BERLIN -- One month after the introduction of an official reporting requirement for Zika virus, authorities have registered 12 cases of the disease in Germany, local media reported on Wednesday.

Newspaper Neue Osnabruecker Zeitung reported the number of officially registered Zika cases in Germany has risen to 56 since October. However, a reporting requirement has been only in force since May 1.

A spokeswoman of the Robert Koch Institute, an organization responsible for disease control and prevention, was quoted as saying: "We believe that each of these patients has been infected while traveling."

There may be a significant number of unreported cases in Germany, since the course of the infection can run mildly and affected persons often do not go to the doctor, she said.

Zika has so far been detected in about 60 countries. Particularly affected are countries in Central and South America.

Zika is mainly spread by mosquito bites. The World Health Organization, however, pointed out that Zika is also sexually transferable.