India and Nigeria accounted for nearly 42 per cent of almost 500,000 child deaths globally due to diarrhoeal disease in 2015, a Lancet study has said.

The study found that diarrhoea is the fourth leading cause of death for children and responsible for 8.6 per cent of all deaths of children aged under five.

It pointed out that 42 per cent of these deaths occur in India and Nigeria alone.

"Chad and Niger had the highest child mortality rates for diarrhoeal disease, with 594 and 485 deaths per 100,000 children each year, respectively.

"However, the largest number of deaths occurred in India and Nigeria, contributing 42 per cent of all 499,000 child deaths in 2015 (105,000 and 103,000 deaths, respectively)," the new Global Burden of Disease study published in The Lancet Infectious Diseases journal said.

It also noted that the number of child deaths caused by diarrhoea reduced by a third between 2005-2015, but mortality rates remain highest in some of the world's poorest countries, with diarrhoea killing almost half a million children under five each year worldwide.

"Diarrhoeal diseases disproportionately affect young children. 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 health care access and the use of oral rehydration solutions," said lead author Ali Mokdad, Institute for Health Metrics and Evaluation (IHME) at the University of Washington, USA.

The study estimates that, in 2015, there were 2.39 billion episodes of diarrhoeal disease globally, with 957.5 million of these being in children.

There were a total of 1.31 million deaths as a result of diarrhoea globally in 2015, including 499,000 child deaths.

While mortality rates have reduced substantially (by 20.8 per cent overall and 34.3 per cent in children), the incidence of the disease has not reduced nearly as fast (5.9 per cent reduction overall and 10.4 per cent reduction in children).

"As a result, diarrhoea still causes a significant amount of disability that mostly affects children under five," the study said.

It noted that diarrhoeal diseases are most common in low-income countries with poor access to clean water, sanitation, and urgent medical care, but are also a frequent cause of hospitalisation in high-income countries making diarrhoea an important health problem globally.

Globally, unsafe water and sanitation were still the leading risk factors for diarrhoea in 2015.

However, better access to clean water, improved sanitation, and fewer cases of malnutrition are likely to be responsible for the reductions in mortality rates for children aged under five, it said.

Rotavirus was the main cause of diarrhoeal death for children, causing 146,000 deaths in 2015 despite mortality rates reducing by 44 per cent since 2005, the study said.

The authors noted that this is the only cause that has seen a reduction and this is likely to be due to the rotavirus vaccine, which had been introduced in 91 countries by March 2017.

Based on this, they suggest that the development of further vaccines may be warranted for example against cryptosporidium, which is the second biggest cause of diarrhoeal death for children under five, causing 60,400 deaths in 2015) and has few treatment options, it said.