Image caption Depression is common across the world

Depression is the second most common cause of disability worldwide after back pain, according to a review of research.

The disease must be treated as a global public health priority, experts report in the journal PLOS Medicine.

The study compared clinical depression with more than 200 other diseases and injuries as a cause of disability.

Globally, only a small proportion of patients have access to treatment, the World Health Organization says.

Depression is a big problem and we definitely need to pay more attention to it than we are now Dr Alize Ferrari, University of Queensland

Depression was ranked at number two as a global cause of disability, but its impact varied in different countries and regions. For example, rates of major depression were highest in Afghanistan and lowest in Japan. In the UK, depression was ranked at number three in terms of years lived with a disability.

Dr Alize Ferrari from the University of Queensland's School of Population Health led the study.

"Depression is a big problem and we definitely need to pay more attention to it than we are now," she told BBC News.

"There's still more work to be done in terms of awareness of the disease and also in coming up with successful ways of treating it.

"The burden is different between countries, so it tends to be higher in low and middle income countries and lower in high income countries."

Policy-makers had made an effort to bring depression to the forefront, but there was a lot more work to be done, she added.

"There's lots of stigma we know associated with mental health," she explained.

"What one person recognises as disabling might be different to another person and might be different across countries as well, there are lots of cultural implications and interpretations that come in place, which makes it all the more important to raise awareness of the size of the problem and also signs and how to detect it."

The data - for the year 2010 - follows similar studies in 1990 and 2000 looking at the global burden of depression.

Commenting on the study, Dr Daniel Chisholm, a health economist at the department for mental health and substance abuse at the World Health Organization said depression was a very disabling condition.

"It's a big public health challenge and a big problem to be reckoned with but not enough is being done.

"Around the world only a tiny proportion of people get any sort of treatment or diagnosis."

The WHO recently launched a global mental health action plan to raise awareness among policy-makers.