Puumala virus
The Puumala virus is a fascinating example of how tiny creatures can carry potent threats across vast landscapes. This silent invader, hosted by the seemingly innocuous bank vole, is a major cause of a kidney-affecting illness across Europe and Russia. Its journey from a rodent's breath to a human's bloodstream reveals a hidden world of viral ecology and public health challenges. Puumala virus, carried by bank voles, is the primary cause of nephropathia epidemica, a mild form of hemorrhagic fever with renal syndrome, across Europe. The virus's segmented RNA genome allows for unique evolutionary strategies like reassortment, impacting its diversity across various European lineages. Transmission to humans occurs primarily through inhalation of aerosolized rodent excretions, with disease incidence fluctuating with bank vole populations and seasonal human-rodent interactions.
AI Summary
The Puumala virus is a fascinating example of how tiny creatures can carry potent threats across vast landscapes. This silent invader, hosted by the seemingly innocuous bank vole, is a major cause of a kidney-affecting illness across Europe and Russia. Its journey from a rodent's breath to a human's bloodstream reveals a hidden world of viral ecology and public health challenges.
- Puumala virus, carried by bank voles, is the primary cause of nephropathia epidemica, a mild form of hemorrhagic fever with renal syndrome, across Europe.
- The virus's segmented RNA genome allows for unique evolutionary strategies like reassortment, impacting its diversity across various European lineages.
- Transmission to humans occurs primarily through inhalation of aerosolized rodent excretions, with disease incidence fluctuating with bank vole populations and seasonal human-rodent interactions.
The Unseen Threat from the Wild
We're talking about Puumala virus, or PUUV, a stealthy pathogen that finds its primary home in a creature you might easily overlook: the bank vole. While often mild, this virus is the leading cause of a specific illness known as nephropathia epidemica in human populations across Europe and Russia. It's a classic example of a zoonotic disease, silently bridging the gap between animals and humans.
The bank vole, a small rodent, is PUUV's natural reservoir. For the vole, the infection is generally persistent and asymptomatic, meaning they carry the virus without showing much sign of illness. They shed the virus throughout their lives through excretions like saliva, urine, and feces, as well as through direct contact during fighting or grooming.
Humans usually contract PUUV by inhaling tiny aerosolized particles contaminated with these rodent excretions. Imagine sweeping a dusty shed where voles have been, or simply spending time in areas they inhabit—that's often enough. Less commonly, bites or scratches from an infected vole can also transmit the virus.
A Mild but Memorable Illness
In humans, PUUV infection can be completely symptom-free in a significant number of cases. However, for those who do develop symptoms, it manifests as nephropathia epidemica, a milder form of hemorrhagic fever with renal syndrome. This disease primarily targets the kidneys, but its reach extends further, creating a distinctive clinical picture.
Initial symptoms often resemble a bad flu: fever, headache, and lower back pain. But then, things get specific. Patients might experience impaired vision, the appearance of small red spots on the skin due to tiny hemorrhages, and critically, kidney involvement. This includes swelling of the kidneys, excess protein or blood in the urine, and even a temporary decrease in urine production, signaling renal dysfunction.
Unraveling the Viral Code
At its core, the Puumala virus is a master of miniature design. Its genome is relatively small, about 12 kilobases long, and uniquely segmented into three negative-sense, single-stranded RNA pieces. This segmentation is key to its adaptability and allows for interesting evolutionary pathways.
Each of these three RNA segments carries vital instructions for the virus's survival and replication. The smallest segment codes for the nucleoprotein, which encapsulates the genome, and a non-structural protein that cleverly interferes with the host's immune response by blocking interferon production. The medium segment dictates the production of the viral spike proteins, crucial for cell entry, and the long segment carries the blueprint for the RNA-dependent RNA polymerase, the enzyme responsible for copying the viral genome.
A Spiky Shield
Imagine a tiny, roughly spherical particle, ranging from 80 to 160 nanometers in diameter. This is the Puumala virion. It's enveloped in a lipid membrane, stolen from the host cell, and studded with prominent spike proteins. These spikes, composed of two viral glycoproteins named Gn and Gc, are arranged in a lattice pattern, forming a crucial interface for infection.
