Image caption The virus is carried by birds and transmitted to humans via mosquitoes

The city of Dallas in the US state of Texas has declared a state of emergency as it struggles to contain an outbreak of mosquito-borne West Nile virus.

At least 14 people have died after contracting the virus in the state so far this year.

Almost 700 cases have been reported across the country - the highest number since 2004.

Texas residents have been urged to use insect repellent and avoid going out at dusk and dawn.

"The city of Dallas is experiencing a widespread outbreak of mosquito-borne West Nile virus that has caused and appears likely to continue to cause widespread and severe illness and loss of life," Michael Rawlings said.

The move clears the way for aerial spraying to kill infected mosquitoes that transmit the disease.

Protective clothing

Aerial spraying is also being used elsewhere, including in New York City - where the virus was discovered on Staten Island last month - and Sacramento, California.

There have been cases of infection reported in people, birds or mosquitoes in 42 US states, with 80% of the cases in Texas, Mississippi and Oklahoma.

In Texas, the worst-affected state, residents have been urged to get rid of stagnant water, where the insect can breed, and to wear protective clothing.

Officials say it is too early to say what has caused the fresh outbreak, though some suggest that a mild winter and wet spring may have been a factor.

The virus was first discovered in 1937 in Uganda. It is carried by birds and spread to humans by mosquitoes.

In extreme cases, symptoms can include high fever, loss of vision and paralysis. Milder manifestations can include headaches and skin rashes.

Those at greater risk of a serious case of the illness are those over the age of 50, or people who already suffer from a major health problem.