Rabies Virus Introduction
Rabies virus is the causative agent of Rabies.
Lyssa = rag or madness ( symbol of Rabies)
It is an acute infection of the central nervous system (CNS) caused by the rabies virus that is almost always fatal. The virus is transmitted to humans through the bite of rabid animals.
Morphology of Rabies virus
- 76× 175 nm
- Helical symmetry nucleocapsid surrounds the spiral-shaped core of nucleic acid that contains a single-stranded RNA genome and viral transcriptase.
- Outer lipid envelope contains haem agglutinating peplomere spikes (10 nm in length)
- The negative-stranded RNA codes for 5 proteins – G, M, N, L, S.
- Rabies virus has a single antigenic type residing in the envelope.
- Resistance to cold, dry, and decay.
- Remains infective for several weeks in the cadaver.
- Sensitive to alcohol, lipid solvent, Chlorohexadine
- Inactivated by Phenol, Formalin, Betapropiolactone (BPL) Sunlight, Ultraviolet, and Heat at 50°C x 1 hr and 60°C x 5 minutes.
Transmission and Pathogenicity:
The natural hosts of Rabies virus are wild animals: Such as;
Dog, Jackel, Cats, Bats
Infected bats are constantly viremic
Through the bite of rabid animal mostly by a rabid dog.
Person to person transmission through corneal transplantation.
Infection with rabies virus causes encephalomyelitis.
Upon entry of Virus
- Multiple into muscles fibers then
- Passes into peripheral nerve and
- Eventually reaches to CNS where it multiples and via peripheral nerve, salivary and other organs. ( During its transport within the nerve, the virus is sheltered from the immune system.)
The heart, skeletal muscles, adrenal gland, and salivary glands become infected. From the salivary glands, the virus enters the saliva. Thus, there is no viremic stage.
Infected neurons contain an eosinophilic cytoplasmic due to the accumulation of ribonucleoprotein – called Negri body.
The antibodies are produced quite late, therefore; these are not useful for protecting against fatal complications of rabies.
However, if a person receives passive immunization before Virus enters to CNS, death may be avoided.
Incubation period: 20 -90 days which depends upon
- dose of inoculum
- severity of wound
- length of the neural path from the wound to the brain.
As soon as CNS is affected, features of furious or dump ( paralytic) rabies develop, depending upon the location of the neuronal infection, either limbic system or neocortex, respectively.
Hydrophobia ( in furious type ): due to painful spasmodic contraction of muscles that control swallowing resulting.
- Increased secretion of saliva
- Electricity disturbances
Death usually occurs within 2.-3 weeks.
in furious type: death occurs due to respiratory or cardiac failure.
In dumb type: death occurs due to extensive respiratory muscle paralysis.
Laboratory diagnosis of Rabies Virus
The diagnosis of animal and human rabies can be made by the following methods:
(2) virus cultivation
(4) virus antigen detection and
(5) molecular test (PCR)
Demonstration of the virus in rabid animals is more important than that inpatients.
A- Diagnosis is man : ( Diagnosis is made during life usually based on clinical findings)
- Corneal impression
- Skin biopsy ( from and neck)
Demonstration of antigen
- Direct Immunofluorescence test
After the death ( Post mortem diagnosis)
- Demonstration of Negribodies in brain tissue ( 80%)
- Take impression smear
- Stain with Seller’s ( Basic fuchsin and Methylene blue in alcohol)
Negri bodies are cytoplasmic inclusion in the nerve cells
Negri bodies appear as round or oval purplish structures ( 3-27 µm) with basophilic granules within the cytoplasm of neurons.
- Isolation of Virus by mouse inoculation with brain tissue, CSF, or saliva and immunofluorescence test in brain tissues of infected mice.
B- Diagnosis in animal:
- Watch for S/S and death
- Brain tissue of rabid animal:
Collect one-piece into Zenker’s fixative for demonstration of Nefribodies
- BHK cell lines
- Chick embryo fibroblast
- Huan diploid cells in primary and continuous cell culture.
