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 the lives of 17 people so far 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

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.

Dallas mayor Rawlings says the city’s West Nile virus outbreak is likely to cause ‘severe illness and loss of life’ [AP]

There have been cases of infection reported in people and birds 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.

David Sanders, the associate professor of biological sciences at Purdue University in West Lafayette, in the US state of Indiana, and an expert on the West Nile Virus, told Al Jazeera the outbreak is “a major concern”.

“We had an extremely mild winter so that probably didn’t reduce the mosquito populations as they tend to get reduced during the winter season, so that’s probably a contribution,” he said.

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.