Rabies

Rabies is an acute viral disease, which causes encephalomyelitis in virtually all the warm blooded animals including man. The causative agent is found in domestic and wild animals, and is transmitted to other animals and to humans through close contacts with their saliva (i.e. bites, scratches, licks on broken skin and mucous membranes).

Rabies is an important zoonotic infection in which man is dead end of the infection and hence does not play any role in its spread to new hosts. In most of the developing countries, dogs are the principal reservoirs of rabies (canine rabies) whereas sylvatic rabies involving animals such as foxes, racoons and coyotes are principal wild animals reservoirs of this disease in developed countries.

Causative agent

Rabies virus belongs to the family Rhabdoviridae and genus Lyssavirus (Lyssa: Greek: rabies). This RNA virus is bullet shaped, round at one end and flat at the other. the virus is covered with a lipid envelope having spike like projections.

Mode of transmission

Rabies virus is predominantly neurotropic and kills the host in short period after it has entered the neurones. Before death, from the brain virus reaches salivary glands and is excreted in saliva. The saliva gains entry into another host through a preexisting breach in skin when mere licking or contamination is adequate or the bite of the rabid animal creates a mechanical breach of skin through which the rabies virus gains entry. Virus may be present in the saliva for many days before clinical signs appear and it may be steadily or intermittently secreted until just before death. Report of pre clinical periods of virus secretion in saliva range from 3 days in cats, 12 days in Mexican free tail bats, 14 days in dogs infected with an Ethiopian virus isolate to 29 days in foxes. Infection has been documented in personnel receiving corneal grafts and organs from rabies cases.

Incubation period

The average incubation period is between 30 - 90 days. Factors which may influence the length of the incubation period include the site of bite, the amount of virus in saliva of the bitting animal, the virus strain, and the age and immune status of the victim. It is shorter in case the bite is closer to brain and massive dose of virus has been inoculated. Incubation period as short as 10 days and as long as 2 years have been reported.

Clinical features in man

The first symptom to appear may be pain and tingling in the affected limb, especially around the site of bite. This is seen in 35-65% cases. Hydrophobia is the best known symptom of this disease and is pathognomonic for rabies.

Post-exposure Treatment in Humans

Because of long incubation period, which is typical of most cases of human rabies, it is possible to institute prophylactic post exposure treatment. This must be started at the earliest to ensure that the individual will be immunized before the rabies virus reaches the Central Nervous System.

In a rabies endemic country like India, where every animal bite is potentially suspected as a rabid animal bite the treatment should be started immediately. To bring out uniformity globally, the WHO recommended classification of animal bite for post-exposure treatment should be followed.

The treatment should be started immediately after the bite. The treatment may be discontinued if animal involved (dog or cat) remains healthy throughout an observation period of 10 days. The observation period is valid for dogs and cats only. Bite by all wild animals should be treated as category III exposure. It should be noted that bites by rats, mice, squirrel, hare and rabbits seldom require treatment. Bat rabies has not been conclusively proved in India and hence exposure does not warrant treatment.

It is re-emphasized that the treatment should be started as early as possible after exposure, but it should not be denied to person reporting late for treatment.

The post-exposure treatment is a three pronged approach. All three carry equal importance and should be done simultaneously.

1. Management of wound
• Since the rabies virus enters the human body through a bite or scratch, it is imperative to remove as much saliva, and thereby the virus, from the wound as is possible by an efficient wound toilet that should not involve additional trauma. Since the rabies virus c an persist and even multiply at the site of bite for a long time, wound toilet must be performed even if the patient reports late. This can be done by prompt and gentle thorough washing with soap or detergent and flushing the wound with running water for 10 minutes. If soap and detergent are not immediately available wash with running water for at least 10 minutes. Avoid direct touching of wounds with bare hands.
• After thorough washing and drying the wound, any on e of the available chemical agents should be applied: Savlon (in appropriate recommended dilution), Dettol (in appropriate recommended dilution), povidone iodine, alcohol etc
2. Passive immunization
3. Active immunization

Control

Any strategy for control of rabies in developing countries shall have following four components:

1. Epidemiolgocial surveillance
2. Mass vaccination
3. Dog population management
4. Community participation

Source : Zoonotic Disease of Public Health Importance