Write a short note on rabies, including post-exposure prophylaxis.

Aetiology

Rabies is an acute progressive viral encephalomyelitis caused by the rabies virus, a bullet-shaped RNA lyssavirus of the family Rhabdoviridae. It is the most lethal infectious disease known to man — once symptoms appear, it is virtually 100% fatal — and yet it is completely preventable.

India bears the largest burden of human rabies in the world, with dogs responsible for the great majority of cases; other sources include cats, monkeys, mongooses, foxes, jackals and bats.

Transmission: by the bite, scratch or lick on broken skin or mucous membrane of an infected animal; virus is present in saliva. Rarely by aerosol in caves or laboratories, and by corneal and organ transplantation.

Pathogenesis

1. Virus deposited in the wound replicates locally in muscle.

2. It binds nicotinic acetylcholine receptors at the neuromuscular junction and enters peripheral nerves.

3. Centripetal spread along axons by retrograde transport to the CNS (about 12–100 mm per day) — during this phase the patient is asymptomatic and the disease is still preventable.

4. Replication in the CNS causes encephalitis, with formation of Negri bodies (eosinophilic cytoplasmic inclusions, classically in hippocampal and Purkinje neurones).

5. Centrifugal spread along nerves to salivary glands, skin, cornea and other organs — making saliva infectious.

Incubation period: usually 1–3 months, but ranges from days to years. It is shorter with bites on the face, head and hands (rich nerve supply and proximity to the CNS), with severe or multiple bites, and in children.

Clinical Features

Death occurs within days of symptom onset, from respiratory or cardiac arrest. There is no effective treatment — the Milwaukee protocol has not shown reproducible benefit, and management of the established disease is palliative, with heavy sedation and analgesia to relieve distress, in a quiet darkened room, with support for the family and protection of staff.

Post-exposure Prophylaxis (pep)

PEP is nearly 100% effective if given correctly and promptly — this is the entire clinical opportunity in rabies.

Step 1 — Categorise the exposure (WHO):

Step 2 — Immediate wound management (the most neglected and most important step):

• Wash the wound thoroughly with soap and running water for at least 15 minutes — this alone substantially reduces the risk by mechanically removing and inactivating virus.
• Apply a virucidal agent — povidone-iodine or alcohol.
• Do NOT suture the wound if possible; if suturing is unavoidable for haemostasis or cosmesis, place loose sutures after infiltrating immunoglobulin and delay closure.
• Do NOT apply irritants — chillies, lime, oil, herbal poultices or cauterisation, which are traditional practices in parts of India and cause harm without benefit.
• Give tetanus prophylaxis and antibiotics for the wound as indicated.

Step 3 — Rabies vaccine:

• Modern cell culture vaccines (human diploid cell, purified chick embryo cell, purified vero cell) are used; nerve tissue vaccines are obsolete and must not be used.
• Given by intramuscular (deltoid — never the gluteal region, where absorption is poor) or intradermal regimens; intradermal regimens are dose-sparing and widely used in India, reducing cost substantially.
• Previously vaccinated individuals need only a short booster course and do not require immunoglobulin.

Step 4 — Rabies immunoglobulin (RIG) — for category III exposures:

• Infiltrate AS MUCH of the calculated dose as anatomically possible directly INTO and AROUND the wound — this is the critical technical point, since RIG works by neutralising virus locally before it enters nerves. Any remainder is given intramuscularly at a site distant from the vaccine.
• Human RIG or equine RIG (cheaper and widely used in India; test dose and observation for anaphylaxis); monoclonal antibody preparations are now available.
• Give RIG at the same visit as the first vaccine dose, and never in the same syringe or at the same site as the vaccine.
• If RIG was not given initially, it may still be given up to 7 days after the first vaccine dose; beyond that the vaccine-induced antibody response makes it unnecessary.

Step 5 — Observation of the animal: a healthy domestic dog or cat may be observed for 10 days; if it remains well, PEP may be discontinued. Never delay starting PEP while observing the animal.

Pre-exposure prophylaxis: for veterinarians, animal handlers, laboratory workers, wildlife officers, travellers to endemic areas, and children in highly endemic settings — it simplifies post-exposure management and removes the need for immunoglobulin.