I was privileged enough to live with a family friend in Accra, Ghana for a period of time. Accra, Ghana is located along the west coast of the African continent. Unfortunately, this particular region has a number of viral infection incidences that have been deemed endemic such as dengue fever and yellow fever. Upon considering the list of endemic viruses as specified by the CDC website, I would like to expound on the yellow fever virus and its characteristics. That is, topics such as the structure, classification, life cycle, disease, treatment, and the extensive public health impact of the yellow fever virus.
Yellow fever virus is categorized as a member of the Flavivirus genus and, by extension, the Flaviviridae family. One of yellow fevers defining characteristics it that it is a single-stranded RNA of " positive polarity genome containing 10,862 nucleotides with a relative molecular mass of 3.75Ã-106. it encodes for three structural proteins, which are synthesized in infected cells. The genome is surrounded by an envelope, which contains a single glycoprotein with type and group specific antigenic determinants." (Yellow fever in Africa: public health impactand prospects for control in the 21st century)
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Given that yellow fever virus is a flaviviruses, it follow the same replication processing as any other plus sense single-stranded RNA of that family. That is, yellow fever virus's genomic RNA is the same sense as mRNA and consequently behaves as mRNA. The aforementioned mRNA can be translated upon infection of the host cell instantaneously. However, the viral protein has to recognize the receptors on the surface of the host cell's membrane. The aforementioned recognition prompts a conformational change in the capsid. Consequently, a channel forms across the cell membrane and the RNA is released into the cytoplasm, where the mRNA will ultimately be translated. Because the entire life cycle of the virus takes place in the cytoplasm, there is no differentiation between early and late expression.
As it pertains to protein synthesis single-stranded RNA "often interfere with host cell methylated cap recognition. Most host cell translation is cap-dependent, so this inhibits a lot of host protein synthesis but not viral protein synthesis - one way in which these viruses can modify the host cell to their advantage. The mRNA is translated into a single polypeptide (polyprotein), which is cleaved. The cleavages occur before translation is complete ( i.e. on the nascent (=growing) chain) and are carried out by virally coded proteases. Some of these proteases can work even while part of the polyprotein." (http://pathmicro.med.sc.edu/mhunt/rna-ho.htm) Once cleave has occurred, RNA polymerases, viral structural components, and proteases are yielded; these constituents facilitate RNA replication.
The RNA replication process is a very systematic one. Viral RNA polymerase then copies plus-sense genomic RNA into complementary minus-sense RNA which incorporates a number of constituents: VPg (or precursor containing VPg), and Viral RNA polymerase (replicase). "The VPg may act as a primer for RNA synthesis; this would explain why it is at the 5' end of all newly synthesized RNA molecules. New minus sense strands serve as template for new plus sense strands (figure 5). Again, poliovirus RNA polymerase and VPg are needed. VPg is linked to the 5' ends of the new plus sense strands (again, it probably functions as a primer)." (http://pathmicro.med.sc.edu/mhunt/rna-ho.htm) Subsequently, the newly synthesized plus strand can proceed to one of three out cones. The new plus strand could function as a template for more minus strands, or it could be packaged into progeny virions, or it could be translated into polyprotein (In which VPg would likely be removed proceeding the translation process.Â "When sufficient plus-sense progeny RNA and virion proteins have accumulated, assembly begins. Particles assemble with VPg-RNA inside and 3 proteins in the capsid [VP0,1 and 3]. VP0 is then cleaved to VP2 and VP4 as the virions mature and these mature virions are infectious. Virions are released following cell lysis. Excess capsids are formed and inclusion bodies may be seen in the cytoplasm." (http://pathmicro.med.sc.edu/mhunt/rna-ho.htm)
Â Yellow fever is a zoonotic infection and, by definition, is transmitted by an animal vector. Specifically it is transmitted by mosquitoes. Once an unsuspecting individual is exposed to the saliva of the mosquito, the viruses' life cycle in the human vessel ensues. Yellow fever virus has a very unique life cycle. In fact, has three distinctive life cycles that function independently from one another. Of the urban, sylvatic, and savannah cycles, the urban cycle is the relevant cycle as it pertains to Ghana. The urban cycle is transmitted between humans by the tiger mosquito, Aedes aegypti.
