Rabies

RABIES

Rabies is a zoonotic disease. Animals act as reservoir of infec-tion. Man gets the infection from the animals

Rabies is a lethal form of encephalitis caused by rabies virus, transmitted through the bite of an infected animal usually a dog.

Properties of the virus

·              The virus is bullet shaped

·              It is covered by a membranous envelope

·              Has protruding spikes 10 nm long

·              Spike is composed of a single gycoprotein

·              The genome is single stranded RNA- is a minus strand RNA

·              Genome contains RNA dependent RNA polymerase

·              When freshly isolated from a case, the virus is called “street” virus

§   It can multiply in neural and non-neural tissues

·              Serial brain to brain passage in rabbits yields a fixed virus

§   It can not multiply in non-neural tissue

Pathogenesis and Pathology

·              Virus is introduced into the tissue by the bite of a rabid animal 

§   Canines

§   Bats etc

·              Virus multiplies in the muscle or connective tissue

·              Infection ascends through the tissue spaces of the sensory nerves to the CNS

·              It multiplies in the CNS

·              Spreads though peripheral nerves to salivary glands and tissues

·              Rabies virus has not been isolated from the blood of an in-fected person

·              The incubation period depends on:

§   Amount of viral inoculum

§   Severityo of the bite

§   The distance the virus has to travel from its point of entry to the brain

§   If bitteno on the face, higher attack rate and shorter incubation period seen

·              There is nerve cell destruction in the cortex, mid brain, basal ganglion, pons and medulla

·              It produces cytoplasmic inclusion bodies in nerve cells called Negri bodies.

Clinical  features

The incubation period is from 4 to 12 weeks, some times it may be much longer. If the bite wound is in the neck or head the incubation period is shorter

The virus spreads from the wound to the central nervous system through nerves.

Symptoms

The symptoms may be either furious or dumb. In the furious type, the patient shows the symptoms of excitement , with tremor, mus-cular contractions and convulsions. When there is spasm of muscle of swallowing, patients show fear of water and hence the name hydro-phobia is also given to rabies.

In the dumb type of rabies, ascending paralysis is seen involving the muscles of swallowing speech and respiration.

·              Virus is present in saliva, skin and eyes and brain

·              The disease is always fatal following convulsions.

Epidemiology

Rabies is a natural infection seen in dogs, cats, bats and carnivo-rous animals such as foxes and wolves. Man acquires the infection generally from infected dogs. Virus is present in the saliva of the in-fected dog. Infected dog dies within ten days. If the dog remain healthy for ten days after biting, it can be regarded as being free of the virus at the time of biting. Aerosol infection has been recorded as a result of laboratory accident. Human patients do not seem to be a source of infection.

Diagnosis

Direct demonstration of virus

Specimens such as hair bearing skin from the back of neck, cor-neal impression smears and brain tissue are examined for virus by im-munofluorescence

Intracytoplasmic inclusion body called Negri body can be dem-onstrated by conventional microscopy

Virus  isolation

Attempt to isolate the virus from brain tissue, saliva, CSF and urine from patients can be made by inoculating into mice and the animal is observed for convulsions, paralysis and postmortem examinations for Negri bodies in the brain tissue.

Vaccination

Rabies vaccine was first developed by Pasteur in 1885. It con-sisted of virus attenuated by drying the spinal cord of infected rabbits for varying length of time over KOH. ild rabies virus is called street virus and attenuated virus is called fixed virus. All vaccines prepared for human use contain inactivated virus.

Now human diploid cell vaccine (HDCV) is used for vaccina-tion. It contains inactivated virus grown on WI38 or MRC-5 diploid human embryo lung cells