Image copyright CDC/ Melissa Brower/SCIENCE PHOTO LIBRARY

Antibiotic-resistant typhoid is spreading across Africa and Asia and poses a major global health threat, warn experts.

Researchers from the Wellcome Trust, who have been tracking the hard-to-treat infection, say it is replacing regular typhoid in many countries.

They analysed bacterial samples from 63 countries - nearly half were impervious to standard antibiotic treatments.

Over-reliance on these drugs is to blame, they say in Nature Genetics.

Increasingly, doctors now need to use other, more expensive and less readily available antibiotics to treat typhoid fever - a disease that kills around 200,000 people each year.

Creeping resistance

The Wellcome Trust work is the largest snapshot of what is happening globally to typhoid resistance.

It shows the problem is widespread.

In many parts of Asia and Africa, where typhoid is endemic, a multidrug-resistant strain of typhoid called H58 has displaced other typhoid strains that have been around for centuries.

Of the 63 countries the researchers looked at, 21 had H58.

Image copyright Science Photo Library Image caption Typhoid growing in the lab

Dr Kathryn Holt, from the University of Melbourne and one of the study authors, said H58 was gaining a firm foothold.

"Multidrug-resistant typhoid has been coming and going since the 1970s and is caused by the bacteria picking up novel antimicrobial resistance genes, which are usually lost when we switch to a new drug.

"In H58, these genes are becoming a stable part of the genome, which means multiple-antibiotic resistant typhoid is here to stay."

The researchers say the global creep of H58 needs urgent international attention.

A third of the world's population is at risk of getting typhoid.

It is caused by the highly contagious bacterium Salmonella typhi, spread from person to person by food and water that is contaminated with traces of infected faeces or urine.

Improved water, sanitation and hygiene could dramatically cut the number of infections, but the investment needed to do this may be our of reach for many developing countries.

There are vaccines against typhoid, but they are not available in all parts of the world, nor are they 100% protective.

A spokeswoman for the World Health Organization said: "Limiting the inappropriate use of antibiotics is part of the solution, and surveillance systems to identify antibiotic-resistant bacteria need to be improved to identify hotspots and intensify prevention and control measures."