WE ALL HAVE UNIQUE BRAINS

VIRAL INFECTIONS

VIRAL INFECTIONS

Viral infections of the brain almost always lead to inflammation of the brain. Some viruses are specifically designed for the brain, but other ones can spread across the whole body. Some of the viruses that infect the brain only tend to occur in people whose immune systems are compromised, but others occur even in immunocompetent (healthy) patients.

Arboviruses

  • Genus: Arbovirus
  • Lipid envelope y/n:
  • Method of transferral: zoonotic transfer (from animals to humans)
  • Symptoms: seizures, confusion, delirium, stupor, coma, reflex assymetry (reflexes present on one side but not the other), ocular palsies (loss of control of eye movements).

Arboviruses are transferred by mosquitoes and they are commonly found in tropical regions. Examples of  infections involving arboviruses include the West Nile virus, dengue fever and the Eastern and Western equine encephalitis. In this case, the disease is not native to humans: rather, animals carry the viruses and the mosquitoes pass them on to human. When they are passed onto humans, they can cause brain inflammation, but not every case in the same: some cases end up more severe than others. In severe cases of encephalitis caused by arboviruses, haemorrhage can occur as necrotizing vaculitis (inflammation of the blood vessels leading to cell death).

Cytomegalovirus (CMV) cytomegalovirus_diagram

  • Genus= Cytomegalovirus
  • Lipid envelope y/n: no
  • Method of transferral:
  • Symptoms: Severe brain damage, microcephaly (brain shrinkage), calcification of the ventricles

Cytomegalovirus is a virus which is common in people that have a compromised immune system or in cases of AIDS (acquired immunodeficiency syndrome, which is caused by HIV). Any type of cell within the central nervous system can be infected, however it tends to infect the ventricles of the brain and causing ventriculoencephalitis (inflammation of the ventricles in the brain).

Herpes Simplex Type 1 and 2 

  • Genus= Herpesviridae 
  • Lipid Envelope y/n: yes
  • Method of transferrance= shedding of cold sores (type 1), sexual contact (type 2).
  • Symptoms= watery blisters in mouth lips and, genital area

Herpes simplex virus is a virus which classically demonstrates the latency cycle. What I meanHerpes Simplex Virus by that is that there is an initial outbreak of herpes and then after a while it goes into remission (disappears). That does not mean it is cured, however: rather, the herpes virus has remained in the body and avoided detection by the immune system by not rupturing the host nerve cells. Eventually, the herpes virus travels down to the axons of those nerve cells, and blisters appears again on the skin in those areas as a consequence.

Despite the fact that herpes does not specifically attack the brain, it can attack the brain and cause herpes encephalitis. Encephalitis (brain inflammation) caused by type 1 herpes usually affects children and young adults, and it focuses on the frontal and temporal lobes, leading to mood swings, and alterations in memory and behaviour. In severe cases, it can necrotize (kill) brain tissue and lead to brain haemorrhage. Type 2 herpes usually affects adults and tends to affect the meninges and cause meningitis. Herpes can also be passed on from mother to child through labour: the child can develop severe encephalitis.

Human Immunodeficiency Virus (HIV)

  • Genus= Lentivirus
  • Lipid envelope y/n: no
  • Method of transferral: sexual contact, blood tranfusion, breast feeding
  • Symptoms: opportunistic infections

HIV is infamous for two reasons: its ability to mutate at a rapid rate and avoid the immune system, thereby preventing the immune system to develop antibodies, and its severe effect on the immune system, leading to early death due to AIDS (acquired immunodeficiency syndrome). As its name suggests, it leaves the host susceptible to all types of opportunistic infections (that would not infect healthy individuals) by attacking the coordinating cell of the immune system, T-helper or CD4+ cells.

HIV can also affect the CNS directly: over half of people that develop AIDS as a result of HIV infection have neurological (brain-originated) symptoms. There are three different subtypes of brain pathology associated with HIV: aspetic HIV meningitis, HIV meningoencephalitis, and vaculoar myelopathy.

  • Aseptic HIV meningitis is lymphocytic in nature (has high numbers of lymphocytes) and also have loss of myelin and inflammation around the blood vessels (otherwise known as perivascular inflammation.)
  • HIV meningitis often leads to dementia, which has the symptoms of memory loss, mood disturbances and loss of mental capacity. There are also giant cells present, which you can find out more about in Abnormal Cells)
  • Vacuolar myelopathy happens within the spinal cord.

Polio Virus

  • Genus= Enterovirus
  • Lipid envelope y/n:
  • Method of transferral= faecal-oral route (from infected poo to mouth, in other words)
  • Symptoms: gastroenteritis (inflammation of the stomach and intestine), flaccid paralysis (floppy muscles), muscle wastage, lack of reflexes, paralysis of the lungs.

Ever since the invention of vaccines, the crutches and iron lungs often associated with polio infection has gone down significantly. It was even declared that polio virus no longer existed in developed parts of the world thanks to vaccines, but unfortunately in recent years it has made a comeback. Polio viruses are not designed specifically to infect the brain, however it can cause paralysis due to damage of the motor neurons in the brain stem, motor cortex or spinal cord.

Rabies Virus 

  • Genus=Lyssavirus 
  • Lipid envelope y/n:
  • Method of transferral: Rabid animal bite
  • Symptoms: foaming at the mouth, fever, headaches, pain from touch, convulsions, mania

Pic: http://images.fineartamerica.com/images-medium-large/rabies-virus-2-russell-kightley.jpg

If Stephen King’s Cujo (both the movie and the book) has taught us anything, it’s that the rabies virus is a bloody scary virus. Unfortunately, it is nearly always fatal, and is specifically designed for the brain. The amount of time between the bite from a rabid animal and infection from the virus is dependent on the location of the bite: the closer to the head, the quicker the infection rate. The amount of time that infection happens is usually a few months. The virus, once it enters the body, transfers up the peripheral nerves near the site and then up to the central nervous system.

 

 

 

Varicella-Zoster Virus 

Pic: http://img.medscape.com/slide/migrated/editorial/cmecircle/2000/228/slide02.gif

  • Genus: Varicellovirus
  • Lipid envelope y/n:
  • Method of transferral:
  • Symptoms: red, itchy spots on the skin

Varicella-Zoster virus is more commonly known as chickenpox, and it commonly occurs in children. It usually only occurs once and then the body cures it, but it can occur again in adults and cause the painful condition of shingles. In people with healthy immune systems, VZ virus doesn’t affect the brain, but in people whose immune system has already been damaged, inflammation caused by VZ virus (or herpes zoster encephalitis) can occur.