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Like a dagger

Poison from the sting of a river stingray causes pain, swelling and necrosis

Reproduction / ALFRED WALLACEFreshwater stingray (Potamotrygon sp): illustration by British naturalist Alfred Wallace, who covered the River Negro between 1850 and 1852Reproduction / ALFRED WALLACE

Those who frequent the beaches of the Tietê River in the interior of São Paulo should pay attention to where they are stepping. For three years, bathers and fishermen have been sharing this place of leisure with river stingrays that sometimes cause dangerous accidents. With its disk-shaped body, this fish of up to 30 kilos likes burying itself in the mud in the shallow regions of the river. When an inadvertent holidaymaker steps on its body, he gets a sting. Not that the stingray is aggressive. But the treading activates an involuntary defense mechanism in the fish, which shakes its long tail and ends up burying its stinger in the bather’s leg or foot – the death in September of the Australian zoologist and TV personality Steve Irwin, a victim of a sting in the chest let fly by a marine stingray, is a rare case.

Almost 10 centimeters long, the stinger of the river stingray is a bone structure in the form of a serrated knife, covered by a glandular tissue which bursts open with the sting and releases the poison into the victim’s organism. The increase of this kind of accident in the last few years led a team of São Paulo researchers to investigate the characteristics of the wounds and the poison itself of this fish, which until a few years ago did not frequent the Tietê.

Analyses coordinated by biophysician Kátia Cristina Barbaro, from the Butantan Institute’s Immunopathology Laboratory, showed that the poison of the river stingray (Potamotrygon falkneri) is more toxic than that of a marine stingray found all along the Brazilian coast: Dasyatis guttata, better known as the longnose stingray. To evaluate the effects of a sting, Kátia’s team applied two equal doses of poison from each one of these species in different groups of mice.

One day after injecting the poison of the river stingray into four mice, only two remained alive. On the second day, they were all dead; whereas the rodents that had been given the poison of the longnose stingray survived. The tests also showed that the poisons of both species cause swelling and intense pain. Just the penetration of the stinger, incidentally, in itself causes a deep wound that burns like fire. But it is the poison that contributes for the pain, comparable to a knife wound, extends for almost 24 interminable hours.

Were these by no means agreeable effects not be enough, the poison of the river stingray also cause the death of the tissue (necrosis) in the region of the sting, besides muscular lesion. In general, up to three months are needed for the complete cicatrisation of the wound. Kátia also noted that the poison of the Potamotrygon falkneri appears to spread more easily in the organism. This is because one of its components is an enzyme called hyaluronidase, which helps the dispersion of the toxins. The hyaluronidase dissolves a gelatinous compound – hyaluronic acid – that keeps the cells of the tissues united.

Studying the morphology of the stinger and the tissue that it is wrapped in, the team from the Butantan, with the assistance of researchers from the Cell Biology Laboratory, found that the river stingray can also release a larger quantity of poison in a sting, because the whole of its stinger is covered by gland tissue – which produces the poison -, whereas in the stinger of the marine stingray, the gland tissue is restricted to just two points.

Marcela da Silva Lira, a biologist from Kátia’s group, is currently trying to produce a serum capable of combating the activity of the freshwater stingray’s poison and reduce its effects. Although they have not yet arrived at an antidote, there is one piece of good news. In the tests done at the Butantan, the poison of the Potamotrygon and that of the Dasyatis stimulated the organism of rabbits to produce antibodies. “It’s a sign that the serum can neutralize the stingrays” poison” Kátia says.

Remote origin
 Despite the visible difference – the Dasyatis guttata has a triangular-shaped body and can reach three times the size of the freshwater stingray ” it is imagined that these two species had a common ancestor that lived in the sea and reached the continent between 20 million and 10 million years back, when the Atlantic Ocean occupied part of Brazil’s present-day South and Center-West regions. Until a short time ago, the almost 20 species of river stingrays existing in Brazil were found only in the Paraná, Paraguay, Araguaia and Tocantins rivers and in the Amazon basin. It is believed that the construction of dams at the hydroelectric power plants on the Paraná and Tietê rivers in the last three decades has favored the migration of the stingrays at least as far as the region of Presidente Epitácio and Castilho, in the interior of São Paulo.

It was accidents with P. falkneri on the Upper Paraná that called the attention of physicians João Luís Costa Cardoso, from Vital Brazil Hospital, and Vidal Haddad Júnior, from the Faculty of Medicine of the São Paulo State University (Unesp) in Botucatu, to the arrival of this fish in the interior of São Paulo. Cardoso and Haddad decided to turn to Kátia, after verifying that the people stung by the river stingray developed a necrosis similar to the kind caused by poison of the brown spider (Loxosceles sp), studied by the researcher. From the first contact to now, Kátia has become one of the coordinators of the network who has been accompanying the accidents with the freshwater stingray in Paraná, in Mato Grosso do Sul and in São Paulo and investigating the environmental impact caused by this fish. In collaboration with Patrícia Charvet-Almeida, she is also studying the poison of the stingrays from Pará.

For as long as no antidote is produced for the poison of the stingrays, the researchers are learning with the riverside inhabitants to alleviate the pain of the stings, who plunge the wounded leg or arm into a basin with hot water. The hot water reduces the pain because the poison of the stingray is sensitive to heat, as Kátia proved in laboratory experiments. This simple strategy, though, does not prevent necrosis in the region of the wound, usually the hand or the arm, hit when they try to free the fish from the net or the hook. Troubled by the accidents or still little accustomed to the recently arrived fish, the riverside inhabitants and those who frequent the Tietê do not appreciate the presence of the stingrays, nor its meat, although it is very flavorsome.

The Project
Comparative studies between poison secreting glands of freshwater stingrays (Potamotrygon genus) and marine stingrays (Dasyatis genus)
Regular Line of Research Grants
Kátia Barbaro – Butantan Institute
R$ 37,750.00 (FAPESP)