Bed bugs, for years linked to unsanitary households, poverty and the darker side of Dickens, are making a comeback.

Infestations of the bugs, which feast on human blood and leave itchy weals on the skin, had been virtually eliminated in prosperous countries but have had a resurgence in the last seven years.

In America, wealthy householders have been horrified to discover that the cause of their bites, often on the arms and shoulders, had been bed bugs. Australia has also reported a recent "dramatic" increase.

Experts are unable to explain why the human bed bug, Cimex lectularius, has returned, but speculate that it may have become resistant to pesticides. Another theory is that pesticides used on other household pests, such as ants or cockroaches, are now species specific and are missing the bed bugs, allowing them to breed and multiply.

Clive Bose, a pest management specialist, told the Institute of Biology's journal, Biologist, yesterday: "Data from some sources indicate that since the mid-1990s the numbers of reported infestations have almost doubled annually, although numbers are nowhere near pre-war levels."

The 5mm bugs are reddish brown and fast runners. They also have a flat shape which means they can hide in narrow cracks in furniture and the seams of bed clothes.

Starving them of blood does not help since the adults can live for about a year without nourishment. The adult female lays two or three eggs a day which she attaches to rough surfaces and which hatch in about 10 days. They do not normally live on humans.

Mr Bose said some experts thought that the increase in long-haul travel might explain the increase but he believes this is unlikely as the recent infestations have not been of the tropical bed-bug, Cimex hemipterus, as might have been expected.

An alternative theory was that the bugs were becoming resistant to pesticides, he said. A study in East Africa had shown a link between increased use of pesticide-treated mosquito nets and increased resistance in bed bugs.

The pesticide involved, pyrethroid, was widely used in bed-bug sprays in developed countries, he said.