The Viral Life Cycle: An Invasion
The Puumala virus begins its invasion by latching onto specific receptors on the surface of host cells, primarily endothelial cells and macrophages, using its glycoprotein spikes. These receptors are often β3-integrins, common proteins involved in cell adhesion. Once bound, the cell mistakenly engulfs the virion in a process called endocytosis, forming a small bubble called an endosome.
Inside the endosome, a critical change occurs: the pH drops. This acidic environment triggers the viral envelope to fuse with the endosome's membrane, much like two soap bubbles merging. This fusion releases the viral RNA genome directly into the host cell's cytoplasm, signaling the start of viral replication.
The viral RNA-dependent RNA polymerase then gets to work, transcribing the negative-sense RNA genome into messenger RNA (mRNA) that the host cell's ribosomes can translate into viral proteins. It also creates positive-sense RNA strands, which serve as templates for producing new copies of the viral genome. New viral components are synthesized, assembled at the endoplasmic reticulum, and bud off, acquiring their protective lipid envelope in the process.
Once fully assembled, these new virus particles, or virions, are transported within cellular vesicles to the host cell's membrane. From there, they are released to infect neighboring cells or shed into the environment, ready to continue their cycle of transmission. This meticulous process ensures the virus's propagation while minimizing immediate harm to the host cell, allowing for persistent infection.
Tracing the Virus's Footprints
The Puumala virus might have been officially identified in 1980, but its impact on humans stretches back much further. A notable epidemic struck soldiers in Finland during World War II, leading to the first description of the disease, then named "nephropathia epidemica." Soldiers living in harsh conditions, often in forests with high vole populations, became unwitting hosts.
The actual virus was isolated from bank voles in Finland by Markus Brummer-Korvenkontio and Antti Vaheri. They used advanced immunofluorescent antibody tests, similar to those that led to the discovery of the Hantaan virus. The name "Puumala" was chosen from a Finnish municipality, a simpler alternative to another proposed name, "Pieksämäki."
A European Story
Puumala virus is found throughout Europe and Russia, closely mirroring the habitat of its bank vole host. Over time, scientists have identified at least eight distinct genetic lineages of PUUV, each typically associated with a specific geographic region. These lineages offer a glimpse into the virus's ancient history.
It's thought that these distinct lineages emerged thousands of years ago, when bank voles were isolated in different "refuges" during the Last Glacial Period. As the ice receded and the climate warmed, these voles, carrying their unique viral strains, recolonized much of Eurasia, spreading the genetic diversity we see today.
The Shifting Sands of Evolution
Like all viruses, PUUV evolves through mutations, small changes in its genetic code. But because of its segmented genome, it has another powerful evolutionary trick: reassortment. This is where different viral segments from two distinct lineages can mix within a single infected cell, creating a brand-new hybrid virus.
Ecology: The Vole, The Virus, The Human
The ecology of Puumala virus is intricately tied to the population dynamics of the bank vole. These vole populations famously fluctuate on a 3-4 year cycle, influencing the prevalence of PUUV in the environment and, consequently, the number of human infections. High vole numbers often mean higher human risk.
Environmental factors also play a huge role. In northern regions, human infections peak in autumn and early winter, partly because voles seek refuge in human dwellings as their outdoor food supply dwindles. In more temperate zones, spring and summer see higher transmission, often linked to seasonal activity patterns of both voles and humans.
Living with the Risk
Fortunately, recovering from a Puumala virus infection likely grants lifelong immunity, as repeated infections are rarely observed. While no vaccine currently exists, the most effective prevention strategy is straightforward: minimize contact with rodents and their excretions, especially in areas where bank voles are common. Good hygiene and caution in forested areas are key.
Article
Puumala virus
Puumala virus (PUUV) is the main cause of hemorrhagic fever with renal syndrome (HFRS) in Europe and Russia. Puumala virus is transmitted by the bank vole (Clethrionomys glareolus). In its natural reservoir, PUUV causes a persistent infection with few symptoms and is spread through excretions, fighting, and grooming. Humans can become infected by inhaling aerosols that contain rodent saliva, urine, or feces, as well as through bites and scratches. In humans, infection is usually asymptomatic but can lead to a mild form of HFRS often called nephropathia epidemica (NE). Symptoms include fever and headache, impaired vision, as well as the appearance of spots on the skin and renal symptoms such as kidney swelling, excess protein in urine, blood in urine, decreased urine production, and kidney failure. The case fatality rate from infection is less than 1%.
