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Home Health Disease Health Yellow Fever
About Yellow Fever

Yellow fever causes 200,000 infections and 30,000 deaths every year, with nearly 90% of these occurring in Africa. Nearly a billion people live in an area of the world where the disease is common. It is common in tropical areas of South America and Africa, but not in Asia. Since the 1980s, the number of cases of yellow fever has been increasing. This is believed to be due to fewer people being immune, more people living in cities, people moving frequently, and changing climate.The disease originated in Africa, where it spread to South America through the slave trade in the 17th century. Since the 17th century, several major outbreaks of the disease have occurred in the Americas, Africa, and Europe.In the 18th and 19th century, yellow fever was seen as one of the most dangerous infectious diseases. The yellow fever virus was the first human virus discovered.

Cause

Transmission

Aedes aegypti feeding
Adults of the yellow fever mosquito Aedes aegypti. The male is on the left, females are on the right. Only the female mosquito bites can transmit the disease.

Yellow fever virus is mainly transmitted through the bite of the yellow fever mosquito Aedes aegypti, but other mosquitoes such as the tiger mosquito (Aedes albopictus) can also serve as a vector for this virus. Like other Arboviruses which are transmitted via mosquitoes, the yellow fever virus is taken up by a female mosquito when it ingests the blood of an infected human or other primate. Viruses reach the stomach of the mosquito, and if the virus concentration is high enough, the virions can infect epithelial cells and replicate there.

Pathogenesis

After transmission of the virus from a mosquito, the viruses replicate in the lymph nodes and infect dendritic cells in particular. From there they reach the liver and infect hepatocytes (probably indirectly via Kupffer cells), which leads to eosinophilic degradation of these cells and to the release of cytokines. Necrotic masses known as Councilman bodies appear in the cytoplasm of hepatocytes.

Fatality may occur when cytokine storm, shock, and multiple organ failure follow.

Yellow fever is caused by the yellow fever virus, a 40 to 50 nm wide enveloped RNA virus, the type species and namesake of the family Flaviviridae.It was the first illness shown to be transmissible via filtered human serum and transmitted by mosquitoes, by Walter Reed around 1900.The positive sense single-stranded RNA is approximately 11,000 nucleotides long and has a single open reading frame encoding a polyprotein. Host proteases cut this polyprotein into three structural (C, prM, E) and seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5); the enumeration corresponds to the arrangement of the protein coding genes in the genome.Yellow fever belongs to the group of hemorrhagic fevers.

After entering the host cell, the viral genome is replicated in the rough endoplasmic reticulum (ER) and in the so-called vesicle packets. At first, an immature form of the virus particle is produced inside the ER, whose M-protein is not yet cleaved to its mature form and is therefore denoted as prM (precursor M) and forms a complex with protein E. The immature particles are processed in the Golgi apparatus by the host protein furin, which cleaves prM to M. This releases E from the complex which can now take its place in the mature, infectious virion.

Diagnosis

Yellow fever is a clinical diagnosis, which often relies on the whereabouts of the diseased person during the incubation time. Mild courses of the disease can only be confirmed virologically. Since mild courses of yellow fever can also contribute significantly to regional outbreaks, every suspected case of yellow fever (involving symptoms of fever, pain, nausea and vomiting six to ten days after leaving the affected area) is treated seriously.

If yellow fever is suspected, the virus cannot be confirmed until six to ten days after the illness. A direct confirmation can be obtained by reverse transcription polymerase chain reaction where the genome of the virus is amplified.Another direct approach is the isolation of the virus and its growth in cell culture using blood plasma; this can take one to four weeks.

Prevention

Personal prevention of yellow fever includes vaccination as well as avoidance of mosquito bites in areas where yellow fever is endemic. Institutional measures for prevention of yellow fever include vaccination programmes and measures of controlling mosquitoes. Programmes for distribution of mosquito nets for use in homes are providing reductions in cases of both malaria and yellow fever

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