By Agencies

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The mayor of Dallas has declared a state of emergency in the ninth largest US city as it struggles to contain an outbreak of mosquito-borne West Nile virus that has claimed 14 people this year.

"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 on Wednesday.

The emergency declaration by Rawlings followed a similar action last week by Dallas County officials and paves the way for aerial pesticide spraying to begin this week.

Nearly half of the 693 human cases of the mosquito-borne West Nile virus infections reported this year to the Center for Disease Control and Prevention (CDC) have been in Texas.

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

Aerial spraying also is being used elsewhere, including in neighbourhoods in New York City and Sacramento, California, to combat the spread of West Nile virus.

Officials say spraying is the most effective way to fight the mosquitoes that carry the disease despite safety concerns about exposing people to chemical pesticides.

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

The number of infections usually peaks around mid-August, the CDC said.

Health officials said the reasons for the increased activity were not clear. Factors such as weather, numbers of mosquitoes and human behaviour contribute to when and where outbreaks happen.

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.