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Microbiology, Fifth Edition
Microbiology, 5/e
Lansing M Prescott, Augustana College
Donald A Klein, Colorado State University
John P Harley, Eastern Kentucky University
Human Diseases Caused by Viruses

Study Outline

  1. Airborne Diseases
    1. Chickenpox (varicella) and shingles (zoster)
      1. Chickenpox
        1. Caused by varicella-zoster virus, a member of Herpesviridae; is acquired by inhaling virus-laden droplets into the respiratory system
        2. Incubation period is 10 to 23 days after which small vesicles appear on face and upper trunk
        3. Can be prevented or infection shortened with attenuated vaccine or the drug acyclovir
        4. Infection confers permanent immunity from chickenpox, but does not rid individual of virus; instead, virus enters a latent stage in the nuclei of sensory nerve roots
      2. Shingles
        1. When an adult who harbors the virus is under stress, the virus can emerge and cause sensory nerve damage and painful vesicle formation, a condition known as shingles
        2. Treated with acyclovir or famciclovir in immunocompromised patients
    2. Influenza (flu)
      1. Caused by orthomyxoviruses that can undergo frequent antigenic variation
        1. Antigenic drift-small variation
        2. Antigenic shift-large variation
      2. Animal reservoirs are important (e.g., chickens and pigs) and contribute to antigenic shifts
      3. Virus is acquired by inhalation or ingestion of virus-contaminated respiratory secretions; it enters host cells by receptor-mediated endocytosis
      4. Influenza is characterized by chills, fever, headache, malaise, and general muscular aches and pains; diagnosis can be confirmed by rapid serological tests
      5. Treatment is focused on alleviating symptoms, but some antiviral drugs have been shown to decrease duration and symptoms of type A influenza
    3. Measles (rubeola)
      1. A skin disease with respiratory spread caused by Morbillivirus, a member of family Paramyxoviridae
      2. After 10-21 day incubation, cold-like symptoms develop, followed by a rash; on rare occasions can develop into subacute sclerosing panencephalitis
      3. MMR (measles, mumps, and rubella) vaccine is used for prevention
    4. Mumps
      1. Caused by mumps virus, a member of the genus Rubulavirus in the family Paramyxoviridae
      2. Spread in saliva and respiratory droplets; portal of entry is the respiratory tract
      3. Causes swelling of salivary glands; meningitis and inflammation of testes are complications, especially in postpubescent male
      4. Therapy is supportive and the MMR vaccine is used for prevention
    5. Respiratory syndromes and viral pneumonia
      1. Acute respiratory syndromes
        1. Caused by a variety of viruses collectively referred to as acute respiratory viruses
        2. Associated with rhinitis, tonsillitis, laryngitis, and bronchitis; immunity resulting from infection is incomplete and reinfection is common
      2. Viral pneumonia is clinically nonspecific, and symptoms may be mild or severe (death is possible)
      3. Respiratory syncytial virus (RSV) is the most dangerous cause of respiratory infection in young children; is a member of the RNA virus family Paramyxoviridae
    6. Rubella (German measles)
      1. Caused by rubella virus, a ssRNA virus of family Togoviridae
      2. Virus is spread by respiratory droplets, and the resulting infection is mild in children (a rash), but disastrous for pregnant women in first trimester; in pregnant women it causes congenital rubella syndrome, which leads to fetal death, premature delivery, and congenital defects
      3. No treatment is indicated; a vaccine (MMR) is available
    7. Smallpox (variola)
      1. Caused by variola virus, a dsDNA virus belonging to the family Poxviridae
      2. Virus is transmitted by aerosol or contact; symptoms include severe fever, prostration, rash, toxemia, and septic shock
      3. Virus was eradicated as the result of a vigorous worldwide vaccination program; eradication was made possible for several reasons
        1. Disease has easily identifiable clinical features
        2. There are virtually no asymptomatic carriers
        3. It infects only humans (there are no animal or environmental reservoirs)
        4. It has a short period of infectivity
  2. Arthropod-Borne Diseases
    1. General features of arthropod-borne diseases
      1. Viruses multiply in tissues of insect vectors without producing disease, and vector acquires a lifelong infection
      2. Three clinical syndromes are common
        1. Undifferentiated fevers, with or without a rash
        2. Encephalitis-often with a high case fatality rate
