The man didn't have a fever when he left Liberia on May 17, or upon arrival at JFK International Airport in New York, the Centers for Disease Control and Prevention said Monday.
The next day, he went to a New Jersey hospital complaining of a sore throat, fever and tiredness. But he didn't tell the staff there about his travel to West Africa and was sent home the same day, the CDC said.
He went back to the hospital Thursday with worsening symptoms and was moved to a treatment center for viral hemorrhagic fevers. A test for Lassa fever came back positive early Monday, according to the CDC. The patient died that evening in isolation.
"Lassa fever is a viral disease common in West Africa but rarely seen in the United States," the CDC said. "There has never been person-to-person transmission of Lassa fever documented in the United States."
Close contacts to be monitored
This latest case is the sixth known instance of Lassa fever among travelers coming back to the United States since 1969, excluding convalescent patients, according to the CDC. The most recent case before this one was in Minnesota last year.
Like Ebola, Lassa fever can produce bleeding symptoms in patients, but it has a fatality rate of only about 1%, compared with around 70% for Ebola. It is also less likely to be spread from person to person, the CDC said.
"The virus is not transmitted through casual contact," it said, "and patients are not believed to be infectious before the onset of symptoms."
The CDC said the risk to other people from the New Jersey case was considered "extremely low." But it's working with public health officials to draw up a list of people who had contact with the patient and will monitor those identified as close contacts for 21 days.
Lassa fever kills about 5,000 people a year in West Africa, where it's carried by rodents and transmitted to humans through contact with the animals' urine or droppings.