Rio de Janeiro has declared a dengue epidemic after diagnosing more than 50,000 cases of the tropical mosquito-borne infection this year and over 500 in the last week alone.

"We have a plan focused on the epidemic and we continue to be in a state of alert," Hans Dohmann, the city's health secretary, said late Tuesday, adding that the number of cases had surpassed 300 per 100,000 residents per month.

State-run Agencia Brasil meanwhile reported that 517 people had been struck with the disease last week alone.

The state of Rio de Janeiro has reported a total of 64,423 confirmed cases thus far this year, with 13 deaths, 12 in the city itself.

Last year authorities recorded 168,242 cases and 140 deaths.

Dengue is caused by any one of four viruses transmitted by Aedes aegypti mosquitoes.

Symptoms include high fever, severe headaches, pain behind the eyes, skin rash and mild bleeding. In its advanced stage the disease causes hemorrhages.

The World Health Organization estimates there are 50-100 million dengue infections worldwide each year.