Genomic study sheds light on ‘Typhoid Mary’ carriers
Researchers have carried out the first large-scale genomic study of enteric fever carriage in England and Wales, revealing that the risk of unknowingly spreading the disease appears to depend more on the individual than on the bacteria itself.
The study, led by researchers at the Quadram Institute in collaboration with the UK Health Security Agency (UKHSA) and Public Health Wales, analysed surveillance data collected between 2004 and 2023.
Published using genomic and epidemiological data, the research examined infections caused by Salmonella Typhi and Salmonella Paratyphi, bacteria responsible for enteric fever.
Although the disease is uncommon in the UK, cases have increased since the pandemic, reaching 652 confirmed infections in 2023.
The researchers estimate that around 2.7% of cases involve long-term carriage, where individuals continue to carry and shed the bacteria despite showing no symptoms.
The phenomenon is best illustrated by the case of Mary Mallon, aka ‘Typhoid Mary’, who unknowingly infected numerous people in early 20th-century New York while remaining healthy herself.
Using bacterial genome sequencing alongside demographic and surveillance data, the team investigated whether specific bacterial strains were associated with carriage.
Instead, they found no genetic signature linking particular strains to long-term carriage, suggesting host factors such as immune responses, age and travel history are likely to play a greater role.
The study also found that older patients and those without a recent history of foreign travel were at greater risk of becoming carriers.
Dr Marie Anne Chattaway, unit head of UKHSA's Gastrointestinal Reference Unit, said the work demonstrates the value of integrated surveillance.
"By applying genomic analysis to long-term surveillance data... we have been able to explore carriage at a scale not previously possible," she said.
Alice Nisbet, a postgraduate researcher at the Quadram Institute, said the findings could help guide future research.
"By shifting focus towards examining the immune profiles of carrier patients... future work will help to better characterise this neglected condition," she said.
The researchers believe the findings could help public health agencies better identify asymptomatic carriers and develop more targeted surveillance strategies.
The research was published in PLOS Neglected Tropical Diseases.
Pic: Brano
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