An escalation of intimidation tactics by animal-rights extremists in the US has prompted scientists to look to Britain for answers.

This is World Laboratory Animal Liberation week, a flashpoint on the calendar for academics involved in animal experimentation.

But whereas demonstrations outside US laboratories have been occurring for decades, recent events suggest an escalation of activities.

In February, arsonists set fire to the home of a researcher from the University of California at Santa Cruz. Jeffrey H. Kordower, chairman of the Committee on Animals in Research at the Society for Neuroscience, suggested that it was part of a wider trend.

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"The attacks are more frequent and they are much more violent, and (targets) include not only the researcher's laboratory but also ... the researcher, his family, and his home," he told The Chronicle of Higher Education.

In an attempt to deal with the threat, an activist who helped to establish a pro-research group in Britain has been drafted in to help.

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Tom Holder, a recent graduate of the University of Oxford, helped run Pro-Test, which staged debates and demonstrations supporting the use of animals in UK research.

He was invited to the US by Jacquie Calnan, president of Americans for Medical Progress, who said many scientists were now too scared to speak up for their work.

Mr Holder will take part in a tour of campuses to try to make support for researchers more vocal.

"In Britain we found that animal-rights groups were having problems gathering new recruits (as a result of pro-research demonstrations)," he said.

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There have been high-profile resignations by US academics using animals in research in recent years.

One scientist shut down a project involving primates at the University of California at Los Angeles in 2006 after a bomb was left near the home of a fellow researcher. Another quit his job at Ohio State University in 2002 after receiving death threats.

But despite concerns about a worsening research climate, analysis suggests that laboratory activity is not being hindered.

Records obtained from the National Institutes of Health through a request under the Freedom of Information Act show the number of research grants involving animals has remained relatively constant over the past two decades.

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A comparison of total numbers of applications and awards by The Chronicle of Higher Education found that the number involving animals has stayed near 42 per cent since 1990. Nevertheless, the US is starting to feel the pressure that scientists in Britain have had to deal with for years, and some claim that researchers are being scared into silence.

Frankie L. Trull, president of the Foundation for Biomedical Research, said: "People are not willing to put their heads up above the parapet for fear that they'll get blown off."

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john.gill@tsleducation.com.