The 69-year-old former firefighter reportedly lost his temper over a barking dog belonging to his two nephews, who are also his neighbours in the town of Castel Gandolfo, Roma Today reported.
As one of the nephews tried to defend himself the pensioner struck him with a machete, cutting off the little finger on his right hand. The victim was transported to Gemelli Hospital for surgery.
The other nephew was hit a few times in the head and was taken to casualty, according to Il Messaggero.
The attacker was arrested and taken to Velletri prison. He now faces charges of grievous bodily harm.
According to the pensioner's family, the dog was loved by all and the row was about another matter.
They claim that he did not mean to hurt anyone and that the nephew was injured in a scuffle.