Shared delusions: Mark Tromp, 51, is still missing while wife Jacoba (right) remains in a Goulburn psychiatric facility with daughter Riana after a mysterious family road trip went bad

Is it mental illness? Some sort of environmental poisoning situation involving a farm-related toxin? Or is someone, as parents Mark and Jacoba Tromp fear, really out to get them?

The couple vanished on Tuesday during an ill-fated roadtrip to the Jenolan Caves in NSW with their three adult children, Ella, 22, Mitchell, 25 and Riana, 29.

Mark Tromp, 51, remains missing after bolting from the car in which the family were travelling while Jacoba, 53, was placed under psychiatric care in Goulburn, where daughter Riana is also being treated after being found in a “catatonic state” on Tuesday.

Mrs Tromp was transferred to the same facility as her daughter after being found in a disoriented state in Yass — some 240km from where she was last seen — yesterday.

The case bears many hallmarks of a rare psychiatric condition known as folie à deux, a French term meaning “madness of two” which almost always occurs in close-knit families, between husband and wife and between siblings.

The syndrome can be shared by more than two people — folie à trois, folie à quatre, folie en famille or even folie à plusieurs — which translates as “madness of many”.

The term was originally coined to describe the case of a French married couple, Margaret and Michael, who began exhibiting paranoid and delusional behaviour.

Riana Tromp, 29, is being treated at the same mental health facility as her mother after being found in a catatonic state in the back seat of a man’s ute in Goulburn.Source:Facebook

Mitchell with his sister EllaSource:Facebook

Doctors were unable to establish which of the two was the first to become psychotic but they worked out that the couple had fallen into a cycle of reinforcing each other’s delusions.

They shared the unfounded belief that their home was being targeted by random people. These unidentified individuals did not steal or damage, rather, they spread dust around the house, scattering lint everywhere as they walked in the couple’s shoes as they slept.

The only factors that need to be present in a case of folie à deux are a hard-to-break attachment and social isolation.

There have been several famous cases of folie à deux, including the 2008 case of Swedish twins Ursula and Sabina Eriksson, who were notoriously captured on CCTV running through traffic in Liverpool, England as they fled from non-existant assailants who they believed were trying to harvest their organs.

In 2010, US actor Randy Quaid and his wife Evi were arrested in Canada in a psychotic and delusional state after attempting to seek asylum for from a group of assassins called the “star whackers,” whom they believe responsible for the deaths of Heath Ledger and David Carradine.

Mitchell Tromp has told police and reporters the family fled their berry farm at Silvan, east of Melbourne, after their parents became convinced somebody was out to get them.

“I’ve never seen anything like it. It’s really hard to explain or put a word on it but they were just fearing for their lives and then they decided to flee,” he said.

“It was a build-up of different, normal, everyday events — just pressure — and it slowly got worse as the days went by.”

Close-knit: Siblings Ella and Mitchell Tromp address the media at Monbulk Police Station after the disappearance of their parents. Picture: Andrew HenshawSource:News Corp Australia

So convinced were Mr and Mrs Tromp that their lives were in danger that they insisted all phones and tablets be left at home, prompting Mitch, who had secretly brought along his mobile, to toss the phone out of the car window once his parents became aware of it.

It emerged earlier today that the Tromps may have been prepared to go as far as feeling the country — but changed their minds at the last minute given they could be traced via their passports.

“In their delusional state, the entire family was planning on going overseas with ... but they decided against it saying ‘passports can be tracked’,” a police source told the Telegraph.

“They thought the safer option would be going into a place where technology cannot reach them.”

Two of the three Tromp children lived with their family on the farm and all three worked there seven days a week. Ella even juggled work on the farm with her own business.

The family’s social media accounts are peppered with references to their close-knit and their work ethic.

When police searched the farm as part of their efforts to track down Mark Tromp, they found the house unlocked and in a state of disarray.

Police told The Daily Telegraph it appeared the Tromps had fled in fear after sifting through years of financial records relating to their farm.

The Tromp family fled their berry farm in Silvan, east of Melbourne in fear of their lives. Picture: Andrew HenshawSource:News Corp Australia

Officers reportedly found “piles and piles” of documents stacked throughout the home as well as passports for every family member alongside credit cards.

“The piles were very ordered and they were clearly looking for something,” one officer told the paper.

Monbulk Police Sergeant Mark Knight, who is leading the search for Mark Tromp and knows the family personally, says the Tromps are not drug users or members of a church or cult. He said the family is not in debt and there is no standover man lurking in the shadows.

Sergeant Knight said when he checked their home after the first reports of Riana being found near Goulburn, the house was open and keys were in car ignitions, but there was no sign of any struggle.

“This is just a massive meltdown, I’m sure of it,” he told the Canberra Times. “Something triggered them.”

When approached by News.com.au about the possibility the family was suffering from folie a deux — or a “shared psychotic disorder” as the syndrome is now more commonly known — he said he was keen to learn more.

“This is something to talk to with the doctors at Goulburn where the mother and the daughter are being treated,” he said.

“That’s interesting, very interesting indeed.”

A map of the family’s bizarre, ill-fated road tripSource:The Daily Telegraph

WHAT WE KNOW

• Mark and Jacoba Tromp and their three adult children Mitchell, Ella and Riana leave their Silvan home on Monday, August 29

• The family left in Ella’s grey Peugeot and headed to NSW

• On Tuesday morning, Mitchell leaves the group near Bathurst and makes his way to Sydney and then catches a train to Melbourne

• Ella, Riana and parents make it to Jenolan Caves, about 150km west of Sydney

• At the caves, on Tuesday, the daughters leave their parents and make their way to Goulburn

• Ella and Riana split up at a Goulburn service station because they want to get home different ways

• Riana found under mental stress on the side of a road near Goulburn and stopped by police in her attempts to go home.

• Riana taken to hospital, where she remains

• NSW Police informs Victoria Police about the family

• Ella acquires a car, drives home and arrives at the family’s Silvan farm on Tuesday night while Victoria Police are there

• Mitchell arrives in Melbourne on Wednesday morning. At one point on the trip Mitchell admitted to having his mobile phone, but he voluntarily threw it out the window at Warburton.

• Ella’s Peugeot found in Victoria, in Wangaratta, on Wednesday night

• A man, believed to be Mark, seen running from the car.

• On Thursday, Jacoba Tromp is taken to Yass hospital by a passer-by who sees her in the NSW town

• Extensive searches of Wangaratta fail to find Mark Tromp

WHAT WE DON’T KNOW

• Where Mark Tromp is

• How Mark and his wife became separated

• What the parents’ movements were after daughters left them

• Why the family was “emotionally traumatised”

For those needing help, contact Lifeline on 13 11 14.