The Would-be Plague Ship – Operation Cauldron and the Carella Incident

The Carella Incident

The Cold War. Just the very mention of it conjures up thoughts of spies, paranoia and political brinksmanship while civilisation itself sat literally minutes away from nuclear Armageddon but there was another, perhaps even more frightening angle to the stand-off between east and west. While the nuclear arms race took centre stage, behind the scenes another just as deadly arms race was on; the development of the perfect biological weapon that could quietly, cheaply and effectively destroy the enemy and Britain was in it from the beginning.

Britain had an active biological and chemical weapons program in place since the First World War situated at Porton Down, Wiltshire. It was established in the wake of the German use of poison gas on the Western Front; an act which opened a Pandora’s Box in terms of biological and chemical warfare to break the stalemate of the trenches. The site was used to develop both new types of poison gas and employment techniques as well as developing countermeasures to an enemy’s weapons. Eventually the site morphed in to the Microbiological Research Establishment which further advanced research in to weaponised nerve agents for use on the battlefield and possibly beyond should British cities ever be attacked first – it was this threat of retaliation that eventually stopped Adolf Hitler’s Nazi Germany from employing biological and chemical weapons against British cities.

The romance between the east and west born out of the need to destroy Nazism died almost as quickly as the war ended and both sides viewed their former allies with suspicion over the future. Without question the spoils of the war lay in the advanced German weapons research leading to both sides scrambling to get their hands on data, equipment and of course the scientists themselves believing they would not only speed up the rebuilding process but also give an advantage over the opposition in the newly gestated Cold War.

MRE Porton Down reseacrh NBRC

Exercise at Porton Down (

German research in to nerve agents such as tabun, sarin and soman was especially sought after by Britain and the United States not just for application in their own inventories but also to prevent the Soviet Union from gaining by the technology. In the wake of the devastation of World War II which saw entire cities and their populations on the brink of annihilation a brutal theory began to develop regarding how the next war could possibly be fought. If one side could develop biological weapons that could eradicate an enemy population, then it would leave all the enemy country’s valuables intact for plundering and all this would be achieved at very little expense. It was hardly a new idea since biological warfare stretched back to antiquity when infected bodies would be dropped down wells or thrown in to besieged towns and villages to contaminate the people but now there was a very real possibility of it being perfected to a science.

The staff at Porton Down wasted no time in its race to further develop the captured German research as well as continue their own research projects. This led to a series of trials carried out in 1952 intended to test the effectiveness of various pathogens released in to the air and study how they would disperse and infect a target area. Called Operation Cauldron, the tests required cooperation between the team at Porton Down, the Royal Navy, the Royal Air Force (primarily for logistical purposes) and the local government in Scotland. The plan required the use of live animals, in this case guinea pigs and monkeys, to be infected out in the open air by clouds of weaponised pathogens released by the researchers in order to study the infection and lethality rates. To do this without risking contamination of the general public the tests were to be conducted at sea off the coast of the Isle of Lewis in the Outer Hebrides.

The extremely secretive tests began in May 1952 and continued until September making use of the calmer weather of summer. The Royal Navy granted the team use of HMS Ben Lomond (below) under the command of Captain Phillip Welby-Everard to act both as the control ship for the tests and as a field laboratory for the scientists. The unfortunate animals used in the tests would be taken from Ben Lomond’s hold and restrained aboard a floating pontoon, in fact a converted mulberry harbour barge from the war, where they would be exposed to various pathogens.

HMS Ben Lomond.jpg

(For more details about how Cauldron was conducted you can view the video at the bottom of this article)

Among the pathogens tested were;

