"Where do pox viruses replicate?
Pox viruses replicate in epithelial-cell cytoplasm, unlike most other dsDNA viruses, which require the host cell's proteins in the nucleus to carry out transcription.
Pox viruses are large enough to carry their own replication proteins into a host cell & thus don't need to hijack the host cell's nucleus.
What is the site of infection of rotaviruses?
Replicates in enterocytes at villi tips in small-intestine lining, especially of piglets
What is the site of infection of influenza viruses, such as Equine Influenza Virus (family Orthomyxoviridae)?
Ciliated epithelial cells in nasal lining, bronchi, bronchioles
What are the site of infection by members of Parvoviridae, such as FPV?
Haematopoetic & Lymphoreticular Systems
Name some viruses that target animals' skin epithelial cells.
Parapoxvirus: Orf (sheep, goats, humans)
Leporipoxvirus: Myxomatosis (rabbits)
What is the target site of the rabies virus (family Rhabdoviridae)?
Central Nervous System:
Name two types of virus that replicate in neurons.
Rabies & Herpes.
Rabies is shed in saliva & herpes is shed from skin
What is the target site of infection for Equine Herpesvirus (EHV-1)?
Infection & destruction of endothelial cells & endothelial-cell layer
What is the target site of infection for Canine Distemper Virus (CDV)?
Multiple tissues (systemic):
Infects lymphocytes then all tissues
Entry via URT via droplets & aerosols ➔ spread to lymphoid organs via blood ➔ spread to all epithelia (respiratory tract, gut, bladder, skin) & CNS
Structure: Non-enveloped ∴ stable in environment
Genome: segmented, dsRNA genome
Target tissue: GIT, enterocytes, villi tips in small-intestine lining; not systemic
Transmission: Ingestion (orofaecal) & Oropharynx (tonsils)
Describe Equine Influenza Virus's
Structure: Enveloped ∴ not stable in environment
Genome: Segmented ssRNA genome
Target tissue: Ciliated epithelial cells of respiratory tract ie., nasal lining, bronchi, bronchioles
Transmission: Direct contact & aerosol via URT
Describe Feline Panleukopenia Virus
Structure: Small, non-enveloped, v stable in environment
Target tissue: Haematopoetic & Lymphoreticular system (macrophages & lymphocytes); lymphoid tissue esp tonsils; rapidly dividing cells in very young animals, so intestinal crypt cells of puppies
Transmission: Mutual grooming, aerosol
Pathogenesis: Virus ingested or inhaled via oropharynx ➔ arrive at tonsils ➔ replicate in lymphoid tissue/tonsils ➔ viraemia & spread to other sites ➔ destruction of lymphoid cells leads to immune suppression, leaving animal vulnerable to secondary bacterial infection ➔ virus hides from immune system in carrier
Structure: Very large (200-400 nm) ∴ encode all enzymes needed for replication themselves; generally enveloped
Target tissue: Skin epithelial cells
Transmission: Entry via skin wounds, insect bites or injections; also via blood in circulation
Pathogenesis: Direct intro of virus thru epithelium via cuts, abrasions, needles, insect bites OR via blood in circulation, escaping blood vessel, possible invasion of neighbouring dermal cells ➔ formation of macule (flat red) ➔ papule (raised, red), more inflammation, vasodilation, ↑ permeability ➔ intraepidermal vesicle (small blister), virus invades epithelium ➔ ULCER: epithelium ruptures, virus discharges & infects other areas
Describe the Rhabdovirus's:
Structure: Enveloped bullet-shaped virions
Genome: Negatove-sense ssRNA
Target tissue: Neurons, CNS
Transmission: Bite wound (shed in saliva)
Pathogenesis: Virus intro via bite wound ➔ travels to salivary glands via neurons ➔ spread to CNS via neurons ➔ lytic infection of neurons leading to necrosis ➔ invasion of inflammatory cells around blood vessels ➔ can be furious form or dumb/paralytic form, which can result in paralysis or spasms of pharyngeal muscles, inhibiting swallowing and leading to ↑ salivation/foaming ➔ virus shed in saliva
Describe Equine herpesvirus (EHV-1)'s:
Structure: Large, enveloped, icosahedral capsid
Target tissue: Vascular endothelial cells
Transmission: Aerosol, contact
Pathogenesis: Enters via respiratory tract ➔ respiratory disease ➔ infects lymphocytes & establishes viraemia in blood
➔ abortion: vasculitis in placental blood vessels ➔ thrombi ➔ haemmorhages
➔ neurological disease: blood vessels of spine & brain infected ➔ ataxia & paralysis ➔ virus becomes latent in lymphocytes & CNS
Describe Canine Distemper Virus (CDV)'s:
Structure: Enveloped so not stable in environment
Target tissue: First lymphocytes then multiple tissues; systemic
Transmission: Aerosols or droplets
Pathogenesis: Inhaled via URT ➔ spread to lymphoid organs via blood ➔ spread to all epithelia (respiratory tract, gut, bladder, skin) & CNS
Describe clinical signs of rotavirus & how it disease is prevented.
