Family Rhabdoviridae. Rabies virus

Teaching objectives State the routes of rabies transmission Describe rabies pathogenesis List the clinical features of rabies virus infection Describe the diagnosis of rabies Outline how rabies is treated and prevented

Rabies is an acute infection of the central nervous system. Transmission bite of a rabid animal. Cornea transplant taken from patient who died from undiagnosed rabies

Non bite transmission have also occurred where transmission is by exposure to aerosol of bat secretions containing rabies virus

Properties of the Virus rod- or bullet-shaped enveloped single-stranded RNA virus family Rhabdoviridae. genus Lyssavirus, Rabies is the only medically important rhabdovirus.

Has a wide host range All warm-blooded animals (foxes, coyotes, wolves, opossums skunks, raccoons, and bats)

Pathogenesis

However, it is also possible for rabies virus to enter the nervous system directly without local replication. It multiplies in the central nervous system and progressive encephalitis develops.

The virus then spreads through peripheral nerves to the salivary glands and other tissues. Other organs where rabies virus has been found include pancreas, kidney, heart, retina, and cornea. * Rabies virus has not been isolated from the blood of infected persons.

Clinical Findings Incubation period depend on the location of the bite range from two wks to 16 wks or longer. Is shorter when bite is sustained on the head rather than the leg where the virus have to travel shorter distance to CNS

The clinical spectrum can be divided into three phases: a prodromal phase, an acute neurologic phase, and coma.

The prodromal phase lasting 2–10 days, may show any of the following nonspecific symptoms: fever, anorexia, headache, photophobia, nausea and vomiting, sore throat, an abnormal sensation around the wound site

Acute neurologic phase patients show signs of nervous system dysfunction such as nervousness, confusion, hallucinations,

pupillary dilatation, and increased salivation and perspiration. A large fraction of patients exhibit hydrophobia

The act of swallowing precipitates a painful spasm of the throat muscles. This phase is followed by convulsive seizures, coma and death. The major cause of death is respiratory paralysis. Recovery and survival are extremely rare.

Rabies should be considered in any case of encephalitis or myelitis (a disease involving inflammation of the spinal cord which disrupts CNS functions linking the brain and limbs) of unknown cause even in the absence of an exposure history. Because of the long incubation period, people may forget a possible exposure incident. People who contract bat rabies often have no recollection of being bitten by a bat.

Laboratory Diagnosis Rabies Antigens Tissues infected with rabies virus is identified by immunofluorescence A biopsy specimen is taken from the skin of the neck at the hairline. or histology staining of Negri bodies

Negri body in brain cell A definitive pathologic diagnosis of rabies can be based on the finding of Negri bodies in the brain or the spinal cord.

Reverse transcription-polymerase chain reaction testing (RT-PCR) can be used to amplify parts of a rabies virus genome from fixed or unfixed brain tissue. sequencing of amplified products allows identification of the infecting virus strain.

Viral Isolation Isolation of virus from sources like saliva spinal fluid and brain tissue or rise in titter antibody to the virus negri body can be demonstrated in cornea scrapings and in autopsy specimen of the brain

Treatment and Prevention There is no treatment for clinical rabies. It is therefore essential that individuals at high risk receive preventive immunization, give individuals exposed post exposure prophylaxis

Because treatment is of no benefit after the onset of clinical disease, it is essential that post exposure treatment be initiated promptly. Post exposure rabies prophylaxis consists of the immediate and thorough cleansing of all wounds with soap and water, administration of rabies immune globulin, and a vaccination regimen.

Preexposure Prophylaxis This is indicated for persons at high risk of contact with rabies virus (research and diagnostic laboratory workers, spelunkers) or with rabid animals (veterinarians, animal control and wildlife workers).

The goal is to attain an antibody level presumed to be protective by means of vaccine administration prior to any exposure. It is recommended that antibody titers of vaccinated individuals be monitored periodically and that boosters be given when required.

Epidemiology Rabies is enzootic in both wild and domestic animals. Worldwide, at least 50,000 cases of human rabies occur each year; however, rabies is grossly underreported in many countries. Almost all rabies deaths (> 99%) occur in developing countries, with Asia accounting for over 90% of all rabies fatalities.

In these countries, where canine rabies is still endemic, most human cases develop from bites of rabid dogs. Children aged 5–15 years are at risk.

Bats present a special problem because they may carry rabies virus although they appear to be healthy, excrete it and transmit it to other animals and to humans. Bat caves may contain aerosols of rabies virus and present a risk to spelunkers.

Migrating fruit-eating bats exist in many countries and are a source of infection for many animals and humans Human-to-human rabies infection is very rare. The only documented cases involve rabies transmitted by corneal and organ transplants.