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36 Human Diseases Caused by Viruses

 

CHAPTER OVERVIEW

This chapter discusses viruses that are pathogenic to humans, with emphasis on those viral diseases occurring in the United States.

 

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

! describe those viral diseases that are transmitted through the air and that directly or indirectly involve the respiratory system

! discuss viral diseases transmitted by arthropod vectors

! discuss viruses requiring direct contact for transmission, because they are so sensitive to environmental conditions that they are unable to survive for significant periods of time outside their hosts

! discuss viral diseases that are food- or waterborne

! discuss slow virus diseases that may be caused by viruses or prions

! discuss viral diseases that do not readily fit into any of the above categories

 

CHAPTER OUTLINE

I. Airborne Diseases

A. Chickenpox (varicella) and shingles (zoster)

1. Chickenpox is acquired by inhaling virus-laden droplets into the respiratory system (epidemic)

2. Incubation period is 10 to 23 days

3. Confers permanent immunity, but the virus enters a latent stage in the nuclei of sensory nerve roots

4. When the person 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 (zoster)

5. Treated with acyclovir or famciclovir in immunocompromised patients

6. Infection can be prevented or shortened by a live attenuated vaccine

B. Influenza (flu)Cundergoes frequent antigenic variation

1. Antigenic driftCsmall variation

2. Antigenic shiftClarge variation

3. Treatment is symptomatic

4. Enters by receptor-mediated endocytosis

5. Can be rapidly identified in clinical specimens by an enzyme immunoassay (EIA)

6. Epidemics vary greatly in severity and mortality rate

C. Measles (rubeola)Cskin disease with respiratory spread; infection confers permanent immunity; MMR vaccine is used for prevention; serious outbreaks are still reported in North America and Europe, especially among college students

D. Mumps

1. Spread in saliva and respiratory droplets

2. Can be serious in postpubescent male (infects testes and causes sterility)

3. MMR vaccine is used for prevention

E. Respiratory syndromes and viral pneumoniaCacute respiratory viruses

1. A syndrome is a set of signs and symptoms that occur together and characterize a particular disease

2. Associated with rhinitis, tonsillitis, laryngitis, and bronchitis

3. Immunity is incomplete, and reinfection is common

4. Viral pneumonia is clinically nonspecific, and symptoms may be mild or severe (death is possible)

5. Treatment is supportive, not curative

6. Respiratory syncytial virus (RSV) is the most dangerous cause of respiratory infection in young children

F. Rubella (German measles)

1. Infection spread by respiratory droplets

2. Mild in children

3. Disastrous for pregnant women in first trimesterCcauses congenital rubella syndrome, which leads to fetal death, premature delivery, and congenital defects

4. A vaccine (MMR: measles, mumps, rubella) is available

G. Smallpox (variola)Cin the past was devastating but has been eradicated

1. It has obvious (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

5. A vigorous worldwide vaccination program was undertaken

6. The last case from a natural infection occurred in Somali in 1977

II. Arthropod-Borne Diseases

A. Viruses multiply in tissues of insect vectors without producing disease

B. Vector acquires a lifelong infection

C. Three clinical syndromes are common

1. Undifferentiated fevers, with or without a rash

2. EncephalitisCoften with a high case fatality rate

3. Hemorrhagic feversCfrequently severe and fatal

D. Permanent immunity; no vaccines available; treatment is supportive

E. Some examples are yellow fever, Ebola viral hemorrhagic fever, various forms of encephalitis, and Colorado tick fever

III. Direct Contact Diseases

A. Acquired immune deficiency syndrome (AIDS)

1. Caused by human immunodeficiency virus (HIV), a lentivirus within the family Retroviridae, which is believed to have evolved in Africa from a virus that infects African green monkeys

