He was at Cascavel Zoo, in the state of Parana, with his father and three-year-old brother when he got into an area just outside the tiger's enclosure and tried to pet the animal through the bars of the cage.
Several visitors tried to alert the father that he was in a prohibited area.
Local teacher Ricardo Espindula, who feared something might happen, recorded footage of the boy with the tiger moments before the attack on his mobile phone.
When the father next looked, he saw his son covered in blood.
Boy's arm torn off by tiger in Brazilian zoo
The boy reaches out to the tiger after entering a prohibited area
First aiders arrived quickly and took the boy to a local university hospital where he received treatment for the deep cuts to his right arm.
He was operated on several times, but lost the injured limb and remains in intensive care.
A biologist at the zoo said the tiger, called Hu and weighing more than 200kg, was a docile animal and had been at the facility for more than two years.
There is a guard usually watching the animal but he had gone on his rounds at the time of the attack.
The tiger is currently in isolation.
Source: www.youtube.com
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