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Rabies virus belongs to the family: Rhabdoviridae. (Greek: Rhabdos: rod). They can infect a variety of animals and plants Worldwide, it is estimated that approximately 55 000 persons die of Rabies each year TABLE 1 - Rhabdoviruses TypeVirusDistributionSpesTypeVirusDistributionSpecies infectedDiseaseVesiculovirusVesicular stomatitis virus (VSV)CarribeanCattle, pigs horseAcute, self limitingLyssavirusRabies virusWorldwideMany mammals including humansSlow, progressivePlant rhabdoviruses Other animal rhabdoviruses Mammals, fish, birds, arthropods
Structure of rhabdoviruses
Rhabdoviruses are negative strand RNA viruses; that is they have a single strand of RNA that is anti-sense to the messenger RNA needed to code for viral proteins. This means that the RNA cannot code directly for protein synthesis and must be copied to positive strand mRNA. As a result, the virus must carry its own RNA-dependent RNA polymerase.
As their name suggests these viruses are rod shaped. They have one end that is rounded and are often referred to as bullet-shaped. Each virus particle is up to 100nm diameter and 400 nm long but this is very variable. They have an envelope derived from the host cell plasma membrane. The virus has only five proteins.G (Surface) Protein. This is the surface glycoprotein spike and exists as trimers. There are about 1200 G proteins (400 trimers) per virus particle. It is a transmembrane protein with an N-terminal signal sequence. The G protein binds to cellular receptors and is the target of neutralizing antibodies. There are three sugar chains that are N-glycosidically attached. Penetration of the virus into the cytoplasm takes place in the endocytic pathway and not at the plasma membrane. This is because the G protein trimer undergoes a change in conformation at pH 6.1 which stabilizes the trimer and probably allows a hydrophobic region of the molecule to become exposed and to embed in the membrane of the cell to be infected.M (matrix) protein. This is a peripheral membrane protein (originally M stood for membrane) that appears to line the inner surface of the viral membrane, though this remains somewhat controversial. It may act as a bridge between the membrane or G protein and the nucleocapsid.Nucleocapsid. This is the infectious ribonucleoprotein core of the virus. It is a helical structure that lies within the membrane. In negative stain electron micrographs, such as seen in figure 1, the nucleocapsid has a striated appearance.
N (Nucleoprotein) protein. This is the major structural protein and covers the RNA genome. It protects the genome from nucleases and holds it in a conformation that allows transcription
L (Large) protein and NS (nonstructural, otherwise known as P (phospho)) protein together form the RNA-dependent RNA polymerase or transcriptase. The L protein has a molecular weight of 240 kiloDaltons and its gene takes up 60% of the genome (figure 3).
Replication (figure 2)
The receptors for rhabdoviruses have yet to be definitively identified but some experiments point to phospholipids, particularly phosphatidyl serine, as the cell surface receptor molecule.
After endocytosis, pH-dependent fusion with the membrane of the endocytic vesicle occurs. The nucleocapsid enters the cytoplasm. All subsequent stages take place here with no involvement of the nucleus of the cell.
First, the polymerase, which is carried in the entering virus, makes five individual mRNAs, one for each viral protein. Note, the RNA must be made before any viral protein synthesis and so the infecting virus must supply the polymerase enzyme. (As might be expected, this primary transcription process takes place in the presence of protein synthesis inhibitors). The mRNAs are capped, methylated and polyadenylated. The sequence of transcription is N, NS(P), M, G and L with synthesis of the mRNAs being attenuated at each gene junction (figure 3). This means that less of the L mRNA is made than any of the others.
In addition, the polymerase transcribes the negative-sense genomic RNA into a positive sense strand. This serves as a template for the transcriptase to transcribe new negative sense genomic RNA molecules. This replicative phase does require protein synthesis and the same polymerase is involved. In the replicative phase, this enzyme must ignore signals that define the individual mRNA species and make one single RNA molecule. The switch between transcription of mRNAs and replication of genomic RNAs seems to be controlled by the level of N protein
The G protein mRNA is translated in association with the endoplasmic reticulum and transported via the Golgi body to the cell surface. Here, it forms patches with which the M protein associates. The genomic length negative strand RNA molecules associate with N, L and NS (P) proteins forming the core nucleocapsids. This, in turn, associates with the M protein at the inner surface of the plasma membrane or perhaps in the cytoplasm. The interaction between nucleocapsid and M protein causes the former to change configuration so that it appears more condensed. The nucleocapsid then buds through the membrane.