- Group A cause diarrhoea (milk scours) in calves, piglets, foals & young birds
- 1-4-day incubation period
- shortens villi (not crypt cells)
- shed in high numbers in faeces; max shedding on days 3-4 post-infection
- survives in faeces for several months & in water so contamination can build up a lot in unhygienic environment
- no systemic spread
- prevented by antibodies (esp IgA) in colostrum & vaccination
Describe clinical signs of Equine Influenza Virus.
Dry cough, nasal discharge, pyrexia, depression
Describe the clinical signs of Feline panleukopenia virus (FPV) aka Feline Parvovirus & how the disease can be prevented.
- After viraemia, virus spreads to all lymphoid organs: thymus, bone marrow, lymph nodes, spleen, Peyer’s patches
- In cats, it destroys all WBCs regardless of type, ∴ panleukopenia ➔ immune suppression
V&D (bloody), dehydration ➔ death possible
- In dogs, targets rapidly dividing intestinal crypt cells, causing diarrhoea
- Can be prevented by vaccine
Describe clinical signs of Poxviruses & how they can be treated or prevented.
- Formation of macule (flat red) ➔ papule (raised, red) ➔ intraepidermal vesicle (small blister), virus invades epithelium ➔ ULCER: epithelium ruptures, virus discharges & infects other areas
- Uncomplicated lesions resolve in 3-4 weeks
What are the clinical signs of rabies and how can it be prevented?
- Can be furious form or dumb/paralytic form, which can result in paralysis or spasms of pharyngeal muscles, inhibiting swallowing and leading to ↑ salivation/foaming
- Prevented by vaccine
What are the clinical signs of Equine Herpesvirus (EHV-1) and how can infection be prevented?
Abortion: vasculitis in placental blood vessels ➔ thrombi ➔ haemmorhages)
Neurological disease: blood vessels of spine & brain infected ➔ ataxia & paralysis ➔ virus becomes latent in lymphocytes & CNS
- Prevented by isolation of mares in late-stage pregnancy
- Vaccination only reduces respiratory disease but incomplete for abortion or CNS disease
What are the clinical signs of Canine Distemper Virus in dogs and how is infection prevented?
- Cough, nasal discharge, conjunctivitis
- Convulsions, seizures due to CNS infection
- Hard-pad disease in dogs
- Up to 50% of cases are subclinical or mild
- Prevented by vaccine
Give an example of a viral diagnostic techniques based on identifying a virus's morphology, and describe its advantages & disadvantages.
Electron microscope: direct visualisation of virus, applying negative staining with heavy metals, ie., stain appears dark and the virus appears light
Advantages: can be used for viruses that can’t be cultured; allows identification of new agents
Disadvantages: requires specialised equipment & experienced personnel; low-sensitivity may require concentration of sample & often purification of sample
Give examples of viral diagnostic techniques based on identifying a virus's antigenicity, and describe their advantages & disadvantages.