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The yellow fever symptoms emerge in stages after infection . Symptoms such as fever, headache, chills, back pain, loss of appetite, nausea, and vomiting can result from infection in the initial stages. However, less probable, but more severe, symptoms manifest in the subsequent stages. Approximately 15 % of infected individuals experience a toxin phase and consequently suffer from jaundice, abdominal pain, black vomit, and internal bleeding. "In the late stages of the disease, patients can develop hypotension, shock, metabolic acidosis, acute tubular necrosis, myocardial dysfunction, and cardiac arrhythmia. Confusion, seizures, and com also occur." (http://www.cdc.gov/ncidod/yellowfever/YF_Symptoms.html) Unfortunately, Secondary bacterial infections as well as kidney failure can develop as a result of yellow fever infections. For those who survive the infection, they incur life long immunity. (http://www.cdc.gov/ncidod/yellowfever/YF_Symptoms.html)
Currently, there is no explicit medical treatment for yellow fever virus. However, yellow fever's symptoms and secondary infections can be treated.
There are various methods of prevention as it pertains to yellow fever. Fortunately, there is a yellow fever vaccine that is readily available in order to prevent infection. In fact, it is a requirement and one has to have had the vaccine in order to enter into Ghana's borders. The yellow fever vaccine is "a live virus vaccine which has been used for several decades. A single dose confers immunity lasting 10 years or more. If a person is at continued risk of yellow fever infection, a booster dose is needed every 10 years. Adults and children over 9 months can take this vaccine." (http://www.cdc.gov/ncidod/bvbid/yellowfever/YF_Prevention.html) Although the vaccine is efficient, there are other precautionary measures that can be taken in order to further avoid infection. It is recommended that one wears clothing that can serve as a barrier against mosquitoes. That is, long sleeve shirts and pants will prevent the vector of yellow fever from infecting an individual. Another way to prevent infection is to wear insect repellent, particularly one that contains a high percentage of deet. A substantial amount of deet as an essential component that well help ward of mosquitoes in an efficient manner An additional way to avoid contracting yellow fever is to be aware of the times in which yellow fever transmitting mosquitoes are active so one could remain indoors in order to minimize contact with them. Although there are many ways to prevent infection, endemic regions still suffer.
Yellow fever virus has a tremendous impact as it pertains to public health. In Ghana, the yellow fever that originates from the aforementioned urban cycle is the post prevalent. That is, urban yellow fever is responsible for substantial outbreaks. "The disease has brought untold hardship and indescribable misery among different populations in Africa. It's one of Africa's stumbling blocks to the economic and social development. Despite landmark achievements made in the understanding of the epidemiology of yellow fever disease and the availability of a safe and efficacious vaccine, yellow fever remains a major public health problem." "Africa contributes more than 90% of global yellow fever morbidity and mortality. Apart from the severity in morbity and mortality, which are grossly under reported, successive outbrakes of yellow fever control measures have disrupted existing health care delivery services, overstreched scarce internal resources, fatigued donor assistance and resulted in gross wastage of vaccines." (Yellow fever in Africa: public health impact and prospects for control in the 21st century)
Yellow fever is diagnosed via extensive analysis of a patients travel history, virus detection, and IgM antibody detection.
In conclusion, yellow fever virus is a precarious virus that is transmitted via mosquito. Although the virus itself cannot be medically treated, its symptoms can be. The severe symptoms of yellow fever include a wide variety of symptoms. Fortunately, all of the aforementioned complications can be avoid using several methods. One could take the yellow fever vaccine or carry out any number of methods to avoid contact with virus transmitting mosquitoes - avoid being outdoors during mosquito activity peak hours, use of appropriate clothing and insect repellent. Although there are many ways to combat yellow fever virus infection, the endemic regions still regular occurrences.
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