Rabies

Rabies is a deadly viral disease causing encephalitis in humans and other mammals. It is primarily transmitted through the saliva of infected animals, commonly via bites. The rabies virus and Australian bat lyssavirus are the main causative agents. Rabies has a nearly 100% fatality rate once symptoms appear, making early intervention very important.

Signs and Symptoms

A man suffering from rabies tied to a hospital bed.

The incubation period for rabies is typically one to three months but can range from less than a week to several years. Initial symptoms are nonspecific, including fever and headache. As the disease progresses, it causes brain and meningeal inflammation, leading to symptoms such as partial paralysis, anxiety, insomnia, confusion, agitation, abnormal behaviour, paranoia, and hallucinations.

Rabies is historically known as hydrophobia due to the intense fear of water observed in infected individuals. This occurs because attempts to drink can cause painful spasms in the throat and larynx, while saliva production increases, leading to foaming at the mouth. There are two forms of rabies:

• Furious rabies: Characterised by irrational aggression and hyperactivity.
• Paralytic rabies: Marked by muscle weakness and paralysis.

Animals with "dumb" rabies appear depressed, lethargic, and uncoordinated.

Causes

Rendering of the rabies virus.

Rabies is caused by lyssaviruses, including the rabies virus and Australian bat lyssavirus. The virus has helical symmetry and is enveloped with a single-stranded RNA genome. It infects cells by binding to acetylcholine receptors and undergoes replication within muscle or nerve cells before travelling to the central nervous system.

An electron microscope image of the rabies virus.

TEM micrograph with numerous rabies virions and Negri bodies.

Transmission

Rabies is typically spread by bites from infected animals, with dogs being the primary source globally. Bats are the main source in the Americas. The virus can also be transmitted through scratches or contact with infected saliva on mucous membranes. Human-to-human transmission is extremely rare and has occurred only through organ transplants.

Diagnosis

Diagnosing rabies can be challenging due to its nonspecific early symptoms. The fluorescent antibody test (FAT) is the gold standard, detecting rabies antigen in brain tissue. Other methods include RT-PCR assays and microscopic examination for Negri bodies. Diagnosis from saliva, urine, and CSF is less reliable than brain tissue analysis.

Prevention

Vaccination is the most effective preventive measure. Pre-exposure vaccination is recommended for high-risk individuals, while post-exposure prophylaxis (PEP) is very important after potential exposure. PEP involves thorough wound cleaning and administering rabies immunoglobulin and vaccines.

A young girl about to receive PEP after being bitten by an animal thought to be rabid.

Treatment

Treatment after exposure includes wound cleaning, rabies immunoglobulin, and a series of rabies vaccinations. HRIG is expensive and primarily administered around the bite. The Milwaukee protocol, involving induced coma and antiviral drugs, has been largely ineffective in treating symptomatic rabies.

Epidemiology

Rabies causes approximately 59,000 deaths annually, predominantly in Asia and Africa. India has the highest incidence, largely due to stray dogs. Rabies has been eradicated in many regions through vaccination and control measures, but it remains a significant health concern in developing countries.

Map of rabies-free countries and territories.

Self-assessment MCQs (single best answer)

What is the primary mode of rabies transmission?

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Which of the following is NOT a symptom of rabies?

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What is the incubation period for rabies typically?

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Which of the following is the gold standard for diagnosing rabies?

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Which form of rabies is characterised by irrational aggression and hyperactivity?

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What is the most effective preventive measure against rabies?

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Which virus is one of the main causative agents of rabies?

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In which part of the world are bats the main source of rabies transmission?

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What is commonly observed in individuals with hydrophobia due to rabies?

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Which protocol involves induced coma and antiviral drugs for treating symptomatic rabies?

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