        3. Hemorrhagic fevers-frequently severe and fatal
      3. Infection provides permanent immunity; for many of the diseases, no vaccines are available; treatment is usually supportive
    2. Colorado Tick Fever
      1. Caused by Coltivirus (RNA virus)
      2. Tick-borne; main reservoirs are ground squirrels, rabbits, and deer
      3. Symptoms include abrupt onset of fever, chills, severe headaches, photophobia, and muscle pain
      4. Serology is used to confirm diagnosis
    3. Yellow Fever
      1. Mosquito-borne; there are two patterns of transmission
        1. Urban cycle-human-to-human transmission
        2. Sylvan cycle-monkey-to-monkey and monkey-to-human transmission
      2. Early symptoms include fever, chills, headache, backache; these are followed by nausea and vomiting; in severe cases jaundice, lesions and hemorrhaging occur
      3. Prevention and control is by vaccination and vector control
  3. Direct Contact Diseases
    1. Acquired immune deficiency syndrome (AIDS)
      1. Caused by human immunodeficiency virus (HIV), a lentivirus within the family Retroviridae; believed to have evolved in Africa from viruses that infect other primates
      2. Disease occurs worldwide, but certain groups are more at risk; these include homosexual/bisexual men, intravenous drug users, transfusion patients and hemophiliacs, prostitutes, and newborn children of infected mothers
      3. Virus is acquired by direct exposure of the person?s bloodstream to body fluids containing the virus; can also be transmitted via breast milk
      4. Virus targets CD4+ cells such as T-helper cells, macrophages, dendritic cells, and monocytes
      5. Precise mechanism of pathogenesis is unknown
      6. Four types of pathological changes may ensue
        1. AIDS-related complex (ARC)-mild fever, weight loss, lymph node enlargement, and presence of antibodies to HIV; can develop to full-blown AIDS
        2. AIDS-antibodies not sufficient to prevent infection; virus establishes itself in CD4 immunocompetent cells, which then proliferate in the lymph nodes and cause the lymph nodes to collapse; leads to depletion of T-cell progenitors, which cripples the immune system; this leaves the person open to opportunistic infections
        3. AIDS dementia and other evidence of central nervous system damage; the virus can cross the blood-brain barrier
        4. AIDS-related cancer-Kaposi?s sarcoma (caused by human herpesvirus 8; HHV-8), carcinoma of the mouth and rectum, B-cell lymphomas
      7. Diagnosis is by viral antigen detection or by viral antibody detection (seroconversion)
      8. Three types of antiviral agents are used to treat HIV disease
        1. Nucleoside anologues that inhibit HIV reverse transcriptase (RT)
        2. Nonnucleoside inhibitors of HIV RT
        3. Inhibitors of HIV protease
      9. Vaccines to stimulate production of neutralizing antibodies are currently under investigation
      10. Prevention and control involves screening of blood and blood products, education, and protected sexual practices (use of condoms)
    2. Cold sores-fever blisters
      1. Caused by herpes simplex type 1 (HSV-1), a dsDNA virus; transmission is by direct contact
      2. Blister at site of infection is due to viral- and host-mediated tissue destruction
      3. Lifetime latency is established when virus migrates to trigeminal nerve ganglion; is periodically reactivated in times of physical or emotional stress
      4. Herpetic keratitis-recurring infections of the cornea; can result in blindness
      5. Drugs are available that are effective against cold sores; diagnosed by cell culture and immunological tests
    3. Common cold
      1. Caused by many different rhinoviruses as well as other viruses; many do not confer durable immunity
      2. Understanding rhinovirus structure has suggested approaches to developing vaccines and drugs
      3. At one time, common cold was thought to be spread by explosive sneezing, but now it is believed to be primarily spread by hand-to-hand contact; treatment is supportive
    4. Cytomegalovirus inclusion disease
      1. Most infections are asymptomatic but infection can be serious in immunologically compromised individuals; virus persists in the body and is shed for several years in saliva, urine, semen, and cervical secretions
      2. Infected cells have intranuclear inclusion bodies
      3. Diagnosis is by viral isolation and serological tests
      4. Some antiviral agents are available for treatment; these are only used in high-risk patients; prevention is by avoiding close personal contact with infected individual and by using blood or organs from seronegative donors
    5. Genital herpes
