BECOME A MEMBER
สมัคร ! สมาชิกชมรมรักสุขภาพ
ฟรี ข่าวสาระความรู้เรื่องสุขภาพ

top
 VIROLOGY - LECTURE 
 TWENTY  
 RABIES  

Virology Lecture 1
BASIC VIROLOGY: DEFINITIONS, CLASSIFICATION, 
MORPHOLOGY AND CHEMISTRY  

Virology Lecture 2
 
 DNA VIRUS REPLICATION 
 STRATEGIES  

 RNA VIRUS REPLICATION
 STRATEGIES  

 ONCOGENIC VIRUSES 
 
 SEVEN  HUMAN  
 IMMUNODEFICIENCY VIRUS  
 AND AIDS  

 PICORNAVIRUSES - 
 PART ONE  
 ENTEROVIRUSES  
Virology Lecture 

 HERPES VIRUSES  

Virology Lecture 9

INFLUENZA VIRUS

Virology Lecture 10

MEASLES (RUBEOLA) 
AND MUMPS VIRUSES  

Virology Lecture 11
RUBELLA (GERMAN 
 MEASLES) VIRUS

Virology Lecture 12
RABIES  




Health Navigation






สนใจรายละเอียดเพิ่มเติม
กรุณาแจ้งให้ทึมงานเพื่อ
จัดเตรียมหาสาระให้



Contact : 
info@thailabonline.com
ชมรมเรารักสุขภาพ 
ไทยแล็ปออนไลน์



 

  VIROLOGY - LECTURE TWENTY  

  RABIES  

Dr Richard Hunt

MBIM lecture 76

Rabies virus belongs to the family: Rhabdoviridae. (Greek: Rhabdos: rod). They can infect a variety of animals and plants

 

TABLE 1 -  Rhabdoviruses

Type Virus Distribution Species infected Disease
Vesiculovirus Vesicular stomatitis virus (VSV) Carribean Cattle, pigs horse Acute, self limiting
Lyssavirus Rabies virus Worldwide Many mammals including humans Slow, progressive
Plant rhabdoviruses        
Other animal rhabdoviruses     Mammals, fish, birds, arthropods  

 

Structure of rhabdoviruses (figure 1)

rna6.jpg (100851 bytes) General structure of a rhabdovirus rabies-wad.gif (67510 bytes) Negative stain electron micrograph of rabies virus  Wadsworth Center, NY Dept of Health

Figure 1 - Rhabdovirus structure

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)

cycle.jpg (99302 bytes)  The cycle of rabies infection and replication CDC

Figure 2 - Replication of rabies virus

Binding
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.

Penetration
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.

Transcription
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.

genome.gif (7785 bytes) The rhabdovirus genome  CDC

Figure 3 - Rhabdovirus genome

Replication
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

Assembly
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.

 

Pathogenesis

Vesicular Stomatitis Virus (VSV)

VSV infects cattle in Carribean and occasionally in US. It is also found in horses and pigs but rarely humans

 

Rabies

WEB RESOURCES
Behavior of a rabid raccoon
North Carolina Depatment of Health and Human Services
Requires RealAudio
The sound of a rabid cow
North Carolina Depatment of Health and Human Services
Requires RealAudio
CDC Rabies Page
 

Rabid animals become aggressive and harbor the virus in saliva and thus transmission is frequently via animal bites, although there is some evidence for spread via aerosols such as in bat urine in caves. The virus has also been transmitted by transplantation of corneas from infected individuals.

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.

 

  CDC  route.jpg (56128 bytes) © Richard Hunt
1. Raccoon is bitten by a rabid animal
2. Virus enters wound via saliva
3. Virus spreads through nerves to spinal cord and brain
4. Incubation period of 3-12 weeks with no symptoms
5. In brain the virus replicates and spreads to other tissues including the salivary glands. Signs of disease occur
6. The animal dies within a week

Figure  4 - Rabies pathogenesis

Epidemiology

Rabies is spread, usually by bites from animals, to other animals and to man. It is thus a zoonotic infection. The most important reservoir as far as humans are concerned is the dog but other animal reservoirs of importance include, in the United States, racoons, bats, skunks and foxes. Rabies is found in most continental countries of the world but not in some island nations such as the United Kingdom and Australia. The incidence of rabies cases in various animals is shown in figure 5.

 

WEB RESOURCES

Bats and Rabies (CDC)

Rabies: Question and Answer (CDC)

 
 

 

Table 2 -  Major animal reservoirs of rabies

North America Skunks, raccoons, bats, foxes
South America Rabid dogs, vampire bats
Europe Badgers, foxes

In many western countries where rabies is endemic, vaccination of animals has reduced the rate of human disease and in the United States there is approximately one case of human rabies per year. In countries such as the United Kingdom, where there is no rabies in the wild animal population, vaccination is not used. In some other countries, rabies is much more of a problem. For example, India records about 25,000 cases of human rabies per year, mainly from dog bites. In South America, rabies transmission by vampire bats is a major problem for the cattle industry (table 2).

