Five people have caught the Ebola virus from pigs. Credit: R. RANOCO/REUTERS

Four more workers at pig farms in the Philippines have contracted the Ebola-Reston subtype of Ebola virus, in addition to a case reported two weeks ago.

All five, identified by the presence of antibodies to the virus in their blood, worked with sick pigs and were probably infected more than six months ago. Ebola Reston was discovered last year in pigs on Luzon, the largest island in the Philippines (see _Nature_ 457, 364–365; 2009). The virus has yet to trigger any symptoms in humans, but could mutate into more virulent forms inside pigs or other animal carriers.

Health authorities are now testing acquaintances of the infected five to see whether human-to-human transmission might have occurred.