  • Brucellosis – A disease known under many local names such as Malta Fever or Gibraltar Fever as it was quite common in the Mediterranean. A highly contagious disease, it can be caused by the ingestion of unpasteurized milk or undercooked meat from infected animals or by close proximity with their secretions. Symptoms include profuse sweating and joint and muscle pain with a 2% mortality rate. A weaponised version could be used to incapacitate an enemy force or population reducing effective resistance.
  • Tularemia – More commonly known as O’Hara’s Fever or Rabbit Fever. Symptoms include very high temperature (fever), lethargy, loss of appetite, signs of sepsis and if left untreated death will occur. It is highly virulent in humans meaning large numbers could be infected quickly and given its long incubation period there is a reduced chance of an enemy becoming aware that their troops have been infected and taking appropriate countermeasures before large numbers of people are contaminated.
  • Pneumonic plague – One of the three main forms of plague caused by the bacterium Yersina pestis it is far more contagious than bubonic plague (v.) but fortunately much rarer. This form of plague aggressively attacks the lungs and is contracted from inhalation of fine infective droplets which can be transmitted from human to human without involvement of fleas or animals. Without urgent treatment death occurs in up to 90% of all cases even with 2015 medical technology to say nothing about 1950s medicine.
  • Bubonic plague – Perhaps the most well-known form of plague thanks in no small part to its place in history where it wiped out millions across Europe in medieval times, bubonic plague is an infection of the lymphatic system and is most predominantly contracted from the bite of an infected flea. Bubonic plague creates a vast array of painful and disabling symptoms including muscle cramps, very high fever, seizures, vomiting blood, gangrene in the bodies extremities such as fingers and toes and the decomposition of skin while the person is still alive.
MRE Porton Down research Operation Cauldron

Preparing the guinea pig boxes (Crown)

Initially the animals were infected on the pontoon by being sprayed with clouds containing the pathogens but in later tests to determine their suitability for military applications, bombs were exploded near the pontoon that contained the pathogens in an effort to disperse their spores over the target animals. These tests eerily echo similar tests carried out by the Imperial Japanese Army’s notorious Unit 731 biological research unit based in China during the 1930s up to the end of World War II except in those instances the Japanese used living humans instead of animals. 3,492 guinea pigs and 83 monkeys were used in the test program almost all of which were killed and then meticulously dissected aboard the Ben Lomond to study the effects on their bodies the weaponised diseases had.

By mid-September 1952 the tests were drawing to a conclusion. The weather was becoming more and more of an issue as a Scottish autumn set in leading to the final day of testing to have to be delayed until September 15th. Captain Welby-Everard and the research team was under increasing pressure to complete the day’s trials or scrap them and attempt again the next day; an undesirable proposition since the weather forecast showed that conditions would continue to deteriorate overnight with an unfavourable wind for the trials on the 16th. Finally, at 1800hrs the first test was allowed to begin with the pathogens being spread across the pontoon at 1809hrs. The infected test animals were then moved away and replaced with “clean” samples. A second test was carried out at 1839hrs and again the animals were replaced with a third and final batch. The final batch were to be used in the testing of an air detonated explosive device fitted with bubonic plague bacteria.

It was as the preparations for detonation were getting under way that Captain Welby-Everard was alerted to the presence of an unauthorised vessel approaching the target area. The vessel was quickly identified as the fishing trawler Carella, a nineteen-year-old, 421-ton vessel of the famed Fleetwood trawler fleet with a crew of eighteen aboard and under the command of skipper E. Harris. The ship had apparently ignored visual warnings that the area was restricted and was nearing the test site north of the target pontoon at a range of two miles. Welby-Everard ordered that the vessel be signalled away by radio and signal lamp and continued on with preparations for the test. A similar occurrence earlier in the test program had ended without incident and he saw no reason why this time it should be any different reasoning that with the Ben Lomond and two supporting vessels all displaying warning flags and hailing the fishing vessel with signal lamps that the trawler was bound to turn away before it could get in to any trouble regarding the test.

carella 2.jpg

The Carella (Fleetwood)

Captain Welby-Everard therefore ordered the test to proceed and at 1902hrs the bomb was detonated engulfing the pontoon in a cloud of plague-carrying particles which was then carried away by the wind. Efforts to signal the Carella continued to fail however forcing one of the supporting craft to intercept the trawler and try to make direct contact. The trawler continued on oblivious to the fact that it was soon two miles downwind of the pontoon and in very real danger of coming in to contact with the invisible cloud of plague particles. Lookouts aboard the supporting craft and the Ben Lomond noted that the entire trawler’s complement were below decks which goes some way to explaining why the signals were not spotted but when the supporting craft made contact with them the crew of the Carella claimed they had attempted to signal the Ben Lomond but without success. This claim has been denied as a lie by the Royal Navy personnel on watch that night.

