Basic Virology
Chapter 2
Virus Replication
Viruses must replicate in living cells
Viruses will uses the cell to replicate its viral nucleic acids, expression of viral genes, create proteins, and the cells energy source
Productive Infection: Any infection that results in the production of more infectious virus at the end than at the start
Burst Size: The actual number of infectious viruses produced in an infected cell - this number can range from less than 10 to over 10,000
Infection can sometimes lead to a state of coexistence btw the cell and virus which can persist for however long the host lives.
Lysogeny: Only in bacterial cells - the accompanying changes to the protein composition of the cell’s surface (the immune “signature” of the cell) and functional changes.
Transformation: Only in animal cells - changes to the protein composition of the cell’s surface & functional changes
results in altered growth properties and can cause cells to have some or many properties of cancer cells
Stages of Virus Replication in the Cell
With simple viruses it is the productive infection cycle
The virus specifically interacts with the cell surface receptors (animals & bacterial) and the viral genome is introduced into the cell.
Viral genes are expressed through synthesis of few or many viral proteins involved in the replication processes.
Viral proteins modify the host cell and allow the viral genome to replicate using host & viral enzymes. This is usually where the cell is irreversibly modified and eventually killed.
New viral coat proteins assemble into capsids and viral genomes are included.
Virus is released where it can infect new cells and repeat the process.
Pathogenesis of Viral Infection
Bacterial Restriction: limitation of viral replication
Inapparent or Asymptomatic infections: Infections with no measurable disease symptoms. Seen in the interaction btw normally virulent viruses and a susceptible host.
Stages of Virus-Induced Pathology
Initial Infection leads to virus replication at the site of entry and multiplication and spread into favored tissues.
The time btw the initial infection and the observation of clinical symptoms of disease is the incubation period
The host will respond to the antigen by adaptive immunity. This will cause a decline in virus replication.
Initial Stages of Infection - entry of the virus into the host
Reservoir: Source of the virus
The entry of the virus will generally follow a specific pattern leading it to a specific site or region of the body
Vector: The means of infection btw individuals
Many viruses must continually replicate to maintain themselves. Therefore, for many viruses they will replicate in the vector and reservoir.
Viruses enter cells via the cooperative interaction btw the host cell and the virus - requires a hydrated cell surface.
Incubation Period and Spread of Virus Through the Host
It takes time for the virus to replicate and lead to symptoms of disease because:
Only a limited amount of virus can be introduced
Cell based innate immune response occur immediately upon infection
The early stage or incubation period can last from a few days to many year depending on the specific virus
Many types of viruses must move or be moved through the host to establish infection at a preferred site. This site is referred to as the target tissue or target organ.
Several modes through which a virus spreads through the host
Viruses will move through the circulatory system, this is viremia. Spread through the bloodstream either passively as a free virus or adsorbed to the surface of cells tat they do not infect, like RBCs.
Direct entry of the virus into the lymphatic circulatory system also can lead to viremia. These viruses that replicate in the gut enter the lymphatic system via Peyer’s patches. These provide a route directly to lymphocytes w/o passage through the bloodstream.
Some viruses will spread through the nervous system. Some neurotropic viruses can spread to the PNS directly to the CNS.
Multiplication of virus to high lvls - occurrence of disease symptoms
Viral replication at specific target tissue often defines symptoms of the disease.
The nature of the target and the host response are of primary importance in establishing symptoms.
One or another viral protein can recognize specific encoded proteins, thus, have a major role in specifying the virus’ tissue tropism.
One major factor in viral tropism is the distribution and occurrence of specific viral receptors on cells in the target tissue.
The Later Stages of Infection - The Immune Response
Infections with virus do not necessarily lead to any or all symptoms of a disease. The severity of symptoms is a function of the virus genotype, the amount of virus delivered to the host, & the host’s general immune response.
Immune response involves B & T cells
Effector T cells: kill cells expressing foreign antigens on their surfaces
B Cells secrete antibodies
Inflammation, macrophage destruction of infected cells, and increases in body temp
The Later Stages of Infection - Virus Spread to the Next Individual
Virus entry is the converse of virus entry at the start
May play a role in how the virus is spread
Bodily fluids, touch, airborne, etc.
The Later Stages of Infection - Fate of the Host
The host either recovers or dies
some resolve the virus with clearance, some result in lfelong latent infection
viruses may result in the appearance of a disease or syndrome