Rift Valley Fever (RVF)

A major zoonotic disease called Rift Valley fever (RVF) is caused by the Rift Valley fever virus (RVFV). It was first identified in the 1930s in Kenya's Rift Valley, and since then, it has been a growing concern in various parts of Africa. The RVF poses a threat to human health and livestock numbers that requires effective control and preventive measures, and BioVenic has been engaged in related research.

Virology

The RVFV belongs to the Phlebovirus genus in the Phenuiviridae family. RVFV is about 90-110 nm in diameter, spherical, and enveloped. The genome is segmented single-stranded RNA, which is divided into L, M, and S segments, with lengths of 6.4 kb, 1.7 kb, and 3.9 kb, respectively. The L and M segments are negative-strand RNAs, and the S segment is ambisense RNA.

Fig. 1 Schematic diagram of the structure of RVFV.¹

Susceptible Animals

Domestic ruminants including cattle, buffalo, sheep, goats, and camels are primarily susceptible to the RVFV. Sheep exhibit higher susceptibility compared to cattle and camels. In cases where pregnant animals are infected, goats experience an abortion rate as high as 85%, while sheep face a 100% abortion rate. The mortality rate among calves ranges from 10% to 70%, while adult cattle have a mortality rate of less than 10%. Additionally, the mortality rate for sheep, goats, and pigs ranges from 10% to 20%.

Transmission

Mosquito-borne organisms with RVFV and infected ruminants are the main sources of infection for this disease. The RVFV is transmitted in ruminants and humans mainly through the following routes,

• Direct contact with contaminated animal tissues, body fluids, or consumption of undercooked meat, milk, etc.
• Mosquito transmission, Aedes, Culex, Anopheles, and many other mosquito species can be transmitted, but mainly Aedes.
• Laboratory infections due to aerosols have been occasionally reported, and human-to-human transmission has not been reported.

Fig. 2 Transmission cycle diagram of RVFV.²

Pathogenesis

The pathogenesis of Rift Valley fever has not been fully elucidated. However, the literature suggests that the virus replicates primarily in the liver, causing hepatic necrosis and subsequent liver dysfunction. Other organs, including the spleen, brain, and lymph nodes, can also be affected.

Epidemiology of RVF

• Regional distribution: RVF is mainly distributed in eastern and southern Africa, and has also been reported in the Middle East of Asia. With the increasing convenience of transportation, the risk of cross-border transmission of RVFV is increasing, and many countries such as Sweden, France, Germany, the Netherlands, and Canada have reported imported cases.
• Population distribution: Infection can occur at any age, but children are less likely to be infected, and men are more likely than women. Animal breeding and slaughter personnel, veterinarians, etc. are high-risk groups.
• Epidemic characteristics: The prevalence of the disease can be observed throughout the year, with its seasonal distribution primarily influenced by the activity of mosquito vectors.

Signs and Symptoms

The initial clinical symptoms of RVF are nonspecific.

• In humans, the manifestations of RVF can vary from mild flu-like symptoms to severe complications. Typical symptoms include fever, headache, muscle pain, and weakness. However, in severe cases, individuals may experience hemorrhagic fever, encephalitis, and ocular symptoms, and, in some instances, it can lead to death. The case fatality rate can reach as high as 50%.
• In animals, RVF causes severe consequences, leading to abortion in pregnant livestock, high mortality in juvenile animals, and disease in adult livestock.

Diagnosis of RVF

The clinical diagnosis of RVF needs to be combined with epidemiological history, clinical manifestations, and laboratory test results to make a comprehensive judgment, and it needs to be identified with influenza, dengue fever, Japanese encephalitis, and Q fever. BioVenic develops customized RVF diagnostics solutions based on immunodiagnostics and molecular diagnostics for customers, for example,

• ELISA: Detects specific IgM or IgG against RVFV in blood, and also detects RVFV antigen.
• RT-PCR: Direct detection of RVFV RNA.
• Immunohistochemistry (IHC): Identification of RVFV in liver sections.

Treatment

• Currently, there is no specific treatment available for either humans or animals affected by Rift Valley fever.
• Most cases of RVF are mild and have a short course of disease. No special treatment is required.
• In cases of severe RVF, the primary approach involves symptomatic and supportive treatment. It is crucial to focus on maintaining proper water, electrolyte, and acid-base balance.

Control and Prevention

Since its discovery, RVFV has caused multiple pandemics, each of which has caused serious losses in animal husbandry and seriously threatened the public health security of endemic countries. The virus has a high pathogenicity and strong transmission ability. To prevent RVF, BioVenic recommends the following measures.

• Control the Source of Infection
  A live attenuated vaccine and an inactivated vaccine can be used to vaccinate livestock before they become infected with RVF.
• Cut off the Transmission Route
  Avoid contact between animals and staff with animal tissues and body fluids suffering from RVF, and remember not to eat undercooked meat or milk. At the same time, pay attention to killing and preventing mosquitoes.
• Protect Vulnerable Groups
  There is currently no human vaccine available. Take strict personal protective measures when contacting and slaughtering sick animals, such as wearing masks, gloves, and protective clothing.

BioVenic has long been at the forefront of diagnostics development for RVF. Working with leading animal disease experts, we provide our clients with the development services of effective vaccines, antiviral therapies, and diagnostics solutions to combat RVF. If you would like to cooperate with us, please feel free to contact us.

References

1. Huiskonen, Juha T., et al. "Electron cryo-microscopy and single-particle averaging of Rift Valley fever virus: evidence for GN-GC glycoprotein heterodimers." Journal of virology 83.8 (2009): 3762-3769.
2. Himeidan, Yousif E., et al. "Recent outbreaks of rift valley fever in East Africa and the Middle East." Frontiers in public health 2 (2014): 169.