What Are Bunyavirales? Transmission, Diseases & More

The Bunyavirales order is a large, diverse group of viruses found worldwide. These viruses infect a wide array of hosts, including humans, animals, and plants, giving them public health and economic importance. Their expansive geographic reach and the variety of illnesses they cause make understanding this viral order a subject of ongoing scientific research.

Characteristics of the Bunyavirales Order

In viral classification, an “order” is a high-level group of virus families sharing fundamental properties. The Bunyavirales order is defined by its genetic structure. These are RNA viruses, meaning their genetic material is ribonucleic acid, and their genome is segmented into three separate strands: large (L), medium (M), and small (S). This segmented nature allows for genetic reassortment, a process where viruses can swap segments if two different strains infect the same cell.

This reassortment can contribute to the emergence of new viral variants. These viruses are also enveloped, meaning the virus particle is enclosed in an outer lipid membrane derived from the host cell it infected. Embedded in this envelope are viral glycoproteins, which the virus uses to attach to and enter new host cells.

Bunyaviruses replicate within the cytoplasm of the host cell, using their own RNA-dependent RNA polymerase enzyme to copy their genome. The L segment of the genome encodes this polymerase, the M segment codes for the surface glycoproteins, and the S segment codes for the nucleoprotein that protects the RNA. New virus particles assemble at the Golgi apparatus before they are released from the cell.

Transmission and Vectors

Most viruses in the Bunyavirales order are transmitted to humans by vectors, which are organisms that carry and pass on an infectious agent. These vectors are primarily arthropods and rodents. Arthropod-borne viruses, often called arboviruses, are spread through the bites of insects such as mosquitoes, ticks, and sandflies. These insects acquire the virus by feeding on an infected animal, a reservoir host, and then transmit it to others during a subsequent blood meal.

Different bunyaviruses are associated with specific arthropod vectors. For instance, Rift Valley fever virus is transmitted by mosquitoes, especially those from the Aedes and Culex genera. Crimean-Congo hemorrhagic fever virus is carried by ticks, particularly those of the Hyalomma genus. The transmission cycle is maintained as these arthropods feed on various animals, which often harbor the virus without showing signs of illness.

An exception to arthropod-borne transmission is the Hantaviridae family. Viruses in this family are carried by rodents, such as deer mice, cotton rats, and rice rats. Humans become infected not through a bite, but by inhaling aerosolized particles contaminated with the urine, droppings, or saliva of infected rodents. Activities that disturb these materials, like cleaning sheds or cabins, can release the virus into the air.

Significant Human Pathogens and Associated Diseases

Several viruses in the Bunyavirales order cause severe human diseases. Hantaviruses, for example, are responsible for two main clinical syndromes depending on the virus and location. In the Americas, Sin Nombre virus and related hantaviruses cause Hantavirus Pulmonary Syndrome (HPS). This illness begins with flu-like symptoms but can rapidly progress to severe respiratory distress as fluid fills the lungs. In Europe and Asia, other hantaviruses cause Hemorrhagic Fever with Renal Syndrome (HFRS), characterized by high fever, hemorrhage, and acute kidney injury.

Another pathogen is the Crimean-Congo hemorrhagic fever virus (CCHFV). This virus causes a severe and often fatal illness known as Crimean-Congo hemorrhagic fever (CCHF). The disease has a sudden onset with symptoms that include:

• High fever
• Headache
• Joint pain
• Vomiting

As the illness progresses, it can lead to a severe hemorrhagic state, with patients exhibiting bruising, nosebleeds, and uncontrolled bleeding. The mortality rate for CCHF is high in endemic areas.

Rift Valley fever virus (RVFV) also impacts both human and animal health. In humans, RVFV infection results in a relatively mild, flu-like illness, though a small percentage of individuals can develop severe complications. These can include ocular disease leading to vision loss, meningoencephalitis (inflammation of the brain and surrounding tissues), or a hemorrhagic fever syndrome. The virus also causes economic losses due to high rates of death and abortion in infected livestock, such as sheep, goats, and cattle.

Geographic Distribution and Outbreaks

The geographic range of bunyaviruses is tied to the habitats of their specific vectors and reservoir hosts, so the diseases they cause are often concentrated in particular regions. For example, Crimean-Congo hemorrhagic fever is found in Africa, the Balkans, the Middle East, and parts of Asia. Hantaviruses are also geographically distinct; HPS is a concern in the Americas, while HFRS is prevalent in Asia and Europe.

Rift Valley fever is endemic to sub-Saharan Africa, with outbreaks periodically occurring on the Arabian Peninsula. Outbreaks of RVFV are often linked to environmental factors. Periods of heavy rainfall and flooding can lead to an increase in the mosquito populations that transmit the virus, triggering widespread transmission among livestock and humans.

The global distribution of these viruses is not static. International travel and trade can introduce viruses to new areas if a suitable vector is present. The emergence of Schmallenberg virus in Europe in 2011, a virus affecting ruminants, demonstrated how quickly a bunyavirus can spread when conditions and host populations are favorable.

Prevention and Treatment Approaches

Preventing bunyavirus infections centers on vector control and avoiding exposure. For arthropod-borne bunyaviruses, this involves personal protective measures such as using insect repellent, wearing long sleeves and pants, and using bed nets. Reducing vector populations around homes by eliminating standing water where mosquitoes breed and clearing brush where ticks reside can also limit transmission risk.

For rodent-borne hantaviruses, prevention focuses on minimizing contact with rodents and their waste. This includes rodent-proofing buildings by sealing holes, setting traps, and storing food in rodent-proof containers. When cleaning areas with evidence of rodent infestation, it is important to avoid stirring up dust by wetting contaminated surfaces with a disinfectant to inactivate the virus.

Currently, there are no specific antiviral drugs approved for most human bunyavirus infections. Medical care is primarily supportive, focused on managing symptoms and complications while the patient’s immune system fights the infection. This can involve fluid management, maintaining oxygen levels, and providing respiratory support. While a few vaccines exist for some bunyaviruses, like Rift Valley fever, they are not widely available for the general human population.