Rabies

                   Rabies

• The best known and most important member of the Rhabdoviridae is rabies virus, a Lyssavirus (Greek lyssa ,rage or fury).
• This viral infection, which affects the central nervous system of most mammals including humans, is invariably fatal.
• Most clinical cases are due to infection with rabies virus (genotype 1).
• Classical rabies caused by genotype 1 lyssavirus is endemic on continental land masses with the exception of Australia and Antarctica.

Epidemiology

Two epidemiologically important infectious cycles are recognized.

         1. urban rabies in dogs  

         2. sylvatic rabies in wildlife.

  • More than 95% of human cases are the result of bites from rabid dogs.

  • Although virus may be transmitted through scratching and licking, transmission usually occurs through bites.

  • Infected animals may excrete virus in their saliva for some time before the onset of clinical signs.

Pathogenesis

• Following introduction into the tissues, virus enters peripheral nerve endings. There may be limited replication locally in myocytes or other tissue cells.

• The virus is transported to the central nervous system by retrograde axoplasmic flow and becomes widely disseminated in nervous tissue by intra - axonal spread.

• Although rabies viral antigens are highly immunogenic, immune detection is delayed because intracellular transport prevents contact with the cells of the immune system in the early stages of infection.

• Clinical signs develop following neuronal damage caused by viral replication.

• Virus spreads centrifugally within nerve cell processes and is released at axon terminals where it infects many non – nervous tissues including the salivary glands.

Clinical signs

• The incubation period  (Normally 3 to 8 weeks), which is highly variable and can be of many months duration, is influenced by various factors including

Ø  host species

Ø  virus strain

Ø  The quantity of virus in the inoculum

Ø  The site of introduction of the virus.

•  The clinical course in domestic carnivores, which usually lasts for days, may encompass prodromal, furious (excitative) and dumb (paralytic) phases.

• In the prodromal phase, affected animals are often confused and disorientated; wild animals may lose their natural fear of humans.

• The furious phase is characterized by an increase in aggressiveness and hyper excitability, and there is a tendency to bite at inanimate objects and at other animals. Affected animals may roam over long distances.

• In dumb rabies, muscle weakness, difficulty in swallowing, profuse salivation and dropping of the jaw are the usual features.

Diagnosis

• History

• Signs and symptoms

• Clinical examination

• Ante mortem diagnostic tests for rabies are not generally used except in humans where saliva is examined by PCR.

• Rabies virus is particularly abundant in Ammon ’ s Horn of the hippocampus, cerebrum, cerebellum and medulla.

• The preferred method of diagnosis is the direct fluorescent antibody test (FAT) on acetone – fixed brain tissue smears.

•CSF (cerebrospinal fluid)analysis.

•Negri bodies

                Negri bodies are eosinophilic, sharply outlined inclusion bodies found in the cytoplasm of nerve cells containing the virus of rabies.

They consist of ribonucleic protein produced by the viruses.