An introduction to Rabies (Hydrophobia)

Dr Asem Suresh Kumar Meitei
Contd from previous issue
After serial intra-cerebral passage in rabbits, the street virus becomes modified; its usual incubation period (the period from the entrance of the causal organism to the first appearance of clinical symptoms of the disease) of 15 to 20 days being reduced to six or seven days. It is, then known as “fixed” virus. Anti-rabies vaccines are made with strains of fixed virus.
Street virus, the form of rabies as it occurs in nature, can be transmitted by several different routes, viz., the intracranial, theintra-neural, the intramuscular, the subcutaneous, the intravenous and the intra-cutaneous. It may be possible to infect rabbits by inhalation, if these animals are exposed to an atmosphere containing pulverized virus. It is also possible for the virus to pass from the mother during late pregnancy via the placenta to the foetus.
The dog is the principal vector and 90 % of the cases in man are from canine sources. Only about 2% of the human cases are transmitted by cattle, horses, sheep, squirrels and cats. Wild animals normally avoid human habitations, but when affected with rabies, they are liable to trespass and enter farmyard of dwelling and then to attack any person or animal that comes within their reach.
The source of infection is always an infected animal, and the method of spread is always by the bite of an infected animal, although contamination of skin wounds by fresh saliva may result in infection..because of the natural occurrence of rabies in animals in caves inhabited by infected insectivorous bats, inhalation as a route of infection came under suspicion. It is now accepted that inter-bat spread, and spread from bats to other species is principally by bites. Ingestion of virus can also lead to infection if the dose is large enough. It is also considered likely that outbreaks occurring naturally amongst carnivores may originate by the eating bats wich have died of rabies.
Under natural condition, rabies is commonly transmitted by the bite of a rabid animal, usually a carnivorous animal. Rarely does infection occurs after a scarification, a scratch, or a lick. However, there is a record of a rabies case in a girl following licks by a rabid dog. Any infected animal, and even man, is capable of discharging virus in the saliva by means of which it is conveyed to a wound or scarification, thus setting up an infection. The virus may be present in the saliva of an infected animal several days before any clinical symptoms of rabies become manifested.
On this account, all bites inflicted by dogs and other carnivores in enzootic areas (India is enzootic) should be treated with suspicion (should be treated for rabies), even though only about 50 % of rabid dogs actually secrete rabies virus in their saliva. The bite inflicted by the wolf is by far the most infectious. This is followed in severity by jackal, fox, dog and cat bite wounds. The pathogenesis of rabies is in direct proportion to the extent of the trauma and the amount of virus deposited.
Transmission of rabies through milk is not clearly known to occur although there is possible occurrence of the virus in milk, but the virus cannot be demonstrated in the blood at any time.
(To be contd)