Chapter 24

Viral Diseases and DNA Viruses

• All DNA viruses are double stranded (ds) except for Parvoviridae (ss)

• All RNA viruses are single stranded (ss) except for Reoviridae (ds)

• The envelope is derived in part from host cell membranes; often contains spikes

• Viral infection are specific to the host: A viral receptor-host cell interaction

• Most cells infected by viruses are destroyed due to severe pathology and loss of functions

• Diseases range from very mild or asymptomatic to severe

• Many diseases are strictly human in origin, others are zoonoses transmitted by arthropods

• Protection is given by interferon, neutralizing antibodies, and cytotoxic T-cells
  

Most DNA Viruses Can Integrate into Human Chromosome

A process leads to oncogenesis (production of cancer cells)

Retroviruses (RNA) are involved in cancer

Viral Diseases Vary in Severity Depending on:

• Virulence
• Age
• Health
• Habitat of the host

Life-long immunity develops to some but not all viral agents

Viral Diseases are Diagnosed by

• Serology

• PCR

• Tissue culture

• Molecular techniques 

• Nature of the genome (ds or ss)

• Existence of an envelope

• Target cells

• Size

There are 6 Groups of DNA Viruses

I. Enveloped Viruses
 

• Poxviruses (no capsid)

  • Smallpox

• Herpesviruses (latent)

  • Cold sores, mononucleosis

• Hepadnaviruses (ds + ss)

  • Hepatitis B

II. Non-Enveloped or 'Naked' Viruses
 

• Adenoviruses (ds)

  • Common cold, keratoconjunctivitis

• Papovarviruses (ds)

  • Genital warts

• Parvoviruses (ss)

  • Erythema contagiosum

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Enveloped DNA Viruses

Poxviruses

Clasification and Structure

• Produce eruptive pustules (pocks or pox) and leaves depressed scars (pockmarks)

• Largest and most complex of animal viruses

• Largest genome of all viruses

• Multiply in cytoplasm in well-defined sites (factory areas)

• Specificity for the cytoplasm of epidermal cells and subcutaneous connective tissue

• Produce typical lesions and stimulate cell growth that can lead to tumor formation

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Facts on Smallpox

The 33^rd World Health Assembly "declares solemnly that the world all its peoples have won freedom from smallpox.... and unprecedented achievement int he history of public health...." (Resolution 33-3, May 8, 1980, Geneva, Switzerland)

• Last smallpox case was reported in 1977

• Vaccination is by scarification with the Vaccinia virus, but is not longer done routinely

• Now only exist in government laboratoriesIts genes have been cloned however, the culture collection have not been destroyed
   

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Other Poxviruses

• Cowpox (udder and teats), rare in humans
• Monkeypox (may affect humans)
• Rabbitpox
• Mousepox
• Camelpox
• Elephantpox
• Buffalopox

A Large Family that Includes

• Type 1 (fever blisters)

• Type 2 (genital infections)

• Herpes zoster

• Chickenpox

• Shingles

• Cytomegalovirus (CMV)

• Epstein-Barr virus (EBV)

• Herpesvirus 6 and 7

Characteristics

• Latency and recurrent infections are typical with potential for oncogenesis

• Among the largest viruses (150 to 200 nm)

• Enclosed within a loosely fitting envelope with glycoprotein spikes

• Icosahedral capsid containing ds-DNA

• Replication within nucleus, viral release with cell lysis

• Deactivated by organic solvents or detergents and unstable outside host

• Humans are the only reservoirs

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HSV-1

• Usual etiologic agent herpes labialis, ocular herpes, gingivostomatitis and pharyngitis

• Transmitted by closed contact, usually of face and mouth

• Latent stage occurs in trigeminal ganglion

• Ocular herpes (herpetic keratitis) when latent virus travels into the ocular rather than the mandibular branch of the trigeminal nerve

• Whitlows (Abscess on the distal portion of finger) -- among personnel working on oral cavity

• Neonatal encephalitis (> 30% of cases)

Identification

• Tissue culture
• Flourescent antibodies
• DNA probes

Therapy

• Acyclovir (Zovirax) since 1980's
• Cuts length of infection and reduce viral shedding
• Also, lysine taken orally at the earliest stage
  

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HSV-2

• Etiologic agent of herpes genitalis

• Transmitted by sexual or close contact

• 3^rd or 4^th most common STD in USA

• Latency stage occurs in sacral ganglia

• Skin lesions on internal, external genitalia, thighs and buttocks (thin walled vesicles)

• Recurrent infections triggered by fever, UV radiation, menstruation, stress, or mechanical injury

• It is more than a source of pain and discomfort: it often has psychological effects that impair sexual relationships

Complications

Whitlows (among obstetric, gynecological personnel)

Neonatal encephalitis (most cases)

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The Varicella-Zoster Virus

• Same virus causes varicella (chickenpox) and a recurrent herpes zoster (shingles) infection

• Kock's postulates showed that the virus is the same in both diseases, and that zoster is a reactivation of a latent varicella virus

• Chickenpox

• Transmitted by droplets

• Skin lesions, macules, pox-like vesicles and itchy rash

• Fever

 

Shingles

• Virus migrates to spinal nerves, and is reactivated by surgery, cancer or other stimuli, travels out to skin causing painful lesions on head and trunk

• Recurs only once or twice

• Use acyclovir

• Vaccine available in Japan

Detection

Fluorescent antibodies of viral antigens in skin infections

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Cytomegalovirus (CMV)

• Among the most ubiquitous or common pathogens of humans

• Causes a cytopathic effect that produces giant cells and formation of nuclear and cytoplasmatic inclusions

• High infection rate in children, pregnant women, drug abusers, and male homosexuals

• Spread through close contact (saliva, mucus, urine, and semen

• Can go systemic and cause death

• Common in AIDS and transplant patients

• Antiviral drugs for therapy

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Human Herpesvirus 6 (HHV-6)

• Opportunistic in leukemia patients

• Connection to Kaposis sarcoma (?)

• Possible role of herpesvirus in cancers?

• First evidence that links herpesvirus to cancer was found in 1994:

  ....viral DNA was found in a majority of Kaposi's sarcoma tumors that was not present in non-cancerous tissues from the same patient

 

 

Luis A. Materon, Ph.D.
lmateron@utpa.edu