Half a million children under the age of five died from diarrhoea-related illnesses in 2015, despite a significant reduction in the number of child deaths from such diseases over the past decade.

The number of deaths fell by 34% between 2005 and 2015 after concerted efforts to improve water and sanitation worldwide, according to a study published in the Lancet Infectious Diseases journal on Friday.

However, 499,000 children under five – and 1.3 million people of all ages – died as a result of diarrhoea in 2015, making it the fourth leading cause of mortality among young children. Diarrhoea was responsible for 8.6% of all deaths among under fives.

The authors of the study, which was funded by the Bill and Melinda Gates Foundation, called for urgent action.

“Diarrhoeal diseases disproportionately affect young children,” said Dr Ali Mokdad of the University of Washington’s Institute for Health Metrics and Evaluation, the lead author.

“Despite some promising reductions in mortality, the devastating impact of these diseases cannot be overlooked. Immediate and sustained actions must be taken to help low-income countries address this problem by increasing healthcare access and the use of oral rehydration solutions.”

Yael Velleman, senior policy analyst on health and hygiene at the NGO WaterAid, said: “It’s always wonderful news when we find out that less children are dying from diarrhoea. But if we’re honest about it, there is still a glass half empty. Half a million deaths of children under five is totally unacceptable, especially when we know these deaths are largely preventable.”

Unsafe water and sanitation are the main causes of diarrhoea, the effects of which are often compounded by a lack of access to medical care.

While deaths from diarrhoea dropped by 34.3% among young children – and 20.8% overall – between 2005 and 2015, there has not been a corresponding fall in the prevalence of the condition. Over the same period, cases have fallen just 5.9% for all ages and 10.4% for children under the age of five. There were 2.4bn cases of diarrhoea reported globally in 2015.

“We still have children getting very ill and that has a long term impact of their development, their susceptibility to other diseases, like pneumonia, and a major impact on their growth,” said Velleman.

The countries with the highest rates of child mortality for diarrhoeal disease were Chad and Niger, with 594 and 485 deaths respectively per 100,000 children. Due to the size of their populations, the largest number of deaths occurred in India and Nigeria. Between them, these two countries accounted for 42% of all diarrhoea-related deaths among children under the age of five in 2015.

The biggest drop in mortality rates occurred in sub-Saharan Africa, where deaths fell from 445 to 277 for every 100,000 children between 2005 and 2015. The study’s authors cautioned, however, that a lack of data could mean figures are underestimated.

The main cause of death among children who suffered from diarrhoea was rotavirus, which caused 146,000 deaths in 2015. This represented a 44% drop since 2005, probably due to the introduction of the rotavirus vaccine in 91 countries, prompting the researchers to suggest that developing vaccines for other diseases – such as cryptosporidium, the second biggest cause of diarrhoeal death – might be warranted.

While deaths from diarrhoeal illnesses are rare in high-income countries, there was a 15.4% increase in the number of reported cases in such regions over the past decade.

In the UK, 30 children under the age of five died in 2015 and there were roughly 1,500 deaths overall from diarrhoeal illnesses, a reduction of 18.5% since 2005.