Rabies

Introduction

Rabies is a disease and it’s caused by a specific virus called the Rabies lyssavirus.
The reason this infection is so famous is because its literally the deadliest disease – it has the highest case fatality rate of any human infectious disease.
This term refers to the proportion of people that die from a certain disease. For some context, the 1918 Spanish flu had a case fatality rate of about 2.5%, untreated bubonic plague sat about 60%, and Ebola virus disease is as high as 90%.
For unvaccinated individuals, rabies will kill 99% of people.

It’s a part of the Rhabdoviridae family. Rhabdo means rod, referring to the bullet shape of the pathogen. Within this family, it’s a part of the genus Lyssavirus. Lyssa in Greek mythology was the personification of “mad rage”, quite telling of the disease’s clinical features.

Rabies is a zoonotic infection that occurs in mammals, usually transmitted to humans by the bite of an infected host, usually a dog.

Now lets track the virus during its pathogenesis which has 5 main steps.

1st, inoculation. A patient is bitten and inoculated with virus. Its incubation period is usually 50 days.

2nd, the virus, now in the muscle, makes its way to the neuromuscular junction where it binds to the nicotinic acetylecholine receptors.

3rd it then travels up the axon of the motor neuron and replicates

4th the virus continues to spread up to the central nervous system to the brain where it rapidly disseminates.

5th the rabies virus exits the CNS. It does this through motor, sensory, and autonomic nerves, and by replicating locally in salivary and lacrimal glands in order to be transmitted to the next host.

For its management, I just want you to think about postexposure prophylaxis. Since there is no effective therapy for rabies, it’s extremely important to prevent the disease after an animal bite.

This involves three things:

local wound care like cleaning and debridement
Active immunization in the form of one of two purified inactivated rabies vaccines available.
Passive immunization in the form of rabies immunoglobulin also known as RIG, which is infiltrated at the site of the bite or via an IM injection distally.

Prophylaxis is not required in some circumstances as per Harrisons Internal Medicine:

If you get bitten by an animal but rabies is not present in the area, you don’t need prophylaxis
If it is and the animal was not captured, you require prophylaxis
If the animal is a normally behaving dog or cat then you can observe it for 10 days. But if it’s another animal like a bat, you require it.
If the animal doesn’t show signs of illness in this observation period, you don’t need it.
Otherwise or if you’re unsure, the animal needs to be sent for examination for the virus, and if positive at autopsy you do need prophylaxis

The clinical features of rabies always begins with a prodrome of nonspecific manifestations.

These include fever, malaise, and headache.

The earliest specific neurological features include paraesthesia and pruritis around the site of exposure, even though the wound is often healed by this time.

After the prodrome, there are two major forms of the disease that might ensue. Encephalitic rabies or paralytic rabies.

Encephalitic rabies is more common at 80%. Its also known as furious rabies because of its MAD features.

M stands for mental state changes. Hallucinations and combativeness is common along with episodes of wild hyperexcitability, typically followed by periods of complete lucidity.

A stands for autonomic dysfunction. Hypersalivation is the most notable feature. Another curious phenomenon is hydrophobia. This involves involuntary, painful contractions of the diaphragm and pharyngeal muscles in response to swallowing liquids. Usually we inhibit strong reflexes that act to protect the respiratory tract in the event of choking. When inhibition is lost due to CNS damage, it results in an exaggerated defence response even when a patient isn’t choking.

If you put hypersalivation and hydrophobia together, you can probably understand why patients get the classic appearance of “foaming at the mouth.”

Our MAD acronym rapidly ends with Death, once the virus starts causing dysfunction in the brainstem.

On the other hand, Paralytic rabies only occurs in 20% of patients. The cardinal features of classical encephalitic rabies are missing as patients have an early flaccid paralysis, often beginning around the area of the bite.

For rabies, I just remember that a fifth get stiff, glad they’re not MAD.

This reminds you that rabies has two forms. Its paralytic 20% of time, otherwise its encephalitic with the classic MAD features: mental state changes, autonomic dysfunction and death.