Argentina is trying to solve a medical mystery after 57 sailors were infected with the coronavirus after 35 days at sea, despite the entire crew testing negative before leaving port.

The Echizen Maru fishing trawler returned to port after some of its crew began exhibiting symptoms typical of Covid-19, the health ministry for the southern Tierra del Fuego province said on Monday.

According to the ministry, 57 sailors, out of 61 crew members, were diagnosed with the virus after undergoing a new test.

However, all of the crew members had undergone 14 days of mandatory quarantine at a hotel in the city of Ushuaia. Before that, they had negative results, the ministry said in a statement.

Two of the other sailors have tested negative, and two others are awaiting test results, the province’s emergency operations committee said.

Two sailors were hospitalised.

“It’s hard to establish how this crew was infected, considering that for 35 days, they had no contact with dry land and that supplies were only brought in from the port of Ushuaia,” said Alejandra Alfaro, the director of primary health care in Tierra del Fuego.

A team was examining “the chronology of symptoms in the crew to establish the chronology of contagion”, she said.

The head of the infectious diseases department at Ushuaia Regional Hospital, Leandro Ballatore, said he believed this is a “case that escapes all description in publications, because an incubation period this long has not been described anywhere”.

“We cannot yet explain how the symptoms appeared,” said Ballatore.

The crew was placed in isolation on board the ship and returned to the port of Ushuaia.

Argentina exceeded 100,000 total cases on Sunday, and the death toll rose to 1,859. Most infections are in the Buenos Aires area.