Captain Welby-Everard couldn’t legally stop the trawler from continuing on its journey, at least not without revealing the very secret nature of the tests to the crew who remained completely oblivious to what was really going on and composed a coded, cryptic message which he sent to the Admiralty regarding the incident and requested instructions;

During Cauldron trails of Agent L at 1900hrs 15th September the steam-trawler CARELLA Number H4 of Hull bound Fleetwood from Iceland(C) disregarded signals and crossed danger area after release of agent. Vessel passed two miles to leeward position of pontoon sixteen minutes after time of release. Wind speed six knots. Consider vessel may have passed through toxic cloud.

Due to the highly secret nature of the operation the message was met with a more lukewarm response than it deserved. Just what “Agent L” referred to was known by only a few individuals even at the Admiralty and it would not be until noon the next day that a response was sent back to Welby-Everard. The Admiralty and the staff at Porton Down assessed the situation and deemed the threat to the Carella to be negligible requiring no further action on the part of Welby-Everard and his people. With a six knot wind recorded over the pontoon at the time of detonation of the weapon then the trawler would have left the danger area before the plague spore-carrying particles could have made contact with it. Even if some of the plague spores did reach the trawler then in all likelihood they would be dead by that point not having a host within which to incubate.

This sigh of relief was short lived.

A follow-up report noted that the original report of a six knot wind over the pontoon did not mean that the wind speed was consistently six knots up to the trawler’s position. It was quickly determined that in actual fact the wind speed at a distance of two miles north of the pontoon would actually be in the region of anywhere between five and nine knots increasing the chances of the plague-carrying spores reaching the Carella. To exacerbate the perceived threat to the trawler a reassessment of the distance of the vessel by the Ben Lomond showed that the crew had been in error of around 400 yards meaning it was that much closer than first thought. This meant that there was now a very real chance that the Carella had indeed come in to contact with the invisible cloud and its deadly contents.

Duncan Sandys

Duncan Sandys (commons.wikimedia)

Alarm bells within the Admiralty rang out and an emergency meeting was held by late afternoon, almost 24 hours after the incident, to assess the danger. In attendance were Duncan Sandys, Minister of Supply (and Winston Churchill’s son-in-law), and representatives of the Admiralty, Ministry of Health and Ministry of Supply who had an umbrella of authority over Porton Down and their activities. During the meeting, Sands was given all the information available and after intense debate they concluded that the likelihood of the Carella’s crew being in any danger was still remote but not beyond the realms of possibility.

The next question therefore was to how best respond to the situation. A plan was considered whereby the crew of the trawler would be ordered to a secure British port and given injections of streptomycin for several weeks under controlled conditions while the vessel itself was detained and thoroughly decontaminated. But there was a problem. If they proceeded on this course of action, then it would prevent any outbreak but it would also almost certainly blow the lid on the intense secrecy of Operation Cauldron and confirm that Britain was developing “plague weapons”. Given the low chance of the crew having been infected it was decided not to implement the plan and thus keep the secret safe from the oblivious trawler crew and the world at large.

This of course begged the question; what if? The Carella was on its way to the rich Icelandic fishing grounds and it was not uncommon for Fleetwood trawlers to dock in Icelandic ports either for rest or repairs. If the crew had been infected, then there was the chance they could infect the people of Iceland which would not only potentially kill countless innocent people but cause a major diplomatic incident and blow the secret anyway. Put simply, Sands and the Admiralty couldn’t just ignore the problem. Therefore, a plan was put in place to monitor the Carella’s crew as they went about their trade in the cold Icelandic waters.

