A major city in Brazil has declared a state of emergency amid an outbreak of dengue fever which has so far claimed the lives of 31 people.

Officials in Sao Paolo took the step as case numbers exceeded 300 per 1,000 inhabitants, following declarations from authorities in Rio de Janeiro and Santa Catarina. Rio has an incidence rate of 700 per 100,000 people, with more than 42,000 cases.

Frequent rains and high temperatures, which accelerate the hatching of mosquito eggs and the development of larvae, make the famously hot city of Rio especially susceptible. Every couple of years, outbreaks become epidemics.

Spread by mosquitoes, dengue fever is usually not serious, but some people can suffer from a more severe type of the infection, according to the NHS. It is often found in tropical parts of Africa, Asia, South America, Central America, the Pacific Islands and some southern US states.

Sao Paolo’s Control Coordinator of Diseases, Regiane de Paula, said: “We worked with data from the monitoring panel: 311 cases per 100 thousand inhabitants. We are in an epidemic, according to what the WHO (World Health Organization) determines from confirmed cases.”

Acting Secretary of Health Priscilla Perdicaris said the decree means officials can hire people and increase their care network.

She added: “Municipalities can use our decree as justification to declare a state of emergency in their municipalities.” The decree also helps unlock resources from the federal government for state and local municipalities.

Ms Perdicaris said: “It’s a limited resource. We don’t even have an estimate of the global value yet, but any resource helps.”

On top of the city’s confirmed death toll, another 122 deaths are being investigated for dengue fever.

Cases have to date reached 138,259, with 169 considered serious, according to local reports citing official figures.

Symptoms of dengue fever can include a high temperature, severe headache, pain behind the eyes, muscle and joint pain, feeling or being sick, swollen glands and a blotchy rash made up of flat or slightly raised spots.

However, dengue does not always cause symptoms, according to the NHS. Symptoms usually start four to 10 days after being bitten by an infected mosquito.

It can’t be caught from an infected person and there are no cases of dengue fever in Britain. To check the health risks for a country you plan travelling to, visit the TravelHealth Pro website.