Drug-resistant typhoid has become epidemic in Africa, according to a new study which examined bacteria from 63 countries.

Drug-resistant typhoid has become an invisible epidemic in Africa, scientists have said after an unprecedented probe into the disease.

Writing in the journal Nature Genetics, the team sounded the alarm after sequencing more than 1800 samples of typhoid bacteria from 63 countries.

This revealed the rise of a multi-drug-resistant (MDR) strain called H58, which does not respond effectively to frontline antibiotics.

H58 has recently acquired mutations that blunt newer drugs such as ciprofloxacin and azithromycin, they added.

H58 is "displacing" antibiotic-sensitive versions of the germ, the study warned.

The now-dominant lineage "has emerged and spread throughout Asia and Africa over the last 30 years," it said.

The paper described "an ongoing, unrecognised MDR epidemic within Africa itself."

According to the World Health Organization, about 21 million people are infected with typhoid every year, and an estimated 216,000 to 600,000 die.

The disease is caused mainly by Salmonella typhi bacteria in food or water contaminated with the faeces or urine of infected people.

Symptoms can include fever, headaches, constipation and diarrhoea, rose-coloured spots on the chest, and an enlarged spleen or liver. Symptoms can be mild or severe, and some people can carry the germ yet not fall sick.

Vaccines against S. typhi do exist, although they are not always effective and are not suitable for young children - the group most at risk.

In many developing countries, the disease is controlled through drugs, and lax use of them is helping the resistance problem.

The team said that of the 1,832 S. Typhi samples they analysed, dated from 1992 to 2013, 47 per cent were of the H58 type, confirming that it was now widely disseminated.