| Rift Valley fever | |
|---|---|
 | |
| TEMmicrograph of tissue infected with Rift Valley fever virus | |
| Specialty | Infectious diseases,veterinary medicine  |
| Symptoms | Fever,muscle pains,headaches |
| Complications | Loss of sight,confusion, bleeding,liver problems |
| Duration | Up to a week |
| Causes | Phlebovirus spread by an infected animal ormosquito |
| Diagnostic method | Findingantibodies or the virus in the blood |
| Prevention | Vaccinating animals against the disease, decreasing mosquito bites |
| Treatment | Supportive care[1] |
| Frequency | Outbreaks inAfrica andArabia |
Rift Valley fever (RVF) is aviral disease of humans and livestock that can cause mild to severe symptoms. The mild symptoms may include:fever,muscle pains, andheadaches which often last for up to a week. The severe symptoms may include: loss of sight beginning three weeks after the infection, infections of thebrain causing severe headaches andconfusion, and bleeding together withliver problems which may occur within the first few days. Those who have bleeding have a chance of death as high as 50%.
The disease is caused by the RVFvirus. It is spread by either touching infected animal blood, breathing in the air around an infected animal beingbutchered, drinkingraw milk from an infected animal, or the bite of infectedmosquitoes. Animals like cows, sheep, goats, and camels may be affected. In these animals it is spread mostly by mosquitoes.[1] It does not appear that one person can infect another. The disease is diagnosed by findingantibodies against the virus or the virus itself in the blood.Prevention of the disease in humans is accomplished by vaccinating animals against the disease. This must be done before an outbreak occurs because if it is done during an outbreak it may worsen the situation. Stopping the movement of animals during an outbreak may also be useful, as may decreasing mosquito numbers and avoiding their bites. There is a humanvaccine; however, as of 2010, it is not widely available. There is no specific treatment and medical efforts are supportive.
Outbreaks of the disease have only occurred inAfrica andArabia. Outbreaks usually occur during periods of increased rain which increases the number of mosquitoes. The disease was first reported among livestock inRift Valley ofKenya in the early 1900s,[2] and the virus was first isolated in 1931.[1]
In humans, the virus can cause several syndromes. Usually, they have either no symptoms or only a mild illness with fever,headache,muscle pains, andliver abnormalities. In a small percentage of cases (< 2%), the illness can progress tohemorrhagic fever syndrome,meningoencephalitis (inflammation of the brain andtissues lining the brain), or affect the eye. Patients who become ill usually experience fever, generalised weakness, back pain, dizziness, and weight loss at the onset of the illness. Typically, people recover within two to seven days after onset. About 1% of people with the disease die of it. In livestock, the fatality level is significantly higher. Pregnant livestock infected with RVFabort virtually 100% of foetuses. Anepizootic (animal disease epidemic) of RVF is usually first indicated by a wave of unexplained abortions.[citation needed]
Other signs in livestock include vomiting and diarrhea, respiratory disease, fever, lethargy, anorexia, and sudden death in young animals.[3]
| Rift Valley fever virus | |
|---|---|
 | |
| Phlebovirus virion and genome | |
Virus classification | |
| (unranked): | Virus |
| Realm: | Riboviria |
| Kingdom: | Orthornavirae |
| Phylum: | Negarnaviricota |
| Class: | Bunyaviricetes |
| Order: | Hareavirales |
| Family: | Phenuiviridae |
| Genus: | Phlebovirus |
| Species: | Phlebovirus riftense |
| Synonyms[4] | |
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The virus belongs to theBunyaviricetes class. This is a class of enveloped negative single-stranded RNA viruses. All bunyaviruses have an outer lipid envelope with twoglycoproteins—G(N) and G(C)—required for cell entry. They deliver their genome into the host-cellcytoplasm via endocytosis of prepackaged virions .[5]
The virus' G(C) protein has a class IImembrane fusion protein architecture similar to that found inflaviviruses andalphaviruses.[6] This structural similarity suggests that there may be a common origin for these viral families.[citation needed]
The virus' 11.5kb tripartitegenome is composed of single-strandedRNA. As aPhlebovirus, it has anambisense genome. Its L and M segments are negative-sense, but its S segment is ambisense.[7] These three genome segments code for six major proteins: L protein (viral polymerase), the two glycoproteins G(N) and G(C), thenucleocapsid N protein, and thenonstructural NSs and NSm proteins.[8]
The virus is transmitted through mosquitovectors, as well as through contact with the tissue of infected animals. Two species—Culex tritaeniorhynchus andAedes vexans—are known to transmit the virus.[9] Other potential vectors includeAedes caspius,Aedes mcintosh,Aedes ochraceus,Culex pipiens,Culex antennatus,Culex perexiguus,Culex zombaensis andCulex quinquefasciatus.[10][11][12] Contact with infected tissue is considered to be the main source of human infections.[13] The virus has been isolated from two bat species: thePeter's epauletted fruit bat (Micropteropus pusillus) and theaba roundleaf bat (Hipposideros abae), which are believed to bereservoirs for the virus.[14]
