The Phenuiviruses

In June of 1912, according to the Colonial Report of the East Africa Protectorate, farmers whose sheep grazed the shores of Lake Naivasha in the Rift Valley region of Kenya described a disturbing development: “…The appearance of an obscure disease among young lambs, causing a heavy mortality…”. Many sheep died suddenly, while others suffered spontaneous abortions.
Some 18 years later, on a Merino sheep farm in the same Rift Valley region, 3,500 lambs and 1,200 ewes succumbed to an outbreak of an apparently very similar disease. “Symptoms were very indefinite, and lambs rarely survived 24 hours after they had become visibly ill. Death also followed very promptly the onset of symptoms in ewes,” scientists Robert Daubney and John R. Hudson wrote in a 1931 edition of the Journal of Pathology and Bacteriology. “Some animals were observed to vomit, in some cases there was a thick mucopurulent nasal discharge, and in a small proportion the faeces contained much blood.”
The scientists went on to explain that their laboratory investigations had revealed two further important discoveries: that the disease was one not only of sheep, but also “of cattle and man”; and that it was caused by a viral pathogen. That pathogen—given the name Rift Valley Fever Virus —is now classified as a member of the Phenuivirus family, one of The Viral Most Wanted.
It’s large, yes—but the Phenuivirus family’s 180 or so members belong to at least 20 separate subgroups and are not all close. Some subgroups, such as the Phleboviruses for example, include several human pathogens like Rift Valley Fever Virus and Dabie Bandavirus that are more closely related to each other than to viruses in the wider family.
Rift Valley Fever, or RVF, is the most infamous and significant human pathogenic threat in the Phenuivirus Family. While it is mostly a disease of animals—primarily infecting livestock—it can and does also infect people, causing serious disease and dangerous outbreaks.
Dabie Bandavirus—a Phenuivirus that can cause severe fever and a blood condition characterised by a low platelet count known as thrombocytopenia syndrome—is another notable human infection culprit in this family, as is Heartland Virus, which causes similar thrombocytopenia symptoms.
The 1931 journal paper that first identified RVF referred to it in its title as “Enzootic Hepatitis or Rift Valley Fever” is also sometimes referred to as Infectious Enzootic Hepatitis of Sheep and Cattle.
Heartland Virus gets its name from the Heartland Regional Medical Center in St. Joseph, Missouri, where it was first identified in 2009.
Dabie Bandavirus is also known as the Severe Fever with Thrombocytopenia Syndrome Virus, or SFTSV. The name Dabie Bandavirus comes from Dabie Mountains area of central China, where the virus was initially discovered.
Phenuivirus virions, or virus particles, are either spherical or pleomorphic—in other words variable in shape. They are relatively small, with diameters ranging from 80 to 160 nanometers. Inside, they contain three segments of single-stranded RNA. Phenuiviruses are typically enveloped viruses, which means they are covered in a cloak or membrane. This viral envelope is covered with glycoprotein spikes, which the viruses use to attach to host cells.
Source: Viral Zone by SwissBioPics
Phenuiviruses gain entry to and infect human cells by first binding to specific receptors on the cells’ surfaces. After attaching and being taken in by the host cell, the virus hijacks its machinery to produce new viral proteins and RNA to form new viral particles. These then go on to infect further cells, replicate and spread.
Rift Valley Fever Virus does not necessarily need accomplices to help it spread disease and death, but it uses them nonetheless. Mosquitoes—particularly Aedes and Culex mosquitoes—are carriers of the disease and can transmit it from infected animals to people through their blood-sucking bites. Blood-feeding or hematophagous flies have also been known to transmit the disease. According to the World Health Organisation, the majority of human Rift Valley Fever infections result from direct contact with the blood or organs of infected animals and to date there has been no known or documented human-to-human transmission.
Dabie Bandavirus and Heartland Virus use ticks as their main accomplices. In the majority of cases of infection in people, they have been transmitted through the bites of infected ticks—in particular Haemaphysalis longicornis or the Asian Long-horned tick, and the Lone Star tick or Amblyomma Americanum. In some cases, however, the viruses can also spread from person to person through direct contact with blood or bodily fluids of someone infected.
The most common victims of Rift Valley Fever are people in sub-Saharan Africa who live or work in close proximity to livestock animals that could be infected. This means farmers, herders, slaughterhouse workers and vets are at highest risk of becoming infected. People in endemic areas who are frequently exposed to mosquitoes, or often bitten by them are also at high risk.
Until the mid-1970s, Rift Valley Fever was thought to be a disease confined to the African continent and one that only really affected animals. Human cases were very rare and usually only caused mild symptoms. But after a season of heavy rain and flooding in South Africa in 1974, a severe outbreak of RVF spread to more than 500,000 animals and into more than 100 people. It was also during this outbreak that the first human RVF deaths were reported, with seven victims unable to fight the viral disease off. This first known deadly human outbreak in Africa was followed by similarly worrying outbreaks in Egypt in 1977, in Mauritania in 1987. Another epidemic in Kenya and East Africa in 1997–1998 was considered among the most devastating in the region, infecting about 27,500 people and killing more than 400 of them. By 2000, RVF had spread beyond the African continent for the first time and was detected in Saud