      1. Caused by herpes simplex type 2 (HSV-2), a dsDNA virus that is a member of Herpesviridae; virus is most frequently transmitted by sexual contact
      2. Disease has active and latent phases
        1. Active phase-the virus rapidly reproduces; can be symptom free or painful blisters in the infected area may occur, as well as other symptoms (fever, burning sensation, genital soreness); blisters heal spontaneously
        2. Latent phase-after resolution of active phase virus retreats to nerve cells; the viral genome resides in the nuclei of host cells and can be periodically reactivated
      3. Congenital (neonatal) herpes is spread to an infant during vaginal delivery; therefore, infected females should deliver children by caesarean section
      4. There is no cure, but acyclovir decreases healing time, duration of viral shedding, and duration of pain
    6. Human herpesvirus 6 infections
      1. Etiologic agent of exanthem subitum (rash) in infants, a short-lived disease characterized by a high fever of 3 to 4 days duration, followed by a macular rash; CD4 cells are the main sites of viral replication and the tropism of the virus is wide and includes CD8+ T cells, natural killer cells, and probably epithelial cells; transmission is probably by way of saliva
      2. Virus produces latent and chronic infections and can be reactivated in immunocompromised individuals, leading to pneumonitis; virus has been implicated in a variety of other diseases, including chronic fatigue syndrome and lymphadenitis; diagnosis is by immunofluorescence or enzyme immunoassay; there is neither treatment nor prevention currently available
    7. Human parvovirus B19 infections
      1. Mild symptoms (fever, headaches, chills, malaise) in most normal adults; erythema infectiosum in children; joint disease in some adults; serious aplastic crisis in immunocompromised individuals or those with sickle-cell disease or autoimmune hemolytic anemia; anemia and fetal hydrops (the accumulation of fluid in the tissues) in infected fetuses
      2. Spread by a respiratory route
      3. Antiviral antibodies are the principal means of defense, and treatment is by means of commercial anti-B19 immunoglobulins; infection is usually followed by lifelong immunity
    8. Leukemia-certain leukemias (adult T-cell leukemia and hairy-cell leukemia) are caused by retroviruses (HTLV-1 and HTLV-2, respectively) and are spread similarly to AIDS; they are often fatal and there is no effective treatment, although interferon (INF-a) has shown some promise
    9. Mononucleosis (infectious)
      1. Caused by the Epstein-Barr virus (EBV), a herpesvirus (dsDNA virus), which is spread by mouth-to-mouth contact (?kissing disease?) or by shared bottles and glasses; virus replicates in lymphatic tissue, eventually infects B cells, and causes enlargement of lymph nodes and spleen, sore throat, headache, nausea, general weakness and tiredness, and a mild fever; disease is self-limited
      2. Treatment is largely supportive and requires plenty of rest; diagnosis is made by serological tests
      3. EBV is also associated with Burkitt?s lymphoma and nasopharyngeal carcinoma in certain parts of the world
    10. Rabies
      1. Caused by a number of different strains of neurotropic viruses of the family Rhabdoviridae (negative-strand RNA viruses)
      2. Transmitted by bites of infected animals; aerosols in caves where bats roost; or by scratches, abrasions, open wounds, or mucous membranes contaminated with saliva of infected animals
      3. Virus multiplies in skeletal muscle and connective tissue, then migrates to central nervous system, causing a rapidly progressing encephalitis
      4. In the past, diagnosis depended on the observation of characteristic Negri bodies (masses of virus particles or unassembled viral subunits); today diagnosis is based on immunological tests, virus isolation, as well as the detection of Negri bodies
      5. Symptoms progress and death results from destruction of the part of the brain that regulates breathing
      6. Vaccines conferring short-term immunity are available and must be given soon after exposure (postexposure vaccination is effective because of the long incubation period of the virus); prevention and control involves annual preexposure vaccination of dogs and cats, postexposure vaccination of humans, and frequent preexposure vaccination of humans at special risk
    11. Viral hepatitides
      1. Hepatitis is any inflammation of the liver; currently nine viruses are recognized as causing hepatitis; some have not been well characterized
      2. Hepatitis B (serum hepatitis)
        1. Caused by hepatitis B virus (HBV), a dsDNA virus with a circular genome
