The worst effect of a bioterrorist attack may be mass psychological illness which could last years, doctors say. Although chemical and biological weapons are intended to maim and kill, the terror they spread is likely to do the most widespread damage to people's health, says Simon Wessely, professor of psychiatry at Guy's, St Thomas' and King's School of Medicine.

Effects range from immediate alarm and anxiety triggering symptoms including nausea, fever and headaches to long-term malaise, distrust of medical authorities and fears about reproductive health.

Terror-induced illness is already starting to materialise in response to the anthrax incidents in America, Professor Wessely and colleagues from the US and Australia say in the British Medical Journal.

In Maryland 35 people were treated for nausea, headache and sore throat after a man sprayed liquid from a bottle on a subway station platform. The liquid turned out to be window cleaner. A school in Washington state was evacuated and 16 students and a teacher were sent to hospital after fumes were detected. The fumes were from drying paint.

Fear has spread around the world. In the Philippines, 1,000 students in Manila overwhelmed local clinics complaining of flu-like symptoms after rumours spread via mobile phone text messages of a bioterror attack.

Measures taken by the authorities to contain the threat may inadvertently amplify the psychological damage. The US government is considering placing chemical agent detectors on the Washington subway but the high probability of false alarms means they may cause more disruption than an attack would. During the 1991 Gulf War about 4,500 alerts to a possible chemical or biological hazard were reported but none was confirmed.

The long-term effects could multiply the damage caused by individual incidents. A decade after the Gulf War thousands of veterans still believe they were injured by chemical or biological agents, or the vaccines given to the military to protect against them, despite growing evidence to the contrary.

The destruction of an Iraqi arms dump by the US at the end of the war led to the accidental discharge of Sarin gas, the nerve agent used in a terrorist attack on the Tokyo subway in 1995 which caused 12 deaths and 5,000 injuries. Although there was no evidence the Iraqi incident caused physical damage to soldiers, many US veterans still believe they were poisoned.

Part of the reason for their concern is that the US government initially denied it had blown up the arms dump. By the time it was admitted, the government had lost credibility.

Professor Wessely said: "It was a pivotal episode and it showed how cohesion can fragment. Today, everyone is pulling together but if something like that happened again it could have a very deleterious effect. Let us assume we beat the terrorists. We may still lose the battle."

Writing in the BMJ the authors say the general level of malaise, fear and anxiety after bioterrorist attacks "may remain high for years" and uncertainty over the long-term effects of low-level exposure to the agents may further increase anxiety.

"Because health officials cannot provide blanket assurances that no harm will result from brief ... exposure to toxic agents, frustration and then a growing distrust of medical experts and government officials may result, robbing state institutions of the trust they need to manage recovery."

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