Genomic characterization of dengue circulation in Togo, 2024

Mosquitoes are more than just a summer nuisance; they are efficient couriers for viruses that can make people very sick. One of the most common of these is dengue fever, a disease that causes high fevers and severe pain, and which has been spreading rapidly across the globe. For a long time, scientists knew that dengue existed in Africa, but they often lacked the tools to see exactly which version of the virus was moving around, where it came from, or how it was traveling between countries. Without this detailed map, it is difficult to stop the spread. To understand the virus, researchers look at its genetic code, which acts like a unique fingerprint for every strain. By comparing these fingerprints from different places and times, they can trace the history of an outbreak, much like a detective tracing a family tree to find where a specific branch began. This kind of work is crucial because it tells public health officials not just that an outbreak is happening, but how it started and where it is likely to go next, allowing them to target their efforts more effectively.

In 2024, Togo faced its first officially declared dengue outbreak, part of a larger surge of cases sweeping across the African continent. While health officials in Togo were tracking the number of sick people, they did not know much about the specific virus causing the illness. To fill this gap, a team of scientists from Togo and South Africa joined forces to study the virus directly. They collected blood samples from nearly five hundred people who were suspected of having dengue between May and September of that year. After testing these samples, they found that about 213 people were indeed infected. The researchers then focused their attention on the most promising samples, those with high amounts of virus, to read the full genetic code of the dengue virus inside them. This process allowed them to identify exactly which type of dengue was circulating and to build a detailed picture of how it moved through the country.

The results revealed that the outbreak was caused almost entirely by one specific type of dengue virus, known as DENV-1, with just a single case of a different type, DENV-3. When the scientists compared the genetic code of the virus found in Togo with sequences from around the world, they discovered a clear connection. The virus in Togo was most closely related to a strain that had caused a large outbreak in Tanzania several years earlier. The genetic evidence suggests that this virus did not appear in Togo out of nowhere; instead, it was likely brought into the region from East Africa, possibly as early as 2020, long before the 2024 outbreak became visible. Once the virus arrived, it seems to have circulated quietly, undetected by health systems, for several years before conditions became right for a major explosion in cases.

The study also showed how the virus spread once it was established. The outbreak did not happen all at once across the country; instead, it moved in waves. The southern regions, particularly the capital city of Lomé and its surroundings, saw the first and largest surge of cases. As the epidemic progressed, the virus traveled northward to other districts, with different areas peaking at different times. The researchers found that this spread was helped by two main factors: the weather and human movement. The climate in Togo during 2023 and 2024 was highly favorable for the mosquitoes that carry dengue, creating a perfect environment for the virus to multiply. At the same time, the virus followed the main roads and transport routes, moving with people as they traveled between the busy southern cities and the northern towns. This combination of a welcoming environment and active human travel allowed the virus to establish itself and spread quickly across the nation.

Despite the success of this study in mapping the outbreak, the researchers noted that there are still large gaps in how dengue is monitored across West Africa. Because so few samples were taken from neighboring countries in previous years, it is difficult to say with absolute certainty exactly how the virus moved between borders or if there were other hidden introductions that were missed. However, the work done in Togo has doubled the number of genetic records available for this specific virus strain in the region, providing a much clearer view than existed before. The findings confirm that the 2024 epidemic was the result of a single introduction followed by rapid local expansion, driven by the right mix of climate and human activity. This detailed understanding highlights the need for continuous, high-quality monitoring of the virus across the entire region, so that future outbreaks can be spotted early and stopped before they grow into large epidemics.