Although many components encoded by RVFV's RNA play an important role in the virus' pathology, thenonstructural protein encoded on the S segment (NSs) is the only component that has been found to directly affect the host. NSs is hostile and combative against the hostinterferon (IFNs) antiviral response.[15] IFNs are essential for the immune system to fight off viral infections in a host.[16] This inhibitory mechanism is believed to be due to several reasons, the first being, competitive inhibition of the formation of thetranscription factor.[15] On this transcription factor, NSs interacts with and binds to a subunit that is needed forRNA polymerase I andII.[15][17] This interaction cause competitive inhibition with another transcription factor component and prevents the assembly process of the transcription factor complex, which results in the suppression of the host antiviral response.[15][17] Transcription suppression is believed to be another mechanism of this inhibitory process.[15] This occurs when an area of NSs interacts with and binds to the host's protein,SAP30 and forms a complex.[15][17] This complex causeshistone acetylation to regress, which is needed for transcriptional activation of the IFN promoter.[17] This causes IFN expression to be obstructed. Lastly, NSs has also been known to affect regular activity of double-stranded RNA-dependentprotein kinase R. This protein is involved in cellular antiviral responses in the host. When RVFV can enter the host's DNA, NSs forms a filamentous structure in the nucleus. This allows the virus to interact with specific areas of the host's DNA that relates to segregation defects and induction of chromosome continuity. This increases host infectivity and decreases the host's antiviral response.[15]
Diagnosis relies on viral isolation from tissues, or serological testing with anELISA.[3] Other methods of diagnosis includeNucleic Acid Testing (NAT),cell culture, andIgM antibody assays.[18] As of September 2016, theKenya Medical Research Institute (KEMRI) has developed a product called Immunoline, designed to diagnose the disease in humans much faster than in previous methods.[19]
A person's chances of becoming infected can be reduced by taking measures to decrease contact with blood, body fluids, or tissues of infected animals and protect against mosquitoes and other bloodsucking insects. The use of mosquito repellents and bed nets are two effective methods. For persons working with animals in RVF-endemic areas, wearing protective equipment to avoid any exposure to blood or tissues of animals that may potentially be infected is an important protective measure.[20] Potentially, establishing environmental monitoring and case surveillance systems may aid in the prediction and control of future RVF outbreaks.[20]
Novaccines are currently available for humans. While vaccines have been developed for humans, it has only been used experimentally for scientific personnel in high-risk environments.[1] Trials of several vaccines, such as NDBR-103 and TSI-GSD 200, are ongoing.[21] Different types of vaccines for veterinary use are available. The killed vaccines are impractical in routine animal field vaccination because of the need for multiple injections. Live vaccines require a single injection but are known to cause birth defects and abortions in sheep and induce only low-level protection in cattle. The live-attenuated vaccine, MP-12, has demonstrated promising results in laboratory trials in domesticated animals, but more research is needed before the vaccine can be used in the field. The live-attenuated clone 13 vaccine was recently registered and used in South Africa. Alternative vaccines using molecular recombinant constructs are in development and show promising results.[20]
A vaccine has been conditionally approved for use in animals in the US.[22] It has been shown that knockout of the NSs and NSm nonstructural proteins of this virus produces an effective vaccine in sheep as well.[23]
RVF outbreaks occur acrosssub-Saharan Africa, with outbreaks occurring elsewhere infrequently. Outbreaks of this disease usually correspond with the warm phases of the EI Niño/Southern Oscillation. During this time there is an increase in rainfall, flooding, and greenness ofvegetation index, which leads to an increase in mosquito vectors.[24] RVFV can be transmitted vertically in mosquitos, meaning that the virus can be passed from the mother to her offspring. During dry conditions, the virus can remain viable for many years in the egg. Mosquitos lay their eggs in water, where they eventually hatch. As water is essential for mosquito eggs to hatch, rainfall and flooding cause an increase in the mosquito population and an increased potential for the virus.[25]
The first documented outbreak was identified in Kenya in 1931, in sheep, cattle, and humans;[26] another severe outbreak in the country in 1950–1951 involved 100,000 deaths in livestock and an unrecorded number of humans with fever.[27] An outbreak occurred in South Africa in 1974–1976, with more than 500,000 infected animals and the first deaths in humans.[28][29] InEgypt in 1977–78, an estimated 200,000 people were infected and there were at least 594 deaths.[30][31] InKenya in 1998, the virus killed more than 400 people.[citation needed] Since then, there have been outbreaks inSaudi Arabia andYemen (2000),[citation needed]East Africa (2006–2007),[32] Sudan (2007),[33] South Africa (2010),[34][35] Uganda (2016),[36] Kenya (2018),[37]Mayotte (2018–2019),[38] Kenya (2020–2021)[39] and Burundi (2022).
Rift Valley fever was one of more than a dozen agents that theUnited States researched as potentialbiological weapons before the nation suspended its biological weapons program in 1969.[40][41]
The disease is one of several identified byWHO as a likely cause of a future epidemic in a new plan developed after theEbola epidemic for urgent research and development toward new diagnostic tests, vaccines and medicines.[42][43]
This article incorporates text from this source, which is in thepublic domain.