2. Requires direct exposure of the person=s bloodstream to body fluids containing the virus; groups most at risk are:

a. Homosexual/bisexual men

b. Intravenous drug users

c. Transfusion patients and hemophiliacs

d. Prostitutes

e. Newborn children of infected mothers

3. The virus targets CD4+ cells such as certain T cells, macrophages, dendritic cells, and monocytes

4. Four types of pathological changes may ensue

a. AIDS-related complex (ARC)Cmild fever, weight loss, lymph node enlargement, and presence of antibodies to HIV; can develop to full-blown AIDS

b. AIDSCantibodies 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

c. AIDS dementia and other evidence of central nervous system damage; the virus can cross the blood-brain barrier

d. AIDS-related cancerCKaposi=s sarcoma (caused by human herpesvirus 8; HVV-8), carcinoma of the mouth and rectum, B-cell lymphomas

4. Co-infection with human herpesvirus-6 (HHV-6) can greatly enhance the development of AIDS

5. Diagnosis is by viral antigen detection or by viral antibody detection (seroconversion)

6. Drugs such as AZT, ddI and ddC target the reverse transcriptase and slow progress but do not reverse or cure the disease, nor do they affect transmission of the virus by infected individuals

7. New drugs being used are protease inhibitors; they also slow progress of the disease but do not cure it

8. Vaccines to stimulate production of neutralizing antibodies are currently under investigation

9. Prevention and control involves screening of blood and blood products, education, and safe sexual practices (use of condoms)

B. Cold sores (fever blisters)

1. Caused by herpes simplex type 1 (HSV-1)

2. Blister at site of infection is due to viral- and host-mediated tissue destruction

3. Lifetime latency with periodic reactivation in times of physical or emotional stress

4. Herpetic keratitisCrecurring infections of the cornea; leads to blindness

5. Drugs are available that are effective against cold sores

C. Common coldCcaused by many different rhinoviruses, many of which do not confer durable immunity; at one time it 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 and symptomatic

D. Cytomegalovirus inclusion disease

1. Most infections are asymptomatic but infection can be serious in immunologically compromised individuals

2. Infected cells have intranuclear inclusion bodies

3. Spread by contaminated saliva and urine

E. Genital herpes

1. Caused by herpes simplex type 2 (HSV-2)

2. Most frequently transmitted by sexual contact

3. Symptoms include fever, burning sensation, genital soreness, and blisters in the infected area

4. Blisters heal spontaneously, but the virus remains latent and is periodically reactivated

5. Congenital (neonatal) herpes is spread to an infant during vaginal delivery; therefore, infected females should deliver children by caesarean section

6. There is no cure, but acyclovir decreases healing time, duration of viral shedding, and duration of pain

F. 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

2. CD4+ cells are the main sites of viral replication

3. Tropism is wide including CD8+ T cells, natural killer cells, and probably epithelial cells

4. Transmission is probably by way of saliva

5. It can lead to pneumonitis in immunocompromised individuals

6. It has been implicated in chronic fatigue syndrome and lymphadenitis in immunocompetent adults

7. Diagnosis is by immunofluorescence or enzyme immunoassay

8. There is neither treatment nor prevention currently available

G. Human Parvovirus B19 Infections

1. Cause mild symptoms (fever, headaches, chills, malaise) in most normal adults

2. Cause erythema infectiosum in children

3. Can cause a joint disease in some adults

4. Can cause serious aplastic crisis in immunocompromised individuals or those with sickle-cell disease or autoimmune hemolytic anemia

5. Can cause anemia and fetal hydrops (the accumulation of fluid in the tissues) in infected fetuses

6. Spread by a respiratory route

7. Antiviral antibodies are the principal means of defense, and treatment is by means of commercial immunoglobulins containing anti-B19 and human monoclonal antibodies to B19

