DNA Viruses

Viruses contain only a protein coat and a central nucleic acid which may be RNA or DNA but
never both. They are obligate intracellular parasites which depend on the genes of their host cell
for their (the viruses) survival.

Most DNA viruses are double stranded (contain double stranded DNA). Parvoviruses are single
stranded.

Most RNA viruses are single stranded (contain single stranded RNA). Reoviruses are double
stranded.

Because viruses can’t be visualized like bacteria, viral infection is diagnosed by:
1) Symptoms
2) Tissue culture and electron microscopy
3) Antigen detection (via serological tests)
4) Nucleic acid detection
I. Enveloped DNA Viruses
Poxviruses: Largest and most complex group of viruses.
Poxviruses that infect humans include smallpox, cowpox and monkeypox
Herpesviruses: Recurrent, persistent, latent viruses, which enter the lytic cycle in response to
stress.
HSV1, HSV2: Infect mucous membranes. HSV2 is sexually transmitted.
Varicella Zoster virus: Chickenpox and potential recurrence as shingles due to latent
infection (invasion of spinal nerves). Treat with acyclovir,
immune serum globulin, vaccine.
Cytomegalovirus
(CMV). Common herpesvirus spread via body fluids. Chronically
carried. Systemic symptoms. Most prevalent viral infection of the
fetus.
Epstein-Barr virus
(EBV). Mononucleosis. Systemic symptoms. May become
oncogenic leading to Burkitts lymphoma (B cell cancer).
Hepadnaviruses:
Hepatitis B (HBV)
Liver disease, jaundice
Increased incidence of liver cancer
Spread by direct contact and body fluids
II. Non-enveloped DNA viruses
Adenovirus:
Infections of eye, gastrointestinal tract, respiratory tract. Colds.
Papillomavirus: Warts. Genital warts are sexually transmitted (leading STD in US).
Polyomavirus: Potential oncogenes (cancer causing)
III. Non-enveloped single stranded DNA viruses
Parvoviruses: Cause disease in cats, dogs and humans.
RNA Viruses

RNA viruses are classified as to whether or not they have a segmented genome. A segmented
genome allows genes to be rearranged easily and leads to a virus which is constantly changing its
antigenic determinants. Hence, no long-term immunity can be created. Genetic rearrangement
falls into two categories:
1) Antigenic drift: Small changes in the antigenicity of a virus that occur as a result of the
virus mutating over time.
2) Antigenic shift: Large changes in the antigenicity of a virus that occur when two viruses
combine parts of the genomes to create a third, unique virus.
IV. Enveloped, segmented, single stranded RNA viruses
Orthomyxoviruses
Influenza virus: Spread both human to human and animal to human
Virulence factors include two types of glycoprotein spikes which enhance host cell
binding, Hemaglutinin and Neuramidase
Bunyaviruses (arboviruses): Spread by insect vectors. Humans are accidental hosts. Viruses
cause encephalitis or hemorrhagic fever
Hantavirus
Arenavirus: Spread by insects
Lassa fever
California encephalitis
V. Enveloped, non-segmented, single stranded RNA viruses
Paramyxoviruses
Paramyxovirus: Parainfluenza mild respiratory disease
Morbillivirus: Measles
Pneumovirus: Respiratory syncytial virus (RSV), croup
Rhabdoviruses
Lyssavirus: Rabies (zoonotic)
Coronavirus: Colds. SARS (sudden acute respiratory syndrome)
Togavirus:
Rubella (German measles)
Flavivirus:
Yellow fever
Dengue fever
Hepatitis C
VI. Enveloped, single stranded, RNA retroviruses: Contain the enzyme reverse transcriptase, which is
used to transcribe RNA to DNA
Human T cell leukemia virus (HTLV I and II): Causative agents of adult T cell and hairy cell
leukemia
Human immunodeficiency virus (HIV)
Causative agent of AIDS
AIDS is defined as being infected with the virus and one of
 CD4 lower than 200/µl blood
 CD4 makes up less than 14% of lymphocytes
 Experience an AIDS defining illness
First infects macrophages then moves to T helper cells.
Dormancy may last up to fifteen years
Death is by opportunistic infection
Most accurate test for HIV infection is the Western blot (immunoassay), which detects
antibodies to HIV. Usually used as a secondary test
VII. Non-enveloped, singled stranded RNA viruses
Picornaviruses: polio, colds, Hepatitis A
Noroviruses: Gastroenteritis
VIII. Non-enveloped double stranded RNA viruses
Reoviruses: upper respiratory infections
Rotaviruses: Intestinal infections