Rabies

Overview of Rabies

Zoonotic diseases are the ones that spread through animals. These diseases may be either viral or bacterial. People having pets at home or working at the animal care centers are at greater risks of these diseases. Rabies is a disease that belongs to the above category. It spreads mainly through dogs (and other warm-blooded domestic animals) and gives rise to hydrophobia. Each year, rabies causes nearly 24,000 to 60,000 deaths worldwide. The pet owners, as well as the farm owners, should make sure that their domesticated animals are properly vaccinated and do not spread diseases to the humans around them.

What is Rabies?

Rabies is a viral disease that causes brain inflammation in humans and other mammals. The patients usually experience fever and tingling sensation at the site of exposure. The most common manifestations are immobility of certain parts of the body, uncontrolled excitement and an unexplained fear of water. The disease is mainly caused by lyssaviruses. In many cases, rabies may lead to loss of consciousness and death. More than 95% of the human deaths due to rabies occur in Africa and Asia.

What causes Rabies?

Rabies is caused by Rhabdovirus, which belongs to the Lyssavirus genus. When this virus comes in contact with the human or an animal host, it attaches to the body of the host. The subsequent penetration and uncoating of the virus take place, that leads to the release of the viral nucleocapsid into the cytoplasm. The virus G protein binds to the cells of the neuron. This phenomenon is known as neurotropism of the rabies virus.

On entering the muscle or the nerve cell, the virus undergoes replication. Right from the point of entry, the virus is neurotropic. It travels along the neural pathways into the central nervous system. The muscle cells close to the site of infection are affected first. Here the virus replicates without being noticed by the immune system of the body. After the completion of replication, the viruses start binding to the acetylcholine receptors at the neuromuscular junction. Thereafter it migrates to the nerve cell body through the axon, replicates in the motor neurons and finally reaches the brain. Once the brain is infected, the virus travels to the autonomic and the peripheral nervous systems. Thereafter it travels to the salivary glands, from where it is ready to be transmitted to the next host.

How does the disease transmit?

The disease transmits when an infected animal scratches, licks or bites another animal or a human. Direct or indirect exposure to the saliva of the infected animal can also cause the disease, especially if the saliva comes in contact with the eyes, mouth or nose of a healthy person (or animal). The transmission of the virus to the new host also depends on the proximity of the nerve cells at the site of introduction of the virus. The virus is not spread through urine, feces or blood. Person to person transmission of the disease is very rare. Care should be taken so that a healthy person does not come in contact with the saliva of the diseased person through broken skin or open sores. People undergoing transplants or blood transfusion should be particularly aware of the transmission of the disease from an infected donor.

What are the signs and symptoms of Rabies?

The disease has different signs and symptoms in animals and humans.

Symptoms in animals

An infected animal usually shows the following symptoms-

• Attacking inanimate things or other animals
• Gnawing or biting at its own limbs
• Tendency to hide
• Looking depressed
• Losing fear of humans
• Constant drooling
• A drooping head
• Paralysis

Symptoms in humans

In humans, the incubation period of the virus is 1-3 months. This means that the symptoms start manifesting after 1-3 months of the initial infections. In some cases, however, incubation periods have been found to be as short as four days or even longer than 6 years. The following clinical features (symptoms) are generally observed in human beings-

• Violent and aggressive movements
• Uncontrolled excitement
• An unexplained fear of water (hydrophobia)
• Immobility of certain parts of the body
• Mental confusion
• Loss of consciousness
• Sleep disorders (insomnia)
• Agitated behavior
• Hallucinations
• Paranoia and trauma
• Coma and death

The most dangerous symptom of a rabid person is hydrophobia, or “fear of water”. It is a collective term for a group of symptoms where a person has difficulty swallowing water and cannot quench his thirst properly. These people show extreme panic when presented with water or other liquids.

The diseased person thereby produces large quantities of saliva. They may experience spasms in the muscles of the throat and larynx while trying to drink. This is often explained by the fact that the virus replicates and accumulates in the salivary glands of the infected host. Nearly 80% of the rabid people experience hydrophobia.

Which diagnostic tests are available?

The diagnosis of rabies should be done very carefully, since in the early stages it is often confused with certain mental diseases which are characterised by aggressive behaviour. The World Health Organisation (WHO) recommends Fluorescent Antibody Test (FAT), which is basically an immunohistochemistry procedure to detect rabies. Certain light microscopy tests are also effective in detecting the disease. Differential diagnosis should be carried out in order to rule out diseases caused by enteroviruses, arboviruses and herpesviruses.

How to treat and prevent Rabies?

Post-exposure treatment can prevent the disease within 10 days of exposure to the virus. Povidone-iodine or alcohol is usually recommended to reduce the spread of the virus. One dose of Human Rabies Immunoglobulin (HRIG) and four doses of rabies vaccine over a period of 14 days can help prevent the disease. The domesticated animals should be immunised on time and taken for regular checkups to a veterinary surgeon in order to detect diseases if any at a very early stage. This will considerably reduce the increasing global burden of zoonotic diseases.

References