Viral Pathogenesis

Virulence and Pathogenesis

• The ability of a virus to cause disease in a host is referred to as pathogenicity.
• The virus that causes disease is called a pathogen.
• The manner of development of a disease is called pathogenesis.
• The term virulence is used as a quantitative or relative measure of the degree of pathogenicity of the infecting virus.
• Virulence is not an absolute property of a virus, but depends on many variables.

Variables of Virulence

• The three main variables of virulence are the virus, the host, and the environment.
• The "tug of war" for virulence involves the interaction between the virus and the host's immune system.
• Variables pertaining to the host that contribute to virulence include the host's immune response, age, and nutrition.

Viral Structure and Composition

• An enveloped virus has a capsid and a lipid envelope.
• The chemical composition of a virus includes nucleic acid (DNA or RNA) and protein.
• A virus can be visualized using electron microscopy, not compound microscopy.

Diagnosis and Detection

• Methods used during diagnosis and detection of a viral disease include virus isolation, serology, and molecular biology techniques.

Virulence and Viremia

• Virulence is measured by the LD50 (the dose of virus required to kill 50% of infected animals).
• Virus 1 with an LD50 of 120 is less virulent than Virus B with an LD50 of 60.
• Viremia occurs when the virus has replicated within organs and then re-entered circulation.
• The name of the host defense mechanism that results in cell lysis but no spread of virus is apoptosis.

Routes of Entry

• Viruses can breach the skin or enter through mucous membranes, respiratory tract, or GI tract.
• If a virus is incapable of breaching the skin, it can enter through cuts, wounds, or insect vectors.
• The route of entry of viruses into the host's GI tract is through ingestion of contaminated food or water.
• Common sites of pathogenesis and virulence for the Respiratory tract include the nose, throat, and lungs.

Local Spread and Systemic Infection

• A local spread of a virus on epithelial surfaces can cause lesions, inflammation, and cell death.
• To reach the subepithelial layer/underlying tissues, the virus must penetrate the epithelial barrier.
• If the virus is successful in reaching the subepithelial layer/underlying tissues, it has access to blood, lymph, and nerves.
• The presence of virus in the blood is known as viremia.
• The initial entry of the virus into the blood is known as primary viremia.
• Primary viremia can occur through direct entry of the virus into the blood or through replication in infected organs.
• Infection that spreads beyond the primary site of infection is known as systemic infection.
• If a number of organs or tissues are infected, this is describing a disseminated infection.

Viral Spread and Neurotropism

• Viruses can spread via nerves in the host through retrograde axonal transport, anterograde axonal transport, or transsynaptic transmission.
• The fourth way a virus can spread via nerves is through neuronal cell-to-cell transmission.
• Viruses that can enter the CNS after infection of a peripheral site are known as neurotropic viruses.
• Viruses that can infect neural cells are known as neurovirulent viruses.
• Viruses that can cause disease of nervous tissue, manifested by neurological symptoms and often death, are known as neuropathogenic viruses.

Tropism and Outcomes

• Tropism refers to the specific infectivity of a virus for certain cells or tissues.
• Pantropic viruses are capable of infecting a wide range of cells or tissues.
• The 5 general outcomes of viral injury are latency, chronic infection, resolution, transformation, and death.
• The shedding of infectious virions is crucial to the maintenance of infection in populations.
• Acute infection usually involves intensive shedding over a short time period.
• Chronic infection can be shed at lower titers for months to years.

Consequences of Systemic Infections

• One consequence of systemic viral infections is a reddening of the skin, known as erythema.