Neuroinfections – what should we know?

Viruses that infect the nervous system are called neurotropic viruses. The best-known diseases caused by viral infections of the nervous system include rabies, poliomyelitis, shingles and TBE.

What are viruses? Viruses are very simple organisms that require a host for reproduction . As they are only about 20 – 300 nm in size, an electron microscope is used to see them. Viruses vary in shape (round, rod-shaped or asymmetrical ). Their structure is relatively simple: they consist of one or a few molecule classes and are sometimes surrounded by a protein envelope (capsid) or consist of several envelopes of proteins and lipids. The capsid contains the DNA or RNA as genetic material. Viruses are neither cells nor do they have their own metabolism. They do not have their own energy production or the ability to synthesize proteins. This is why viruses – in contrast to other pathogens such as bacteria – are not regard ed as being alive.

How do viruses enter the nervous system? Viral infections of the nervous system are usually only accidental . There are only very few viruses that specialize in the nervous system. One example of this is the rabies virus, which is transmit ted from animals to humans and causes rabies. Natural protective barriers very effectively prevent viruses from entering the nervous system or the brain. Nevertheless, some human pathogens are able to invade the nervous system. The central nervous system (CNS) has the blood-brain and the blood-cerebrospinal fluid barriers, two natural barriers against pathogens. Immunological protective mechanisms ensure that invading microorganisms are harmless .

The CNS is physically protected from injury by bone structures and the meninges. Homeostasis is regulated by two cellular barriers that separate the CNS from the systemic circulation: the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). They protect the brain from pathogens in the blood stream or the cerebrospinal fluid while still delivering nutrients and removing metabolites. The BBB is a dense endothelium with very low permeability that physically separates the systemic circulation from the brain parenchyma. Among the specialized microvascular endothelial cells of the brain, a selective passage exists through which small lipophilic molecules, cytokines and immune cells can pass.

What are the infection routes? To enter the CNS, pathogens must cross the BBB or the BCSFB. This happens via three possible path s:

DIRECT PENETRATION of the BBB/BCSFB

NEUROGENIC SPREAD

Viruses, e.g., herpes virus B, can enter the brain from the nose or mouth via the olfactory and/or trigeminal nerve (main entry route into the CNS). They enter the olfactory nerve via the olfactory epithelial tissue into the olfactory receptor cells and then into the brain via axonal transport.

The rabies virus uses a different route. When bitten by an infected animal, the virus first enters the muscle and from there passes into the motor nerves. From these, the pathway leads into the spinal cord and upwards into the brain.

HEMATOGENOUS SPREAD

Viruses can infect leukocytes. In these infected leukocytes („ Trojan horse s“) they pass through the BBB and spread further in the brain.

Some pathogens, such as flaviviruses, use the „Trojan horse“ mechanism to enter the CNS. Here, phagocytes infected with a pathogen then cross the BBB paracellularly .

In summary, the major entry routes for viruses into the nervous system are as follows:

1. Passive diffusion (the passive-aggressive route): Viruses diffuse between endothelial cells

2. Endothelial cell infection (the energetic route): Virus replication within endothelial cells

3. Viral transcytosis (the shuttle route): Viral transport via endothelial vesicles

4. Cell-associated viral transport (Trojan horse route): In leukocytes through the endothelium.

EXERCISE:

1. Viruses are living organisms. A) True B) False

2. To enter the CNS, pathogens must cross the BBB or the BCSFB. This happens via three possible paths: A) Direct penetration B) Droplet infection C) Neurogenic spread D) Smear infection E) Hematogenous spread

3. There are only very few viruses that specialize in the nervous system, because an infection is only accidental. One example of this is the rabies virus. A) True B) False

You’ll find the answer below.

BIBLIOGRAPHY 1. www.asklepios.com

2. www.dict.cc

3. www.mayoclinic.org

4. www.neurology.org

5. www.springermedizin.de

6. www.webster-dictionary.org

CARMEN LOBITZ Carmen Lobitz ist freie Trainerin, Lehrerin und MTL. Sie gründete 1998 in Berlin lobitz seminare, die unter anderem Englischseminare für Health Professionals anbieten. Kontakt: mail@lobitz-seminare.de www.lobitz-seminare.de

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Lösung für Exercise:

1. B

2. A, C, E

3. A

Neuroinfections – what should we know?

What are viruses?

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Neuroinfections – what should we know?

What are viruses?

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