The genome of PUUV is about 12 kilobases (kb) in length and segmented into three negative-sense, single-stranded RNA (-ssRNA) strands. The small strand encodes the viral nucleoprotein, the medium strand encodes the viral spike protein, which attaches to cell receptors for entry into cells, and a non-structural protein that interferes with interferon production, and the long strand encodes the viral RNA-dependent RNA polymerase (RdRp), which replicates and transcribes the genome. Genome segments are encased in nucleoproteins to form ribonucleoprotein (RNP) complexes that are surrounded by a viral envelope that contains spikes emanating from its surface.
Puumala virus replicates first by binding to the surface of cells with its envelope spikes. Virus particles, called virions, are then taken into the cell by endosomes, where a drop in pH causes the viral envelope to fuse with the endosome, which releases viral RNA into the host cell. RdRp then transcribes the genome for translation by host cell ribosomes and produces copies of the genome for progeny viruses. New virions are assembled at the endoplasmic reticulum and bud from its surface to obtain their viral envelope. Progeny viruses are then transported by a cellular vesicle to the cell membrane, where they leave the cell by exocytosis.
PUUV was discovered in 1980 after being extracted from bank voles. The virus was subsequently linked to a past outbreak among soldiers in Finland during World War Two, when the disease received the name nephropathia epidemica. A similar outbreak occurred among soldiers in Croatia during the Balkan Wars. PUUV is found throughout Europe and Russia where the vole bank lives. Finland is the country most affected by PUUV and experiences thousands of infections each year. While infection is typically associated with HFRS, Puumala virus can cause hantavirus pulmonary syndrome on rare occasions.
Genome
Puumala virus
The genome of Puumala virus is about 12 thousand nucleotides in length and segmented into three negative-sense, single-stranded RNA (-ssRNA) strands. The segments form into circles via non-covalent bonding of the ends of the genome. The small segment, about 1.83 kilobases (kb) in length, encodes the viral nucleoprotein and a non-structural protein that inhibits interferon production. The medium segment, about 3.65 kb in length. encodes a glycoprotein precursor that is cleaved into the two spike proteins Gn and Gc during virion assembly. The large segment, about 6.55 kb in length, encodes the viral RNA-dependent RNA polymerase (RdRp), which is responsible for transcribing and replicating the genome. The ends of each segment contain untranslated terminal regions (UTRs) that are involved in the replication and transcription of the genome.
Structure
Puumala virus
Virions are mostly spherical or pleomorphic in shape and range from 80 to 160 nm in diameter. They contain a lipid envelope covered in spike proteins made of the two viral glycoproteins, Gn and Gc. The spike proteins extend about 10 nm out from the surface and are tetrameric, consisting of four copies each of Gn and Gc with helical symmetry, in which Gn forms the stalk of the spike and Gc the head. Spikes are arranged on the surface in a lattice pattern. Inside the envelope are the three genome segments, which are encased in nucleoproteins to form a ribonucleoprotein (RNP) complex. Attached to each RNP complex is a copy of RdRp.
Life cycle
Puumala virus
PUUV primarily infects endothelial cells and macrophages. It enters cells by using β3-integrins as receptors. Virions are taken into a cell via an endosome. Once pH is lowered, the viral envelope fuses with the endosome, which releases viral RNA into the host cell's cytoplasm. The small segment is transcribed by RdRp first, then the medium segment, and lastly the large segment. Once the genome has been transcribed, RdRp snatches caps from host messenger RNA (mRNA) to create viral mRNA that is primed for translation by host ribosomes to produce viral proteins.
For replication of the genome, a complementary positive-sense strand is produced by RdRp. Copies of the genome are made from this complementary strand. Progeny RNA strands are then encapsidated by nucleoproteins. During replication, the glycoprotein is cleaved in the endoplasmic reticulum by the host signal peptidase during translation. This produces Gn at the N-terminus and Gc at the C-terminus of the protein. Spike proteins are expressed on the surface of the endoplasmic reticulum. Viral RNPs are transported to the endoplasmic reticulum where they bud from the surface, thereby obtaining their envelope. Progeny viruses are then transported by a cellular vesicle to the cell membrane, where they leave the cell via exocytosis.
