The reality of biological warfare may sound to some like the most irrational of fears, but it is one being taken with increasing seriousness in the United States. The appearance of a single dead crow on the streets of Baltimore has been enough to revive fears among some members of the public that the West Nile virus which killed seven New Yorkers over the summer was the work of bio-terrorists. Health authorities yesterday confirmed that the crow was carrying the mosquito-borne virus - never found before outside northern Africa - but the Government has yet to investigate the matter. The West Nile virus causes swelling of the brain and brings about a severe fever that puts the elderly, sick and weak at greatest risk. So far no cases in humans have been found outside New York but autopsies are now being carried out on more than 30 dead crows found across Baltimore, a northeast port city in the state of Maryland. A probe by health authorities and intelligence agencies at the time of the New York deaths linked the appearance of the virus to wooden cargo crates and increased shipping movements through the Third World rather than a more sinister conspiracy. Even so, preparations in Government are gradually moving ahead for the day when the US faces attacks from what is now being billed in America as the 'threat of the 1990s', given America's post-Cold War enemies. A Senate subcommittee heard warnings last week that seed companies and researchers at universities and the Agriculture Department were on alert after anonymous threats that foreign agents could target the nation's food supply. 'There is tremendous potential for surprise here and it's entirely possible that a biological event could occur without us knowing it because we don't really have the tools in place to detect [bio-terrorism],' department administrator Floyd Horn told the hearing of the Armed Services Committee. 'In terms of foreign terrorist threats, there is significant information . . . suggesting an attempt to attack agriculture at some point in time. 'But we do not have any strong evidence of such an attack at the moment.' The CIA, Defence Intelligence Agency and FBI also gave classified briefings in an indication of how serious the problem is considered. The US intelligence community claims it has evidence that scientists in more than a dozen countries, including Russia, Iraq and North Korea, have the ability to produce weapons carrying diseases that can hit farms, such as anthrax, stem rust and rice blast. Subcommittee chairman Senator Pat Roberts of Kansas said he was particularly worried about the possibility of hard-up Russian scientists taking their skills to Iraq or Iran or of gangsters seeking to build their own biological arsenals. However, the military's special biological warfare forces are generally focused on attacks against humans rather than supplies, the committee heard. A federal health agency issued a grim warning this year about how comparatively easily nationwide chaos could be caused. The Centres for Disease Control and Prevention issued a report showing how America's health agencies would be left virtually powerless in the initial stages of an attack. It created fictional scenarios in which crowds in average northeast cities were exposed to smallpox or anthrax - in one a single terrorist sprays germs from an aerosol can at a football game. The report showed how both diseases could spread rapidly across the nation within weeks, creating vast uninhabitable wastelands and killing tens of thousands of people. The report also showed how the pressure on social and health authorities could quickly turn to civil chaos and political crisis.