New Yorkers and tourists tempted to cool off during the current heatwave by plunging into the Hudson River have been warned: it's never been more dangerous.

Researchers at Columbia University reported Thursday that antibiotic-resistant bacteria was widespread in the river that flows down the western side of the city, and said that raw sewage was the most likely cause.

"If you find antibiotic-resistant bacteria in an ecosystem, it's hard to know where they're coming from,' said Columbia micro-biologist Andrew Juhl, one of the authors of the study. "In the Hudson we have a strong case to make that it's coming from untreated sewage."

The researchers tested ten locations on the river and found bacteria resistant to the most common antibiotic ampicillin 84 percent of the time, as well as a correlation between the presence of these microbes and high levels of sewage-indicator bacteria.

Campaigners say sewage is regularly dumped in the Hudson because treatment plans cannot cope with the volumes of waste water they have to deal with at times of heavy rain. City authorities are trying to address the problem by introducing porous pavements and by planting vegetation on rooftops to reduce runoff.

The spread of bacteria resistant to antibiotics, has been blamed on the drugs being over prescribed for minor infections and their widespread use in livestock production. In the US alone, 100,000 people die every year as a result of hospital-acquired infections with antibiotic-resistant pathogens.

New York has been sweltering all week in temperatures in the high 90s Fahrenheit (up to 36 degrees Celsius), increasing the lure of the nearest water for residents and visitors alike.

Despite pollution concerns, swimming in the Hudson is common. The waterway hosts the annual "8 Bridges", a 120-mile swim marathon, and the Great Hudson River Swim. Ironically, in light of the current weather conditions, the latter event, a fun swim off lower Manhattan, had to be cancelled this year because of unseasonably cold conditions at the end of May.