Yellow Fever VIrus

What is yellow fever?

Yellow fever is a serious viral infection, transmitted by mosquitoes in tropical regions. It has both an urban cycle and a jungle cycle that relies on monkeys as carriers ('sylvatic cycle').In mild cases the symptoms are similar to influenza, but serious cases develop a high temperature and may have a series of after effects, such as internal bleeding, kidney failure and meningitis.A classic feature of yellow fever is hepatitis, which is the reason for the yellow colouring of the skin (jaundice) and the name of the disease.Yellow fever can cause sudden epidemics, with a mortality rate of almost 50 per cent. Although a safe, efficient vaccine has been available for the last 60 years, epidemics still occur, constituting a health risk in tropical regions. The disease is covered by the International Quarantine Regulations, which are taken very seriously by authorities everywhere. Therefore, the vaccine has to be administered by a specially authorised doctor. In the UK you can only obtain the yellow fever vaccination from a designated Yellow Fever Clinic.

What causes yellow fever?

Yellow fever virus belongs to the Flaviviridae family, other members of which cause dengue fever and Japanese encephalitis. The virus is introduced into the bloodstream via the saliva of the mosquito as it bites. The virus can then be transported around the body and reproduce itself in a variety of the body's cells, usually the liver, kidneys and blood vessels. In serious cases, these cells may become damaged themselves.In addition, the cells of the immune system are affected and release large quantities of signalling substances. These substances are the cause of the normal disease symptoms, such as muscular pain and fever, which are also observed in influenza.

How is yellow fever passed on?

The virus is transmitted among humans by a couple of species of mosquito, including Aedes egyptii, which can also transmit dengue fever. It is an unexplained fact that despite the presence of the Aedes mosquito in Asia, yellow fever is limited to Africa and South America. In its original jungle cycle, the mosquito sucks the blood of an infected monkey. The mosquito develops a permanent infection, in which the virus accumulates in its salivary glands. Then the mosquito bites another monkey, which then also becomes infected with the virus. A person travelling through the jungle may also become infected by an infected mosquito. When this person returns to urban areas, a new cycle begins. Urban cycles start when an infected traveller returns from the jungle. A mosquito bites the traveller, who then becomes infected and passes the virus on to other people, and either an epidemic breaks out, or an endemic situation is perpetuated.

Orthomyxoviridae

Influenza virus; a member of the Orthomyxoviridae family

Description and Significance
The orthomyxovirus gets its name from the Greek word 'myxa' that means mucus.
Influenza virus types A and B are both common causes of acute respiratory illnesses. Both virus types may cause epidemics of considerable morbidity and mortality but influenza B infections are often limited to localized outbreaks whereas influenza A viruses are the principal cause of larger epidemics including worldwide pandemics. Influenza occurs in winter epidemics that affect 1-5% of the population in temparate regions. Influenza can be contracted throughout the year in tropical regions and its contribution to overall morbidity and mortality is less well defined. (source: World Health Organization: Immunization, Vaccines and Biologicals)

Genome Structure
The genome of the orthomyxovirus consists of six segments to eight segments of linear, negative-sense, single-stranded RNA. The complete genome is 10000-14600 nucleotides long. Segment 1 is fully sequenced and the complete sequence is 2300-2500 nucleotides long. Although sequenced and of the same length as Segment 1, segment 2 only has an estimate of the sequence so far. Segment 3 is also sequenced, but only estimated, and the complete sequence is 2200-2300 nucleotides long. Segment 4 has been completely sequenced and the complete sequence is 1700-1800 nucleotides long. Segment 5 has been sequenced, but only estimated, and is 300-1900 nucleotides long. Segment 6 has been sequenced, but only estimated, and is 1400-1500 nucleotides long. Segment 7 has been sequenced, but only estimated, and the complete sequence is 800-1100 nucleotides long. The genome has terminally redundant sequences and the sequence is repeated at both ends. The nucleotide sequences at the 3'-terminus are identical. The 5'-terminal sequence has conserved regions and repeats complementary to the 3'-terminus; terminal repeats at the 5'-end are 11-14 nucleotides long. The 3'-terminus has conserved nucleotide sequences; is 11-13 nucleotides long; in the genera of same family. The sequence has conserved regions in all RNA species or some RNA segments. The multipartite genome is encapsidated with each segment in a separate nucleocapsid, and the nucleocapsids are surrounded by one envelope. Each virion contains defective interfering copies.

Virion Structure of an Orthomyxovirus
The virions of an orthomyxovirus consist of an envelope, a matrix protein, a nucleoprotein complex, a nucleocapsid, and a polymerase complex. The virus capsid is enveloped. The virions are spherical to pleomorphic and filamentous forms occur. The virions are 80-120 nm in diameter and 200-300(-3000) nm long. The surface projections are densely dispersed distinctive hemagglitinin-esterase (HEF) spikes, or spaced widely apart hemagglutinin (HA) spikes. Clusters of neuramidase (NA) irregularly inerpose the major glycoprotein in a ratio of HA to NA about 4-5 to 1. There are about 500 spikes evenly dispersed or clustered and are covering the surface comprising hemagglutinin, or neuraminidase, or esterase-esterase. The surface projections are composed of one type of protein or different types of proteins and are 10-14 nm long and 4-6 nm in diameter. The nucleocapsid is elongated with helical symmetry and is segmented with loops at one end. The segments have different sized classes with clear predominate lengths with a length of 50-130 nm (in differnent class sizes) and a width of 9-15 nm.

ref:http://microbewiki.kenyon.edu/index.php/Orthomyxoviridae