A view through a microscope of Salmonella enterica Typhi, which causes typhoid

CDC/Armed Forces Institute of Pathology, Charles N. Farmer

Scientists say a drug-resistant strain of the bacterium that causes typhoid fever may be heading from Asia for Africa, suggesting the need for a shake-up in ways of combating the lethal disease.

The research, published in the journal Science today (24 November), examined the genetic diversity of over 100 strains of the bacterium Salmonella enterica Typhi, revealing its evolutionary history.

Every year 21 million people contract typhoid fever and 200,000 people in the South die from it — mainly in southern Asia, Africa and South America.

One of the study's authors, Philippe Roumagnac, explained that a strain from Asia resistant to chloramphenicol and other recommended antibiotics has become prevalent in several regions, including Africa.

Roumagnac, a molecular biologist at the Max Planck Institute for Infection Biology in Berlin, Germany, said that while using the right antibiotic can reduce the current fatality rate from ten to one per cent, there is a need to develop new vaccines.

"A complementary way to combat typhoid is to promote the development and introduction of new and improved vaccines," he told Scidev.Net.

The study reveals that for millennia the bacterium has lurked in people who show no symptoms, called asymptomatic carriers.

Asymptomatic carriers provide a safe reservoir for the bacteria to multiply and infect other people, indicating that treatment of the acute form of the disease, including vaccination, will not be enough to eradicate typhoid. Further, if an asymptomatic carrier is exposed to antibiotics, the bacteria can acquire resistance.

The results of the study therefore emphasise the importance of an effective treatment regime for typhoid that aims not only to cure the patient, but also to prevent the development of the carrier state and of drug resistance. 

For example, the report shows, the use of fluoroquinolone antibiotics has led to the development of drug-resistant strains, although they have not yet been replaced by more effective drugs.

Clean water, hygiene and good sanitation are an effective prevention against the spread of typhoid, which is present in faeces.