        2. Virus is transmitted by blood transfusions, contaminated equipment, unsterile needles, or any body secretion; also transplacental transmission to fetus occurs
        3. Most cases are asymptomatic; sometimes fever, appetite loss, abdominal discomfort, nausea, and fatigue develop; death can result from liver cirrhosis or HBV-related liver cancer
        4. Control measures involve excluding contact with contaminated materials, passive immunotherapy within seven days of exposure, and vaccination of high-risk groups
      3. Hepatitis C
        1. Caused by hepatitis C virus (HCV) an ssRNA virus within the family Flaviviridae
        2. Virus is spread by intimate contact with virus-contaminated blood, in utero from mother to fetus, by the fecal-oral route, or through organ transplants
        3. Diagnosis is by serological tests
        4. Has reached epidemic proportions
        5. Treated with interferon
      4. Hepatitis D
        1. Is caused by hepatitis D virus (HDV) (formally called the delta agent), which only causes disease if the individual is coinfected with hepatitis B virus; coinfection may lead to a more serious acute or chronic infection than that normally seen with HBV alone
        2. Diagnosis is by serological tests; treatment is difficult and often involves administration of alpha interferon; prevention and control is by the use of the hepatitis B vaccine
      5. Recently, hepatitis F and hepatitis G have been identified and are currently being investigated
  4. Food-Borne and Waterborne Diseases
    1. Gastroenteritis (viral)-acute viral gastroenteritis
      1. Caused by Norwalk and Norwalk-like viruses, rotaviruses, caliciviruses, and astroviruses
      2. Main transmission route is fecal-oral route; disease is leading cause of childhood death in developing countries
      3. Seen most frequently in infants; disease severity may range from asymptomatic infection, to mild diarrhea, to severe and occasionally fatal dehydration
      4. Viral gastroenteritis is usually self-limited; treatment is supportive
    2. Hepatitis A-caused by the hepatitis A virus (HAV)
      1. Spread by fecal contamination of food or drink, or by infected shellfish that live in contaminated water
      2. Caused by the hepatitis A virus (HAV), a plus-strand RNA virus of family Picornaviridae
      3. Mild intestinal infections sometimes progress to liver involvement; most cases resolve in four to six weeks and produce strong immunity
      4. Control is by hygienic measures and sanitary disposal of excreta; a killed vaccine (Havrix) is now available
    3. Hepatitis E
      1. Implicated in many epidemics in developing countries in Asia, Africa, and Central and South America
      2. Caused by hepatitis E virus (HEV), an ssRNA virus
      3. Infection is associated with fecal-contaminated drinking water; HEV enters the blood from the gastrointestinal tract, replicates in the liver, is released from hepatocytes into the bile, and is subsequently excreted in the feces
      4. HEV, like HAV, usually runs a benign course and is self-limiting; can be fatal (10%) in pregnant women in their last trimester
      5. There are no specific measures for prevention other than those aimed at improving the level of health and sanitation in affected areas
    4. Poliomyelitis
      1. Caused by poliovirus, a member of the family Picornaviridae; is a plus-strand RNA virus that is stable and remains infectious in food and water
      2. Once ingested, virus multiplies in throat and intestinal mucosa; subsequently enters bloodstream and causes viremia (99% of viremia cases are transient with no clinical disease); can enter central nervous system (less than 1% of cases), leading to paralysis
      3. Vaccines have been extremely effective (less than 10 cases per year; no endogenous reservoir) in preventing and controlling the disease; global eradication may be possible in the next few years
  5. Slow Virus and Prion Diseases
    1. Progressive pathological process caused by a virus or a prion that remains clinically silent for months or years; this is followed by progressive clinical disease, ending in profound disability or death
    2. Four of the six recognized human diseases are caused by prions; these diseases are called spongiform encephalophathies
  6. Other Diseases
    1. Diseases that do not fit into any of the previous categories; includes diabetes mellitus, viral arthritis and warts
    2. Warts are caused by papillomaviruses; treatment involves removal of warts, physical destruction, or injection of interferon; some papillomaviruses play a major role in the pathogenesis of epithelial cancers of the male and female genital tracts








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