A super-virulent strain of tuberculosis that targets a specific ethnic group has been identified by scientists, who say that new treatments may be needed to combat the spread of the disease.

The strain was responsible for the largest outbreak of TB in a British school, when at least 254 children were infected at Crown Hills community college in Leicester five years ago.

Scientists working on Mycobacterium tuberculosis say the virulent strain, known as CH, spreads preferentially through people originating from the Indian subcontinent and is one of six that transmits more easily through different ethnic groups. The other strains include European-American, two West African, an East Asian and Indo-Oceanic varieties.

Research by Mike Barer, a clinical microbiologist at Leicester University, and colleagues at Imperial College, London, uncovered a mutation in the CH strain that weakens the immune system of those it infects. The scientists fear that by evolving to target specific ethnic groups the disease will become harder to control.

"If it is adapting to different groups, we may need new approaches to treatment and prevention, such as different types of vaccine for each group," said Professor Barer, whose study was published in the Proceedings of the National Academy of Sciences yesterday.

The discovery was made by chance during an investigation to understand the severity of the 2001 outbreak. Half of the infected children had received the BCG vaccine to protect against the disease.