Diversity
Puumala virus
There are eight lineages of PUUV: Central European, Alpe-Adrian, Danish, South-Scandinavian, North-Scandinavian, Finnish, Russian, and Latvian. The Central European lineage is found in France, Belgium, Germany, Slovakia, and the Netherlands; Alpe-Adrian in Austria, Slovenia, Croatia, and Hungary; Danish in Denmark; South-Scandinavian in Norway and southern Sweden; North-Scandinavian in northern Sweden; Finnish in Finland, Karelia, and western Siberia; Russian in Estonia, Latvia, and central Russia; and Latvian in Latvia and northeast Poland. These lineages likely emerged from isolation of bank voles in glacial refuges during the last glacial period, 28–23 thousand years ago. After the last glacial period, bank voles recolonized much of Eurasia and carried the different lineages with them.
Evolution
Puumala virus
The most common way that hantaviruses evolve is through mutations of individual nucleotides being inserted, deleted, or substituted. Because Puumala virus has a segmented genome, it is possible for recombination and reassortment of segments to occur, whereby segments from different lineages mix in a single host cell and produce hybrid progeny. This has been observed in Europe, where different lineages of Puumala virus that have overlapping ranges frequently reassort. These reassorted Puumala viruses are not competitively superior to Puumala viruses that have not undergone reassortment. The M segment of the genome is the segment most commonly involved in reassortment.
Ecology
Puumala virus
The bank vole, the natural reservoir of Puumala virus.
Puumala virus is carried by bank voles (Clethrionomys glareolus), found throughout Europe and Russia. Incidence of NE changes based on the bank vole population, which fluctuates on a 3–4 year cycle. Prevalence of PUUV in bank voles is highest in late autumn during increase and peak years. In more northern environments, bank vole populations fluctuate primarily due to predation, whereas in more temperate environments fluctuation is driven by deciduous tree masting. PUUV transmission and PE incidence likewise vary based on geography: transmission in boreal zones is greatest in autumn to early winter and in temperate zones greatest in spring to summer. During winter, the food supply of bank voles declines significantly, which leads them to enter human dwellings in search of food. This increases the likelihood of transmission to humans during winter in colder climates.
In its natural reservoir, PUUV causes a persistent and mainly asymptomatic infection. Wild bank voles shed the virus for the rest of their life after they become infected. High levels of maternal antibodies in newborns may delay the time at which they get infected, which indirectly delays the incidence of human infections in various years. Bank voles infected with PUUV have a lower winter survival rate than uninfected ones, possibly due to effects on metabolism. PUUV infection also affects the body condition of the mother, which affects the body condition of offspring. Transmission of Puumala virus occurs through inhalation of aerosols that contain saliva, urine, or feces, as well as through contaminated food, bites, and physical contact with bank vole excretions. Antibodies to Puumala virus have been detected in cattle, deer, and rabbits, but the role of these animals as hosts is unknown.
Disease
Puumala virus
Puumala virus infection is subclinical 70–80% of the time, with no apparent symptoms. When symptoms appear, PUUV usually causes a mild form of hemorrhagic fever with renal syndrome (HFRS) often called nephropathia epidemica (NE). Symptoms usually occur 2–6 weeks after exposure and come in five phases: fever, hypotension, low urine production, high urine production, then recovery. HFRS is hallmarked by acute kidney disease with kidney swelling, excess protein in urine, and blood in urine. Early symptoms include fever, headache, lower back pain, nausea, vomiting, diarrhea, bloody stool, and the appearance of spots on the skin. During the hypotensive phase, there is a sudden lowering of blood pressure and shock due to microvascular leakage. Low urine production then occurs as a result of renal failure. As renal function recovers, urine production increases. In addition to standard HFRS symptoms, PUUV infection frequently causes ocular and central nervous symptoms during the early phase of the disease, including blurry vision, near-sightedness, vertigo, and rarely encephalitis. In more mild cases, the different phases of illness may be hard to distinguish, or some phases may be absent, while in more severe cases, the phases may overlap.
