Louisiana's struggling residents, forced from their homes by once-in-500-year floods, are being warned against disease, animal infestations and snake bites as they contemplate returning to their devastated communities.

On Monday 20,000 people had been evacuated, and President Barack Obama had declared a major disaster in four parishes - including East Baton Rouge, the district where Alton Sterling was shot by a white officer on July 5, and three policemen murdered in retaliation a fortnight later.

Sterling's aunt, Sandra Sterling, was among those who volunteered to go house by house, by boat, to evacuate stranded people. She said it was only natural given what people had done for her in recent weeks.

"This is my giving back," she said. "And thank you all for helping me."


John Bel Edwards, governor of Louisiana, said the flooding was unprecedented, with 12,000 people staying in shelters. He and his family were even forced to leave the Governor's Mansion, in central Baton Rouge, when chest-high water filled the basement and electricity was shut off.

In Denham Springs, 19km east of Baton Rouge, coffins unearthed by the rains floated down the streets.

Anna Johnson, who took photos of the macabre scenes, described the freak flooding as "worse than Hurricane Katrina" - the 2005 disaster that wreaked havoc in the state, particularly in New Orleans.

As much as 27cms of rain fell on Baton Rouge in just 24 hours this weekend, with 17 inches of rain reported in nearby Livingstone.

The worst of the rain is now over, but rivers continue to rise. Water levels in some rivers are not expected to recede for two days. Any additional rain in the next couple of days could trigger more flash flooding.

Mr Edwards said that 40,000 businesses and homes were without power, and 1,400 bridges needed inspecting before they could be opened to traffic.

And those preparing to return to their flooded homes have been warned of the risk of tetanus, E-coli and Norovirus - with anyone suffering a puncture wound in the debris told to go to hospital for an emergency tetanus vaccination.

When residents return home to find houses affected by flood damage, they may develop respiratory issues, including asthma. Allergens may increase, especially if mould, dust mites and cockroaches are in the area after a flood.

Mosquitoes pose a significant problem - with added potency from the Zika epidemic, which arrived in neighbouring Florida last month.

"Swarms of mosquitoes may be seen in the affected regions several weeks after the storm," the Louisiana Health Department said online, noting that mosquito eggs "can lie dormant for years without water and these eggs will now hatch."

And snakes are more prone to enter abandoned homes, cars or even furniture if their nests are flooded, according to the Louisiana Health Department.

"I'm still asking people to be patient. Don't get out and sightsee," said Mr Edwards. "Even when the weather is better, it's not safe."