Chapter 25 Clinical Virology

1
Chapter 25 Clinical Virology

• MLAB 2434 Clinical Microbiology
• Cecile Sanders Keri Brophy-Martinez

2
General Characteristics

• Obligate intracellular parasites
• Identified by either cell culture OR rapid tests
  from clinical specimens
• FA detection
• EIA
• Optical immunoassay (OIA)

3
Structure of Viruses

• Contain a viral genome of either RNA OR DNA
• Protein coat (capsid)
• Genome can be double stranded (ds) OR single
  stranded (ss)
• Genome protein coat called a virion
• Some viruses have an envelope

4
Classification of Viruses

• DNA OR RNA
• Number of strands (ds or ss)
• Morphology
• Presence or absence of envelope
• Approximately 3600 species of viruses

5
Viral Reproduction (Replication)

• Unique to viruses
• Virus attaches to surface of susceptible cell by
  specialized structures on specific receptors on
  the cell surface
• Virus enters cell by endocytosis (fusion of viral
  membrane cell membrane) OR lysis of cell
  membrane

6
Viral Reproduction (Replication) (contd)

• Inside the cell, virus loses protein coat,
  releasing DNA or RNA
• Viral genome directs host cell to make viral
  proteins and genome
• Virus-coded proteins and genome re-assemble in
  host cell
• New virions released by host cell lysis OR
  budding from host cell membrane

7
Viral Reproduction (Replication) (contd)
8
Specimen Collection and Transport

• Viral shedding greatest during early stages of
  infection, so specimens should be collected as
  early as possible
• Aspirates are best, but swabs are acceptable if
  cotton, dacron or nylon (Calcium alginate swabs
  inhibit growth of some viruses)
• Commercial viral transport systems available

9
Specimen Processing

• Optimal to process viral cultures immediately
• If impossible, store in refrigerator up to 4 days
• If longer, freeze at -70 C (-20 C will cause
  crystal formation, which disrupts host cells and
  results in significant loss of viruses)

10
Methods in Diagnostic Virology

• Three major methods to diagnose viral infections
• Direct detection of virus in clinical specimen
• Serologic antibody assays to detect viral
  antibodies
• Isolation of virus in culture

11
Direct Detection

• Advantages
• Rapid diagnosis
• Detection of nonculturable viruses
• No need for culture
• Disadvantages
• Confined to specific virus
• Costly and labor intensive
• Dependent of specimen adequacy and quality

12
Direct Detection (contd)

• Methods include
• Immunostaining
• EIA
• OIA
• Nucleic acid probes
• Gene amplification assays
• Electron microscopy
• Light microscopy (looking for cell inclusions or
  cytopathic effects on cells)

13
Serologic Assays

• Problems with serologic assays
• Measures host response rather than detect virus
• Antibody-producing capabilities of humans vary
• Antibody levels do not necessarily correlate with
  acuteness of infection

14
Serologic Assays (contd)

• Indications for serologic assays
• Diagnosis of infections with nonculturable
  organisms like hepatitis
• Determination of immune status (i.e. rubella,
  etc.)
• Monitoring immunosuppressed or transplant
  patients
• Epidemiologic studies

15
Viral Isolation

• Gold standard
• Three methods
• Cell culture
• Animal inoculation
• Embryonated eggs
• Most cell cultures done for herpes and genital
  and respiratory viruses

16
Cell Cultures

• Cell culture tissue culture
• Three categories
• Primary cell cultures
• Uses tissue from animals
• Seeded onto surface to form a monolayer
• Limited cell division

17
Cell Cultures (contd)

• Diploid cell cultures
• Cells can divide up to 50 times
• Human neonatal lung (HNL) is an example
• Continuous (heteroploid) cell cultures
• Cells are capable of unlimited cell division
• Derived from human cancer cells

18
Cell Cultures (contd)

• Advantages of cell culture
• Sensitive
• Can identify broad spectrum of viruses
• Etc.
• Disadvantages
• Time required for isolation and identification
• Viable organisms required
• Specialized resources and personnel needed

19
Cell Cultures (contd)

• Once viruses are grown in cell culture, cells are
  examined microscopically for cytopathic effects
  (CPE) on cells
• Some viruses, such as influenza, do not cause
  CPE, so changes must be demonstrated with
  hemagglutionation or immunofluoresence tests

20
Respiratory Viruses

• Influenza Viruses
• ssRNA virus
• Causes crucial health problems (epidemics and
  pandemics)
• Antigenic drifts and shifts
• Attack ciliated epithelial cells of respiratory
  tract

21
Respiratory Viruses(contd)

• Parainfluenzae Viruses
• Enveloped RNA
• Major cause of respiratory disease in young
  children
• Respiratory Syncytial Virus (RSV)
• Most common virus isolated from infants with LRT
  infections
• Enveloped RNA

22
Respiratory Viruses(contd)

• Adenoviruses
• Half of all infections are asymptomatic
• Causes 10 of all pneumonia cases
• Causes 5 - 15 of all gastroenteritis in
  children
• dsRNA

23
Respiratory Viruses (contd)

• Rhinoviruses
• Major cause of common cold
• Infect nasal epithelial cells and activate
  inflammatory response
• ssRNA, small and naked
• No cure
• Coronaviruses
• ssRNA
• Cold-like infections

24
Exanthemas

• Definition skin eruption accompanying certain
  infectious diseases
• Mumps
• Swelling of parotid glands, testes, ovaries and
  pancreas
• ssRNA
• Related to parainfluenza viruses
• Vaccine available