Detecting virus using antibodies in immunoassays; can use monoclonal antibody specific for one antigenic epitope or polyclonal antibodies, ie., a mix of antibodies to detect several epitopes
Advantages: fast, don't require live or infectious virus, specific
Disadvantages: no amplification, relatively low sensitivity
Antigen ELISA - plate covered with antibody in sandwich in capture assay to detect antigen
Immunohisto/cytochemistry (tissue or cells) - viral antigen in cells is detected by specific antibody, which carries fluorescent label (immunofluorescence assay) or enzyme label that reacts with colour substrate (immunoperoxidase assay)
Immunochromatography (fluids) - eg. Rapid ImmunoMigration Witness FeLV test for finding p27 antigen in blood sample, uses antigen-specific antibody labeled with colloidal gold
Give examples of viral diagnostic techniques based on detection of antibodies.
Neutralisation assay: serum sample mixed with virus ➔ antibodies in sample bind to virus ➔ mixture added to cells ➔ if antibodies present, virus won’t infect cells ➔ if no antibodies, viruses affect cells, which show cytopathic effects such as Negri inclusion bodies
Haemagglutination inhibition assay: based on some viruses' ability to agglutinate RBCs, esp influenza virus. Either a big blob of blood in the well or a “button” or “dot” of blood in the case of no reaction ie., presence of antibody (positive result for antibody response to virus)
Antibody ELISA - opposite of antigen ELISA; plate is coated with antigen ie. whole virus or purified virus protein ➔ antibody in serum sample binds to antigen ➔ second antibody that recognises IgG is added ➔ second antibody carries enzyme label that changes colour substrate when bound
Give examples of viral diagnostic techniques based on genome composition, and describe advantages & disadvantages.
This involves identifying a virus based on detection of viral nucleic acid. There are two techniques:
1. PCR - polymerase chain reaction:
- amplification of viral sample
- if not a DNA virus but RNA, then RNA needs to be converted to cDNA for PCR analysis
- PCR is based on ability of Taq heat-stable polymerase to amplify DNA, by replicating from short primers specific for virus sequence; Taq can survive repeated cycles of heating to 94 C
- generation of amplicon (viral sequence) of predicted size indicates presence of virus nucleic acid in sample; ie., produces large number of copies that are easy to detect
- separated on agarose gel using electrophoresis
- with electrophoresis must have right positive control
2. Real-time quantitative PCR - fluorescence-tagged:
- labelled DNA provides quantitative measure of amt of viral DNA in sample, eg measure of viral load; no need for electrophoresis
Advantages of both: very sensitive, can use v small amt of starting material; doesn’t require live virus; specific as primers designed for target DNA
Disadvantages: very sensitive, danger of contamination & false positives
What are the 10 mechanisms viruses use to avoid detection and/or destruction by the host immune system?
1. Interference with Interferon (IFN) of the innate immune system
2. Hiding out in immune-privileged sites such as CNS, eye & gonads
3. Syncytium formation
5. Viral production of antigen decoys
6. Antigenic DRIFT
7. Antigenic SHIFT
8. Interfering with MHC Class I Pathway of antigen presentation
9. Avoiding detection by NK cells
How do viruses interfere with Interferon (IFN) to avoid detection by the innate immune system?
(Type 1 IFNs are produced by virus-infected cells to make neighbouring cells more resistant to infection by:
↑ degradation of virus mRNA to block viral transcription
↓ synthesis of virus protein by blocking ribosomes
↑ MHC Class I presentation to cytotoxic CD8+ Killer T-cells, degranulation/caspase cascade/Fas/TNF-alph & apoptosis)
Some viruses have evolved to circumvent Type I IFNs (α, β, Ω):
- virus produces early proteins that interfere with the signalling cascade produced by the infected cell’s toll-like receptors. This viral interference means the cell can’t produce IFN & its viral-resistance genes are NOT switched on
- virus’s early proteins could BLOCK IFN-receptors, so the cell can’t respond to IFN & its viral-resistance genes are NOT switched on
Give an example of a virus that interferes with Interferon (IFN) of the innate immune system as an immune-evasion mechanism.