Orthobunyaviruses are a large genus of viruses in the Peribunyaviridae family. They are RNA viruses, with their genetic material organized into three segments inside a roughly spherical structure that includes a protective outer lipid envelope. With over 170 identified species, this group of viruses is a concern for human and animal health globally.
The name Orthobunyavirus is derived from Bunyamwera, Uganda, where the first virus of this kind was identified. These pathogens are zoonotic, meaning they can be transmitted from animals to humans, leading to a range of illnesses. The genetic diversity within the genus is vast, partly due to genomic reassortment, where co-infecting viruses can swap genetic segments, creating novel strains.
Transmission Mechanisms of Orthobunyaviruses
Orthobunyaviruses are arboviruses, a term for viruses transmitted by arthropods. The primary method of transmission to humans and other vertebrates is through the bite of an infected insect. Mosquitoes are the most common vectors, but biting midges and ticks also spread these viruses.
The transmission cycle begins when a vector, such as a mosquito, feeds on an infected vertebrate host. This host, often a mammal or bird, acts as a reservoir, maintaining the virus in nature. After an incubation period within the vector, the virus can be transmitted to a new host, including humans, during a subsequent blood meal.
These viruses have adapted to a wide array of vertebrate hosts. For instance, certain species of rodents and marsupials can serve as amplifying hosts, meaning the virus replicates to high levels within them, increasing the likelihood of transmission to feeding vectors. The specific vectors and reservoir hosts vary depending on the virus and geographic region.
Notable Orthobunyaviruses and Associated Diseases
La Crosse virus, found in North America, is a leading cause of pediatric encephalitis, or inflammation of the brain, in the United States. Initial symptoms include fever, headache, nausea, and fatigue, which can progress to more severe neurological disease, especially in children under 16.
Another example is the Oropouche virus, which causes epidemics of a febrile illness in Central and South America. Oropouche fever is characterized by the abrupt onset of high fever, joint pain, and headache. While it is a self-limiting illness lasting about a week, some patients may experience a recurrence of symptoms and, in rare instances, it can lead to meningitis.
In Europe, the Schmallenberg virus is a pathogen of veterinary concern, affecting ruminants like cattle, sheep, and goats. In adult animals, it causes a mild disease, but the primary impact is on pregnancies. Infection during specific gestational periods can lead to stillbirths and congenital malformations, resulting in economic losses for farmers. Other viruses, such as the California encephalitis virus and Jamestown Canyon virus, also cause human neurological diseases in North America.
Global Distribution and Outbreak Patterns
Orthobunyaviruses are found across the globe, with particular diversity in Africa and Oceania, though they are present on nearly every continent. Their geographic range is linked to the distribution of their specific arthropod vectors and vertebrate hosts. For example, the viruses that cause California encephalitis are found where their specific mosquito vectors and chipmunk or squirrel hosts co-exist.
Outbreaks show distinct seasonal and geographic patterns. In temperate climates, infections are more common during the summer and fall when mosquito populations are at their peak. The emergence of some orthobunyaviruses has been linked to environmental changes, such as deforestation, which can alter vector habitats and bring humans into closer contact with infected animals and insects.
The emergence of the Schmallenberg virus in Europe demonstrated how a previously unknown virus can spread rapidly across a continent, affecting livestock over a wide area.
Prevention and Management Approaches
Preventing orthobunyavirus infections centers on avoiding the bites of infected arthropods. Public health strategies focus on vector control, which involves reducing populations of mosquitoes, midges, and ticks. These measures include eliminating standing water where mosquitoes breed and applying insecticides. For personal protection, individuals should use insect repellents with ingredients like DEET, wear long-sleeved shirts and pants, and ensure window and door screens are intact.
Currently, there are no specific antiviral treatments or commercially available vaccines for most human orthobunyavirus diseases. Medical care is supportive, focusing on managing symptoms and complications. This can include rest, fluid intake, and medications to reduce fever and pain. Severe cases, such as encephalitis, may require hospitalization for intensive care.