An introduction to Rabies (Hydrophobia)

Dr Asem Suresh Kumar Meitei
Contd from previous issue
But the actual number of loss from rabies among livestock in India cannot be ruled out as the reporting system is grossly deficient.
Transmission Dynamics
All warm blooded animals are susceptible to rabies. Opossums (Marsupial) are the possible ex- ception. There is no variation of susceptibility with age (pigs one day old contracted the disease although the younger are said to be more susceptible. Variation in susceptibility between species is noticeable. Foxes, Cotton rats, and coyotes are extremely susceptible, cattle, rabbits and cats are highly susceptible, dogs, sheep and goats are moderately susceptible. The ques- tion of immunity after natural infection does not arise, but immunity can be produced artificially by vaccination.
 It is also possible that the infection can pass the placental barrier to the foetus during late pregnancy. A predominant animal reservoir is the principal source of rabies virus transmission (e.g, infected dogs are the main reservoir for disease transmission in Asia and Africa).
All animal reservoirs are also vectors of the virus; however, not all vectors are reservoirs. For example, although cats can effectively transmit rabies virus, no cat-to-cat transmission of rabies virus perpetuates, and no unique feline rabies virus variant has been documented.
Nonetheless, cats are the most commonly reported rabid domestic animal in the US. Virus is present in the saliva of rabid cats, and humans have developed rabies after being bitten by rabid cats. Reported cases in domestic cats have outnumbered those in dogs in the US every year since 1990.
Transmission of rabies virus almost always occurs via introduction of virus-laden saliva into tissues, usually by the bite of a rabid animal. Although much less likely, virus from saliva, salivary glands, or neural tissues can also cause infection by entering the body through fresh wounds or intact mucous membranes.
Saliva is infectious at, or before, the time clinical signs occur. Domestic dogs, cats, and ferrets may shed virus up to 10 days before onset of clinical signs; viral shedding in wildlife has been reported for several weeks before onset of signs. Rabies virus has not been isolated from skunk musk (spray). Hematogenous transmission does not occur.
According to the US Advisory Committee on Immunization Practices, under most circumstances, there is no danger of aerosol transmission of rabies virus.  However, aerosol transmission has occurred under very specialized conditions in which the air contained a high concentration of suspended particles or droplets containing viral particles. Such conditions have been responsible for laboratory transmission under less than ideal containment situations. There has been a suggestion of very rare natural aerosol transmission in a cave inhabited by millions of bats. Oral and nasal secretions containing virus were probably aerosolized from tens of thousands of rabid bats in a confined roost space within a cave. Aerosol infection may occur via direct attachment of the virus to olfactory nerve endings. Rabies virus has been transmitted by transplantation of tissues and organs from infected humans.
As per WHO-APCRI study it was estimated that 20,562 human deaths are due to Rabies occurs annually in India. The annual incidence of animal bites was estimated to be 1.7% (17.5 million per year). As per the Million Deaths Study 2012, India has an estimated 12,700 deaths due to furious Rabies. The cases occur throughout the year. Majority of the rabies cases are associated with dog bites. Cats, wolf, jackal, mongoose and monkeys are other important animals transmitting rabies in India. Bat rabies has not been conclusively reported from India. However, serological evidences of exposure of bats to rabies or related lyssavirusare available.
Sylvatic rabies: Rabies is perpetuated in the nature in the form of “Sylvatic” rabies (rabies of the forest or jungle). Sylvatic rabies involving animals such as foxes, racoons and coyotes are principal reservoirs of this disease in developed countries. Sylvatic rabies is the major problem in much of Europe. The disease is still spreading from a focal point which developed in Poland in the mid -30s. The disease has spread westward to East and West Germany., Denmark, Belgium; Cze-choslovakia, Austri, Switzerland and France and  the threat to the United Kingdom.
It is being maintained in a never-ending cycle primarily among the carnivores in the jungle. Surprisingly, there is an ecological balance and we do not see all the carnivorous animals dead or extinct due to rabies. Also, it should be remembered that some of the exposed animals act as reservoirs.
Urban rabies: Rabies as we see in man and domesticated animals is called “Urban” or “Domestic” rabies. Some domesticated animals, carnivores, are bitten by the wild rabid animals; also usually carnivores, and later the domestic animal suffer from apparent or in-apparent rabies and spread it by biting human or other domestic animals. Urban rabies has a dead end, which sylvatic rabies does not have. Therefore, we repeatedly see outbreaks of rabies in domestic populations.
The Strain of virus that occurs in animals under natural condition is known as “Street” virus.
(To be cont)