Zika virus—a Flavivirus that is causing a pandemic, “Zika fever,” in the Americas and the Pacific. The outbreak began in early 2015 in Brazil, then spread to other parts of South and North America.
The Zika virus was first isolated in 1947 from a rhesus macaque in the Ziika Forest of Uganda. For years human beings were not a significant host for the virus. In 1956 an experiment was reported in which a volunteer was inoculated with the virus but there was no evidence of transmission from the human volunteer to mosquitos. Zika spread eastward infecting only an occasional human. The strain of virus that finally reached the Americas did not come from Africa as one might, suspect, but from Asia, apparently crossing the Pacific Ocean to Brazil.
The flavirus genome is a single strand of RNA that mimics the host cell’s mRNA. The viral RNA (vRNA) is copied in the host cell by a reverse transcriptase making duplicate copies of itself. It is also “read” like an mRNA by the ribosomes of the cell’s rough endoplasmic reticulum producing a peptide that is enzymatically cleaved to yield ten virus proteins. Three of these proteins will go to make up new virus particles enclosing the vRNA strand. The other seven assist in the viral processing, but are not incorporated into the virus particle. The change in the Zika virus that made it more compatible with a human host seems to have been an alteration in one of the non-structural coding sequences of the viral RNA, specifically those coding for the viral protein, NS1.
Sequences of three nucleotides (codons) in an RNA strand specify each amino acid in a protein. There are 64 possible codons, but only about 20 amino acids. There may be as many as four codons for each amino acid. Codons that specify the same amino acid are known as “synonymous” codons. The mutations changing one codon to a synonymous codon are called “silent” mutations because the changes are not expressed in the protein. So the proteins coded for by the African and Asian strains are the same, the difference is in the codons that specify the amino acids that make up the NS1 protein. In some way the change has made the Zika virus more transmittable from human to mosquito.
Word origin: The Zika virus gets its name comes from Ziika Forest of Uganda, where the virus was first isolated in 1947. Ziika means "overgrown" in the Luganda language. The Ziika forest is seventeen miles south of Kampala on Lake Victoria. It is an area devoted to research and owned by the Uganda Virus Research Institute (UVRI). It is the site of identification of Chikungunya virus and the West Nile flavivirus. Other well-known flaviviruses are those responsible for Yellow fever and Dengue fever “Yellow fever” got its name from the yellow jaundice it caused and Flavivirus comes from the Latin flavus meaning "yellow."
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