Detection of Plasmodium falciparum harbouring pfhrp2 deletions in six border states of Sudan

In the vast, sun-drenched landscapes of sub-Saharan Africa, malaria remains a relentless force, claiming more lives than any other infectious disease. For decades, the frontline defense against this parasite has relied on a simple, portable tool: the rapid diagnostic test. These small plastic strips work like a specialized sensor, designed to detect a specific protein called HRP2 that is produced in abundance by the deadliest form of the malaria parasite, Plasmodium falciparum. When a drop of blood is applied, the presence of this protein triggers a visible line, confirming an infection and allowing for immediate treatment. However, nature is constantly evolving. In recent years, scientists have discovered that some malaria parasites have learned to hide. By deleting the very genes that instruct them to make the HRP2 protein, these parasites become invisible to the standard tests, leading to false negatives and leaving infected people untreated. This biological trickery has been documented in neighboring countries, raising a critical question for Sudan: has this invisible threat crossed the border?

A team of researchers from Sudan's National Malaria Control Program and various scientific institutes set out to answer this question by looking directly at the parasites circulating in six states along the country's international borders. These states—Kassala, White Nile, Sennar, West Darfur, South Darfur, and Khartoum—were chosen because they share boundaries with nations where these gene-deleted parasites were already known to exist. Between July 2019 and December 2020, the team enrolled 523 patients who had come to clinics with fever and were suspected of having malaria. The process began with the familiar sight of a blood smear under a microscope, the traditional gold standard for diagnosis. Every patient who tested positive by microscope then had their blood analyzed with the standard rapid test strips to see if the HRP2 protein was detected. Crucially, the researchers also used a more sensitive molecular technique, a type of genetic testing called PCR, to identify exactly which species of parasite was present in every single sample, regardless of what the rapid test said.

The results painted a clear and concerning picture. Out of the 513 patients confirmed to have P. falciparum through genetic testing, the standard rapid test correctly identified 477 of them. However, in 36 cases, the rapid test failed to show a positive result, even though the genetic test proved the parasite was there. This meant that roughly one in every fourteen infections was being missed by the primary diagnostic tool. The researchers then investigated these 36 "invisible" parasites to see if they had indeed lost the HRP2 protein. They found that 11 of these parasites had completely deleted the gene responsible for making the protein, rendering them undetectable. The remaining 25 had partially deleted the gene, removing specific sections of the genetic code. This confirmed that the parasites were not just rare anomalies but a present and measurable reality within Sudan's border regions.

To understand how these invisible parasites were spreading, the team examined the genetic makeup of the deleted genes, essentially creating a family tree to see if the parasites in different states were related. They discovered a wide variety of genetic patterns, with no single group dominating a specific area. The parasites from Khartoum were not clustered together, nor were those from the Darfur region. Instead, the different genetic types were mixed together across the map. This pattern suggests that these parasites are not the result of a single introduction event, but rather that they have been crossing the borders multiple times and circulating widely among the population. The study did not find evidence that these gene deletions were linked to a specific gender or age group, nor did it find that the frequency of missed diagnoses varied significantly from one state to another, though the numbers were slightly higher in Khartoum and lower in White Nile.

The implications of these findings are direct and urgent for public health. The study provides molecular proof that P. falciparum parasites capable of evading the standard rapid diagnostic tests are already established in Sudan. Because the country relies heavily on these tests to guide treatment, the presence of these "invisible" parasites poses a significant risk of misdiagnosis, which could allow the disease to spread unchecked. The researchers emphasize that this is not a problem limited to a single village or a fleeting moment in time; the genetic diversity found suggests a complex, ongoing exchange of parasites across the region. To protect the progress made in malaria control, the study calls for a national strategy that includes monitoring for these gene deletions and considering the use of alternative diagnostic tools that do not rely solely on the HRP2 protein. Without adapting their detection methods, health systems risk losing their ability to see the enemy, leaving patients vulnerable to a disease that has learned to hide in plain sight.