The Zika virus has been found alive in a man's sperm after a record 93 days, according to a new report that adds to the many unknowns surrounding the foetus-harming germ.

The 27-year-old Frenchman's semen tested positive for Zika in March this year, three months after he experienced symptoms of an infection picked up while travelling in Thailand last October and November.

The case was reported in The Lancet medical journal this week.

The previous longest recorded virus survival in semen was 62 days after the onset of symptoms.

Benign in most people, Zika has been linked to microcephaly -- a shrinking of the brain and skull -- in babies, and to rare, potentially-fatal adult-onset neurological problems.

It is transmitted mainly though the bites of infected mosquitoes, in rare cases via sex, but also through the placenta to unborn children.

In an outbreak that started last year, about 1.5 million people have been infected with Zika in Brazil, and more than 1,600 babies born with microcephaly, according to the World Health Organization.

The new case highlights that people returning from areas where Zika is non-endemic, such as Thailand, can also be infected, said the report authored by health specialists from Toulouse in southern France.

The possibility of "protracted" virus presence should be kept in mind when people plan to have children, it added.

The existing six-month period for monitoring virus survival in infected people "should be expanded to patients returning from non-epidemic areas," wrote the team.