RABIES
INTRODUCTION
Rabies is an infectious disease of animals caused by a bullet-shaped, enveloped
RNA virus, 180 x 75 nm. Man is occasionally infected, and once infection
is established in the CNS, the outcome is almost invariably fatal.
HUMAN RABIES
Is acquired from virus in saliva entering a bite wound caused by an infected
animal, usually a rabid dog. The severity of the bite determines the
risk of infection. The disease does not usually spread from man to man.
Incubation
After inoculation, the virus enters small nerve endings at the site of the
bite. The virus slowly travels up the nerve to reach the CNS where
it replicates and then travels down nerves to the salivary glands where there
is further replication. The time it takes to do this depends upon the length
of the nerve - a bite on the foot will have a very much longer incubation
period than a bite on the face. The incubation period may last from
two weeks to six months. Very often the primary wound is healed and
forgotten by the time of clinical presentation.
Clinical Presentation
A) Furious Rabies
When the virus reaches the CNS the patient presents with headache, fever,
irritability, restlessness and anxiety. This may progress to muscle
pains, salivation and vomiting. After a few days to a week the patient
may experience a stage of excitement and be wracked with painful muscle
spasms, triggered sometimes by swallowing of saliva or water. Hence they
drool and learn to fear water (* Hydrophobia). The patients are also
excessively sensitive to air blown on the face. The stage of excitement lasts
only a few days before the patient lapses into coma and death.
Once clinical disease manifests, there is a rapid, relentless progression
to invariable death, despite all treatment.
B) Dumb Rabies
Starts in the same way, but instead of progressing into excitement, the subject
retreats steadily and quietly downhill, with some paralysis, to death.
Rabies diagnosis may easily be missed.
Wild animals may be abnormally tame or appear sick - beware of approaching or picking up such an animal ("dumb rabies").
Epidemiology
1. The disease is endemic in wild animals in most parts of the world although some countries (UK, Australia) are rabies free through vigorous control. The wild animal cycle constitutes the natural reservoir.
2. Wild animals may bite and infect domestic animals (cattle, horses, pigs, dogs and cats) which in turn may infect man. Occasionally wild animals may infect man directly.
3. In recent decades, a separate form of dog rabies (spread from dog to dog) has been recognised as spreading from West Africa eastwards and southwards in Africa. Via Mozambique, it reached Natal Kwazulu in the late 1970's and early 80's. Semi-wild dogs in Natal have formed the highest endemic rabies reservoir and source for human cases in the whole of the RSA.
Note: As in man, an infected animal becomes sick and dies. There is no substantial evidence of a true carrier status in apparently well animals, except perhaps in bats.
2. Animal
3. Man
Prophylaxis
High-risk persons, eg. veterinarians, may be immunised before exposure, and
then merely require one or two booster doses if they should be exposed to
rabies.
Animal Vaccines
A range of live and killed virus vaccines are available for
domestic animals (farm animals, cats and dogs).
Experimental vaccination of wildlife by using recombinant vaccinia vaccine (live) in bait for foxes in Europe and North America has been quite promising.
Illustrations and layout by Linda Stannard, 1995 ©