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Microbiology

Viral Pathogens

Serodiagnosis is pivotal in identifying and understanding viral infections. By detecting antibodies or viral antigens in a patient’s serum, we can determine their exposure history, disease stage, and immune status. This section focuses on the serodiagnosis, clinical significance, and epidemiology of several key viral pathogens: hepatitis viruses (A, B, C), Epstein-Barr virus (EBV), human immunodeficiency virus (HIV), cytomegalovirus (CMV), rubella virus, and measles virus

Hepatitis A Virus (HAV)

  • Serodiagnosis
    • Anti-HAV IgM: Indicates acute HAV infection. Appears early in the infection and declines within a few months
    • Anti-HAV IgG: Indicates past HAV infection or vaccination. Provides long-term immunity
  • Clinical Significance
    • HAV causes acute, self-limiting hepatitis. It does not cause chronic infection
    • Symptoms range from mild (fatigue, nausea) to severe (jaundice, liver failure)
    • Serodiagnosis confirms the diagnosis and helps distinguish it from other causes of hepatitis
  • Epidemiology
    • Transmission: Fecal-oral route (contaminated food or water)
    • Risk factors: Poor sanitation, travel to endemic areas
    • Prevention: Vaccination (highly effective) and good hygiene practices
    • Globally distributed, with higher prevalence in areas with poor sanitation

Hepatitis B Virus (HBV)

  • Serodiagnosis
    • HBsAg (Hepatitis B surface antigen): Indicates current HBV infection (acute or chronic). It’s the first marker to appear
    • Anti-HBs (Antibody to HBsAg): Indicates immunity to HBV (either from vaccination or past infection). It appears after HBsAg clears
    • Anti-HBc IgM (Antibody to Hepatitis B core antigen IgM): Indicates acute or recent HBV infection. Appears shortly after HBsAg
    • Anti-HBc IgG (Antibody to Hepatitis B core antigen IgG): Indicates past or chronic HBV infection. Persists for life
    • HBeAg (Hepatitis B e antigen): Indicates high viral replication and infectivity
    • Anti-HBe (Antibody to HBeAg): Indicates lower viral replication and infectivity
  • Clinical Significance
    • HBV can cause acute or chronic hepatitis
    • Chronic HBV infection can lead to cirrhosis, liver failure, and hepatocellular carcinoma
    • Serodiagnosis helps determine the stage of infection, guide treatment decisions, and assess immunity
  • Epidemiology
    • Transmission: Parenteral (blood), sexual contact, mother-to-child (vertical transmission)
    • Risk factors: Injection drug use, unprotected sex, healthcare workers, infants born to infected mothers
    • Prevention: Vaccination (highly effective), safe injection practices, screening of pregnant women
    • Globally distributed, with higher prevalence in certain regions (e.g., Southeast Asia, Africa)

Hepatitis C Virus (HCV)

  • Serodiagnosis
    • Anti-HCV (Antibody to HCV): Indicates exposure to HCV. Does not necessarily mean current infection (some people clear the virus spontaneously)
    • HCV RNA: Detects active HCV infection. Used to confirm chronic infection and monitor treatment response
  • Clinical Significance
    • HCV often causes chronic hepatitis
    • Chronic HCV infection can lead to cirrhosis, liver failure, and hepatocellular carcinoma
    • Serodiagnosis is used to screen for HCV infection, confirm chronic infection, and monitor treatment response
  • Epidemiology
    • Transmission: Primarily parenteral (blood)
    • Risk factors: Injection drug use, blood transfusions (before 1992), healthcare workers, tattoos
    • Prevention: Safe injection practices, screening of blood products
    • Globally distributed, with varying prevalence depending on risk factors

Epstein-Barr Virus (EBV)

  • Serodiagnosis
    • Viral Capsid Antigen (VCA) IgM: Indicates acute EBV infection (infectious mononucleosis). Appears early and declines within a few months
    • VCA IgG: Indicates past EBV infection. Persists for life
    • Early Antigen (EA) IgG: Indicates acute or reactivated EBV infection. Usually disappears after acute infection, but can persist in some individuals
    • EBV Nuclear Antigen (EBNA) IgG: Indicates past EBV infection. Appears later than VCA IgG and persists for life
  • Clinical Significance
    • EBV causes infectious mononucleosis (“mono”), characterized by fatigue, fever, sore throat, and lymphadenopathy
    • EBV is also associated with certain cancers (e.g., Burkitt lymphoma, nasopharyngeal carcinoma)
    • Serodiagnosis helps diagnose acute infection and determine past exposure
  • Epidemiology
    • Transmission: Saliva (“kissing disease”)
    • Most people are infected with EBV during childhood or adolescence
    • Ubiquitous worldwide

Human Immunodeficiency Virus (HIV)

  • Serodiagnosis
    • HIV Antibody/Antigen Combination Assay: Detects both HIV antibodies (IgG and IgM) and the HIV p24 antigen. Used for initial screening
    • HIV Antibody Differentiation Assay: Distinguishes between HIV-1 and HIV-2 antibodies. Used to confirm positive screening results
    • HIV RNA (Viral Load): Measures the amount of HIV RNA in the blood. Used to monitor disease progression and treatment response
  • Clinical Significance
    • HIV causes acquired immunodeficiency syndrome (AIDS), characterized by a weakened immune system and opportunistic infections
    • Early diagnosis and treatment with antiretroviral therapy (ART) can prevent disease progression and improve survival
    • Serodiagnosis is crucial for identifying infected individuals, monitoring disease progression, and guiding treatment
  • Epidemiology
    • Transmission: Sexual contact, parenteral (blood), mother-to-child (vertical transmission)
    • Risk factors: Unprotected sex, injection drug use, healthcare workers, infants born to infected mothers
    • Prevention: Safe sex practices, safe injection practices, screening of blood products, ART for pregnant women
    • Globally distributed, with higher prevalence in certain regions (e.g., sub-Saharan Africa)