Molecular test: RT-PCR is now the test of choice for a quick diagnosis of rabies infection. It can be applied to samples of skin and saliva.
- Successful treatment is not applicable in clinical use.
Once rabies has been established, there’s nothing much that can be done except intensive supportive care.
Postexposure prophylaxis: In cases of animal bites, dogs and cats in the endemic rabies area should be observed for 10 days. If signs develop, they should be killed and their tissues examined in the laboratory. Wild animals are not observed, but the animal should be killed and examined if captured. Local treatment of wounds and active and passive immunization are essential components of post-exposure prophylaxis. Wound treatment-Surgical debridement should be performed. The wound should not be sutured. Exp Passive immunization-human rabies immunoglobulin around the wound area; to be complemented by an i.m. Dose to provide short-term protection. There is convincing evidence that combined treatment with rabies immunoglobulin and active immunization is far more effective than active immunization alone. Equine rabies immunoglobulin (ERIG) is available in many countries and is significantly cheaper than HRIG. experimentally, the incidence of rabies in animals may be reduced by local treatment alone.
Active immunization-The best preparation available is the human diploid cell vaccine. The vaccine is usually given in the deltoid region, and 5 doses are usually given.
Preexposure prophylaxis: People who are regularly at high risk of exposure, such as vets, laboratory workers, animal managers, and wildlife officers, should be considered pre-exposure prophylaxis by active immunization with a cell culture vaccine.
Neural vaccine : ( Suspension of nervous tissues of animals infected with fixed rabies virus)
- Pasteur ( 1885) Initial vaccine was Pasteur’s crowd vaccine, prepared from infected rabbit spinal cord.
Types of neural vaccine
1- Semple vaccine : ( Semple 1911 at CRI, Kasauli, India)
- 5% suspension of infected sheep brain with fixed virus and
- Inactivated with phenol at 37°C.
2- Betapropiolactone (BPL) Vaccine:
Same as Semple type, inactivated with BPL in place of Phenol
3- Infant brain vaccine
The vaccine developed with the use of infant mice, rats of the brain.
Encephalitogenic factor in brain tissue is a basic protein associated with the myelin which is scanty or absent in the nonmyelinated neural tissues in the newborn animal.
- Egg vaccines:
a) Duck egg vaccine:
Inactivated with BPL and Poorly immunogenic
b) Live attenuated chick embryo vaccine : ( made from flurry strain)
- Low egg passage vaccine ( 40 -50 eggs passage for immunization of dog)
- High egg passage vaccine (180 passage) for cattle and cats.
2. Tissue culture vaccine:
- Human diploid cell vaccine:
Prepared from human diploid cell lines and inactivated with BPL or Trinitrobutyl phosphate
-Free from serious side effects.
Other vaccines: ( equally effective)
Prepared by growing fixed Rabies virus with into:
Chick embryo fibroblast
BHK cell lines
Dog kidney cell lines
Vero cell line
Vervet or Africa green monkey kidney cell lines
3- Subunit vaccine:
The glycoprotein subunit of the virus surface ( protective antigen) has been cloned and a recombination DNA vaccine has been produced. Which is still on trial.
Failure of prophylaxis: HDCV has been used to treat thousands of people exposed to possible rabid animals over the past 12 years and has been proven to be effective. At least 16 people treated with HDCV have died of rabies following exposure. All patients had major exposures and, in the majority of cases, the incubation period was short, 21 days or less. Treatment was frequently not started immediately within 24 hours and only half of the patients received a combination of serum and vaccine. Despite optimal treatment, at least one person died.
- Human Diploid Cell Vaccine (HDCV)—HDCV was introduced in 1978. It is grown in WI-38 (US) or MRC-5 (Europe) cells. The vaccine is highly effective, antibodies have been shown in 100 percent of all recipients in several studies. Serious adverse reactions to HDCV have been extremely rare.
- Neural Vaccine is unsatisfactory and poorly immunogenic as they contain mostly nucleoprotein and a small quantity of glycoprotein. It may contain residue of live virus.