The virus that causes herpes is likely the main cause of multiple sclerosis (MS), a groundbreaking study has suggested.
Harvard academics found that the Epstein-Barr virus (EBV) - one of the most common viruses in the world - increases the chance of multiple sclerosis 32-fold.
Scientists have long speculated that the virus may be linked to the condition, but have been unable to provide any evidence to back this up.
“This is the first study providing compelling evidence of causality,” said Alberto Ascherio, professor of epidemiology and nutrition at Harvard Chan School and senior author of the study.
“This is a big step because it suggests that most MS cases could be prevented by stopping EBV infection, and that targeting EBV could lead to the discovery of a cure for MS.”
There is currently no cure for multiple sclerosis (MS), which is a condition that affects around 110,000 people in the UK.
The National Multiple Sclerosis Society says the risk of developing MS in the general population is around one in 1,000.
MS is a chronic inflammatory disease of the central nervous system that attacks the myelin sheaths protecting neurons in the brain and spinal cord, and its cause remains elusive.
The symptoms are unpredictable and can range from physical disabilities like mobility problems to mental health conditions, such as depression.
Many people with MS report feeling an overwhelming sense of exhaustion, making simple tasks a struggle. In around a quarter of cases, the first noticeable symptom is a problem with one of the patient's eyes, including colour blindness and double vision.
In contrast, Epstein-Barr Virus infection is astoundingly common, and can be found in around 95 per cent of people.
EBV infection can leave people feeling tired and sore, and is also known as mono or herpes. After a person contracts the pathogen, it remains in their system forever, often without any symptoms but occasionally reactivating.
Prof Daniel Davis, professor of immunology at the University of Manchester, who was not involved with the research, said: “Well over 9 in 10 people are infected with this virus worldwide, usually in childhood, and only very rarely does a problem arise.
“We already knew that this virus increases the risk of some cancer types, and now we know that it is also possibly a factor in multiple sclerosis, although it’s important to note for most people that have the virus, it will not cause them any problems.
“Crucially, we do not know why only a small fraction of people infected with this virus develop a problem.”
He added that other factors will be at play, such as genetics, and a cure may only be found once these have been identified.
The US-based researchers studied the medical records of more than ten million people enlisted in the US military, of which 955 developed multiple sclerosis during their period of service.
Blood samples taken twice a year by the military were analysed by the researchers to determine if the person had the virus, and this was cross-referenced against a later MS diagnosis, which normally only begins to manifest ten years after the identification of EBV.
The risk of a person was found to be 32 times greater in people who tested positive for the herpes virus than in the minority of people who do not have EBV.
A protein and biomarker for nerve degeneration called NfL only increased in those who had been infected with the virus, the researchers write in their study, published in the journal Science.
Their findings cannot be explained by any other known risk factors for MS and suggest EBV may therefore be the leading cause.
Professor Ascherio said: “Currently there is no way to effectively prevent or treat EBV infection, but an EBV vaccine or targeting the virus with EBV-specific antiviral drugs could ultimately prevent or cure MS.”