Characteristics and Structure of Viruses

General Properties of Viruses

Viruses are obligate intracellular parasites that require a host cell for replication. They are too small to be seen with a light microscope and must be visualized using electron microscopy. Viral classification is based on the type of nucleic acid (DNA or RNA), and their host range is determined by specific interactions between viral surface proteins and host cell receptors.

• Core of nucleic acid: Either DNA or RNA, but never both.

• Capsid: Protein coat made of subunits called capsomeres.

• Envelope: Some viruses possess a lipid envelope derived from the host cell membrane; others are naked.

• Spikes: Glycoproteins on the envelope that facilitate attachment to host cells.

Host range is determined by the compatibility between viral surface proteins and host cell receptors. The envelope is acquired from the host cell membrane during viral budding. Outside a cell, a virus is called a virion.

Viral Replication Cycles

Bacteriophage Replication

Bacteriophages are viruses that infect bacteria. Their replication involves several steps:

• Attachment (Adsorption): Phage binds to specific receptors on the bacterial surface.

• Entry (Penetration): Injection of viral nucleic acid into the host cell.

• Synthesis (Replication): Host machinery synthesizes viral components.

• Assembly & Maturation: New virions are assembled.

• Release: Host cell lyses, releasing new phages.

Lysogenic conversion occurs when a temperate phage integrates its DNA into the host genome, potentially altering host phenotype.

Animal Virus Replication

Animal viruses follow similar steps but with differences in entry and release mechanisms:

• Attachment: Viral proteins bind to host cell receptors (e.g., HIV gp120 binds CD4).

• Entry: By direct penetration, endocytosis, or membrane fusion.

• Uncoating: Removal of capsid to release viral genome.

• Synthesis: DNA viruses replicate in the nucleus; RNA viruses in the cytoplasm. Retroviruses use reverse transcriptase.

• Maturation: Assembly of viral components.

• Release: By budding (enveloped viruses) or cell lysis (naked viruses).

Persistent infections involve continuous release of virus without cell death. Latency refers to dormant viral genomes within host cells.

Viruses and Cancer (Oncoviruses)

Oncogenic Viruses

Certain viruses can induce cancer by disrupting normal cell cycle regulation. Examples include:

• Human papillomavirus (HPV): Cervical and other cancers.

• Hepatitis B and C viruses (HBV, HCV): Hepatocellular carcinoma.

• Epstein-Barr Virus (EBV): Burkitt lymphoma, nasopharyngeal carcinoma.

HPV E6 and E7 proteins inactivate tumor suppressors such as p53, leading to uncontrolled cell division.

Diagnostic Virology

Laboratory Techniques

• Tissue or Cell Culture: Growth of viruses in cell monolayers; observation for cytopathic effect (CPE).

• Chicken Eggs: Used for vaccine production (e.g., influenza).

• ELISA: Detects viral antigens in patient samples.

• PCR/DNA Probes: Amplifies and detects viral nucleic acids.

• Serological Testing: Detects patient antibodies against viruses.

Antigen: Substance that induces an immune response. Antibody: Protein produced by B cells that binds specific antigens.

Prions

Characteristics of Prions

Prions are infectious proteins that lack nucleic acids. They cause disease by inducing abnormal folding of normal cellular prion proteins (PrP), especially in neural tissue.

• Human prion diseases: Variant Creutzfeldt-Jakob disease (vCJD).

• Animal prion diseases: Bovine Spongiform Encephalopathy (BSE), Chronic Wasting Disease (CWD).

Pathogenic DNA Viruses

Poxviridae Family

Smallpox was eradicated in 1980 due to effective vaccination and lack of animal reservoirs. Remaining stocks are held in secure labs.

Herpesviridae Family

Includes several important human pathogens:

• HSV-1 (HHV-1): Oral herpes; remains dormant in trigeminal ganglia.

• HSV-2 (HHV-2): Genital herpes; remains dormant in sacral ganglia.

• Varicella Zoster Virus (HHV-3): Chickenpox and shingles; latency in dorsal root ganglia.

• Epstein-Barr Virus (HHV-4): Infectious mononucleosis; associated with certain cancers.

• Cytomegalovirus (HHV-5): Asymptomatic in healthy individuals; severe in immunocompromised and congenital infections.

Human Papillomavirus (HPV)

HPV causes warts and is linked to cervical and other cancers. Diagnosis is by Pap test and HPV DNA testing. Prevention is via vaccination (Gardasil-9).

Hepatitis B Virus (HBV)

HBV is transmitted via blood and body fluids, infects liver cells, and is linked to liver cancer. Prevention is by vaccination.

Pathogenic RNA Viruses

Picornaviridae Family

• Rhinovirus: Common cold.

• Enteroviruses: Polio, Coxsackie viruses, Echovirus.

• Hepatitis A Virus (HAV): Fecal-oral transmission; vaccine-preventable.

Arboviruses

• West Nile Virus: Mosquito-borne; birds are natural hosts.

• Dengue Fever: Mosquito-borne; causes severe flu-like symptoms.

• Zika Virus: Mosquito-borne; associated with birth defects.

Coronaviruses

Coronaviruses are enveloped, positive-sense ssRNA viruses. Notable diseases include SARS, MERS, and COVID-19. The spike (S) protein binds to ACE2 receptors for cell entry.

COVID-19 symptoms range from mild respiratory illness to severe systemic disease. Diagnosis is by PCR or antigen testing.

Retroviridae Family (HIV)

HIV is a retrovirus with two copies of ssRNA, reverse transcriptase, and an envelope. It targets CD4+ T cells and leads to AIDS if untreated. Antiretroviral therapy (ART) combines drugs from multiple classes to suppress viral replication.

Parasitic Protozoa, Helminths, and Arthropod Vectors

Protozoan Life Stages

• Cyst: Dormant, infective stage; survives in the environment.

• Trophozoite: Active, feeding, and reproductive stage in the host.

Transmission and Life Cycles

• Giardia intestinalis: Transmitted via contaminated water/food; causes giardiasis (diarrhea, cramps). Diagnosis by cysts/trophs in stool or ELISA.

• Leishmania: Transmitted by sand flies; causes cutaneous, mucocutaneous, or visceral leishmaniasis.

• Plasmodium (Malaria): Transmitted by Anopheles mosquitoes; infects red blood cells. Diagnosis by blood smear.

• Toxoplasma gondii: Transmitted via undercooked meat or cat feces; can cause congenital infections.

• Cryptosporidium parvum: Causes diarrhea; transmitted via contaminated water. Diagnosis by oocysts in stool.

Helminths

• Ascaris lumbricoides: Largest nematode infecting humans; transmitted via ingestion of eggs. Diagnosis by eggs or adult worms in stool.

• Enterobius vermicularis (Pinworm): Transmitted via ingestion of eggs; causes perianal itching. Diagnosis by tape test for eggs.

Summary Table: Key Viruses and Their Features