PUUV infection is mainly observed in northern and central Europe and Russia, where it is the main cause of HFRS. Incidence of hantavirus infection in Europe has been growing over time and is highest in Finland, where PUUV causes 1,000–3,000 HFRS cases yearly. The main risk factor for infection is visiting forested and suburban areas and, in the cold season, staying indoors during that time period. Smoking is a risk factor for more severe renal symptoms. Increased amounts of protein, blood, and glucose in urine and increased levels of inflammatory markers such as interleukin-6 are strong predictors of increased disease severity. Symptoms are less severe in children than in adults. The case fatality rate from PUUV infection is low, at 0.08–0.4%, and nearly everyone recovers fully without long term consequences. Death usually only occurs among the elderly. On rare occasions, individual cases and small clusters of hantavirus pulmonary syndrome have been observed in Europe and in Turkey.
PUUV infection is diagnosed based on observation of symptoms and testing for hantavirus nucleic acid, proteins, or hantavirus-specific antibodies. Because of how mild illness caused by PUUV can be, it is often misdiagnosed. Treatment is supportive in nature and includes intravenous hydration, electrolyte therapy, platelet transfusions, and, in cases of kidney injury or failure, intermittent dialysis and continuous renal replacement therapy. No vaccines exist to protect against Puumala virus infection; the main way to prevent infection is to avoid or minimize contact with rodents. Repeated infections of hantaviruses have not been observed, so recovering from infection likely grants life-long immunity.
Classification
Puumala virus
Puumala virus is classified into the species Orthohantavirus puumalaense in the genus Orthohantavirus, which is in the family Hantaviridae, the family that all hantaviruses belong to. Other members of Orthohantavirus puumalaense include Hokkaido virus and Muju virus. The CG1820 isolate of Puumala virus is the exemplar virus of the species. This taxonomy is shown hereafter:
• Family: Hantaviridae • Genus: Orthohantavirus • Species: Orthohantavirus puumalaense • Hokkaido virus, found in grey red-backed voles (Craseomys rufocanus) in Hokkaido, Japan and Siberia, Russia and in northern red-backed voles (Clethrionomys rutilus) in Siberia, Russia • Muju virus, found in royal voles (Craseomys regulus) in South Korea • Puumala virus
History
Puumala virus
In 1942, an epidemic of disease occurred among soldiers in Salla, Eastern Lapland, Finland during the Second World War. Around 1,000 German and 60 Finnish soldiers were affected. Symptoms included fever, headache, abdominal pain, back pain, nausea, and vomiting. Notably, temporarily impaired vision was noted in many patients, as were increased protein in urine, blood in urine, and reduced renal function. The novel disease was named "nephropathia epidemica". Reports at the time suspected leptospirosis and noted that it could not spread from person to person. No cases during the outbreak resulted in death. Both bank voles and Norway lemmings were suspected as having caused the epidemic, but hantaviruses have not been discovered in Norway lemmings, which left just the bank vole, and thus Puumala virus, as likely responsible for the Salla epidemic. Another epidemic occurred in 1995 in Croatia during the Balkan Wars. As with the 1942 epidemic, the outbreak mainly occurred in soldiers who lived in poor conditions such as trenches and wooden huts in forests. This exposed them to rodents that carried the virus. In addition to PUUV, Dobrava virus was also responsible for the 1995 epidemic.
In 1980, Markus Brummer-Korvenkontio and Antti Vaheri tested for hantavirus infection in the lung tissue of bank voles using immunofluorescent antibody tests, a method similar to what was used to discover the first hantavirus, Hantaan virus. Subsequent testing of the Finnish population showed that many Finns had been infected with the virus. Two names were considered for this newly discovered virus, both names of Finnish municipalities: Puumala and Pieksämäki, the former of which was chosen since the latter would be more difficult for English speakers to pronounce. Puumala virus was accepted as a species by the International Committee on Taxonomy of Viruses in 1987 and has undergone a series of changes to its species name, first changing to Puumala hantavirus, then Puumala orthohantavirus, and most recently to the current Orthohantavirus puumalaense.