What is encephalitis and its Causes?

Encephalitis | Johns Hopkins Medicine

Encephalitis is an inflammation of the brain caused most of the time by viruses. It can go unnoticed and heal independently, or it can occur as a severe disease that leaves significant neurological sequelae.

Its leading cause is viral infections (by herpes viruses, enteroviruses, or arboviruses). It is a disease whose frequency is difficult to determine; This is because most of the time, encephalitis causes mild and very diffuse symptoms, similar to the flu or a cold. That is why it is suspected that some flu symptoms are mild encephalitis that heals on their own.

Most patients present as the most common manifestations high fever (in 90% of cases), headaches (80%), disorientation (70%), some language disorder (60%), behavioral disturbances (40 %), and seizures (30-60% of cases).

Origin and incidence of encephalitis

The frequency of encephalitis in the general population is not very high. Only three or five cases appear for every million inhabitants. The population groups that suffer the most from the disease are children, with five or 10 cases per 100,000 inhabitants, especially children under one year of age. The sequelae of encephalitis are much more dramatic in this age group since they have a whole life ahead of them that can be conditioned by a handicap.

In general, encephalitis is caused by viruses. However, other microorganisms such as bacteria and other agents are capable of triggering it. Among the viruses, those of the herpes virus family (herpes simplex, varicella-zoster virus, cytomegalovirus), enteroviruses, and viruses transmitted by animals (such as mosquitoes, ticks, animals with rabies) are the culprits of most cases.

There is also immune-mediated encephalitis caused by alterations in the immune system of the affected person, which require specialized neurological surveillance and treatments other than infectious encephalitis.

The importance of vaccine protection

Encephalitis can heal itself. You need to treat the symptoms and let time do its work. Fortunately, there are more and more specific drugs to eliminate the responsible viruses. But if something has modified the impact of this disease on society, it is vaccines. Many of the viral infections that cause encephalitis ( measles, enterovirus, polio, rubella, or mumps) can be prevented today with safe vaccines and mandatory in most countries. 

Causes of encephalitis

The cause of encephalitis is infections by neurotropic viruses, which have a particular affinity for the central nervous system. These viruses can be transmitted in different ways; some of them through the respiratory route (such as the measles virus ), some through the fecal-oral course (such as the poliovirus ), and others even sexually (such as the herpes simplex virus type 2 ). The main viruses that can cause encephalitis are :

  • Herpes simplex virus types 1 or 2 (HSV): These viruses cause innocent cold sores or genital herpes. The virus remains in the nerve ganglia for life, occasionally migrating to the skin, causing blisters and itching. In a few cases, the virus can make a mistake and migrate to the central nervous system causing encephalitis.
  • Other herpes viruses: within the group of herpes viruses, others can cause encephalitis, in addition to other diseases.
  • Mosquito-borne viruses: these are called arboviruses. They appear mainly in specific regions of the world, such as the West Nile virus or the Japanese encephalitis virus (also found in Southeast Asia). Other viruses that cause encephalitis are transmitted by animal bites, such as the rabies virus.
  • HIV: HIV can cause encephalitis from first contact (rare) or in any reactivation of the virus abandonment of antiretroviral therapy.

Coming in contact with any of these viruses does not mean that you will develop encephalitis for sure. Many of these viruses are known for other, more specific diseases. The appearance of encephalitis also depends on other factors such as age (the very young and the elderly are at greater risk) and the state of the immune system (an alteration of the same can cause immune-mediated encephalitis ).

Among other less common causes of encephalitis, we can find other microorganisms, such as bacteria (those that cause tuberculosis, Lyme disease, or syphilis ), or parasites (nematodes, cysticercosis, and toxoplasmosis ) in patients with diseases or a weakened immune system. They can even be due to an allergic reaction to the vaccination.

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