The disease that caused the 1991 and 2008–2009 Ethiopian wolf outbreaks in Bale was rabies.
The outbreaks were linked to infected domestic dogs entering or living near the Ethiopian wolf’s Afroalpine habitat. Rabies spread through wolf populations that were already small and fragmented, making each outbreak especially destructive. Together, the two Bale outbreaks caused the death or disappearance of about 75% of known animals.
The Ethiopian wolf is Africa’s most endangered carnivore and the world’s rarest canid. Conservationists responded with emergency vaccination campaigns for domestic dogs and, in some cases, wolves. These efforts helped Bale’s populations recover to pre-outbreak levels about a decade after the major epidemic.
Canine distemper is another disease recorded in Ethiopian wolves, including outbreaks in Bale, but it was not the disease identified with the major 1991 and 2008–2009 outbreaks. That distinction makes rabies the canonical answer.