Rabies

Rabies is a serious zoonotic disease caused by lyssaviruses. This term refers to diseases that spread from animals to humans. The virus causes acute and severe encephalitis, which is an inflammation of the brain and the protective membranes surrounding it. Historically, the disease was often called hydrophobia, or the fear of water. This name came from the intense panic victims feel when offered liquids. Once the physical symptoms of rabies appear, the disease is almost always fatal. It is currently classified as a neglected tropical disease.

The mechanism of the virus is a complex biological process. It begins when an infected animal bites or scratches a human. The virus can also enter through the eyes, nose, or mouth via infected saliva. It can even be transmitted through tears or nervous tissues. Once inside, the virus is neurotropic, meaning it specifically targets nerve cells. It usually starts by replicating in muscle cells near the site of the wound. This allows the virus to multiply without being noticed by the host's immune system.

To move through the body, the virus must travel to the central nervous system. It binds to specific receptors, likely the acetylcholine receptor, at the neuromuscular junction. The virus then uses a process called retrograde transport to move along the nerve cell axon. It achieves this by using a protein called P to interact with dynein, a protein in the cytoplasm. Once it reaches the cell body, it moves rapidly into the brain. After infecting the brain, the virus travels outward to the salivary glands. This prepares the virus to be transmitted to a new host through biting.

There are two main clinical forms of the disease. The first is the furious form, which affects about 80% of infected people. This version causes irrational aggression and can lead to a "foaming at the mouth" effect. This occurs because saliva accumulates in the mouth. The second is the paralytic form, which affects the remaining 20%. This form is marked by muscle weakness, loss of sensation, and paralysis. Unlike the furious form, the paralytic version does not usually cause hydrophobia.

The timeline of the disease is measured by the incubation period. This is the time between the initial infection and the first symptoms. This period usually lasts between one and three months. However, it can be as short as one week or as long as one year. The exact timing depends on how far the virus must travel along the nerves to reach the brain. Once symptoms like fever, headache, or confusion begin, death usually occurs within two to ten days.

Globally, the impact of rabies is significant and widespread. The virus is present in more than 150 countries on every continent except Antarctica. It causes approximately 59,000 deaths every year. About 40% of these deaths occur in children under the age of 15. More than 95% of these human deaths happen in Africa and Asia. Dogs are the most common source of infection globally. In areas where dogs carry the disease, they are responsible for more than 99% of human cases.

In different parts of the world, the primary source of rabies changes. In the Americas, bat bites are the most common cause of infection. In these regions, dog bites account for less than 5% of cases. While most mammals can be infected, some animals are rarely involved. Rodents, such as mice and squirrels, are almost never infected. This is often because larger rabid animals kill them before they can spread the virus. Some animals, like the Virginia opossum, are resistant due to their lower body temperatures.

Prevention and medical intervention are vital for managing the risk. If an exposure occurs, washing the wound with soap and water for 15 minutes can help. This may reduce the number of viral particles. Doctors can use the rabies vaccine and rabies immunoglobulin to prevent the disease. These treatments are effective only if they are administered before symptoms start. Vaccination programs for animals and people at high risk are also essential. These efforts have successfully removed rabies from dogs in places like Japan and Australia.