Rabid animals become aggressive and harbor the virus in saliva and thus transmission is frequently via animal bites. In rare cases, Rabies has been transmitted by corneal transplant or transplant of other tissues, or through contact of infected saliva with mucosal membranes or an open wound in the absence of a bite. The CDC states: “Inhalation of aerosolized Rabies virus is also a potential non-bite route of exposure, but other than laboratory workers, most people are unlikely to encounter an aerosol of Rabies virus”. It has been suggested that people in infected bat caves may be exposed to aerosolized virus. Most bats are not infected.
The virus binds to nerve or muscle cells at the site of the inoculation via nicotinic acetylcholine receptors. Here the virus can remain for a prolonged period of time (up to several months). The virus can replicate in muscle cells at the site of the bite with no obvious symptoms. This is the incubation phase.
The virus then moves along the nerve axons to the central nervous system using retrograde transport. The virus arrives at the dorsal root ganglia and the spinal cord. From here, spread to the brain occurs. A variety of cells in the brain can be infected including in the cerebellum, the Purkinje=s cells and also cells of the hippocampus and pontine nuclei. This is the prodromal phase. Infection of the brain leads to encephalitis and neural degeneration although elsewhere the virus seems to cause little in the way of a cytopathic effect. Involvement of the brain leads to coma and death. This is the neurological phase and during this period, the virus can spread from the central nervous system, via neurons, to the skin, eye and various other sites (adrenals, kidneys, pancreatic acinar cells) and the salivary glands (figure 4).
There are various factors that determine the timing of the onset of symptomatic Rabies but most important are the number of virus particles in the infection and how close the bite is to the brain. The immunological status of the patient is also important. It should be noted that the immune response to naturally acquired virus is slow and a good neutralizing response is not seen until the virus has reached the brain which is too late for survival. Cell-mediated immunity plays little role in a Rabies infection. Rabies is almost always fatal and only three survivors of symptomatic Rabies have been documented. Nevertheless, a good immune response that eliminates the infection, can be achieved using a vaccine even after infection because of the long incubation phase.
Rabies is usually transmitted by an animal bite. Worldwide most cases arise from a dog bite. Canine Rabies is prevalent in Latin America, Asia and Africa.
In recent years (1990 - 2004), in the US the majority of cases (35 out of 47) have been associated with bat rabies; of the remaining cases, two were acquired in the US (one dog/coyote like-strain and one raccoon strain) and 10 were acquired outside the US (all dog/coyote like strains).
Many animals in the US are infected with Rabies viruses, including raccoons (especially along the eastern seaboard states), skunks, coyotes, and foxes. Small rodents are rarely infected, but there have been cases reported, especially in woodchucks. Dogs, cats and cattle are potential vectors - in the US immunization of pets has lessened the risk of pets acquiring Rabies from wild animals. Bats also carry rabies, although most bats are not infected. Bats have very small, sharp teeth, and people who are bitten may not be aware of the bite, or do not bother to do anything about it. With most bites from other rabid animals, the victim normally seeks treatment because the bite is more serious and also because the animal appeared to behave in a suspicious fashion; the level of awareness seems to be lower for suspiciously behaving bats. Immunization of pets and prompt response for bites from most suspicious animals may explain why bat-transmission of Rabies has been the predominant mode of transmission in recent years.
In many cases of bat-associated rabies, there is no record of a bite. In some cases, the victim or their family may be aware that they handled a bat or that an oddly behaving bat was found (e.g. a bat which is active by day, is easily approached, is unable to fly, is in a room in a house or on a lawn). However, if the victim is not able to answer questions it may be difficult to obtain a history of bat contact since they may not have found the incident worth mentioning to anyone.
Human to human transmission has occurred in a few cases of corneal transplants (when it was not realized that the encephalitis was due to rabies). This has led to stricter criteria in screening of potential donors for encephalitis so that those who might have Rabies (or Creutzfeld-Jakob disease) are not accepted. Recently (2004) an organ donor who died of a brain hemorrhage also had Rabies and it was transmitted to 4 recipients. Apart from transplant cases, no human-human spread of the disease has ever been documented.
