The European Union’s health agency (ECDC) said on Wednesday there was nothing to suggest that the Andes strain of hantavirus had mutated following a deadly outbreak of the illness on a cruise ship.

The deaths of three passengers from a rare hantavirus outbreak on a cruise from Argentina to Cape Verde sparked international alarm.

Seven other passengers are confirmed to have the virus, including a French woman in a critical condition, while an eighth case is considered “probable”, according to an AFP tally. All of the passengers have been evacuated and are now in quarantine.

“Preliminary investigations based on the whole genome sequencing that is available to us suggest that there are no indications that this virus is acting any differently from the known virus circulating in some regions of the world,” Andreas Hoefer, of the Stockholm-based European Centre for Disease Prevention and Control, told journalists.

“All sequences obtained to date are virtually identical, which means that there is likely only a single transmission event from an infected animal to a human,” Hoefer, a microbiologist and molecular epidemiologist, added.

The disease is usually spread from infected rodents, typically through urine, droppings and saliva.

Laboratory testing in South Africa and Switzerland confirmed the virus to be the Andes strain — the only hantavirus strain known to pass between humans.

There are no vaccines or specific treatments for hantavirus. Both ECDC and World Health Organisation guidelines include a 42-day quarantine and constant monitoring of high-risk contacts because the incubation period can take six weeks.

“Due to the long incubation period, it is still possible that more cases among the passengers who are now actually in quarantine will occur,” ECDC director Pamela Rendi-Wagner said.

“This cannot be excluded.” Gianfranco Spiteri, ECDC’s head of global epidemic intelligence and health security, said the risk of transmission was greatest for those who showed symptoms of the illness.

But he acknowledged that people might be contagious in the first couple of days before symptoms appear.

“So in terms of taking a preventive and highly precautionary approach, we recommend, for example, that contact tracing should be done for two days before [the] onset of symptoms as well,” he said.