, New Jersey, tells Donald G. McNeil Jr. of the New York Times.

More surprising still was that the ticks did not belong to any domestic American species. They were later identified as Asian long-horned ticks (Haemaphysalis longicornis), a highly fecund species that poses a serious risk to animals and has been known to transmit fatal diseases to humans. Rainey’s sighting marked the first time in half a century that a new tick species had been seen in America. And over the past year, Asian long-horned ticks have spread to seven U.S. states.

The Asian long-horned tick is widespread in Japan, China and the Korean Peninsula, and has also been found in Australia and New Zealand. In recent months, the blood-sucking critters have been reported in New Jersey, New York, Arkansas, North Carolina, Virginia, West Virginia and Pennsylvania. Officials don’t know how the ticks came to the U.S., but the United States Department of Agriculture posits that they may have hitched a ride on domestic pets, horses, livestock or even humans.

Though they aren’t much bigger than a poppy seed, Asian long-horned ticks are voracious eaters. They descend in groups on warm-blooded host animals, sucking so much blood that their bodies swell to the size of peas. Because they feed in large infestations, the ticks can cause host animals to die of blood loss. And unlike some ticks, which will feed only on specific animals, long-horned ticks are not particularly discriminating about what they eat. In the United States, they have been found feasting on horses, dogs, deer, an opossum and a calf.

“[The tick] is an aggressive biter,” the North Carolina Department of Agriculture warned in a statement.

It is also a prolific breeder. As Rachael Rettner of Live Science reports, female long-horned ticks reproduce asexually, meaning that they do not need a male involved. After gorging itself on blood, a single female can lay up to 2,000 eggs—enough to establish a tick population in a new location.

At the moment, Asian long-horned ticks in the United States primarily pose a threat to livestock and other animals. Andrea Egizi, an entomologist at Rutgers University, tells the Times’ McNeil Jr. that she has tested more than 100 specimens found in New York and New Jersey for six diseases that ticks transmit to humans—Lyme, relapsing fever, anaplasmosis, babesiosis and two types of ehrlichiosis—and none were found to harbor these dangerous pathogens. The Center for Disease Control and Prevention screened another 100 ticks for three viruses—Bourbon, Powassan and Heartland—and the ticks came up negative for these as well.

In Asia, however, long-horned ticks are known to carry deadly pathogens. The most serious is a virus that causes severe fever with thrombocytopenia syndrome (SFTS), which can lead to internal bleeding and organ failure. SFTS has an overall fatality risk of 15 percent, McNeil Jr. reports, but it kills 50 percent of people over 60 who contract the syndrome.

For now, U.S. health officials are more concerned about diseases transmitted by domestic ticks, which are steadily increasing in frequency. But the USDA notes that livestock and pet owners can keep long-horned ticks at bay by making sure that the grass and brush on their property is trimmed short. Humans can protect themselves by taking the same precautions that are recommended for domestic ticks: use insect repellant, steer clear of tall grasses and check your body and clothing for ticks after coming indoors.