A. STRUCTURE / COMPOSITION
|Structure of Rabies Virus|
- Members of family Rhabdoviridae are rod or bullet- shaped (75 x 180 nm); one end conical, other planar ( concave).
- Membranous envelope with protruding spikes or peplomers (10 nm) which are composed of trimers of viral glycoprotein (G) spikes, doesn’t cover the planar end.
- Beneath the envelope is the membrane or matrix (M) protein layer. Membrane may project outwards from planar end of some virion forming a bleb.
- The core of the virion consists of helical RNP ( group specific antigen). Genome is unsegmented, linear, ss(-)sense RNA ( 12kb, MW 4.6 X 106) and the virion contain an RNA dependent RNA polymerase.
- survives at 4°C for weeks and at -70°C for years.
- Sensitive to ethanol, iodine- preparations, 40– NH4+ compounds, soap, detergents and lipid solvents (ether, chloroform, acetone).
- Inactivated by phenol, formalin, ß- propiolactone( BPL), UV or sunlight; thermal inactivation at 50°C in 1 hr, and at 60°C in 5 minute.
- Inactivated by CO2, so, for storage in dry ice, it should be sealed in glass vials.
- Attachment: Rabies virus attaches to host cells via its glycoprotein spikes; nicotinic acetylcholine receptor may serve as a cellular receptor (in neurons).
- Entry: Entry is by endocytosis.
- Replication and Assembly:
- Viral RNA polymerase transcribes viral genome into 5 mRNA species; the monocistronic mRNA species code for 5 virion proteins: nucleocapsid (N), polymerase proteins (L,P), matrix (M), and glycoprotein (G).
- Genome RNP is a template for complementary (+) sense RNA, which is responsible for a generation of (-) sense progeny RNA. Same viral protein serves at polymerase for viral RNA replication as well as transcription.
- newly replicated genomic RNA associates with viral transcriptase and nucleoprotein to form RNP cores in the cytoplasm; M- protein is important in packaging the RNA.
- Release: The newly formed virus particles acquire an envelope by budding through the plasma membrane.
- Rabies virus has a wide host range; all warm-blooded animals including human can be infected. Susceptibility varies; high (fox, wolves, bats, cats), to moderate ( dogs, sheep) to low ( opossums).
- Virus is widely distributed in the nervous system, saliva, urine, lymph, milk and blood of infected animals.
- When freshly isolated in lab, strains are referred to as “ street virus”; these cause fatal encephalitis in lab animals following inoculation by any route, after a long and variable incubation period ( 1-12 weeks) and regularly produce intracytoplasmic inclusion bodies (Negri bodies).
- Serial brain to brain passage in rabbits of the street virus yields a fixed virus that no longer multiplies in extraneural tissues; after intracerebral inoculation, it produce fatal encephalitis after a short and fixed incubation period of 6.7 days; negri bodies usually not demonstrable.
There is a single serotype of rabies virus; however there are strain differences among viruses isolated from different species.
- substitution at amino acid position 333 of gp results in loss of virulence, where Arg has replaced Gln or Ile in the more pathogenic variant.
- purified spikes containing viral gp elict neutralizing( protective) antibody; thus provide a safe and effective subunit vaccine.
- antiserum prepared against purified nucleocapsid Ag used in diagnostic IFT, the antibody is not protective and are group specific.
- virus possesses haemagglutinating activity ( property of gp spike), optimally seen with goose erythrocytes at 0-4°C and PH 6.2.
- GP spikes are inactivated by heat ( 56°C for 30-60 min), ether, trypsin, deoxycholate or Tween 80, but not by BPL.
- Inhalation while in bat- infested cave
- aerosols released during centrifugation of infected materials in the lab
- Ingestion of flesh of rabid animals (high dose would be necessary)
- short prodrome.
- acute neurologic (encephalitic) phase
- coma / death
prodrome lasting 2-10 days may show malaise, anorexia, headache, photophobia, nausea and vomiting, sore throat, fever; there is profound sense of apprehension, anxiety, agitation, irritability, insomnia or depression.
During the acute neurologic phase( 2-7 days), there are signs of nervous system dysfunction which begins with hyperactivity → bouts of bizarre behaviour, agitation or seizures.
• pathognomic feature is difficulty in drinking, together with intense thirst; attempts to drinking may bring painful spasms of pharynx / larynx producing choking and gagging, patients develop a dread of even the sight or sound of water ( hydrophobia).
• generalized convulsions follow death occurs with in 1-6 days due to respiratory arrest during convulsion.
– furious rabies→ predominantly encephalitic disease with neurologic dysfunction.
– dumb rabies → paralytic illness; with symmetrical ascending paralysis followed by coma and death, occurs in about 20% patients.
- 99% infection who develop symptoms end fatality.
- It is essential that individuals at high risk receive protective immunization
- Introduction of cell culture vaccines which are free from serious complication has made pre –exposure immunization in humans safe and feasible.
• local treatment : prompt cauterization of wound destroys the virus
– wound should be thoroughly cleaned with soap solution / detergent and running water for 5 minute, followed by application of 40-70% alcohol or tincture of iodine or 40– NH4 compound ( cetavlon).
– scrubbing should be avoided and suturing delayed if possible.
active immunization: with inactivated whole virus vaccine ( cell – culture grown), containing atleast 2.5 IU/ dose, given into deltoid muscle.
All vaccines for human use contain only inactivated rabies virus.
– associated with serious risk of neurological complications (Postvaccination encephalitis) due to the presence of encephalitogenic factor ( a basic protein associated with myelin)
– low potency per dose; poor immunogen.
– complete treatment involves up to 23 painful injections.
1.egg vaccine: duck embryo vaccine → discontinued due to poor immunogenicty
live attenuated vaccine → prepared in chick embryos ( eg flury strain)
– used for animals but not for humans
– low egg passage of 40-50 (LEP) for dogs.
– High egg passage of 180 (HEP) for cattles and cats.
- Human diploid cell vaccine (HDCV): virus adapted to growth in WT- 38 human normal fibroblast cell line.
- virus preparation concentrated by ultrafiltration and inactivated with BPL
- highly antigenic and free from serious side effects; high cost.
- Rabies vaccines, adsorbed ( RNA): made on diploid cell line derived from fetal rhesus monkey lung cells; inactivated with EPL and concentration by adsorption to aluminum phosphate.
- Purified chick embryo cell vaccine ( PCEC): prepared from fixed rabies virus strain fury LEP grown in chicken fibroblasts; inactivated with BPL and further purified by zonal centrifugation.
- Rabies immunoglobulin Human ( HIRG)
- prepared by cold ethanol fractionation from plasma of hyperimmunized humans
- fewer adverse reaction than to equine IG.
- Antirabies serum, equine
- concentrated serum from horses hyper immunized with rabies virus.
- Indicated for persons at high risk ( researchers, lab workers, veterinarians )
- To attain an antibody level presumed to be protective by means of vaccine administration prior to any exposure.
- Two injections of 1 ml vaccine given into deltoid muscle 4 weeks apart; reinforcing dose given at 12 months.
POST EXPOSURE PROPHYLAXIS
The decision to administer rabies antibody, rabies vaccine or both depend on
- no successful treatment for clinical rabies; symptomatic treatment may prolong life, but the outcome is almost always fatal.
- preexposure vaccination is desirable for all persons who are not at high risk of contact with rabid animals, but this does not eliminate the need for prompt post exposure prophylaxis if exposure to rabies occurs.
- In countries where dog rabies exists, stray animals should be destroyed and vaccination of pet dogs and cats should be mandatory.