The true story of how The Pale Horse caught a criminal and saved lives

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At the time of The Pale Horse's publication in 1961, thallium poisoning was not widely known. While there is an abundance of the element on the planet, it's so thinly distributed that the chances of anyone being accidentally harmed by it are incredibly small. It must have seemed like the perfect weapon to Christie, while plotting her novel. Little did she know that, in bringing it to wider attention, she would both save lives and draw the ire of the national press...

The Teacup Killer

Thallium gained notoriety in the 1970s when it was revealed to be the weapon of choice for serial killer Graham Young. Born in Neasden in 1947, Young had experimented with poisons from a young age - testing them on members of his own family, eventually killing his stepmother, Molly. He served eight years in Broadmoor for his crimes, though he may have continued to experiment while behind bars - he claimed to have been responsible for the death of fellow inmate John Berridge, though Berridge's passing was officially recorded as suicide.

On his release in 1971 Young found work at John Hadland Laboratories in Bovingdon, Hertfordshire. Soon after, numerous employees were taken mysteriously ill. Initially this was blamed on a virus, the "Bovingdon bug" but it soon became apparent that something more sinister was afoot. Following the death of his foreman, Bob Egle, and a continued spate of illnesses on site, Young was arrested and sentenced to life in prison.

It wasn't long before the press picked up on the link between Christie's novel and Young's crimes. The Daily Mail, in particular, criticised the author for highlighting the use of thallium as a poison, suggesting that Young had been inspired by the book, a myth that persists to this day. There's little evidence of this and, when asked directly about the novel, Young denied having ever read it.

What is known, however, is that one of the doctors working with Scotland Yard recognised the symptoms of thallium poisoning precisely because he had read Christie's novel. If anything, The Pale Horse helped Young's eventual capture.

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Saving lives

In the years that followed The Pale Horse is known to have saved at least two more lives. In 1975, Christie received a letter from a reader in South America who claimed that she had helped a man after reading the book.

She wrote: "Of this I am quite, quite certain - had I not read The Pale Horse and thus learned of the effects of thallium poisoning, X would not have survived; it was only the prompt medication which saved him; and the doctors even if he had gone to hospital, would not have known in time what the trouble was." In a detail worthy of Christie herself, the author of the letter claimed that "X" had been poisoned by his young wife.

Another case came in 1977, a year after Christie had passed away. The British Journal of Hospital Medicine reported that a 19-month-old girl was flown to Hammersmith Hospital from Qatar for emergency medical treatment. She was immediately put into intensive care, but her mysterious ailment left doctors baffled and her condition worsened.

Enter Marsha Maitland - a nurse at the hospital who had been making her rounds. By coincidence, Maitland had recently read The Pale Horse and commented on the similarities between the symptoms as described in the novel and the condition afflicting the infant. Unable to test for thallium in the hospital, the case was taken Scotland Yard where urine tests confirmed the hypothesis.

The child, it turned out, had accidentally ingested some insecticide containing the lethal ingredient. After receiving the appropriate treatment, she made a slow but full recovery. The author of the journal's report thanked Christie for raising awareness of the dangers of thallium and Marsha Maitland for "keeping us up to date on the literature".

So there you have it. While Agatha Christie may well have been fondly nicknamed "the Queen of Crime", she was also something of a life-saver!

The Pale Horse is available to watch now on BBC iPlayer