RABIES:
Rabies
is an acute encephalitis caused
by a virus and is
infectious to most mammals. It is spread
from animals to humans. Transmission of rabies is via an infected
animal’s
saliva usually through a bite though transmission may also occur when
an open
wound, scratch or mucous membrane is exposed to saliva. The disease
manifests
itself in two phases, the paralytic and excitable phase.
In non-vaccinated animals, rabies is almost
always invariably fatal. In Western Canada,
it is found mostly in carnivores and in several wild species acting as
reservoirs for the virus.
The rabies virus: (1)
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Classification
Order Mononegavirales
Family
Rhabdoviridae
Genera
(3 important genera)
Ephemerovirus
Vesiculovirus
Lyssavirus
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www.ivis.org
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This last genera includes
the rabies virus, Lagos bat, Mokola
virus, Duvenhage virus, European bat virus 1 and 2 and the Australian
bat
virus.
Rhabdoviridae
are non-segmented and characteristically bullet shaped viruses. (See
Figure 1). They are negative-stranded RNA (Ribonucleic acid) viruses
which requires an RNA
polymerase enzyme to convert it to a positive stranded RNA which can
then be transcribed and translated into viral protein (See Figure 3
below)
Structure : (1)
Viruses of the Rhabdoviridae
are 180 nm
long and 75 nm wide.
- its genome encodes for
five
proteins:
Nucleoprotein
(N)
Phosphoprotein
(P)
Matrix
protein (M)
Glycoprotein
(G)
Polymerase
(L)
Figure 2 - Longitudinal
view and cross-section of rabies
virus
Pathogenesis: (4)
Rabies is a viral zoonosis
and
therefore
can be passed between humans and animals, which is another reason why
vaccination protocols are crucial (See Vaccination Protocols and
Legislature). Most infections result from a
bite or scratch of an infected animal. Transmission is through the
animal's saliva which is contaminated with the rabies virus, and when
it comes into contact with the victim's mucous membrane, skin lesions,
etc... the virus is spread.
The rabies virus has a
selective affinity for nervous tissue but it first binds itself to
receptors
present on the muscle cells of the bite wound. The time of infection to
the onset of clinical
signs
varies with i) the amount of virus acquired through the bite, ii) how
much the tissue is
innervated
and iii) its proximity to the brain. The
larger the dose received and the closer the to the central nervous
system, the
faster the onset of signs. It can vary from 4 days to several years,
though on
average it is between 20 and 90 days. This is
another reason why quarantine and adherence to vaccination protocols
are important for future travels. (See VACCINES
AND PROTOCOLS)
Figure 3
- Viral incorporation and replication
http://0-www.cdc.gov.mill1.sjlibrary.org/ncidod/dvrd/rabies/the_virus/images/cycbig.jpg
Steps in Virus Infection and
Replication:
(See Figure 3 above)
Adsorption:
This is the initial step where the virus through the interaction of
cell receptors and viral proteins, the virus adheres to the host cell's
surface.
Penetration:
Viral entry into the host cell.
Uncoating:
Removal of viral envelope exposing its genetic material (NB:
Rhabdoviridae are negative stranded RNA viruses)
Transcription:
Rhabdoviridae are negative stranded RNA viruses which requires a RNA
polymerase enzyme to convert it to a positive stranded segment of mRNA
prior to translation.
mRNA:
RNA that serves as a template for protein synthesis. xray.bmc.uu.se/~kenth/bioinfo/glossary.html
Translation:
the process by which the mRNA code is converted to a sequence of amino
acids (a protein). genome.pfizer.com/glossary.cfm
Replication:
amplification of the virus genetic material in the host cell.
Assembly:
assembly of viral components
Budding: complete viruses
emerge from host cell.
(reference: 10 and 18)
