Rabies virus: Properties, life cycle, pathogenesis, diseases and diagnosis

What is Rabies?

Rabies virus is also known as street virus.
It causes rabies which is an acute infection of CNS and is always fatal in untreated cases.
The virus is transmitted to human from the bite of rabid animals especially from the saliva.

It is a Rhabdovirus which is large rod or bullet shaped.
It belongs to Rhabdoviridae family and lyssa virus genus.
Shape and size:
- These are bullet shaped virus measuring 7 X 180nm diameter (50-95 nm diameter, 130-139nm length).
- The virion is composed of RNA, protein, lipid, and carbohydrate.
Genome:
- The viral genome consists of non-segmented -ve sense single stranded RNA.
Nucleocapsid:
- The nucleocapsid is spiral or helical composed of RNA, phosphorylated nucleoprotein and large RNA dependent RNA polymerase I.
Envelope:
- The lipid bilayer envelope derived from the host cell membrane contains virus encoded glycoprotein (G-spikes).
- These glycoproteins are found to bind specifically to cellular receptor and confirm neurotrophism in infected cells.
- These are the major factors for neuro-invasiveness and pathogenicity on the inner surface of viral envelope, closely associated with G-protein is a second membrane known as matrix protein which is thought to play role in viral budding.

Virus replication can broadly be categorized into following:
- Attachment, penetration and uncoating
- Transcription, translation and replication of viral genome.
- Maturation and release

Step I: Attachment, penetration and uncoating:

Rabies virus attaches to cell surface via glycoprotein.
Attachment of virus by glycoprotein takes place at neuromuscular junction possessing acetylcholine receptor, phosphatidyl serine receptor, neuronal cell adhesion molecule or P75 neurotropin receptor.
After the attachment virus enters the host cell by endocytosis and fuses with the endosome thereby releasing ribonucleoprotein complex (RNP) into the cytoplasm.
The fusion of virus with endosome takes place as the pH decreases below 6.2

Step II: Transcription, translation and reproduction:

The negative sense ssRNA is transcribed by virion associated RNA polymerase to positive sense mRNA which codes for structural protein and polymerase complex.
The positive sense RNA serves as template for the synthesis of viral genome and +mRNA that gives rise to different viral proteins.

Step III: Maturation and release by budding:

Virus matures by budding from the cytoplasmic membrane.
In this process the newly replicated RNA associates with viral polymerase protein to form coiled and condensed RNP core in the cytoplasm.
RNP then assembles with matrix protein of cell surface and finally acquires the envelope by budding through the cell surface.

Rabies virus is excreted in saliva of rabid animal so, human acquire virus by the bite of rabid animals.
Virus multiplies in muscle or connective tissue at the site of inoculation and then enter peripheral nerves via motor end plates at neuromuscular junction and spread upto the CNS.
Virus, however, can enter directly into CNS without local multiplication.
It multiplies in the grey matter in brain and propagates through efferent nerves to salivary gland and other tissues like kidney, heart, cornea, retina, pancreas.
The highest titre of virus is seen in submaxillary gland.
Rabies virus produces a specific eosinophilic cytoplasmic inclusion with basophilic granules called the Negri bodies.
The Negri body is filled with viral nucleocapsid and forms the pathognomonic basis of rabies diagnosis.
The absence of negri body however doesn’t rule out the absence of rabies virus.
Susceptibility to rabies infection and incubation period may depend on host age, genetic makeup, immunity, viral strain involved, inoculum size and the distance virus has to travel from the point of entry to CNS.

Incubation period in humans is 1-3 months but may be as short as 1 weeks to as long as many years.
It is usually shorter in children than in adults.
The clinical spectrum of rabies can be divided into three phases.

It lasts for 2-10days and is characterized by any of the following non-specific symptoms, malaise, anorexia, headache, photophobia, nausea and vomiting.
Unusual sensation at the wound site is also observed.

2. Acute neurological phase:

It lasts for 2-7 days and is characterized by the presence of nervous system disorder like nervousness, apprehension, hallucination, lacrimation, pupillary dilation, and increased salivation, hydrophobia and painful throat muscle while swabbing.

Specimens:

Acute mortem – hair follicles, saliva
Post mortem- salivary gland, brain stem, hippocampus, cerebellum.
Rabies virus can cause severe disease and is dangerous for many person in contact.
Because of this risk from contact and handling, The British Advisory Community on danger pathogen together with WHO has classified Rabies virus in hazard group III pathogen and should be handled in biosafety level 2.
In Rabies endemic area animals captured should be sent for laboratory confirmation of Rabies but without any delay post exposure treatment of the bitten person should be done.
Domesticated dogs and cats, particularly if previously vaccinated against Rabies should be observed in isolation for upto 10-14 days.
If they survive for that time, it is unlikely they were incubating rabies virus at the time of incident.
If they succumb or die anti-rabies treatment of bitten person should be strated.

Microscopy/ Histological examination:

This involves the examination of tissue infected with rabies virus rapidly and accurately using direct immunofluorescence which uses anti-rabies monoclonal antibodies.
Impression preparation of brain and cornea tissue is often used.
Similarly, a definite pathological diagnosis is based on the finding of Negri bodies in the brain or spinal cord.
Negri bodies are found in impression preparation or histological section.
These are sharply demarcated more or less spherical and 2-10 μm in diameter, consisting basophilic granules yield in an eosinophilic matrix.
Negri bodies are infiltrated with viral antigens and can be demonstrated by immune-fluorescence technique.