Viruses

Friday, January 9, 2009 12:23 PM

Herpesviridae

Herpesviridae is a large family of DNA viruses that cause diseases in animals and human.

It is an enveloped DNA virus and it can cause latent or lytic infections. It has large double-stranded, linear DNA genomes encased within an icosahedral protein cage called the capsid which is itself wrapped in a lipid bilayer membrane called the envelope. This whole particle is known as the virion.

There is no vaccine is currently available to prevent or eliminate herpes. However, treatments are available to reduce viral reproduction and shedding, prevent the virus from entering the skin, and alleviate the severity of symptomatic episodes.

There are 3 different group of Herpesviridae, they are classified as:

Alphaherpes: Herpes simplex 1, Herpes simplex 2 and Varicella Zoster virus
Herpes Simplex viruses

Herpes Simplex viruses are classified into 2 groups; Herpes Simplex virus 1 and Herpes Simplex virus 2. They enter and hide in the human nervous system, accounting for their durability in the human body. HSV-1 is commonly associated with herpes outbreaks of the face known as cold sores or fever blisters, whereas HSV-2 is more often associated with genital herpes.

Varicella Zoster virus

Varicella Zoster virus (Human Herpes 3) are also classified into 2 groups; Varicella which is chicken pox-respiratory tract and the other group is Herpes zoster is Shingles. Shingles is found in the second stage of chicken pox and it will occur many years after the initial infection.

Girl infected with chicken pox

Guy infected with shingles

Betaherpes: Cytomegalovirus, Human herpes type 6, Human Herpes type 7

Cytomegalovirus (Human herpes 5) infections are frequently associated with salivary glands, though they may be found throughout the body. HCMV infection can also be life threatening for patients who are immunocompromised (e.g. patients with HIV, organ transplant recipients). This virus can also be found in urine, semen, cervial secretion and even breast milk. It is also a virus most frequently transmitted to a developing fetus.

Gammaherpes: Epstein Barr Virus, Human herpes type 8

Epstein Barr Virus (Human Herpes 4) is one of the most common viruses in humans. Most people become infected with EBV, which is often asymptomatic but commonly causes infectious mononucleosis. It has surface receptors known as glycoprotein H (gH) which is essential for penetration of B cells but it also plays a role in attachment of virus to epithelial cells.

Life Cycle of Herpesviridae

(A) Herpes simplex virus (HSV) is shown undergoing the lytic cycle (entry, uncoating, and viral transcription and DNA replication in the nucleus, particle assembly, and exit from the cell) in epithelial cells of the skin to cause a primary infection.

(B) Some virus enters the sensory neuron terminals and travels retrogradely to the nucleus where it establishes latency.

(C) Periodic reactivation results in anterograde transport of viral particles, shedding from the neuron, and re-infection of epithelial cells, which leads to asymptomatic shedding or recurrent lesions.

Clinical Features of Varicella Zoster

Primary infection

- Fever
- Lesion all over the body
- Scratched lesion which will lead to secondary infection

Secondary Infection
- Scarring
- Dangerous in Pregnant women-unborn child will born limbless
- May affect the nervous system- Guillain Barre syndrome

Lab Diagnosis

- Commonly used-Antigen test-EIA
- Blood test
- Virus culture-most difficult

Control

- Stop Risky Behaviour-mainly by sexual contact
- Don’t kiss

By: Toh Wan Nee