RABIES VIRUS & POLIO VIRUS

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

What’s this?

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

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

STRUCTURECore -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

Rabies virus structure

EnvelopeMatrix protein

Source: http://www.cdc.gov

Glycoprotein

Nucleocapsid protein

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.

REPLICATIONAdsorptionPenetrationUncoating

BiosynthesisAssembly and release

Rabies Virus

•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

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

Rabies Virus

• 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

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+%)

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

Negri bodies

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/

Human rabies

Photo courtesy of David Warrell, UK

• Fever

• Headache

• Fatigue

• Loss of appetite

• Nausea/ vomiting

• Sore throat

• Nonproductive cough

• Irritability

• Malaise

• Pain/ paresthesia (abnormal burning sensation)

• 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

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

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

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:SalivaSerumSpinal fluid Hair folliclesSkin biopsy

Amplification of sample by virus isolation and reverse transcriptase polymerase chain reaction (RT-PCR)

•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?

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)

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.

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.

Clinical ManifestationsThere 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.

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.

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.

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.