Overt symptoms clearly define symptomatic Rabies in people who suffer animal bites but by this time, therapeutic intervention is too late. After a bite, laboratory tests can determine whether an animal is indeed rabid. The presence of Rabies virus in an animal or an infected person is determined by multiple tests:
Serology (neutralizing serum or cerebrospinal fluid antibodies in an unvaccinated person are diagnostic but usually are only detectable late in disease).
Immunofluorescence antigen determination using biopsy skin, brain or corneal specimens (figure 8). A full thickness nuchal skin biopsy (skin biopsy from the nape of the neck in which the observer looks at the nerves at the base of the hair follicles) or brain biopsy can be examined for Rabies antigen using a direct fluorescent antibody test.
Saliva may be tested for Rabies virus RNA by RT-PCR (reverse transcription-polymerase chain reaction) or by isolation of the virus.
Histologically very characteristic is the presence of Negri bodies. These are eosinophilic intracytoplasmic inclusions formed by aggregates of nucleocapsids in neurons of about 50 to 80% of infected humans (table 3 and figure 7). They are typical of rabies, but the results need to be read by someone experienced with Rabies and there can be false positives - so all such results need to be confirmed by another method.
Other tests include the growing of virus in the brains of mice or in culture, after which antigen tests are used to determine the presence of virus. Also anti-rabies antibodies can be detected BUT only very late in the disease. Polymerase chain reaction (PCR) can also be used to detect virus
Table 3Histopathologic evidence of Rabies encephalomyelitis (inflammation) in brain tissue and meningesMononuclear infiltration1= Perivascular cuffing of lymphocytes or polymorphonuclear cells or inflammation around a blood vessel CDC Lymphocytic foci 2= Babes nodules consisting of glial cells Image: CDC Negri bodies (see below)
Negri body in infected neuron CDC
Negri body in brain cell
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Rabies virus budding from an inclusion (Negri body) into the endoplasmic reticulum in a nerve cell. A. Negri body.
B. Notice the abundant RNP in the inclusion.
C. Budding Rabies virus. CDC
PREVENTION AND TREATMENT OF A PERSON WHO MAY HAVE BEEN EXPOSED
The wound should be immediately and thoroughly washed with soap and water, then treated with 40-70% ethyl alcohol or an antiseptic such as benzyl ammonium chloride. The State Health authorities should be promptly informed. The risk of exposure to Rabies and whether prophylactic treatment should be given are determined in consultation with the State Health Department. If the animal is available, the brain should be examined for Rabies virus antigen by fluorescent antibody. (In some cases, if the bite was from a domesticated cat or dog, the animal may be kept under close observation).
Rabies vaccine. This is an inactivated vaccine and is strongly immunogenic. It is grown in human diploid cells or rhesus monkey lung cells and is more potent and has fewer side effects than the vaccine used in the early 1980's. A purified chick embryo cell grown vaccine is also available. The vaccine is administered as a series of injections over a 4-week period. HRIG (human Rabies immunoglobulin) is also given:
Human Rabies immunoglobulin (HRIG). HRIG is prepared from the plasma of hyperimmune donors. Up to half of the recommended dose is infiltrated into the wound area if possible. The remainder is given as an intramuscular injection. A separate syringe and a separate site are used for the HRIG and the vaccine so that the HRIG does not neutralize the vaccine.
So far there has never been a case of someone who received appropriate post-exposure prophylaxis in the US developing rabies. (About 40,000 people per year are treated in the US).
Post exposure prophylaxis
People at risk for Rabies infection may be vaccinated as a preventive measure. Such individuals include
certain people in areas with enzootic Rabies who are at risk for exposure to rabid animals: veterinarians and their staff, wildlife control workers, spelunkers (mainly those cave explorers who go into undeveloped caves with bat colonies); travelers who will be spending more than a month in areas with enzootic rabies.
People at high risk for exposure to rabid animals should have regular serologic testing and booster vaccinations when necessary.
If a vaccinated person is exposed to rabies, they still need to get post-exposure prophylaxis, but the number of post-exposure vaccination shots is reduced and HRIG is not used.
If symptoms are localized to the site of the bite, aggressive antiviral therapy (vaccine, HRIG, ribavirin, interferon, monoclonal antibodies, etc) may be tried. There is no specific anti-viral treatment once CNS symptoms develop. Intensive supportive care is given. Five of the six known survivors of Rabies infection received prophylaxis prior to developing clinical symptoms
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