UPPSALA, Sweden — A potentially life-threatening virus is on the move in northern Europe. Researchers have discovered that local rodents in Sweden are carrying a pathogen that can jump to humans and turn into hemorrhagic fever. Even more concerning is the fact that doctors are finding cases of this illness hundreds of miles from where health officials typically see this virus. So, the next question is: will this virus spread even further?
The findings, in a nutshell:
In 2018, doctors in southern Sweden’s Scania County were baffled when they diagnosed a case of nephropathia epidemica (NE) – a rare illness that can potentially cause hemorrhagic fever in people. This disease, caused by the Puumala virus carried by bank voles, had never been seen so far south before, over 300 miles (500 km) from its home territory in northern Sweden. Two years later, a second local case emerged, confirming the virus had spread into a new region.
Through genetic testing, published in the journal Emerging Infectious Diseases, researchers made a stunning discovery – the virus infecting voles in Skåne was not the same strain found in northern Sweden. Instead, it was a distinct variant closely related to Puumala viruses from Finland or Russian Karelia, hundreds of miles away. Somehow, this foreign strain of the hemorrhagic fever-causing virus had jumped into southern Sweden’s bank vole population, likely within the last decade or so. While only two human cases have been identified so far, this emerging viral strain represents an uncertain new public health threat whose prevalence and severity must be closely studied.
Hemorrhagic fever refers to a group of viral diseases that affect the body’s vascular system (network of blood vessels) and cause increased fragility of blood vessels, leading to internal bleeding and other complications. The main symptoms of hemorrhagic fevers include sudden fever, bleeding, organ dysfunction, and shock.
Some well-known examples of hemorrhagic fever viruses include Ebola virus, Lassa fever virus, Crimean-Congo hemorrhagic fever virus, Yellow fever virus, and Dengue virus. These viruses are typically transmitted to humans through contact with infected animals or their bodily fluids or through insects like mosquitoes or ticks. Some of these viruses can also spread from person to person through contact with the bodily fluids of infected individuals. Hemorrhagic fevers are often severe and can be life-threatening, especially without proper treatment and supportive care.
How did scientists make the discovery?
To track down the source of the mysterious nephropathia epidemica cases in Skåne, researchers took several key steps. First, they acquired the proper permits to humanely trap rodents in the area near where the first patient lived. Over multiple trips in 2020 and 2021, they caught 74 rodent-like animals, including 48 bank voles — the small reddish-brown rodents known to carry Puumala virus.
Next, they carefully dissected the animals and tested their lung tissues using a special reverse transcription PCR technique designed to detect hantaviruses like Puumala. This screening revealed nine of the 48 bank voles were infected with a distinct strain of Puumala virus. Through genetic sequencing and phylogenetic analysis, the researchers were able to trace this viral strain back to regions of Finland and Russian Karelia rather than the strains normally circulating in northern Sweden.
To gain higher-resolution genetic data, the team also performed advanced RNA sequencing on a subset of the positive samples. This provided full viral genome sequences, allowing deeper analysis of this foreign strain’s origins and relationships to other known Puumala viruses. Comparing the viruses’ genes also revealed unique mutations that could impact how transmissible and dangerous the virus may be. In conjunction with testing the voles’ mitochondrial DNA, this comprehensive genetic evidence enabled the researchers to reconstruct how this novel strain was likely introduced into Sweden’s southernmost rodent populations in recent years.
What do the researchers say?
“We were surprised that such high proportion of the relatively few voles that we caught were actually carrying a hantavirus that makes people ill. And this was in an area more than 500 km south of the previously known range of the virus,” says Elin Economou Lundeberg, an infectious diseases doctor at Kristianstad Central Hospital, who is one of the study’s first authors, in a media release.
“If the virus has existed in the area for a long time and has simply not been discovered, why haven’t more people become ill? Or, has it become established in Skåne recently and only just begun to spread? And how did it get there?” wonders Professor Åke Lundkvist of Uppsala University, a co-author of the study. “Unfortunately the COVID-19 pandemic intervened, which considerably delayed the completion of this study. These findings are very interesting and show how important it is to investigate the causes as quickly as possible when we see an infectious disease in a new geographical area.”