25
Exanthemas (contd)

• Measles (Rubeola)
• Once very common childhood illness
• Incidence dropped dramatically with advent of
  vaccine
• ssRNA virus
• Abrupt onset with sneezing, runny nose and cough,
  red eyes and high fever, followed by
  maculopapular (flat discolored area of skin with
  raised bump) rash on head and trunk (white spots
  on red background)
• Easily diagnosed clinically lab requests rare

26
Exanthemas (contd)

• Rubella
• Mild febrile illness with rash and
  lymphadenopathy many cases asymptomatic
• Rash starts on face and spreads to trunk and
  limbs no rash on palms and soles
• Causes birth defects in first trimester
• Enveloped ssRNA
• Vaccine strongly recommended
• Serologic titer for immune status

27
Exanthemas (contd)

• Hand, Foot and Mouth Disease
• Seen in young children spread by oral-fecal
  route
• Malaise, headache and abdominal pain, followed by
  sores on tongue, buccal mucosa and soft palate
• Maculopapular rash on hands, feet and buttocks
• Bullae on soles and palms
• ssRNA virus

28
Exanthemas (contd)

• Parovirus B19
• ssRNA
• Causes Erythema Infectiosum, also known as Fifth
  Disease (because it is the fifth infectious rash
  after rubeola, rubella, varicella and roseola)
• Slapped cheek appearance, spreading to trunk
  and limbs

29
Immunodeficiency Viruses

• Human Immunodeficiency Virus Type 1
• AIDS
• ssRNA
• Target cells are CD4 T cells
• Destruction of these cells results in
  opportunistic infections

30
Central Nervous System Viruses

• Enteroviruses
• ssRNA virus
• Includes poliovirus, among others
• Variety of infections and conditions, including
  paralysis
• Viruses initially in lymph tissues and spread to
  spinal cord, heart and skin

31
Agents of Gastrointestinal Infections

• Rotaviruses
• dsDNA with double-layer protein capsid
• Most common cause of gastroenteritis in infants,
  children and elderly
• Oral-fecal route

32
Agents of Gastrointestinal Infections (contd)

• Norwalk and Norwalk-Like Agents
• Gastroenteritis in older children and adults
• Outbreaks in camps, schools, and on cruise ships
• Nausea, vomiting, diarrhea and low-grade fever
• Highly infectious

33
Agents of Gastrointestinal Infections (contd)

• Enteric Adenoviruses
• Epidemics of gastroenteritis in young children
• Other Gastrointestinal Viruses
• Coronaviruses
• Caliciviruses
• Astroviruses

34
Arboviruses

• Derive name from mode of transmission (arthropod
  borne)
• Humans are dead-end hosts
• Four families
• Bunyaviridae family
• Basic fever
• Hemorrhagic fever, including Hantaan virus
• Encephalitis

35
Arboviruses (contd)

• Togaviridae family
• Encephalitis
• Flaviviridae family
• Most common cause of arboviral encephalitis in
  the world, including St. Louis encephalitis (SLE)
• Dengue fever (Classic and hemorrhagic)
• Yellow fever

36
Arboviruses (contd)

• Reoviridae family
• Colorado tick fever

37
Family Arenaviridae

• Causes hemorrhagic fevers
• Infect rodents and humans exposure by inhaling
  aerosolized virus particles
• Includes Lassa fever

38
Family Filoviridae

• Includes Ebola and Marburg viruses
• Rarely cause human infections
• Human infections may result from contact with
  infected monkeys
• High mortality rates
• Unknown reservoirs in nature

39
Rabies

• Transmitted by bite or scratch from infected
  animal
• Pain at site of infection, followed by flu-like
  symptoms
• CNS system changes, followed by death
• Vaccine and postexposure prophylaxis available
• Detected in brain of source animal

40
Human Papillomaviruses

• Causes
• Common and plantar warts
• Epidermodysplasia verrruciformis (rare autosomal
  disease)
• Genital warts
• Associated with cervical cancers

41
Hepatitis Viruses

• Hepatitis A (HAV) oral-fecal Hepatitis B (HBV)
  blood and body fluids
• Hepatitis C (HCV) blood and body fluids
• Hepatitis D (HDV) blood and body fluids
  requires HBV for replication
• Hepatitis E (HEV) oral-fecal

42
Herpesviruses

• HSV Type 1
• Oral herpes
• Cold sores
• Can cause encephalitis
• Recurrent
• HSV Type 2
• Genital herpes
• Neonatal herpes
• Can cause encephalitis
• Recurrent

43
Herpesviruses (contd)

• Varicella-zoster
• Varicella causes chicken pox
• Zoster is clinical manifestation of reactivated
  varicella virus, which can be latent in nerve
  tissue (Shingles)
• Epstein-Barr
• Mononucleosis
• Associated with Burkitts lymphoma and
  nasopharyngeal carcinoma

44
Herpesviruses (contd)

• Cytomegalovirus (CMV)
• Most common congenital infection in U.S.
• Most adults have antibodies to CMV
• Herpsevirus 6 causes Roseola Infantum or
  Fourth Disease

45
Herpesviruses (contd)

• Herpsevirus 7
• Infects CD4 cells
• Viruses present in 75 of adult saliva
• Herpsevirus 8
• Detected in Kaposis sarcoma
• Not culturable

46
Antiviral Therapy

• Some viral infections are treatable, especially
  if therapy is given early in infection
• Antivirals must be designed to target a viral
  replication mechanism without destroying host
  cells
• Like bacteria and antibiotics, the use of
  antivirals can result in virus resistance