For the first time in 50 years, a new tick species has arrived in the United States — one that in its Asian home range carries fearsome diseases.

The Asian long-horned tick, Haemaphysalis longicornis, is spreading rapidly along the Eastern Seaboard. It has been found in seven states.

At the moment, public health experts say they are concerned but not alarmed.

Although domestic U.S. ticks are a growing menace and transmit a dozen pathogens, no long-horned ticks here have yet been found with any human diseases. In Asia, however, the species carries a virus that kills 15 percent of its victims.

For now, the new arrivals are considered a greater threat to livestock.

Known in Australia as bush ticks and in New Zealand as cattle ticks, long-horned ticks can multiply rapidly and suck so much blood from a young animal that it dies.

They were reported in Pennsylvania for the first time last week, and have been sighted in Arkansas, North Carolina, Virginia and West Virginia.

In East Asia, the ticks carry a phlebovirus that causes SFTS, for severe fever with thrombocytopenia syndrome. (Thrombocytopenia means abnormally low levels of platelets, which help the blood clot; a severe drop triggers internal bleeding and organ failure.)