I had never thought that giant otters would be that aggressive, but I can imagine they could be relatively dangerous if provoked. Do you know of any circumstances in which the species has attacked humans?
This species can definitely be highly dangerous when it comes to defending their territory or family groups whether under captive conditions or the wild. Part of their natural behaviour under wild conditions is the mobbing of predators such as jaguar , puma , caiman and humans and in this they are fearless.
In regards to big cats it is mostly just noisy and threatening displays from the otters and I haven't heard of any recorded physical fights between them (but who knows ? , so much of their behaviour and ecology is understudied and so goes undocumented). In terms of caimans these reptiles are often physically engaged and end up either wounded or killed.
Occasionally you hear contemporary reports of the giant otter mobbing people sitting in canoes or fishing on riverbanks in the Pantanal and Amazonian regions. Of course there are also many historical reports that exist within indigenous folklore and many accounts made by colonists and explorers of this kind of behaviour occuring.
Nevertheless, I haven't heard of any people being killed by these creatures in the wild. Giant otters are most definitely not an animal with a ready propensity for aggression to humans and in most circumstances they are typically extremely shy , cryptic, and avoidant of people. In terms of zoos , things are slightly different. Under these conditions they are restricted within an enclosure which they show a strong territorial instinct to defend. As such, when a person falls into this space it is interpreted as a threat and the animals respond very aggressively.
In my limited personal experience with this species I've never encountered any aggression towards me, just grouchiness and snappy behaviour especially when they are waiting for their tillapia. However, I have heard keepers talk of giant otters being vicious and unpredictable at times.
There have actually been several attacks by these species in zoos in Brazil , the most famous example ended in the death of a man :
1978 - An off duty Brazilian police officer visiting the zoo of Brasilia jumped into an enclosure to rescue a child who had falled in.This particular enclosure housed a family group of giant otters and included pups, which may go explain why the man was attacked so badly. The man was severely attacked and later died in hospital from an infection of the wounds he sustained.
This is a retrospective tv news report about the event :
Jovem é resgatado de fosso de ariranha, no DF - G1 Distrito Federal - Vídeos - Catálogo de Vídeos
Here is a retrospective newspaper article with photos about the case :
Morte de sargento que salvou menino no Zoo completa 40 anos