The parents of a two year-old girl diagnosed with childhood dementia face a race against time to save her as they call for more funding into the rare disease.

Gus and Emily Forrester were told the devastatingnews five months ago that their daughter Leni has a rare genetic neurodegenerative disorder known as Sanfilippo disease.

The family have now revealed they are running out of time for treatment options for the terminal condition which, with no cure, limits life expectancy to anywhere from the teenage years to the early 20s.

Leni, who they describe as "a bundle of joy", looks like any other smiling little girl and according to her parents, loves to make others laugh.

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Learning to walk and talk, every milestone she reaches is even more poignant for her family that usual as they know that within the coming years, she could gradually lose all her physical and cognitive abilities.

Emily and Gus, who spoke exclusively to ITV News, have described the situation as "every parent's worst nightmare".

"All your dreams for your child's future are taken away," Emily old ITV. "To be told that she has this condition, and there is no treatment and no cure and no support… it’s completely earth-shattering."

The disease creates a deficiency of an enzyme which usually breaks down harmful molecules in the body. Without this vital process, these chemicals build up in the body, causing progressive damage to the brain as a person ages.

Although children appear healthy at a young age and initially show signs of typical development, at around the age of three, the irreversible damage usually begins. Overtime, those affected will lose their ability to walk, talk, eat and drink and eventually require around the clock care.

There is currently no cure and no UK-approved treatment.

Emily said: "Every day that passes without treatment, this toxic waste is building up in our child’s body."

"If we can’t get treatment, she will suffer the most awful physical and mental decline you can imagine and then die in her early to mid-teens."

However, a clinical trial for a new treatment for the disease will begin in the US later this year. Leni's parents are urging the UK government to help fund the study so that children in the UK can participate.

Emily told ITV news that intervening in the condition early is the key to giving Leni a chance and that the "science" behind a cure is just out of reach for them.

"Early treatment is key for these children. The damage cannot be reversed once it’s done."

"If she has to wait six months, that could mean she can no longer talk. If she waits 12 months, that could mean she loses the ability to walk," she added.

"It’s extremely frustrating because the science is there. The data is there. It’s proven to be effective, and yet we cannot access it."

Professor Brian Badger of the University of Edinburgh has developed a therapy which he believes could help young sufferers of Sanfilippo disease by directly introducing a missing gene into the patient's blood stem cells.

But beginning a clinical trial of the therapy is difficult as it requires significant funding, which is why, the Professor says, government investment is vital.

"We’re racing against time,” he told ITV News. "She will start to lose brain cells and when that happens we’re not going to get them back."

"Charities typically can’t fund this kind of thing. It would be really good if we could see more commitment from government towards these kinds of therapies. There are hundreds of kids like Leni out there who don’t have any therapy, and these treatments have the potential to be transformative."

He added: "All of us have a family member who’s been affected by dementia. Childhood dementia is no different. It’s just much, much worse - because it’s your kid."

Leni's parents are also calling for wider newborn screening to detect a range of rare genetic conditions such as Sanfillipo and urge the government to put more money behind research into treatments.

Dad Gus said: As parents, your role is to protect your children and provide every opportunity you can. Without any treatment, her future and her reality is very, very dark."

"When you group all these rare conditions together, they suddenly don’t become that rare. They affect a lot of people and a lot of families."

For Mum Emily, their battle for their daughter's future feels "unfair".

She told ITV News: "We shouldn’t have to fight for our child’s life She is as valuable as any other child. But no one is fighting for her."

A Department of Health and Social Care spokesperson told ITV News: "Our thoughts are with all those living with dementia and rare conditions including Sanfilippo syndrome.

"We are making sure patients with rare diseases, like Sanfilippo syndrome, get a definite diagnosis faster, while improving access to specialist care, treatment and drugs.

"At the same time we are working hard to find new ways to slow down the progress of the dementia, speed up diagnosis and improve our understanding of the disease."

The full report will be available to watch on ITVX and ITV Evening News on Monday 23rd March.