Three kilometres away, upwind of the ship, lurks a slow-moving fishing trawler. It is spotted by the crew of the warship. It does not appear to be a threat. The boat passes on, beyond the horizon.

This chilling scenario is not new. It was envisioned back in 1970, at the height of the Cold War, by Canadian Forces experts at the Defence Research Establishment at Suffield, Alberta.

Declassified documents obtained by CBC News under the Access to Information Act outline how a warship might be attacked and what it could do to defend itself.

The author of the report, J. F. Currie, saw three situations in which an enemy might want to use biological weapons against warships, instead of trying to sink them.

• Ships in harbours where the port facilities were desired by the enemy.
• A large convoy or task force where few onboard could be killed in a single attack with conventional weapons.
• Ships carrying land forces.

The report warned that a single strike could produce casualties over an area as large as 1,000 square miles.

But in 1970 they didn't seem worried about small-boat suicide attacks using high explosives such as those carried out against the USS Cole in October 2000, and the tanker Limburg in October 2002.

Clandestine attack

In Currie's 1970 example, the fishing trawler releases 120 litres of a biological weapon into the wind. "The plume of the aerosol would not be visible from the target," he notes.

With the invisible, airborne agents, it might be some length of time before crew onboard a ship even realized they had fallen victim to a biological attack. The crew on the deck would be exposed to a large dose, but eventually everyone on the ship, even in the lowest decks would be affected.

"Until such a time as a device for detection of biological agents is available, the first evidence of such an attack is likely to be the onset of illness. Even then, one would question whether the disease were of natural or manmade origin."

He also warned that proving who had carried out the attack would be difficult and that any evidence would likely be too flimsy to support any political or military retaliation.

Weather factors

The success of such an attack is not guaranteed, however. The report notes that factors such as sunlight, relative humidity, atmospheric pollutants and the wind could all play a role in determining how effective the attack is.

Anthrax simulation

After the Second World War, the world's major powers redesigned their warships so that much of the interior of the vessels became what was called a "citadel." The citadel was designed to be sealed against biological and chemical attacks. It was protected by a series of air filters and by "positive" air pressure that would prevent outside air from reaching inside the ship.

Starting in 1961, the United States began to test how well ships could defend themselves from nuclear, bacterial and chemical attacks. Canada started its own program soon after.

Both countries began tests using a substitute for anthrax, a somewhat similar bacteria called bacillus globigii, or BG. Authorities maintain BG is harmless.

Declassified reports obtained by CBC News show the first Canadian test took place off Halifax in June 1961, when the destroyer escort HMCS St. Croix was sprayed with BG.

A major American test took place off Argentia, Nfld., in January and February 1965. Called Operation Copper Head, it was designed to test ships' systems in "frigid weather." Ships were sprayed with BG as well as with zinc cadmium sulfide, which was used to simulate a chemical attack.

The last major Canadian test took place off Vancouver Island in the summer of 1967, involving both HMCS St. Croix and HMCS Yukon. The ships were exposed to both BG and smoke as part of the trial.

The Canadian documents obtained by CBC News indicate that sailors suffered no ill effects from the trials. American sailors, however, have complained about some health hazards from BG and other simulants. The U.S. Defense Department's Gulf War Syndrome office is now studying the reports. The information obtained by CBC News indicates that, while Canada only used ships with airtight citadels for the tests, some of the ships used in the early American tests were Second World War vintage ships with no protection.

Civilian attack

Documents from the Department of National Defence indicate that work with bacillus globigii has continued to the present day. More modern ships have updated – and still classified – protection and detection equipment to protect those inside.

Civilian ships and shore areas are not routinely protected from biological attacks in the same way.

In the Cold War scenario, the author of the report, J. F. Currie, assumed "the probability of an attack can perhaps be assumed to be minimal if the wind direction and a force's distance from the enemy is such that his own civilian population would be at risk" – a factor that might not necessarily deter a terrorist attack.

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