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Foot-and-mouth disease virus (FMDV) is the pathogen that causes foot-and-mouth disease. It is a picornavirus, the prototypical member of the genus Aphthovirus. The disease, which causes vesicles (blisters) in the mouth and feet of cattle, pigs, sheep, goats, and other cloven-hoofed animals is highly infectious and a major plague of animal farming.

Impact
Mortality associated with this virus is usually low, but FMD decreases livestock productivity and countries affected with it are not allowed to partake in international trade of animals and animal products. FMD is preventing economic development in many regions of Asia, Africa, and South America due to its enzootic nature there. This disease ranks first in the A list of infectious diseases of animals published by the Office International des Epizooties.

Outbreaks
2001/2002 European Outbreak

Affected mainly the UK and had an estimated cost of 600 million Euros. This outbreak facilitated the penetration of FMDV of Asiatic origin in Europe, contributing to concerns on further spread in the face of an increasingly global economy. As a consequence of the massive killing of animals during this outbreak, there is presently a vivid debate towards whether a vaccination strategy may not be preferable to the stamping-out and non-vaccination procedure in operation in the EU since 1991. This regulation was adopted for economic considerations, but left susceptible animals in a vast territory defenseless in the face of an accidental introduction of the virus in the region, a situation that historically has been known to favor the occurrence of large epizootics

FDMV Particles
The virus particle (25-30 nm) has an icosahedral capsid made of protein and without an envelope. The particle is composed of 60 copies of four capsid proteins called VP1, VP2, VP3, and VP4. VP1, VP2, and VP3 are external proteins while VP4 is an internal proteins that remains in association with RNA and is modified by a myristate group at is amino terminus.

One copy of each of the capsid proteins are used to assemble a protomer. Five protomers form a pentamer with 12 total pentamers forming the complete capsid in FDMV. The surface capsid proteins consist of eight-stranded B-barrel which surface loops connecting the B-strands. These surface loops include antigenic determinants involved in antibody formation in the host.

There have been four major antigenic sites identified in FDMV. Most notably, there is a major, immunodominant site within the G-H loop of VP1 that is highly disordered as seen in X-ray diffraction patterns of crystal of native virions. A structure consists of a short β-strand which precedes an Arg-Gly-Asp which adopts an open-turn conformation, followed by a short helical region at the carboxy site of the Arg-Gly-Asp. This loops exhibits hinge type movements on the surface of the capsid and it changes position depending upon which antibody is is bound to. The fact that it is exposed to the outside environment and it is mobile may contribute to its immunodominance. The Arg-Gly-Asp region also functions in recognition of integrins that are known to be cellular receptors for FDMV and in host cell antibody binding.

Genome
The genome contains a positive-sense (mRNA sense) single-stranded ribonucleic acid (RNA) genome composed of about 8,500 nucleotides. The RNA is polyadenylated at the 3'-end and has a small protein known as Vpg that is covalently bonded to the 5'-end of the RNA. There are three main functional regions of the genome which include the 5' non-coding regulatory region, the protein-coding region, and the 3' non-coding, regulatory region.

The 5' non-coding region has an S fragment of about 370 residues followed by a polyribocytidylate (polyC) tract of variable length (usually 100-400 residues). There is a pseudoknot. region downstream of the polyC tract. After the pseudoknot is an internal ribosome entry site (IRES). This is a stretch of about 440 residues that functions as the internal initiation site of protein synthesis in a CAP-independent fashion.

Protein synthesis starts at AUG codons separated by about 80 nucleotides in a long open-reading frame that code for a polyprotein of about 2330 amino acids. Differences in length of coding and non-coding regions are observed among natural isolates of FMDV and some times among viruses with a different passage history in cell culture.

Viral Proteins
Double initiation of protein synthesis occurs, which causes two form of the protein protease L to be made. Both proteases catalyze their own cleavage from the rest of the polyprotein they are in. They also catalyze the cleavage of eIF-4g of the CAP complex, which functions to inhibit protein synthesis of the host cell.

The P1 region of the genome encodes the four capsid proteins (structural proteins). The P2-P3 regions encode the non-structural proteins that are involved in genome replication and maturation of the virus. A lot of the non-structural proteins do not have understood functions, although poliovirus protein homologs have been studied in detail.

The protein 3C is a serin protease that catalyzes most of the cleavages needed to process the polyprotein. The protein 2C is involved in RNA synthesis and is usually the site where mutations occur that result in FMDV resistance to guanidine hydrochloride. The protein 3B encodes for three copies of the VPg protein that is linked to the 5'-end of the RNA genome. The protein 3D is the viral RNA-dependent RNA polymerase. The 3' non-coding region of about 90 residues is most likely the site of interaction with viral and host proteins for replication of the RNA. The poly A tract i s also located at the 3'-end.

Replication
The viral RNA of the FDMV is infectious and it replicates using the complementary minus strand of the RNA. Formation of a double stranded replicative form and partially double stranded intermediates are formed during the copying of the genome. Each minus strand serves as a template for the synthesis of many plus strands because minus strands are found at a much lower concentration in the host cell.

