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Viral Disease Examples: Common Cold, Influenza, and HIV/AIDS (MBY161 Theme 12)

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Viral Disease Examples

Learning Outcomes

This theme covers selected examples of viral diseases relevant to microbiology, focusing on the common cold, influenza, and HIV/AIDS. Students should be able to discuss the etiology, pathogenesis, epidemiology, and treatment/prevention strategies for these diseases.

The Common Cold

Etiology and Clinical Features

  • Over 200 different viruses can cause the common cold, including rhinoviruses, coronaviruses, and others.

  • Symptoms: Sneezing, nasal secretion, congestion. May lead to laryngitis and otitis media.

  • Fever is typically absent.

  • Antibiotics are ineffective because the disease is viral, not bacterial.

  • Symptomatic relief is provided by cough suppressants and antihistamines.

  • Vaccine development is not feasible due to the large number of causative viral agents and their antigenic diversity.

Example: Rhinovirus is the most common cause, responsible for about 50% of cases.

Influenza (Flu)

Virology and Structure

  • Influenza virus (Influenzavirus) is an enveloped virus with eight RNA segments.

  • Key surface proteins: Hemagglutinin (HA) spikes (attachment) and Neuraminidase (NA) spikes (release).

  • Outer lipid bilayer and capsid layer protect the viral genome.

Clinical Features

  • Symptoms: Chills, fever, headache, muscle aches. No intestinal symptoms.

Strains and Epidemiology

  • Avian, swine, and mammalian strains exist.

  • Swine act as "mixing vessels" for new strains due to their susceptibility to both avian and human influenza viruses.

Antigenic Variation

  • Antigenic drift: Minor changes due to mutations in HA and NA genes, leading to seasonal epidemics.

  • Antigenic shift: Major changes due to genetic reassortment, often resulting in pandemics.

Prevention and Treatment

  • Mortality rate: ~1%, highest in the very young and very old.

  • Multivalent vaccines are formulated annually to target the most prevalent strains.

  • Diagnosis is difficult based on symptoms alone.

  • Antiviral drugs: Zanamivir (Relenza) and oseltamivir (Tamiflu) inhibit neuraminidase, preventing viral release.

Table: Human Influenza Viruses (Main Types)

Type

Antigenic Subtype

Host

Disease Severity

A

H1N1, H3N2, etc.

Humans, swine, birds

Severe

B

None

Humans

Moderate

C

None

Humans, swine

Mild

Additional info: Table inferred from slide and standard influenza classification.

HIV/AIDS (Human Immunodeficiency Virus / Acquired Immunodeficiency Syndrome)

History and Origin

  • 1981: First cluster of AIDS cases in the US (Pneumocystis pneumonia, Kaposi's sarcoma).

  • 1983: Discovery of HIV as the causative agent.

  • Origin: Cross-species transmission from SIV (Simian Immunodeficiency Virus) in chimpanzees to humans in Africa (~1921).

Subtypes of HIV

Subtype

Origin

Prevalence

Pathogenicity

HIV-1

Chimpanzees, gorillas

99% of cases; Group M = 90%

High

HIV-2

Monkeys, sooty mangabeys

Mostly Africa

Lower than HIV-1

Structure of HIV

  • Genus: Lentivirus (a retrovirus)

  • Genome: Two identical RNA strands

  • Enzymes: Reverse transcriptase

  • Envelope: Phospholipid with gp120 glycoprotein spikes

Pathogenicity and Infectiveness

  • Spread by dendritic cells to lymphoid organs, contacts activated T cells.

  • gp120 binds to CD4+ receptor and CCR5/CXCR4 coreceptors on T helper cells, macrophages, and dendritic cells.

  • Virus fuses and enters the cell.

  • Inside the cell, viral RNA is reverse transcribed into DNA, which integrates into host chromosomal DNA.

  • Active infection: New viruses produced.

  • Latent infection: DNA remains hidden as a provirus; some cells become memory T cells, serving as reservoirs.

  • Rapid antigenic changes and high mutation rate complicate immune response and vaccine development.

Stages of HIV Infection

  • Phase 1: Asymptomatic or lymphadenopathy.

  • Phase 2: Steady decline in CD4+ T cells; few symptoms (persistent infections, fever, oral leukoplakia).

  • Phase 3: AIDS develops; CD4+ count below 200 cells/μl; indicator conditions appear.

Resistance to HIV Infection

  • Initial strong immune response; cytotoxic T lymphocytes (CTLs) suppress viral numbers.

  • Latently infected CD4+ T cells make it impossible to clear infection.

  • Vaccine development is challenging.

Impact of Age and Genetics

  • Older adults and young children are more susceptible.

  • Some exposed individuals remain uninfected due to CCR5 mutation.

  • Long-term survivors have low viral loads and effective CTLs.

Diagnostic Methods

  • Blood is the most common sample; urine and saliva can also be used.

  • Rapid test kits detect antibodies or antigens.

  • Seroconversion (development of detectable antibodies) can take up to 3 months (window period).

  • Virus RNA detected by nucleic acid tests (NATs).

  • Plasma viral load (PVL) measured by PCR to detect viral RNA.

Transmission of HIV

  • Transmitted via: non-sterile syringes/needles, pregnancy/breastfeeding, blood transfusion, organ transplant, unprotected sex.

  • Not transmitted via: food/drink, insect bites, casual touch, clothes/towels, toilet/shower.

Global Epidemiology

  • 39 million infected worldwide (2022).

  • 70% in sub-Saharan Africa; 8.45 million in South Africa.

  • 630,000 deaths worldwide in 2022.

  • Incidence could exceed 1 million cases/year.

  • Key populations: men who have sex with men, people who inject drugs, people in prisons, sex workers, transgender people.

  • More women/girls infected in sub-Saharan Africa.

Preventing and Treating AIDS

Prevention Strategies

  • Use of condoms and discouraging sexual promiscuity.

  • Use of sterile needles.

  • Availability of medications; AIDS is now considered a treatable chronic disease in developed countries.

  • Pre-exposure prophylaxis (PrEP) reduces transmission risk.

Antiretroviral Therapy (ART)

  • Fusion/cell entry inhibitors: Target gp41 region.

  • Reverse transcriptase inhibitors: Nucleoside (NRTIs) and non-nucleoside (NNRTIs).

  • Highly active antiretroviral therapy (HAART): Uses combinations of drugs.

  • Integrase inhibitors: Inhibit HIV integrase.

  • Protease inhibitors: Inhibit viral proteases.

  • Maturation inhibitors: Prevent virus assembly.

  • Tetherins: Tether viruses to cells, preventing release and spread.

Mechanisms of Action: NNRTI vs NRTI

  • NRTIs: Mimic natural nucleosides, are incorporated into viral DNA by reverse transcriptase, causing chain termination.

  • NNRTIs: Bind directly to reverse transcriptase, inhibiting its activity without being incorporated into DNA.

Challenges in HIV Vaccine Development

  • No model of natural immunity to mimic.

  • Lack of suitable research animals.

  • Poor understanding of retrovirus mechanisms.

  • High mutation rate leads to resistant strains.

  • An ideal vaccine would induce immunity before latent reservoirs are established, stimulate CTLs, and be affordable.

Key Equations and Concepts

  • CD4+ T cell count threshold for AIDS diagnosis:

  • Viral load quantification by PCR:

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