More than 40 people have been hospitalised in Las Vegas since Friday, as a heatwave blistered parts of the US.
Las Vegas fire and rescue spokesman Tim Szymanski said: "We will probably start to see a rise in calls Sunday and Monday as the event prolongs.
"People's bodies will be more agitated the longer the event lasts and people may require medical assistance".
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Should the motion go in favour of the anti-hunting lobby, the National Trust’s board will have to decide whether to adopt the ban.
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Felipe’s intervention, the second during the constitutional crisis, came as Madrid plans to impose direct rule on Catalonia.