As the USA in the grips of possible BSE (Bovine Spongiform Encephalopathy) cases among its cattle, prices have dropped to a one-year low, falling 2%. Cattle futures for August delivery have dropped to 80.35 cents. A cow, which had been given a clean bill of health, was found to have BSE after a second test. Authorities say more tests are needed before a confirmation is issued. McDonald's shares have been falling.

Many fear US beef exports industry, worth around $3 billion a year, could be badly hit again.

What is BSE?

Bovine Spongiform Encephalopathy (BSE) is a disease that affects adult cattle. BSE attacks the brain and central nervous system of the animal and eventually causes death. Commonly known as \'Mad-Cow Disease\', BSE has a long incubation period. This means that it usually takes four to six years for cattle infected with BSE to show signs of the disease, such as disorientation, clumsiness and, occasionally, aggressive behaviour towards other animals and humans.

BSE is one of a group of diseases that affect a number of different mammals.

These diseases, known as Transmissible Spongiform Encephalopathies (TSEs), or Prion Diseases result from the build-up of abnormal prion proteins in the brain and nervous system. TSEs gets their name from the spongy appearance in the infected brain, and the fact that they are transmissible via infected material.

A commonly occurring prion disease is scrapie.

This disease affects sheep and goats and is found in many different countries.

Scrapie has been seen in UK flocks for over 250 years, and while some of the symptoms of the disease are very similar to BSE - it attacks the nervous system of sheep and goats and causes death - studies have not shown any link between scrapie and human illness.

What danger is BSE to people?

BSE only develops in cattle, but it belongs to a family of prion diseases, several of which can affect humans. The most commonly known disease in this group among humans is Creutzfeldt-Jakob Disease (CJD). This is a rare and fatal form of dementia that mainly occurs in individuals between the ages of 40 and 80.

CJD is not a new disease among humans, but in 1996, scientists discovered a new strain of CJD that occurs predominantly in younger people.

More recent evidence has shown that the protein that accumulates in the brains of individuals with this new form of CJD is similar to the protein found in cattle infected with BSE, rather than that found in classical CJD. Because of this newly discovered difference, the new illness in humans is known as variant CJD or vCJD.

The occurrence of a new form of CJD in the UK, where there was a high incidence of BSE, suggested that there might be a direct link between BSE and vCJD .Some individuals who have developed vCJD are known to have eaten potentially BSE-infected meat products. Researchers concluded that the most likely origin of this new disease was human exposure to the BSE agent. Like BSE in cattle, vCJD is always fatal in people. As of June 2003, 135 cases of definite and probable vCJD have been diagnosed in the UK.

Since vCJD first appeared in the UK, an extensive programme of research has been put in place to answer questions about vCJD, such as how people are affected by the disease and how long it takes for individuals exposed to the illness to be affected by it.