Rabies virus is an enveloped, negative-sense single stranded RNA virus that causes rabies disease. It encodes 5 structural proteins - N, P, M, G, and L - and replicates in the cytoplasm. Rabies virus enters through breaks in the skin via animal bites. It spreads from peripheral nerves to the central nervous system, causing encephalitis. This leads to symptoms like anxiety, hallucinations, and eventually paralysis and death. Laboratory diagnosis involves direct fluorescent antibody testing, PCR, or histopathology of brain tissue to detect Negri bodies. There is no treatment after symptoms start, making vaccination important for prevention.

1. RABIES VIRUS &
POLIO VIRUS

2. LEARNING
OBJECTIVES
• Describe the disease caused by Rabies
virus
• Describe the important morphological
characteristics & virulence factors of Rabies
virus and their clinical significance
• Describe the pathogenesis of Rabies virus
• Describe the lab diagnosis of Rabies virus
infection

3. What’s this?

4. Background Information
• Family: Rhabdoviridae
• Enveloped (-) ssRNA virus
• Rod or “bullet” shaped
• Approximately 70x180 nm
• Coiled nucleocapsid into cylindrical structure
• RNA genome codes for N, P, M, G, L
• Replication occurs in the cytoplasm inside Negri Bodies
• Genus: Lyssavirus

5. Morphology
bullet-shaped;
45-100 nm in diameter; 100-430 nm long.

6. STRUCTURE
Core -ssRNA
L-protein (large protein)
---RNA dependent RNA polymerase RNP
Capsid N-protein
---protect RNA from hydrolysis
P-protein
Envelope
Bilipid layer
M-protein (matrix protein)
G-protein (glucoprotein)
---spike
---bind to the receptor of the host cell

7. Rabies virus structure
Envelope
Matrix protein
Source: http://www.cdc.gov
Glycoprotein
Nucleocapsid protein

9. Rabies is an RNA virus. The genome encodes 5
proteins designated as N, P, M, G, and L. The order and
relative size of the genes in the genome are shown in
the figure. The arrangement of these proteins and the
RNA genome determine the structure of the rabies
virus.

10. REPLICATION
Adsorption
Penetration
Uncoating
Biosynthesis
Assembly and release

11. RabiesVirus
• Rabies virus replication
• Spike protein mediates attachment (nicotinic
acetylcholine receptor)
• Viral RNA polymerase transcribes a
monocistronic mRNA
• Five polypeptides are encoded by the genome
• N
• L (polymerase)
• P (polymerase)
• M
• G

12. Contd…
• The N assembles with the
polymerase and RNA in progeny
virus (spiral configuration)
• Virus exits by budding
• G protrudes from plasma
membrane
• M binds to inner PM leaflet

14. RabiesVirus
• Pathogenesis
• Requires several weeks for infection to
become apparent
• Transmission through bite or scratch
from infected animal
• Replication in muscle and connective
tissues at site of inoculation
• Enters peripheral nervous system at
neuromuscular junctions
• Spreads up the peripheral nerves to the
central nervous system

15. Pathogenesis Contd…
• Encephalitis
• Virus grows to high titers in the salivary glands
• Rabies patients must be restrained
• Negri bodies appear in neuron cell bodies
• Clinical spectrum
• Prodrome - nausea, headaches, fever, sore
throat, photophobia
• Acute neurologic phase - apprehension,
nervousness, hallucinations, behavioral
anomalies, salivation, perspiration,
hydrophobia, photophobia
• Coma - seizures and death (99+%)

16. Transmission and Mechanism of Infection
Inoculation of virus
at site of wound
 Replication occurs in
muscle tissue near the
bite
Entry into sensory
nerve endings of PNS
Advances to CNS,
reaching dorsal root
ganglia and spinal
cord
 Causes trademark
encephalitis
Spreads to salivary
glands and other
organs

18. Negri bodies

19. Clinical Manifestations
• Five general stages:
• (1) incubation
period
• (2) prodrome
• (3) acute neurologic
period
• (4) coma
• (5) death (or rarely,
recovery)
http://www.ncbi.nlm.nih.gov/books/NBK8618/

20. Human rabies
Photo courtesy of David Warrell, UK

22. • Fever
• Headache
• Fatigue
• Loss of appetite
• Nausea/ vomiting
• Sore throat
• Nonproductive cough
• Irritability
• Malaise
• Pain/ paresthesia (abnormal burning sensation)

23. • Animal susceptibility
• All warm-blooded animals can be infected with varying
susceptibility
• High - wolves, coyotes, foxes, dogs
• Intermediate - skunks, raccoons, bats
• Low - opossums
• Virus occurs in saliva, nervous system, urine, lymph,
milk
• Recovery is rare and only occurs in bats; fatal in
nearly all others
• Vampire bats can transmit virus for months
Rabies Virus

24. Laboratory diagnosis
• PCR
• Serology (IFA)
• Animal control
• Rabid or suspected rabid animals are
killed and examined by histopathology
for Negri bodies and viral antigen
• Vaccination of pets is required by law in
most states
• Immunity and protection