8. Infection is usually followed by lifelong immunity

H. LeukemiaCcertain 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-I) has shown some promise

I. Mononucleosis (Infectious)

1. Caused by the Epstein-Barr virus (EBV), a herpesvirus

2. Spread by mouth-to-mouth contact (Akissing disease@)

3. Occurs frequently in late adolescence

4. Infects B cells

5. Disease is self-limited and requires supportive treatment with plenty of rest

6. EBV is also associated with Burkitt=s lymphoma and nasopharyngeal carcinoma in certain parts of the world

J. RabiesCa disease of carnivorous and insectivorous animals

1. Transmitted by bites or infected animals, aerosols in caves where bats roost, or contaminated scratches, abrasions, open wounds, or mucous membranes with saliva of infected animals; most are caused by dog bites in countries where canine rabies is still endemic

2. Multiplies in skeletal muscle and connective tissue

3. Migrates to central nervous system; produces characteristic Negri bodies (masses of virus particles or unassembled viral subunits)

4. Symptoms progress and death results from destruction of the part of the brain that regulates breathing

5. 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)

6. Prevention and control involves annual preexposure vaccination of dogs and cats, postexposure vaccination of humans, and frequent preexposure vaccination of humans at special risk

K. 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) is transmitted by blood transfusions, contaminated equipment, unsterile needles, or any body secretion; also transplacental transmission to fetus occurs

a. Most cases are asymptomatic

b. Symptomatic cases involve fever, appetite loss, and fatigue

c. Death can result from liver cirrhosis or HBV-related liver cancer

d. Control measures include

(1) Excluding contact with contaminated materials

(2) Passive immunotherapy within seven days of exposure

(3) Vaccination of high-risk groups

3. Hepatitis C is spread by intimate contact with virus-contaminated blood, in utero from mother to fetus, by the fecal-oral route, or through organ transplants; disease is milder than hepatitis B with no jaundice

4. Hepatitis D 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

5. Treatment is supportive, to allow the liver damage to resolve and the liver to repair itself, and the use of the hepatitis B vaccine

6. Recently, hepatitis F and hepatitis G have been identified and are currently being investigated

IV. Food-Borne and Waterborne Diseases

A. Gastroenteritis (viral)Cacute viral gastroenteritis

1. Caused by Norwalk and Norwalk-like viruses, rotaviruses, caliciviruses, and astroviruses

2. Disease severity may range from asymptomatic infection, to mild diarrhea, to severe and occasionally fatal dehydration

3. Leading cause of childhood death in developing countries where malnutrition is common

4. Seen most frequently in infants

5. Treatment is supportive and symptomatic

B. Hepatitis ACcaused by the hepatitis A virus (HAV)

1. Spread by fecal-oral contamination of food or drink, or by infected shellfish that live in contaminated water

2. Mild intestinal infections sometimes progress to liver involvement

3. Low mortality rate (less than 1%); resolves in four to six weeks; infections in children

4. Produces strong immunity

5. A killed vaccine (Havrix) is now available

C. Hepatitis E

1. Implicated in many epidemics in developing countries in Asia, Africa, and Central and South America

2. Infection is associated with fecal-contaminated drinking water

3. 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

5. Can be fatal (10%) in pregnant women in their last trimester

6. There are no specific measures for prevention other than those aimed at improving the level of health and sanitation in affected areas

D. Poliomyelitis (polio, infantile paralysis)

1. Virus is stable and remains infectious in food and water

2. Multiplies in throat and intestinal mucosa

3. Enters bloodstream and causes viremia (99% of viremia cases are transient with no clinical disease)

4. Enters central nervous system (less than 1% of cases), leading to paralysis

5. Treatment is supportive, but vaccines have been extremely effective (less than 10 cases per year; no endogenous reservoir)

V. Slow Virus Diseases

A. Progressive pathological process caused by a virus or a prion

B. Clinical silence for months or years, followed by progressive clinical disease, ending in profound disability or death

VI. Other Diseases

A. Diabetes mellitus (juvenile onset) is suspected to have a viral etiology

B. Viral arthritis does not cause permanent joint damage; associated with various viruses that primarily cause other diseases

C. Warts are caused by papillomaviruses; treatment involves removal of warts, physical destruction, or injection of interferon

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