Herpes virus can increase the likelihood of developing Alzheimer's disease, according to a study which also found that antiviral drugs can significantly reduce the risk of the neurodegenrative disorder. In a research published in the journal Neurotherapeutics in February this year, Taiwanese epidemiologists looked at subjects who suffered severe herpes infection and who were treated aggressively with antiviral drugs.

Scientists from University of Manchester and University of Edinburgh in the UK say that the study shows that herpes simplex virus type 1 (HSV1) leads to an increased risk of developing the disease.

It also supports the viability of a potential way to reduce the risk of Alzheimer's disease.

"This article and two others by different research groups in Taiwan provide the first population evidence for a causal link between herpes virus infection and Alzheimer's disease, a hugely important finding," said Ruth Itzhaki, a professor at the University of Manchester in the UK.

The new findings, published in the Journal of Alzheimer's Disease, argues that the previous study provides the strongest evidence yet for a causal link between herpes infection and Alzheimer's disease.

"I believe we are the first to realise the implications of these striking data on this devastating condition which principally affects the elderly. No effective treatments are yet available," Itzhaki said.

"Almost 30 million people worldwide suffer from it and sadly, this figure will rise as longevity increases," he said. "But we believe that these safe and easily available antivirals may have a strong part to play in combating the disease in these patients. It also raises the future possibility of preventing the disease by vaccination against the virus in infancy," he said.

Most Alzheimer's disease researchers investigate its main characteristics - amyloid plaques and neurofibrillary tangles; however, despite the vast amount of research, the causes of their formation are unknown.

HSV1 infects most humans in youth or later and remains lifelong in the body in dormant form within the peripheral nervous system. From time to time the virus becomes activated and in some people it then causes visible damage in the form of cold sores.

The Taiwanese study identified 8,362 subjects aged 50 or more during the period January to December 2000 who were newly diagnosed with severe HSV infection.

The study group was compared to a control group of 25,086 people with no evidence of HSV infection. Researchers then monitored the development of dementia in these individuals over a follow-up period of 10 years between 2001 and 2010.

The risk of developing dementia in the HSV group was increased by a factor of 2.542. However, when researchers compared those among the HSV cohort who were treated with antiviral therapy versus those who did not receive it, there was a dramatic tenfold reduction in the later incidence of dementia over 10 years.

"Not only is the magnitude of the antiviral effect remarkable, but also the fact that - despite the relatively brief duration and the timing of treatment - in most patients severely affected by HSV1 it appeared to prevent the long-term damage in brain that results in Alzheimer's," said Richard Lathe, from University of Edinburgh.