GENEVA, Switzerland (AFP) — The hantavirus behind the deadly outbreak on a cruise ship is most infectious as soon as symptoms appear, the World Health Organization (WHO) said Monday, explaining the need to quarantine contacts.

The outbreak of the rare virus on the Dutch-flagged vessel MV Hondius has claimed three lives and spurred global concern over its potential spread as evacuees head back to their countries.

“The first few moments of illness is when the infectiousness is highest,” Olivier Le Polain, who heads WHO’s epidemiology and analytics for response division, told a social media event.

The WHO has recommended that all the nearly 150 people who were on-board the ship when it arrived early Sunday in the waters off Tenerife for disembarkment be quarantined for six weeks.

That corresponds to the maximum incubation period for the Andes virus — the only hantavirus strain known to spread between humans — at the heart of the outbreak, which is around 42 days.

On average though, those infected with the virus begin developing symptoms after about three weeks, Polain said.

He stressed that it was important not to wait for symptoms to appear before isolating potential contacts.

If WHO recommends “quarantine it is because actually people are infectious at the very start of illness”, he said, adding that this was “also sometimes when it’s a bit difficult to recognise the symptoms”.

That is because “initial symptoms could be mild…some fatigue, maybe mild fever”, he said adding though that those symptoms then “sometimes deteriorate”.

Polain said the hantavirus’s long incubation period “means that we can see cases again coming up in the next few days, perhaps even next week”.

“That’s why we need to remain vigilant, make sure that at the first signs and symptoms… are being recognised, isolated and taken care of.”

While WHO makes recommendations, it is up to each country to implement the health protocols they think are most appropriate.

Since contact tracing and the repatriation of passengers and crew from the MV Hondius began, most countries have followed WHO guidelines and implemented 42-day quarantines.

Several countries, including Germany, Britain, Switzerland, and Greece, have opted for a 45-day quarantine; Australia and France have announced minimum observation periods of three and two weeks, respectively, which can then be extended.

The United States, in contrast, has indicated that the 17 returning American passengers would not necessarily be quarantined — something WHO chief Tedros Adhanom Ghebreyesus warned “may have risks”.

The ship presented an “environment that was conducive for transmission… It’s an environment where people live together in a fairly enclosed space”, Polain said.

“That’s why also we likely see more spread than we would see otherwise,” Polaid added.

World Health Organization (WHO) Director-General Tedros Adhanom Ghebreyesus speaks to reporters as he arrived to coordinate evacuation efforts of the passengers and crew of the hantavirus-stricken cruise ship MV Hondius (Photo: AFP)