Dozens of bathers in the southern states of Sao Paolo and Parana have been bitten in the past few weeks, including in the rivers of Pereira Barreto, which had not recorded such an attack in more than 20 years. According to Brazilian-based Rede Globo news outlet, more than 30 people in the previously quiet area have sought medical help for bites over the past few days. And during a wave of attacks in Pereira Barreto last week one man ended up losing part of their toe.
While the majority of attacks on humans do not prove fatal, there have been cases in which victims have died.
Red-bellied piranhas are the largest living species among the group and can extend up to 20 inches long.
Piranhas have one of the strongest bites of any fish due to their sturdy jaws and serrated teeth.
Piranhas are attracted to splashing, therefore children are more vulnerable to attacks than adults.
The freshwater fish dwell in rivers, floodplains, lakes and reservoirs in South American.
The increase in attacks has prompted authorities in Sao Paolo to alert people who plan on cooling off with a swim in rivers.
Signs have been erected on riverside beaches warning bathers about the risks.
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Five swimmers were set upon by the fish while relaxing in the rivers of Pereira Barreto on New Year’s Day.
Officials in the region say they want to team up with biologists to study the fish and conduct research in a bid to understand their behaviour better.
One of the theories they will look into is whether a recent ecological imbalance could be the cause of the rise in attacks.
Biologist Murilo Queiroz said consuming foods close to the water will encourage piranhas to swim closer.
Brazil remains a popular holiday destination for British sun-seekers during the winter months.
The average temperature in the South America nation in January is around 27C.