Scientists have identified a new class of drugs that could lead to the first treatment for the human form of mad cow disease.

The compounds have been shown to slow the development of a similar disease in mice.

However, they only work during the early stages of infection.

Researchers believe the drugs may be effective against all types of a disease collectively known as transmissible spongiform encephalopathies (TSEs).

These include BSE in cattle, Scrapie in sheep and CJD in humans.

The drugs may also halt or slow the progression of non-prion diseases that involve an accumulation of dangerous protein in the brain, such as Alzheimer's.

TSEs are thought to be caused by abnormal proteins known as prions.

The diseases occur when prion proteins fold into the wrong shape, causing them to aggregate in the brain.

Prions alter their normal neighbours and make them harmful, causing a chain reaction.

The new compounds, called cyclic terapyrroles, include drugs already used in cancer treatments.

Researchers at the National Institute of Allergy and Infectious Diseases (NIAID) in Hamilton, Montana, USA, tested the compounds on mice injected with Scrapie.

Given at the time of the injection, or mixed in with it, the drugs extended survival time by as much as 300%.

However, when they were administered at a later stage once the disease had taken hold, the effect was minimal.

Early stages

The researchers, whose work is published in the journal Science, suggest that the compounds might delay onset of the disease - but only when administered prior to or in the early stages of infection.

They might also one day be used to treat blood products to deactivate infectious prions.

Lead researcher Dr Suzette Priola said: "Since diagnosis of CJD in humans can be made only after symptoms appear, we are trying to identify a compound that works later in the disease.

"The extremely wide variety of these compounds available for testing makes our current search for one that can affect the disease after the onset of symptoms much more promising."

A new variant of CJD appeared in the UK in 1996 which most scientists now agree is merely "mad-cow" disease, or BSE, in a different guise.

It was probably acquired by the consumption of beef infected with the cattle disease.

The latest figures from the Department of Health show that up to 31 December last year a total of 52 people in the UK had died from new variant CJD.