General
Information
One of the most feared
diseases, rabies is an acute viral encephalomyelitis (affecting
the brain), that particularly affects carnivores and insectivore
bats although it can affect any mammal.
Once the clinical signs appear, the disease is almost invariably
fatal.
Many countries have managed to become free of rabies through
successful eradication programmes.
Transmission is by the introduction of the virus laden saliva
into the tissues. The incubation period maybe prolonged
and variable.
After entering the muscle cells, the virus replicates itself,
and travels via the peripheral nerves to the spinal cord,
and ascends to the brain. From here it once again travels
through the peripheral nerves to the salivary glands.
Rabid animals of all species exhibit typical signs of CNS
disturbance.
Signs
& Symptoms:
The most reliable
signs are unexplained behavioural changes and paralysis.
Clinical disease has 3 forms.
Prodromal
Stage: Lasts for 1-3 days. Typical signs are vague
CNS symptoms progressing rapidly.
Furious form:
This is classical Mad Dog Syndrome. The animal becomes irrational
and with the slightest provocation, viciously and aggressively
uses its teeth, claws (and horns or hooves in the case of
other animals). The posture gives an appearance of alertness
and anxiety with dilated pupils. Such dogs when allowe to
roam, frequently attack people, or any moving object. They
chew the wire and frames of their cages. Muscular incoordination
and seizures develop. Death is the result of progressive
paralysis.
Paralytic
form: This presents with paralysis of the throat
and muscles of trhe jaw, with profuse salivation and inability
to swallow. The lower jaw drops down. The paralysis progresses
to all parts of the bodyand coma and death follow in a few
hours.
Features Of The Various Anti-Rabies
Vaccines Used Currently
a) Modified Live Vaccines
(MLV)
b) Killed Vaccines
c) Subunit vaccines
Modified
Live Vaccines:
Micro organisms used
in the vaccine are altered in such a way that they are no
longer virulent to the majority of host species. But
they still retain their antigenic properties to stimulate
a protective immune response.
Strong systemic immunity and local immune response are produced
by local administration of certain MLV vaccines.
MLV must replicate after innoculation to produce enough
antigen to induce interferon in the next few days after
immunization, providing
early protection against some virulent viral infections.
Certain MLV can induce immunosuppression, may be shed into
the environment and may revert to virulence or cause vaccine
induced disease.
Killed
Vaccines:
- Safer than MLV
To induce a protective immuno response, killed vaccines
require a large antigenic dose, multiple immunizations and
use of an adjuvant.
-
More costly than MLV
Local and systemic vaccine reactions may occur.
Provide a strong immune response and a long duration of
immunity.
Killed vaccines work
by using more antigen (since killed products can not grow
in the body of the recipient) and contain an adjuvant which
mimics a live replicating vaccine by releasing the antigen
slowly over a period of time and by stimulating WBC's to
produce a better immune response.