Cytomegalovirus (CMV)

  • Serodiagnosis
    • CMV IgM: Indicates acute or recent CMV infection. Can also be present during reactivation
    • CMV IgG: Indicates past CMV infection. Persists for life
    • CMV DNA (PCR): Detects active CMV infection. Used in immunocompromised patients and newborns
  • Clinical Significance
    • CMV infection is often asymptomatic in healthy individuals
    • CMV can cause serious disease in immunocompromised patients (e.g., transplant recipients, HIV-infected individuals) and newborns (congenital CMV infection)
    • Serodiagnosis is used to determine past exposure, diagnose acute infection, and monitor disease in at-risk populations
  • Epidemiology
    • Transmission: Close contact (saliva, urine, breast milk, sexual contact)
    • Most people are infected with CMV during childhood or adulthood
    • Ubiquitous worldwide

Rubella Virus

  • Serodiagnosis
    • Rubella IgM: Indicates acute rubella infection. Can also be present during congenital rubella syndrome
    • Rubella IgG: Indicates immunity to rubella (either from vaccination or past infection). Used to assess immunity in women of childbearing age
  • Clinical Significance
    • Rubella (German measles) is usually a mild illness, but can cause serious birth defects if a pregnant woman is infected (congenital rubella syndrome)
    • Serodiagnosis is used to assess immunity in women of childbearing age and diagnose acute infection
  • Epidemiology
    • Transmission: Respiratory droplets
    • Prevention: Vaccination (MMR vaccine - measles, mumps, rubella)
    • Rubella has been largely eliminated in many countries due to vaccination efforts

Measles Virus

  • Serodiagnosis
    • Measles IgM: Indicates acute measles infection
    • Measles IgG: Indicates immunity to measles (either from vaccination or past infection)
  • Clinical Significance
    • Measles is a highly contagious viral illness characterized by fever, rash, cough, and conjunctivitis
    • Complications can include pneumonia, encephalitis, and death
    • Serodiagnosis is used to diagnose acute infection and assess immunity
  • Epidemiology
    • Transmission: Respiratory droplets
    • Prevention: Vaccination (MMR vaccine)
    • Measles is still a significant public health problem in many parts of the world, particularly in areas with low vaccination rates

Summary Table

Virus Serological Markers Clinical Significance Epidemiology
HAV Anti-HAV IgM (acute), Anti-HAV IgG (past/immune) Acute hepatitis, no chronic infection Fecal-oral, poor sanitation, vaccination available
HBV HBsAg (current), Anti-HBs (immune), Anti-HBc IgM (acute), Anti-HBc IgG (past/chronic) Acute/chronic hepatitis, cirrhosis, liver cancer Parenteral, sexual, vertical, vaccination available
HCV Anti-HCV (exposure), HCV RNA (active infection) Chronic hepatitis, cirrhosis, liver cancer Parenteral, no vaccine
EBV VCA IgM (acute), VCA IgG (past), EA IgG (acute/reactivation), EBNA IgG (past) Infectious mononucleosis, associated with certain cancers Saliva, ubiquitous
HIV HIV Antibody/Antigen Combination Assay, HIV Antibody Differentiation Assay, HIV RNA AIDS, opportunistic infections Sexual, parenteral, vertical, ART available
CMV CMV IgM (acute/recent), CMV IgG (past), CMV DNA (active infection) Asymptomatic in healthy individuals, serious disease in immunocompromised/newborns Close contact, ubiquitous
Rubella Rubella IgM (acute), Rubella IgG (immune) Mild illness, congenital rubella syndrome Respiratory droplets, vaccination available, largely eliminated in many countries
Measles Measles IgM (acute), Measles IgG (immune) Fever, rash, cough, conjunctivitis, complications (pneumonia, encephalitis) Respiratory droplets, vaccination available, still a significant public health problem in some areas

Key Terms

  • Serodiagnosis: The diagnosis of disease based on serological tests, which detect antibodies or antigens in serum
  • Epidemiology: The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems
  • Hepatitis: Inflammation of the liver, often caused by viral infection
  • Cirrhosis: Scarring of the liver, often caused by chronic hepatitis
  • Hepatocellular Carcinoma: A type of liver cancer
  • Infectious Mononucleosis: An illness caused by the Epstein-Barr virus, characterized by fatigue, fever, sore throat, and lymphadenopathy
  • AIDS (Acquired Immunodeficiency Syndrome): A condition caused by the human immunodeficiency virus (HIV), characterized by a weakened immune system and opportunistic infections
  • Congenital Infection: An infection that is passed from a mother to her baby during pregnancy, labor, or delivery
  • Vaccination: The process of administering a vaccine to stimulate the immune system and provide protection against a specific disease
  • MMR Vaccine: A vaccine that protects against measles, mumps, and rubella