Disease spread by rats is likely to become an increasing problem as the developing world becomes ever more urbanised, experts have warned.

Every day nearly four million rats are born world-wise - ten for every human.

They are known to carry nearly 70 diseases, but it is suspected actually harbour a lot more.

Rats are a reservoir of disease, the sleeping giants of disease in the world. Dr Lyn Hinds

A meeting of experts in Canberra, Australia, heard on Monday that the rodents pose a growing threat to public health.

Dr Lyn Hinds, of Australia's Commonwealth Scientific and Industrial Research Organisation, told the conference: "Rats are a reservoir of disease, the sleeping giants of disease in the world.

"It is more likely than ever that we will have greater levels of debilitating diseases caused by rats with the increased density of human populations."

Among the diseases that rats are known to carry are cholera, typhus, bubonic plague and leptospirosis, a bacterial illness spread by their urine contaminating water or food.

Dr Hinds said 6,000 cases of leptospirosis were diagnosed in Thailand alone in 2000, killing 350 people.

However, researchers say many more deaths should have been attributed to rats but were classified as unknown fever.

The same disease killed nearly 60 people in the Indian state of Kerala last summer.