Bites from rabies-carrying vampire bats killed 13 people in a north-eastern Brazilian state last month, the health ministry said on Friday.
Vampire bats feed on mammals' blood
The ministry said the record number of deaths was matched by an increase in bat attacks to about 300.
A programme to vaccinate domestic animals and people deemed at risk will begin in the area next week.
One report suggests scientists believe deforestation may be a factor behind the increase in attacks.
Vampire bats feed on mammalian blood, and can pass on rabies from animals to humans.
Laboratory tests have proven that six deaths in Para state were due to human rabies linked to bat bites, said health official Fernando Dourado, speaking to reporters in Belem on Friday.
Test results have not yet arrived for the seven other victims, but they displayed similar symptoms and had also been bitten.
Three people remain ill in hospital, one critically.
The cases appear to be concentrated in areas close to Marajo - the world's biggest estuarine archipelago.
A health ministry spokesman told Reuters news agency government scientists believed the more aggressive behaviour of the bats could be linked to deforestation.
Greater availability of livestock, coupled with less vegetation for fruit-eating bats, could have favoured an increase in the numbers of vampire bats.
Deforestation could also force changes in bats' migration patterns, affecting their population and behaviour.
A doctor at the hospital where the patients died, Carmem Andrea Freitos, has also noticed changes in the incubation period of rabies in her recent patients.
Typically, patients with human rabies die an average of 20 days after being bitten, reported the regional newspaper O Liberal, but Dr Freitos says some of her patients were bitten up to six months ago.