classifications
What determines the structure of viruses
Types of nucleic acids within the virus - nucleic acid may be circular or linear, each virus will contain only one type of nucleic acid = double or single stranded DNA, double or single stranded RNA
CAPSID - the protective shell of proteins which surrounds the nucleic acid of the virus
Virus envelope - an outer covering surrounding the capsid
What is adsorption in the life cycle of a virus
Requires receptors on eukaryotic cell membrane
Receptors are specifically recognized by viral surface proteins
Spike + receptor interaction determines virus host range and tissue specificity
Blocked this receptor + spike interaction may prevent infection
What is release in the life cycle of a virus
- Virus particles assembled and accumulate in cytoplasm
Host cell biosynthesis stops
host cell dies and is lysed - virion released
What is serology
Looking for antibodies produced against the virus in animals blood stream - immune system has recognized and responded with antibodies to virus, often a 2-3 week gap before antibodies appear post infection or post vaccine
Cant tell the difference between current or past infection - just know previous exposure
Can tell the difference between antibodies produced because of a vaccine or in response to an infection
how are viruses an agent of animal disease
Viruses can cause symptoms of disease in a number of ways
- Virus replication leads to host cell lysis = localized tissue damage, inflammation
- Virus infection alters host cell metabolism - cell weakens and dies
- Viral glycoproteins inserted into host cell membranes = host cell surface altered, triggers host immune response which kills cells
- Production of virokines during virus infection = virus encoded proteins which are toxic to host cell or block normal cell functions
What are some examples of RNA viruses
Rhino virus
Coronavirus
Influenza virus
Bovine respiratory syncytial virus
Hepatitis C virus
West nile virus
Measles
Rabies virus
Ebola Virus
What is penetration in the life cycle of a virus
Attachment triggers process to bring virion into cell
Three mechanisms
1) direct penetration through host membrane
2) fusion of virion envelope with host cell membrane - parts of viral envelope left behind
3) endocytosis - use existing phagocytic mechanism of the cell
What are the possible outcomes of viral infections
Lytic infection - host cell killed as new virions released
Persistent infection - host cell survives, virus shed for long periods of time, infectious virions always patient
Latent infection - host cell survives, viral genome inserts into host cell chromosome and replicates, no or limited expression of viral genes, no infectious virus released, Viral gene expression can be triggered at a future time
Host cell transformation - formation of cancerous host cells by tumour viruses, 15% of human cancers may be caused by viruses
Fluorescent antibody test
Collect patient specimen likely to contain infected cells
add labeled antibodies that recognize viral surface proteins
examine tissue sample microscopically with ultraviolet microscope binding of fluorescent antibody to cell
Rapid test but need lots of infected cells
What is rotavirus
Common cause of viral diarrhea in calves
RNA virus with double layered capsid
Virus ingested by calf - enters GI tract, infects villi cells of small intestine, viral replication leads to cell lysis, loss of absorptive capacity in GI tract results in diarrhea
Clinical signs last as long as viral replication
no antiviral treatments
What are some examples of DNA viruses
Herpes virus - IBR in cattle, cold sore virus
Parvovirus - dogs
Pox viruses - Smallpox
Papilloma viruses - warts, cervical cancer
Circovirus - pigs
What is uncoating in the life cycle of a virus
Exposes viral nucleic acid to host cells biochemical machinery
Can occur during penetration or after penetration
usually due to host hydrolytic enzymes which degrade viral capsid
What is enzootic novine leukosis
Retrovirus
Family oncoretrovirus
Inserts RNA into DNA of host cell
Changes genome of host cell
many cattle infected
mostly transmitted via blood contamination
Sporadic cases of lymphosarcoma occur in cattle
How do you try to grow and identify virus directly from host
Requires use of live tissue culture cells - kidney, spleen, skin cells, enbyonated eggs
Add patient specimen to TC cells and allow virus to infect cells
Look microscopically for cytopathic effect on cells - virus growth in cells causes changes to cell morphology, cell lysis, rounding fusing of cells, inclusion bodies inside cells
Slow process
Some viruses do not grow well in TC
requires high technical expertise
What is bovine coronavirus
RNA virus
Very similar to rotavirus
Cell lysis in villi of small intestine
Diarrhea
Treatment is fluids
More severe than rotavirus
Similar control methods
What is the taxonomy of viruses
Genus - letivirus
Species - human immunodeficiency virus
Approx 61 families and approx 2000 known species
What is Gene expression in the life cycle of a virus
Viral nucleic acid is transcribed, translated, replicated
usually completely dependent on host cell enzymes - some virions carry own DNA or RNA polymerases into cell
Can be very complex, depending on type of nucleic acid in virion
What are the problem, approaches and result of anti viral agents
problem - viral life cycle is closely linked to host cell processes = hard to target one without damaging the other
Approaches - try to target = host functions used by virus but not critical for host cell viability, viral activity or structure essential for completion of its life cycle
Result - a small number of agents are available with moderate effectiveness, no true broad spectrum agents exist, many viruses still have no effective anti-viral agents
What is PCR? how is it used?
method of in vitro DNA replication which can amplify DNA segments by more than a million fold enabling minute numbers of DNA segments of a virus or other micro-organism to be multiplied to quantities that can be detected by standard methods
What is bovine herpes virus 1
Infectious bovine rhinotracheitis virus
Enveloped DNA virus
Replicates in host cell nucleus
Often remains latent in infected host cells
Transmission via aerosols, direct contact, contaminated water troughs
Replicates in cells of mucous membranes of nasal passages, upper trachea, sometimes in conjunctiva of eye
large amounts of virus shed in nasal secretions
What is the life cycle of viruses
Outside of host cell, viruses exists as virions - particles unable to replicate independently
All viruses undergo same basic life cycle
- Adsorption, penetration, uncoating, gene expression, assembly, release
What is assembly in the life cycle of a virus
Building of new intact virus from component parts
Process varies depending on type of virus - some spontaneous self-assembly of capsid proteins, sometimes requires enzymes, may occur in cytoplasm or nucleus
How does the lab diagnose viral infections
problems - cant easily be grown in lab like bacteria = symptoms of some viral disease mimic other microbial diseases, diagnostic methods based on culture and growth are more difficulty
Solutions: serology, identifying virus infected cells in host, try to grow and identify virus directly from host, PCR, newer genomic methods
What are the limitations of PCR
PCR is a very sensitive technique but it is very susceptible to contamination. Great care must be taken to avoid contamination with extraneous DNA that may give rise to false positive results
The technique does not differentiate between live or dead viruses or bacteria it merely detects the DNA
Highly dependent on finding a sequence of DNA that is unique to that microorganism used as a target
What are the clinical signs of IBR
1-4 day incubation period
Nasal discharge, decreased appetite, high temp
muzzle and nasal passages inflamed
Conjuctivitis
Most animals recover in 7-10 days, however deaths occur due to increased susceptibility to pneumonia
Note infected animals that survive have latent infections and can shed virus later
effective vaccinations for control of disease