Transmission of drug-resistant typhoid is more local than global

Rising drug resistance, where currently available treatments for an illness are no longer effective, is a major public health concern. A myriad of research has documented increased rates of drug-resistant typhoid throughout the globe. This is a worrying trend; many countries with high drug-resistant typhoid burdens are least likely to have robust health systems with final-line antibiotics available. Many of these studies have focused on the spread of drug-resistant typhoid between countries and continents. But what about drug-resistant typhoid infections from local transmission? In a new study, we set out to quantify the relative proportion of drug-resistant typhoid infections attributable to local transmission of resistant strains versus imported strains.

Transmission and drug-resistance patterns in Bangladesh, Malawi, and Nepal

Our study sequenced samples from Dhaka, Bangladesh, Blantyre, Malawi, and Kathmandu, Nepal. Analysis found that drug resistance was common at all sites. Resistance to first-line antimicrobials was a striking 98% in Blantyre; quinolone-resistant strains were similarly shocking at 99.8% in Dhaka and 89% in Kathmandu. The high levels of resistance to these types of antibiotics are not surprising; they are commonly used in these countries for treatment.

Following further analysis, our research showed that most cases of drug-resistant typhoid came from pre-existing, locally-established strains. Nearly all drug-resistant typhoid infections resulted from local circulation of drug-resistant strains, not imported variants or recent new emergence. These variants persisted throughout more than 2 years of surveillance, causing infections across all age groups.

This study, in simple terms, shows that drug-resistant typhoid is being spread mostly within the community. Drug-resistant typhoid is infrequently spread by from travelers or further evolution of drug-resistance.

These results are the first to show the extensive nature of locally-transmitted drug-resistant typhoid infections. They also shows that while the disease burden is highest in children, the same typhoid variants circulate across age groups. This finding supports the idea that vaccinating children through routine immunization programs may also reduce transmission in older age groups.

Data for decision-making

The data from this study shows that the vast majority of typhoid cases at all three study sites was due to locally-established variants. Furthermore, drug-resistant infections were overwhelmingly caused by transmission of local drug-resistant strains. This study highlights the importance of decision-makers taking strategic action to protect communities in their country based on the transmission dynamics of circulating pathogen variants. It is crucial that decision-makers act for what is needed in their own country.

These results support the hypothesis that interrupting transmission through typhoid conjugate vaccine (TCV) introduction in children would not only protect kids but may also help to reduce drug-resistant typhoid infections across a country’s entire population. Each prevented drug-resistant typhoid case also prevents further spread of these strains, decreasing cases and therefore the need for antibiotics for treatment. TCVs are one of the most effective ways to prevent typhoid infections. They can also slow the emergence and spread of drug-resistant typhoid strains, preserving treatment options for those who get sick.

Photo: PATH