By William Schomberg  

MADRID, Feb 20 (Reuters) - Spanish scientists said on Tuesday they had developed a way of detecting the use of animal-based feed in live cattle and in beef products, which could be an important step in the fight against mad cow disease.  

Animal-based feed is widely believed to transmit bovine spongiform encephalopathy (BSE), or mad cow disease, which has spread across Europe. Eating infected beef products is believed to cause the fatal human variant of the disease.  

The new testing system is likely to be launched commercially later this year, said Antonio Delgado, one of the two scientists from the Spanish government's Superior Council of Scientific Investigation who developed the method.  

It seeks to determine the diet of cattle by examining samples of tissue for nitrogen isotopes, Delgado said.  

A predominance of one type of isotope points to a non-meat diet, while higher levels of another type show consumption of animal products, he said. 

"The most important thing is that a vet can take a sample of bone or hair or other material containing proteins and know what it has been fed without having to destroy the animal," Delgado told Reuters.  

"Supermarkets can also do the test on meat from their cold rooms," he said. 

The method does not test for BSE itself. Existing tests for the disease require examination of an animal's brain once it has been killed. Hundreds of thousands of cows are due to be slaughtered across Europe in an attempt to control the disease, costing governments and taxpayers billions of euros.  

Spain confirmed its first BSE cases last year, leading to a tightening of controls in the livestock industry. But police recently impounded 500 tons of illegal feed and arrested seven people.  

During a test phase, the new diet-control system found traces of animal-based feed in 20% of several hundred samples of Spanish beef.  

The technique has previously been used by archaeologists to determine the diet of ancient humans and animals. But its use to detect the consumption of animal feed by cattle represents a new application, Delgado said.  

About 90 people in Europe have died from variant Creutzfeldt-Jakob Disease (vCJD), the human form of BSE that is linked to eating meat contaminated with BSE. Most of the cases are in Britain. No cases of vCJD have been detected in Spain.  

Patents for the new testing procedure were held by Delgado and his fellow CSIC scientist, Nicolas Garcia, as well as Spanish research company Fisintec Innovacion Tecnologica.