Understanding rabies: Part One …a potentially violent zoonotic disease

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ENDEMIC to Zimbabwe, central, east and Southern Africa is the zoonotic viral disease commonly known as rabies, or ‘imbwa mupengo’ – literally meaning ‘mad dog disease’ in the Shona language, because of its potentially violent nature. 

According to my dictionary, the term is derived from the Latin word ‘rabies’, meaning ‘madness’.  

Whereas the Greeks used the word ‘lyssa’, derived from ‘lud’ or ‘violent’; this root is used in the name of the genus of rabies lyssavirus.

Heedful that September 28 is ‘World Rabies Day’ to promote information on prevention and elimination of the disease, we examine its existence and effects, worldwide, including in Zimbabwe.

Rabies is an infectious viral zoonotic disease that causes acute encephalitis in warm-blooded animals, including humans, that may become infected with the rabies virus. 

The disease is zoonotic, meaning it can be transmitted from one species to another, such as from dogs to humans, commonly by a bite from an infected animal. For a human, rabies is almost invariably fatal if post exposure prophylaxis is not administered prior to the onset of severe symptoms. 

Most animals can be infected by the virus and can transmit the disease to humans.  

Although birds have only been known to be infected in experiments, the virus has been adapted to grow in cells of ‘cold-blooded’ vertebrates (poikilothermic).  

Infected bats, monkeys, cattle, wolves, skunks, foxes, mongooses, raccoons, coyotes, dogs or cats present the greatest risk to humans.  

Rabies may also spread through exposure to infected domestic farm animals and other wild carnivores.  Small rodents, such as squirrels, hamsters, guinea pigs, gerbils, chipmunks, rats, mice and lagomorphs, such as rabbits and hares, are almost never infected with rabies and are not known to transmit rabies to humans. 

The virus is usually present in the nerves and saliva of a symptomatic rabid animal.  

Infection is usually, but not always, caused by a bite. 

In many cases, the infected animal is exceptionally aggressive; may attack without provocation and exhibits uncharacteristic behaviour. 

This is an example of a viral pathogen modifying the behaviour of its host to facilitate its transmission to other hosts. 

Transmission between humans is extremely rare. 

After a typical human infection by bite, the virus enters the peripheral nervous system. 

It then travels along the nerves toward the central nervous system, but cannot be easily detected within the host. 

Once within a muscle or nerve cell, the virus undergoes replication.   

When the virus reaches the brain, it rapidly causes encephalitis, the prodromal phase, and is the beginning of the symptoms. 

Once the patient becomes symptomatic, treatment is almost never effective and mortality is over 99 percent. The rabies virus infects the central nervous system.  Rabies may also inflame the spinal cord, producing transverse myelitis, ultimately causing disease in the brain and death.

Three stages of the rabies virus are: 

 one-to-three-day period characterised by behavioural changes and is known as the prodromal stage. 

the excitative stage, which lasts three-to-four days. 

This stage is often known as ‘furious rabies’ for the tendency of the affected animal to be hyper-reactive to external stimuli and bite at anything near. 

the paralytic stage and is caused by damage to motor neurons. 

Inco-ordination is seen owing to rear limb paralysis, and drooling and difficulty swallowing is caused by paralysis of facial and throat muscles. 

Death is usually caused by respiratory arrest. 

From the point of entry, the virus is neurotropic — travelling quickly along the neural pathways into the central nervous system and then to other organs. 

The salivary glands receive high concentrations of the virus, thus allowing further transmission.  

The rabies virus travels to the brain by following the peripheral nerves. 

The incubation period of the disease is usually a few months in humans, depending on the distance the virus must travel to reach the central nervous system. 

Once the rabies virus reaches the central nervous system and symptoms begin to show, the infection is virtually untreatable and usually fatal within days.

Symptoms of rabies in the early stage are general malaise, headache and fever, progressing to acute pain, violent movements, uncontrolled excitement, depression and hydrophobia.  

A victim may experience periods of mania and lethargy, eventually leading to a coma; usually the primary cause of death is respiratory insufficiency.  

Death almost invariably results two-to-10 days after the first symptoms appear. 

Once symptoms have appeared, survival is rare, even with the administration of proper and intensive care.

Dr Tony Monda BSc, DVM, is currently conducting veterinary epidemiology, agronomy and food security and agro-economic research in Zimbabwe. For views and comments, e-mail: tonym.MONDA@gmail.com

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