25. Contd…
• Vaccines
• First one developed by Pasteur by using
spinal cords from infected dogs
• Today’s principal vaccine is the human
diploid cell vaccine (HDCV) made in the
WI-38 fibroblast cell line
• Post-exposure prophylaxis
• One dose of hyperimmune antiserum
• Five immunizations over 28 days

26. Laboratory Diagnosis
Direct fluorescent antibody test (dFA)
Confirm the presence or absence of rabies antigen in
tissue or saliva
Tests are performed on samples of:
Saliva
Serum
Spinal fluid
Hair follicles
Skin biopsy
Amplification of sample by virus isolation and reverse
transcriptase polymerase chain reaction (RT-PCR)

27. • Rabies differs from other
neurological viruses because it
has:
• a complete different strategy for
virus pathogenesis and spread
• Rabies is spread by animal bites, thus
behavioral changes (i.e. excitability,
irritability, etc.) induced by the virus are
important for spread
• High replication of rabies in salivary
glands of rabid host makes the infected
animal a walking “time bomb”
• 100% mortality rate (if not
treated)
4. How do the clinical aspects of rabies differ from those of
other neurological viral diseases?

28. 4. How do the clinical aspects of rabies differ from those of
other neurological viral diseases?Many common viruses that cause encephalitis have low neuroinvasiveness, but high neurovirulence
 Neuroinvasive: can enter the CNS after infection of a peripheral site
 Neurovirulent: can cause disease of the nervous tissue
Rabies has both high neuroinvasiveness and high neurovirulence
 Neuronal spread is the definitive characteristic of pathogenesis
Examples of neurological viral diseases
 Mumps virus
▪ High neuroinvasiveness, low neurovirulence
▪ Enters the CNS, but has mild symptoms
 Herpes simplex virus
▪ Low neuroinvasiveness, high neurovirulence
▪ Enters the PNS (rarely CNS)

29. Poliovirus
• 3 serotypes of poliovirus (1, 2, and3) but no common
antigen.
• Have identical physical properties but only share 36-52%
nucleotide homology.
• Humans are the only susceptible hosts.
• Polioviruses are distributed globally. Before the
availability of immunization, almost 100% of the
population in developing countries before the age of 5.
• The availability of immunization and the poliovirus
eradication campaign has eradicated poliovirus in most
regions of the world except in the Indian Subcontinent
and Africa.

30. Pathogenesis
• The incubation period is usually 7 - 14 days.
• Following ingestion, the virus multiplies in the
oropharyngeal and intestinal mucosa.
• The lymphatic system, in particular the tonsils and
the Peyer's patches of the ileum are invaded and
the virus enters the blood resulting in a transient
viraemia.
• In a minority of cases,the virus may involve the CNS
following dissemination.

31. Clinical Manifestations
There are 3 possible outcomes of infection:
• Subclinical infection (90 - 95%) - inapparent subclinical
infection account for the vast majority of poliovirus infections.
• Abortive infection (4 - 8%) - a minor influenza-like illness
occurs, recovery occurs within a few days and the diagnosis can
only be made by the laboratory. The minor illness may be
accompanied by aseptic meningitis
• Major illness (1 - 2%) - the major illness may present 2 - 3 days
following the minor illness or without any preceding minor illness.
Signs of aseptic meningitis are common. Involvement of the
anterior horn cells lead to flaccid paralysis. Involvement of the
medulla may lead to respiratory paralysis and death.

32. Laboratory Diagnosis• Virus Isolation
• Mainstay of diagnosis of poliovirus infection
• poliovirus can be readily isolated from throat
swabs, faeces, and rectal swabs. It is rarely
isolated from the CSF
• Can be readily grown and identified in cell culture
• Requires molecular techniques to differentiate
between the wild type and the vaccine type.
• Serology
• Very rarely used for diagnosis since cell culture
is efficient. Occasionally used for immune status
screening for immunocompromised individuals.

33. Prevention (1)
No specific antiviral therapy is available. However the disease may be
prevented through vaccination. There are two vaccines available.
• Intramuscular Poliovirus Vaccine (IPV)
• consists of formalin inactivated virus of all 3 poliovirus serotypes.
• Produces serum antibodies only: does not induce local immunity and
thus will not prevent local infection of the gut.
• However, it will prevent paralytic poliomyelitis since viraemia is
essential for the pathogenesis of the disease.
• Oral Poliovirus Vaccine (OPV)
• Consists of live attenuated virus of all 3 serotypes.
• Produces local immunity through the induction of an IgA response as
well as systemic immunity.
• Rarely causes paralytic poliomyelitis, around 1 in 3 million doses.

34. Prevention (2)
• Most countries use OPV because of its ability to induce
local immunity and also it is much cheaper to produce
than IPV.
• The normal response rate to OPV is close to 100%.
• OPV is used for the WHO poliovirus eradication
campaign.
• Poliovirus was targeted for eradication by the WHO
and to this end, an extensive monitoring network had
been set up.
• Poliovirus has been eradicated from most regions of the
world except the Indian subcontinent and sub-Saharan
Africa. It is possible that the WHO target may be
achieved.