Swarms of snakes are attacking people and cattle in southern Iraq as the Euphrates and Tigris rivers dry up and the reptiles lose their natural habitat among the reed beds.

"People are terrified and are leaving their homes," says Jabar Mustafa, a medical administrator, who works in a hospital in the southern province of Dhi Qar. "We knew these snakes before, but now they are coming in huge numbers. They are attacking buffalo and cattle as well as people." Doctors in the area say six people have been killed and 13 poisoned.

In Chabaysh, a town on the Euphrates close to the southern marshland of Hawr al-Hammar, farmers have set up an overnight operations room to prevent the snakes attacking their cattle.

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"We have been surprised in recent days by the unprecedented number of snakes that have fled their habitat because of the dryness and heat," Wissam al-Assadi, one of the town's vets said. "We saw some on roads, near houses and cowsheds. Farmers have come to us for vaccines, but we don't have any."

The plague of snakes is the latest result of an unprecedented fall in the level of the water in the Euphrates and the Tigris, the two great rivers which for thousands of years have made life possible in the sun-baked plains of Mesopotamia, the very name of which means "between the rivers" in Greek. The rivers that made Iraq's dry soil so fertile are drying up because the supply of water, which once flowed south into Iraq from Turkey, Syria and Iran, is now held back by dams and used for irrigation. On the Euphrates alone, Turkey has five large dams upriver from Iraq, and Syria has two.

The diversion of water from the rivers has already destroyed a large swathe of Iraqi agriculture and the result of Iraq being starved of water may be one of the world's greatest natural disasters, akin to the destruction of the Amazonian rainforest. Already the advance of the desert has led to frequent dust storms in Baghdad which close the airport. Yet this dramatic climatic change has attracted little attention outside Iraq, overshadowed by the violence following the US-led invasion in 2003 and the overthrow of Saddam Hussein.

The collapse in the water levels of the rivers has been swift, the amount of water in the Euphrates falling by three-quarters in less than a decade. In 2000, the flow speed of the water in the river was 950 cubic metres per second, but by this year it had dropped to 230 cubic metres per second.

In the past, Iraq has stored water in lakes behind its own dams, but these reservoirs are now much depleted and can no longer make up the shortfall. The total water reserves behind all Iraqi dams at the beginning of May was only 11 billion cubic metres, compared to over 40 billion three years ago. One of the biggest dams in the country, on the Euphrates at Haditha in western Iraq, close to the Syrian border, held eight billion cubic metres two years ago but now has only two billion.

Iraq has appealed to Turkey to open the sluice gates on its dams. "We need at least 500 cubic metres of water per second from Turkey, or double what we are getting," says Abdul Latif Rashid, the Iraqi Minister of Water Resources. "They promised an extra 130 cubic metres, but this was only for a couple of days and we need it for months." His ministry is doing everything it can, he says, but the most important decisions about the supply of water to Iraq are taken outside the country – in Turkey, Syria and Iran. "In addition there has been a drought for the last four years with less than half the normal rainfall falling," says Mr Rashid.

Large parts of Iraq that were once productive farmland have already turned into arid desert. The Iraqi Ministry of Agriculture says that between 40 and 50 per cent of what was agricultural land in the 1970s is now being hit by desertification.

Drought, war, UN sanctions, lack of investment and the cutting down of trees for firewood have all exacerbated the crisis, but at its heart is the lack of water for irrigation in the Tigris and Euphrates. Farmers across Iraq are being driven from the land. Earlier this month, farmers and fishermen demonstrated in Najaf, a city close to the Euphrates, holding up placards demanding that the Iraqi government insist that foreign countries release more water.

"The farmers have stopped planting and now head to the city for work to earn their daily living until the water comes back," said Ali al-Ghazali, a farmer from the area.

"We pay for our seeds at the time of the harvest, and if we fail to harvest, or the harvest has been ruined, the person who sold us the seeds still wants his money." Najaf province has banned its farmers from growing rice because the crop needs too much water.

The drop in the quantity of water in the rivers has also reduced its quality. The plains of ancient Mesopotamia once produced abundant crops for the ancient Sumerians. From Nineveh in the north to Ur of the Chaldees in the south, the flat landscape of Iraq is dotted with the mounds marking the remains of their cities. There is little rainfall away from the mountains of Kurdistan and the land immediately below them, so agriculture has always depended on irrigation.

But centuries of irrigating the land without draining it properly has led to a build-up of salt in the soil, making much of it infertile. Lack of water in the rivers has speeded up the salinisation, so land in central and southern Iraq, highly productive 30 years ago, has become barren. Even such rainfall as does fall in northern Iraq has been scant in recent years. In February, the Greater Zaab river, one of the main tributaries of the Tigris, which should have been a torrent, was a placid stream occupying less than a quarter of its river bed. The hills overlooking it, which should be green, were a dusty brown.

Experts summoned by the Water Resources Ministry to a three-day conference on the water crisis held in Sulaimaniyah in April described the situation as "a tragedy".

Mohammed Ali Sarham, a water specialist from Diwaniyah in southern Iraq, said: "Things are slipping from our hands: swathes of land are being turned into desert. Farmers are leaving the countryside and heading to the city or nearby areas. We are importing almost all our food, though in the 1950s we were one of the few regional cereal-exporting countries."

The experts recommended that, in addition to Turkey releasing more water, there should be heavy investment to make better use of the waterways such as the Tigris and Euphrates. But this year Mr Rashid says that his budget for this year has been cut in half to $500m (£300m) because of the fall in the price of oil.

The outcome of the agricultural disaster in Iraq is evident in the fruit and vegetable shops in Baghdad. Jassim Mohammed Bahadeel, a grocer in the Karada district, says that once much of what he sold came from farms around the Iraqi capital. "But today, the apples I sell come from America, France and Chile; tomatoes and potatoes from Syria and Jordan; oranges from Egypt and Turkey. Only the dates come from Iraq because they do not need a lot of water."

Rightly feared: Iraq's deadly reptiles

*Saw-Scaled Viper (Echis carinatus) About 2ft long, this viper is blamed for more deaths than any other species in the world. Its bite causes extensive internal haemorrhaging in its victims. Recognisable by an arrow-shaped marking on the head.

*Desert Horned Viper (Cerastes cerastes) The Desert Horned Viper is typically found in sandy terrain and is a common sight in Iraq's southern deserts, identified by the bony horns over its eyes. It lurks in sand, only eyes, nostrils and horns above the surface.

*Desert Cobra (Walterinnesia aegyptia) Like most cobras, it is easily adaptable to various habitats. But locations occupied by humans are a particular favourite where shelter and rodents are on offer. Whilst this glossy snake does not actively seek confrontation, it can move with lethal speed when provoked.