Virology--BIO

Rabies & The Rhabdovirus

Rabies virus - Rabies was first "isolated" by Dr. Louis Pasteur and coworkers in the 1880's after passage from rabbit to
rabbit using neural tissue. By drying rabbit CNS in sunlight, Pasteur et al prepared a vaccine since rabies virus was
inactivated by drying and sunlight (ultraviolet irradiation).

Virus Morphology

The most outstanding characteristic of the rhabdovirus is the bullet-shaped virion.  Such a shape is caused by a lipid envelope, embedded with glycoprotein peplomers, surrounding a helically wound nucleocapsid.  The virions tend to be approximately 70 nm wide and 170 nm long.  Matrix proteins can also be found
under the lipoprotein envelope.

Electron micrograph of the rhabdovirus at 64,000X magnification.  Notice the bullet-shaped virions surrounding the cell.  The dark circle in the cell is the Negri body.

Genome & Replication

     The genome of the rhabdovirus is a single linear molecule of minus sense ssRNA, 11-12 kb in size.  It encodes for five major proteins:
     N = structural nucleoprotein
     NS = phosphoprotein, binds promoter sequence and L gene
     M = matrix protein
     G = glycoprotein peplomer (site for Ab binding)
     L = transcriptase, 5'-cap methylase, 3'polyA polymerase, and
           protein kinase

The virus particle initiates it's replication cycle by the binding of its G protein spikes to receptors of the host cell surface.  Entry then occurs by endocytosis.  The virion fuses with a
vesicle where the uncoating process occurs.  Replication takes place in the cytoplasm; the genes are first transcribed,
processed to functional mRNAs, and finally translated into their final protein products.  Progeny viral RNA is synthesized from a + sense intermediate.  The new genome particles then get inserted into the preformed capsids at which time the
completed virions bud from the plasma membrane.  It is
characteristic of Rhabdoviruses that all these events occur in a portion of the cytoplasm that acts as a "virus factory."  These characteristic inclusion bodies of the Rhabdovirus is known as Negri bodies.

Rabies is transferred to humans through the bite of an
infected animal. In the case of humans, the most common mode of transmission is through dog bites. Rabies is endemic throughout the world except in Australia, Japan, Great
Britain, Hawaii, and many smaller islands including most of the islands of the Caribbean. Transmission from wildlife is becoming more common in countries such as the United States and Canada.

Rabies is spread most effectively when the bite of the
infected animal is deep enough to allow the virus to reach the deep skeletal muscles. Rabies, if untreated is almost
always fatal. This is due to the fact that the virus is
sequestered from the immune system in the deep muscle layers and nerves. However, in its early infectious stages it is accessible to antibody action, hence the foundation for the post-exposure vaccine.

After initial infection and replication in the muscles or subepithelium the virus spreads to the nerves by binding to the Acetylcholine receptors or other neurotransmitter
receptors. Movement of the virus up the axons of the
peripheral nerves eventually leads the virus to the Central Nervous System (CNS). It is here where the classical
symptoms of rabies are observed.

Photo of rabid dog displaying symptoms.

Clinical Presentation in Humans

Classical Rabies- Most cases go through three stages. The first stage is the Prodromal Stage- stage lasts from 2-10 days and presents in the form of fever, headache, malaise, fatigue, and also localized pain around area of initial
infection. The second stage is the
Sensory Excitation Phase- hyperactivity, hallucinations, disorientation, seizures and bizarre behavior. Also about 50% of infected
individuals developed painful spasms of the pharynx and larynx resulting in a fear to eat or drink. Because of this fear they drool rather than swallow. Increased salivation is another symptom of the disease, thus adding to the drooling problem. This phase persists for 2-7 days. The third phase is the
Coma and Paralysis Phase- as the
disease persists deterioration of CNS tissue leads to
paralysis and respiratory problems.

Dumb Rabies- About 20% of cases have only two phases where the patient skips the sensory excitation phase and progresses right to the coma and paralysis phase. Dumb rabies is almost 100% fatal with only three known cases of
survival. In each of these cases the patients had high titers of antibody in the CSF.

Diagnosis

Animals are diagnosed through histological examination of the CNS for Negri bodies. The cytoplasmic Negri body is a diagnostic of rabies encephalitis. It is about the same size (7 micrometers) and color as a mature RBC. However, if you have to wait to make the diagnosis by brain biopsy, you are too late. Rabies can only be
successfully prevented, not treated. Mortality is
essentially 100%, once the virus reaches the brain. In
humans, if the vaccine and anti-rabies serum therapy is given, the serological tests are of no value. However, if samples are taken before treatment, the rabies' antibody is utilized for diagnosis. Indirect immunofluorescence is most often used to detect rabies antigen. The antigen may be obtained from an impression smear, oro-nasal
mucosa scrapings, or hair follicles of the neck. If a death is suspected to be caused by rabies, the CNS, saliva, and nasal secretions are searched during autopsy for the
virus.

A cytoplasmic Negri body.

Treatment

Immediate First Aid- The virus remains localized at the site of the wound for a period of time. To help recovery, wash the wound with soap and water as soon as possible, and follow with application of an antiseptic.

Vaccine- A killed virus vaccine is used after the infection. Rabies is the only human disease that can be prevented by active immunization after infection. This is possible due to the long
incubation period of the virus, and to the fact that infection is from a recognizable event. The vaccine is called "HDCV", for Human Diploid Cell Vaccine. This is a new vaccine and caused milder side affects than the old vaccines of nervous tissue vaccine, or duck embryo vaccine. HDCV should be given intramuscularly on days 0, 3, 7, 14, and 28, followed by a booster dose on day 90. The vaccine is given with HRIG, Human
Rabies Immune Globulin. In the past the serum was from immunized horses, but with the use of HRIG serum sickness does not occur.

Pre-Exposure Immunization- This route of protection is only practical for persons at high risk of exposure to rabies. These include veterinarians, animal handlers, certain laboratory workers, and persons living in or visiting countries where
rabies is a constant threat. Anyone at high risk of contact with rabid animals may seek pre-exposure prophylaxis.

The following table provides pertinent information when deciding whether or not an animal bite warrants treatment for rabies.

WWW Rabies/Rhabdovirus Links

All the Virology on the WWW

The Association of Medical Microbiology

KidsHealth

Dog Owner's Guide

National Center for Infectious Disease

Center for Disease Control

Department of Rabies

World Health Organization

Contact Authors:   Cam McCormack
                      Chad Hissong
                      Dan Thomas

References

Fenner, Frank J. and White, David O.  Medical Virology.  San Diego: 
     Academic Press, 1994.

Teague, Lewis, dir. 
Cujo.  Perf. Dee Wallace, Daniel Hugh-Kelley, and
     Christopher Stone.  Warner Bros., 1983.