The site of the RNA genome replication is a membrane-bound replication complex at the cell cytoplasm. When the virus comes in contact with the membrane of a host cell, it binds to a receptor site and triggers a folding-in of the membrane. Once the virus is inside the host cell, the capsid dissolves, and the RNA gets replicated, and translated into viral proteins by the cell's ribosomes using a cap-independent mechanism driven by the internal ribosome entry site element.

The synthesis of viral proteins include 2A 'cleavage' during translation. They include proteases that inhibit the synthesis of normal cell proteins, and other proteins that interact with different components of the host cell. The infected cell ends up producing large quantities of viral RNA and capsid proteins, which are assembled to form new viruses. After assembly, the host cell lyses (bursts) and releases the new viruses.

FMDV RNA genome replication is error-prone and the viral replicase does not have any proofreading or repair activity. Between 0.2 and 1 mutations are introduced every time a plus or minus strand is copied into an RNA. Because of this, the FMDV populations is composed of different but related non-identical genomes. The consensus nucleotide sequence is an average of many different sequences and a genome with a sequence identical to the consensus sequence most likely does not exist in nature.

Recombination
Recombination can occur within host cells during co-infections by different FMDV strains. Recombination is common and a key feature of FMDV evolution.

Diversity
This replication process is error prone, which can generate many different mutant variations of the virus (quasispecies). One defined genomic sequence is not generally used to describe FDMV and it is known to be genetically and antigenically diverse in nature. Replication most likely always results in the creation of an FMDV quasispecies, When an animal gets infected, mutants subjected to positive selection, negative selection, and random drift within that animals will occur. Transmission of FMDV from an infected animals to a susceptible host will initiate a new round of evolutionary results. This short-term intra-host evolution is the result of differential results of subpopulations of mutant FMDVs. There is also overlap between some antigenic sites and some cell receptor recognition sites, which may result in coevolution of antigenicity and host range.

Types, subtypes, and isolates
Isolates found in the 20th century were grouped into seven serotypes: A, O, C, Asia 1, STAT1, STAT2, and STAT 3. Assigning to serotypes is based on the lack of cross-protection following infection or vaccination. Viruses showing partial cross-protection were assigned to the same serotype to but to different subtypes.

There are 65 FMDV subtypes currently defined, but about two decades ago it was realized that with the use of increasing numbers of monoclonal antibodies, virtually each isolate could be regarded as an antigenic variant.

Classification of isolates based on phylogenetic methods are gradually replacing the traditional groupings based on serological criteria. Phylogenies are based on RT-PCR amplification of genomic FDMV RNA and nucleotide sequencing of specific regions of the genome, usually the region that encodes the capsid protein VP1.

Diagnosis
ELISA and genetic typing techniques have replaced older techniques for diagnosis. ELISA uses serotype-specific sera of monoclonal antibodies. For trade purposes, diagnosis procedures should distinguish animals that have been vaccinated from those that have been infected with FMDV. This distinction can be done by detecting antibodies against some of the non-structural proteins by ELISA, in particular antibodies against 3AB and 3ABC. These antibodies are found in animals which at some time have been infected with FMDV but not in vaccinated animals.

RT-PCR amplification is increasingly used as a tool for an efficient and rapid diagnosis of FMD. Coupled to automated nucleotide sequencing and phylogenetic analysis, these techniques can provide a detailed virus identification to trace the origin of FMDVs associated with new outbreaks.

Control
Control is based on the slaughtering of affected and contact animals or the regular vaccination of the major host species for FMDV (cattle and swine). Both strategies can be used so that a ring vaccination can be established around a focus area where affected and contact animals have been slaughtered.

Vaccination
Manufacturing more effective and safer vaccines that the ones currently available could replace current vaccines in territories where a vaccination policy is currently present. Things that need to be considered for the development of new anti-FMD vaccines are that they should include multiple B cell and T cell epitopes to produce humoral and cellular immune responses, especially the T helper r esponse for antibody production. This response is required to delay and eventually eliminate the possible selection of virus variants showing partial resistance to the immune response evoked by the vaccine.

FMDV infection is usually acute. Viremia and lesion occur within days after infection. Ruminants be asymptomatic carriers where the virus replicates in the oesopharyngeal region. The presence of carriers can jeopardize animal trade because they can infect other animals and initiate an acute infection when in contact with susceptible animals. Classical vaccines cannot prevent the establishment of persistence FMDV in cattle. A new vaccine should be created to target systemic and mucosal immunity, which may minimize the chances of carriers arising and infiltrating a susceptible population.

Vaccine manufacturing should not require the handling of virus because there is a danger of it escaping from vaccine factories. Inactivated whole-virus vaccines can cause outbreaks if inactivation prior to vaccine formulation is not complete. There is good evidence that dome outbreaks most likely had vaccine origin.

Vaccine must be marked to distinguish vaccination from infection for diagnostic purposes. A vaccine can be marked positively by using a genetic tag or negatively by omitting a gene product found during a natural infection.