Symptoms

Vaccination, even after exposure, is extremely effective at preventing disease. Without such treatment, rabies is almost invariably fatal. During the incubation/prodromal period, symptoms include: pain or itching at the site of the wound, fever, headache and gastrointestinal problems. After this period (usually of up to two weeks), CNS infection is apparent. In up to half of patients, hydrophobia is seen. This fear of water is the result of the pain associated with drinking. There are also seizures and hallucinations. In some patients paralysis is the only symptom and this may lead to respiratory failure. Following the neurological phase, the patient becomes comatose. Because of the neurological problems including respiratory paralysis, death ensues.

Diagnosis

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 can be determined by serology and immunofluorescence antigen determination using biopsy skin, brain or corneal specimens (figure 8). Histologically very characteristic is the presence of Negri bodies. These are intracytoplasmic inclusions formed by aggregates of nucleocapsids in neurons of about 90% of infected humans (table 3 and figure 7). 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 (figure 6).

Table 3
Histopathologic evidence of rabies encephalomyelitis (inflammation) in brain tissue and meninges
Mononuclear infiltration perivasc.jpg (28725 bytes)  Perivascular cuffing of lymphocytes or polymorphonuclear cells or inflammation around a blood vessel  CDC
 Lymphocytic foci  Babes2.jpg (18435 bytes) Babes nodules consisting of glial cells  Image: CDC
 Negri bodies  (see below)

 

PCRgel.jpg (9317 bytes)  PCR test results for the presence of rabies virus. The arrows indicate positions of positive bands CDC 

Figure 6

 

negri1.jpg (11993 bytes) Neuron without Negri bodies  CDC negri2.jpg (8458 bytes) Negri body in infected neuron   CDC
negri-bris.jpg (494040 bytes) Negri body in brain cell © Bristol Biomedical Image Archive. Used with permission negri-perk.jpg (120461 bytes)  Histopathology of rabies, brain. Characteristic Negri bodies are present within a Purkinje cell of the cerebellum in this patient who died of rabies.  CDC/Dr. Makonnen Fekadu  maf1@cdc.gov 
abc2.jpg (15325 bytes)  Rabies virus-infected neuronal cell with intracytoplasmic inclusions (Negri bodies). The red stain indicates areas of rabies viral antigen by using IHC or avidin-biotin complex CDC  
negri3.jpg (24339 bytes)  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
RNP.jpg (19739 bytes)  Ribonucleoprotein. Notice the abundant strands of coiled RNP (almost
everything in the image is RNP). CDC

Figure 7

 

 

fapos.gif (4413 bytes) fanggood.jpg (1480 bytes) Direct fluorescent antibody test (dFA)
The dFA test is based on the principle that an animal infected by rabies virus will have rabies virus protein (antigen) present in its tissue. Because rabies is present in nervous tissue (and not blood like many other viruses), the ideal tissue to test for the presence of rabies antigen is brain. The most important part of a dFA test is fluoresecently-labeled anti-rabies antibody. When labeled antibody is added to rabies-suspect brain tissue, it will bind to rabies antigen if it is present. Unbound antibody can be washed away and the areas where the antigen has bound antibody will appear as a bright fluorescent green color when viewed with a fluorescence microscope. If rabies virus is absent there will be no staining. The rabies antibody in the dFA test is primarily directed against the nucleoprotein  of the virus. Rabies virus replicates in the cytoplasm of cells, and infected cells may contain large round or oval inclusions containing collections of nucleoprotein (N) or smaller collections of antigen that appear as dust-like fluorescent particles if stained by the dFA procedure   CDC

Figure 8

Therapy

Post-exposure prophylaxis is the major form of treatment when it is suspected that the patient may have come in contact with a rabid animal. Such contact is most often a bite but could also include contact with tissue from a rabid animal. In the United States up to 20,000 rabies inoculations are given to humans every year. After a bite, it is recommended to wash the wound and to instill anti-rabies serum into the wound. After this, the vaccine is given, accompanied by either horse anti-rabies serum or human rabies immune globulin. This suffices until the patient develops an immune response to the virus. Four more vaccinations are then given over a period of a month (days 3, 7, 14 and 28 after exposure to the virus). Pre-exposure vaccination is recommended for persons who may come in contact with tissue of rabid animals and seems effective for up to two years.

The vaccine

Vaccination against rabies has a long history, the first vaccine having been developed by Louis Pasteur. His vaccine was an attenuated form of the virus produced by inoculation of rabbit spinal cord. The present (HDCV) rabies vaccine is a chemically inactivated one that is produced in human diploid tissue culture cells. Previous vaccines were produced in the brains of animals but these gave more side effects.

 

 


 






We subscribe to the
HONcode principle
of the Health on the
Net Foundation

 
About Us | Add URL I Privacy Policy | Member Register | Health Shop | Contact Us | Health Board | Advertising
Disease / Condition | Head Line News | Healthcare | Diagnostic | Alternative Medicine |
Health Game Zone


1999-2000 Thailabonline.com. All rights reserved. 
By using this information service,    you accept the terms of our Visitor Agreement. Please read it. 
The material on Thailabonline.com and iHealthsite.net are for informational purposes only and is not 
a substitute for medical advice or treatment for any medical conditions.   You should promptly seek 
professional medical care if you have any concern about your health, and you should always consult 
your physician before starting a fitness regimen.
”Thailabonline.com” and “ihealthsite.net” are trademarks of Crystal Diagnostics Co.,Ltd.