CHAPTER 66

• Antiviral Resistance


  When a virus becomes less susceptible to the effects of a drug, leading to reduced effectiveness in preventing illness
• Drug Resistance


  Small changes or mutations in a virus can lead to drug resistance
• Clinical Resistance


  When an infection fails to respond to antiviral therapy despite proper treatment, indicating the virus has developed resistance to the drug being used
• Immunologic Status


  The immune system plays a crucial role in fighting viral infections, and the effectiveness of antiviral therapy can be influenced by the patient's immunologic status
• Pharmacokinetics


  How a drug is absorbed, distributed, metabolized, and excreted in the body, which can affect treatment outcomes
• Potential Antagonism


  When drugs interact in a way that reduces their effectiveness when used together
• Standardization in antiviral susceptibility testing


  • Ensures consistency and reliability
  • Lack of standardized protocols can lead to variability in test results and interpretations
  • The Clinical and Laboratory Standards Institute (CLSI) plays a significant role in developing standards
• Determinants of Antiviral Susceptibility Testing


  • Cell line
  • Viral inoculum titer
  • Incubation time
  • Concentration range of antiviral agent
  • Reference strains
  • Assay method
  • Endpoint criteria
  • Calculation and interpretation

  
• Phenotypic Testing


  Measures viral replication or inhibitory effect in the presence of antiviral agents
• Phenotypic Testing Methods


  • Inhibition Assays
  • Recombinant Virus Assays
• Genotypic Testing


  Detects genetic mutations associated with drug resistance
• Genotypic Testing Methods


  • PCR-Based Assays
  • Sequencing
• Phenotypic Susceptibility Testing Methods


  • Plaque Reduction Assay
  • Dye-Uptake (DU) Assay
  • DNA Hybridization
  • Enzyme Immunoassay (EIA)
  • Flow Cytometry
  • Neuraminidase (NA) Inhibition Assay
• Genotypic Susceptibility Testing Methods


  • PCR-Based Detection of Resistance Genes
  • Quantitative Monitoring of Viral Loads (Quantitative PCR)
  • Pyrosequencing
  • Solid Phase Pyrosequencing
  • Liquid Phase Pyrosequencing
• When to Conduct Susceptibility Testing for Patients with HIV


  • Before Initiation of Therapy
  • When Changing Antiretroviral Regimens
  • In Cases of Suboptimal Viral Load Reduction
• Susceptibility Testing Methods for Patients with HIV


  • Phenotypic Susceptibility Testing
  • Recombinant Virus Assays (RVAs)
  • Genotypic Susceptibility Testing
  • Sequencing
  • Selective PCR and Hybridization Tests
• Antigenic Drift


  The gradual accumulation of mutations in the genes that code for the surface proteins of the influenza virus, particularly hemagglutinin (HA) and neuraminidase (NA), which allows the virus to evade immunity from previous infections or vaccinations
• To combat antigenic drift and ensure maximum efficacy, influenza vaccines need to be reformulated annually based on global surveillance data

  
• Types of Influenza Vaccines


  • Inactivated Influenza Vaccine (IIV)
  • Live Attenuated Influenza Virus Vaccine (LAIV)
  • Recombinant Hemagglutinin Vaccine (RIV)
• Adamantane Inhibitors


  Antiviral drugs that work by blocking the ion channel formed by the M2 protein of the influenza virus, preventing viral uncoating and replication
• Adamantane Inhibitors


  • Primarily effective against influenza A viruses
  • Their effectiveness has decreased over time due to the emergence of drug-resistant strains
• Neuraminidase Inhibitors


  Antiviral drugs that work by inhibiting the activity of the neuraminidase enzyme on the surface of the influenza virus, preventing the release of newly formed virus particles
• Neuraminidase Inhibitors


  • Effective against both influenza A and influenza B viruses
  • Considered the mainstay of treatment for influenza, reducing the duration and severity of symptoms when initiated within 48 hours of symptom onset
• Oseltamivir phosphate (Tamiflu) is one of the most commonly used neuraminidase inhibitors for the treatment of influenza
• Antiviral resistance


  Occurs when a drug is less effective in preventing illness
• Drug resistance


  • Small changes or mutations in a virus can reduce the effectiveness of a drug
  • Clinical resistance occurs when the infection fails to respond to therapy due to immunologic status, pharmacokinetics of the drug in an individual patient, or potential antagonism with one or more drugs
• Standardization


  Very few standards exist, developed by the Clinical and Laboratory Standards Institute (CLSI), determined by cell line used to grow a virus, viral inoculum titer, incubation time in culture, concentration range of the antiviral agent, reference strains, assay method, endpoint criteria, calculation of the endpoint, and interpretation of the endpoint
• Methods of testing


  • Phenotypic (measures viral replication in the presence of antiviral agents, measures the inhibitory effect of an antiviral agent on an entire virus population)
  • Genotypic (uses PCR to detect genes known to cause resistance, uses nucleic acid of the virus to determine whether a virus possesses mutations that can cause drug resistance)
  • Recombinant virus assays (monitors the phenotypic behavior of specific genes in the virus genome in the presence of the antiviral drug)
• Phenotypic susceptibility testing


  • Uses endpoint measurements to determine whether the virus is inhibited by the drug or demonstrates drug resistance, plaque reduction assay is the standard method, other methods include dye-uptake assay, DNA hybridization, enzyme immunoassay, flow cytometry, and neuraminidase inhibition assay
• Genotypic susceptibility testing


  • Uses PCR to detect genes responsible for resistance, quantitative monitoring of viral loads (quantitative PCR) also measures the response to an antiviral agent, pyrosequencing is a rapid and accurate method for quantifying sequence variation
• Susceptibility testing for HIV


  Should occur before initiation of therapy, when changing antiretroviral regimens, and in cases of suboptimal viral load reduction, using phenotypic (recombinant virus assays) and genotypic (sequencing, selective PCR, hybridization tests) methods
• Influenza


  • Vaccines must be reformulated each year due to antigenic drifts, global surveillance provides data to determine and predict circulating strains, types of vaccines include inactivated, live attenuated, and recombinant hemagglutinin
• Influenza treatment


  Two classes of antiviral drugs: adamantane inhibitors (prevent viral uncoating, effective for influenza A) and neuraminidase inhibitors (inhibit NA, preventing viral release, effective for influenza A and B)
• Diseases and vaccine types

  • Yellow fever (attenuated-live)
  • Poliomyelitis (attenuated-live and inactivated)
  • Measles (attenuated-live)
  • Mumps (attenuated-live)
  • Rubella (attenuated-live)
  • Hepatitis B (inactivated)
  • Influenza (inactivated and recombinant)
  • Smallpox (attenuated-live)
  • Chickenpox (attenuated-live)
  • Hepatitis A (inactivated)
  • Rabies (inactivated)
  • Rotavirus (attenuated-live)
• Circumstances of use for vaccines

  • Hepatitis A (traveler to developing country)
  • Hepatitis B (newborns of infected mothers or unimmunized laboratory worker following needlestick)
  • Rabies (after bite from potentially rabid animal)
  • Measles (unimmunized close contact with infected individual)
  • Varicella (newborns of infected mothers at time of delivery)
  • Respiratory syncytial virus (infants younger than 2 years of age with underlying lung disease)
  • Lassa fever (during disease to reduce severity)