HMS Zambesi z-class destroyer

HMS Zambesi (IWM)

A Royal Navy destroyer, HMS Zambesi, was tasked to clandestinely track the Carella with only the senior medical officer onboard knowing the exact nature of the operation. In order to maintain the secret of Cauldron’s tests even the Zambesi’s captain was kept in the dark as to why he was tracking a British trawler although he was told that there was a chance the crew were infected with a contagious virus. Zambesi’s orders required the Royal Navy ship to stay out of sight of the crew so as to not alert them to the fact they were being tracked. To keep track of their quarry therefore the ship monitored the trawler’s regular communications with the owners in Fleetwood who had been instructed to keep in contact with them by the Ministry of Supply but again weren’t told why.

The Zambesi was to only break its cover on the following conditions;

  1. If the crew signalled for medical assistance indicating that a crewmember had been infected and was showing symptoms. The senior medical officer onboard had been given a consignment of antibiotics and instructions on how to treat the crew should this happen.
  2. If the vessel attempted to dock at an Icelandic port regardless of the medical condition of the crew.
  3. The crew attempted to make direct contact with another vessel.

These instructions were memorised by the officers and then promptly destroyed.

With regards to timescale the decision was taken that the danger would not pass until October 6th by which time any symptoms would have manifested itself amongst the crew. If no symptoms presented themselves by that time, then it was clear that either the crew weren’t infected or if some of the spores did reach the trawler then they had long since died and were no a longer a threat.

HMS Truelove Algerine-class minesweeper

HMS Truelove (IWM)

The Zambesi trailed the Carella as it reached the rich Icelandic fishing grounds until on September 22nd another vessel, HMS Truelove, took over the surveillance operation. Truelove was one of a number of Algerine-class minesweepers built in Canada during World War II for the British and Canadian navies. Like a few of her sisters she had been relegated to the fisheries protection role and had become a common sight to British trawlers. Truelove was therefore a less suspicious vessel to be roaming the fishing grounds at the same time as the Carella should the trawler crew have spotted her. Truelove operated under the same orders as Zambesi and monitored the Carella’s radio transmissions and movements through until the end of September and in to October 1952 all the while maintaining a distance of around 50 miles.

In the meantime, the government was looking for someone to blame and Captain Welby-Everard found himself in the firing line. He was accused of committing an error of judgement regarding continuing the test while aware that the trawler had not yet cleared the area. He was also criticised for not highlighting the urgency of the situation in his initial dispatch, however some blame for the latter has to go to the Admiralty itself for not having officers on duty to receive the dispatch and know exactly what it meant since Welby-Everard did inform them of what had happened using legitimate coded language for the operation.

Fleetwood trawlers

Fleetwood trawlers (Fleetwood)

September gave way to early October and the Truelove reported that they were now following the Carella back to its home port at Fleetwood. They estimated that the trawler would arrive in port between the 4th and 5th of October and requested instructions since technically the trawler was not yet in the clear. With no sign of any illness amongst the trawler’s crew there was a general consensus that given the low probability of infection in the first place and with no symptoms having manifested yet that the trawler should be allowed to make port unmolested unless the situation aboard changed during the transit home. Thus the Carella arrived home with her catch on October 5th 1952. Covert measures were taken by the Ministry of Health to monitor the whole crew in the days after their return but it was clear to all concerned that the danger had passed.

All that remained now was for the Admiralty and the Ministry of Health to cover up the mess to avoid any embarrassment. Their efforts were so successful that the crewmembers of the Carella would only learn the truth of what they nearly exposed to and the military operation to track them when the BBC made a documentary on the incident some fifty years later!


Here is an official film made by the research team involved in Operation Cauldron that was not released to the public until 2012. Even then the MoD wanted part of it restricted showing just how secret the operation was.

If I have included it for those who would like to know more about Cauldron BUT BE WARNED if you are upset by the sight of animals being used in testing. It does show the animals being prepared for the tests and while it doesn’t show them getting infected there is a section showing the dissection of the poor creatures. Other than that it is quite dry in places but quite fascinating for those who have an interest in the Cold War and biological weapon research.








9 responses to “The Would-be Plague Ship – Operation Cauldron and the Carella Incident

  1. Pingback: Defence of the Realm – Royal Navy | Defence of the Realm

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