"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.
Why do some viruses hide out in immune-privileged sites such as CNS, eye & gonads as a way of evading immune detection?
These sensitive areas could be irreparably damaged by lymphocytes’ inflammatory & destructive immune processes; the BBB, for example, protects not only from infection but also from lymphocytes.
Some viruses, such as Rhabdoviruses, deliberately migrate to CNS, gonads & eyes via neurons, for example, to evade immune detection
How do some viruses use syncytium formation as a way of evading immune detection?
Viruses produce fusion protein, which enables cells to form syncytium ie., fusion of cells, which faciliates continguous spread of virus from cell to cell without having to exit into the ECF, where they’re susceptible to neutralisation by antibodies.
NB microscopically, viral syncytia appear as a clump of cells with many nuclei
Name a virus that uses syncytium formation as a way of evading immune detection -- specifically, as a mechanism for evading adaptive immunity’s virus-neutralising antibodies in ECF.
The clue is in the name:
Bovine Respiratory Syncytial Virus (BRSV)
How do some viruses use transformation as a way of evading the immune system -- specifically, as a mechanism for evading adaptive immunity’s virus-neutralising antibodies in ECF?
Oncogenic (cancer-causing) retroviruses integrate their DNA into host genome of infected cell ➔ Cell undergoes malignant transformation & proliferates out of control ➔ Viral genome, with oncogenes, replicated with each cell division
Potential risk for xenotransplantation as endogenous retrovirus sequences could become activated to create pathogen
Give an example of a virus that uses transformation as a way of evading the immune system -- specifically, as a mechanism for evading adaptive immunity’s virus-neutralising antibodies in ECF?
FeLV - Feline Leukemia Virus
In FeLV-induced thymic lymphoma, the thymus becomes enlarged due to infection
How do some viruses use antigen decoys as a mechanism for evading immune detection - specifically evading virus-neutralising antibodies?
1. Virus causes infected cell to secrete decoy antigen produced under direction of viral genome ➔ soluble antigen then complexes most of the virus-neutralising antibodies in the ECF ➔ virus can bind to target cell without interference.
2. Some viruses express decoy antigens on surface that are much more immunogenic (more attractive to antibodies, maybe bigger, for example) than the protein used for attachment ➔ antibodies bind to these decoy antigens ➔ virus’s functional antigens enable it to bind to target receptor
Name a virus that produces a decoy antigen to avoid detection by virus-neutralising antibodies.
- produces decoy antigen similar to its structural protein but soluble, secreted by infected cell to saturate antibody binding sites.
How do viruses use antigenic DRIFT to avoiding immune detection - specifically as a mechanism to evade adaptive immunity’s virus-neutralising antibodies?
Virus genes are subject to mutation, so virus-neutralising antibodies specific for defined epitopes of viral antigen can no longer recognise the virus if epitope changes.
This is a slow process of natural mutation.
Natural mutation ➔ protein sequence altered ➔ antibody doesn’t recognise epitope/antigen ➔ virus evades immune system
Name two viruses for which antigenic DRIFT has helped avoid immune detection - specifically as a mechanism to evade adaptive immunity’s virus-neutralising antibodies?
1. Feline calicivirus (FCV) strains
2. Feline panleukopenia virus (FPV) - mutations enabled FPV spike antigen to bind to transferrin receptor on canine enterocytes
How do viruses use antigenic SHIFT to avoiding immune detection - specifically as a mechanism to evade adaptive immunity’s virus-neutralising antibodies?
Whole genes such as RNA segments are swapped between viral strains in a secondary host, such as a pig, to radically change antigens expressed
- virus acquires novel antigenic gene that host has never seen before
This is a rapid process.
Name a type of virus for which antigenic SHIFT has enabled it to avoid immune detection.
Influenza viruses: the HA & NA genes have swapped between humans and animals.
How do viruses avoid immune detection by CD8+ Killer T cells?
They interfere in three ways with MHC Class I Pathway of antigen presentation to CD8+ Killer T-cells:
1. Viral proteins block TAP transporters that convey degraded antigen peptide from cytoplasm to MHC 1 molecule in ER for processing and surface presentation
2. Viral proteins cause MHC 1 to be retained in ER
3. Viral proteins cause dislocation of MHC into cytoplasm, where it gets degraded by proteosomes
NB: This impairment of MHC I also ↑ susceptibility of virus to innate killing response mediated by NATURAL KILLER NK cells, which target cells that show deficient/defective MHC production
How does a virus avoiding detection by NK cells as a mechanism to evade host immunity?
Virus causes infected cell to produce “fake” MHC molecules that “fool” NK cells into “thinking” the cell is ok ➔ virus avoids NK killing
How do viruses use superantigens to avoid cell-mediated immune detection (eg., T-cell-mediated apoptosis, antibody production)?
Some retroviruses produce “superantigens” that “glue” MHC and T-cell receptors together non-specifically, ie., even if they don’t recognise the peptide ➔ polyclonal instead of antigen-specific T-cell response stimulated ➔ every T-cell activated in presence of infection, so T-cells are not effective against the viral pathogen
What strategy do Retroviruses deploy to evade or subvert host immune response, and give an example.
Strategy: Direct immunosuppression - targeted infection & destruction of cells of the immune system, esp. T-helper cells, macrophages & dendritic cells
Example: Feline Leukemia Virus (FeLV) - oncogenic retrovirus causes neoplasia (lymphoma), myelosuppression (anaemia - stem cells in bone marrow infected) and immunosuppression
What strategy do Lentiviruses deploy to evade host immune response, and give an example.
Lentiviruses use both Direct & Indirect Immunosuppression
Direct: Targeted infection & destruction of cells of the immune system, esp. T-helper cells, macrophages & dendritic cells
Example: Feline Immunodeficiency Virus (FIV) - the fighting kind; lymphopenia develops so WBC count low; CD4 T-cell numbers reach critically low level & immunosuppression symptoms become clinically evident
Indirect: Subversion of cytokine responses; viruses produce proteins that bind to cytokines, acting as antagonists that neutralise cytokine reaction; viruses produce FAKE cytokines, acting as agonists
Example: Epstein-Barr virus (glandular fever) in humans & Orf virus in sheep - produce FAKE interleukin-10 (IL-10) that causes infected host cell to “think” there’s no infection, so immune response is very weak.
What strategy do Pestiviruses deploy to evade host immune response to infection, and what is an example?
Example: Bovine Viral Diarrhoea Virus (BVDV) - Transplacental spread of NON-CYTOPATHIC BIOTYPE of virus from dam to foetus ➔ antigen present in primary lymphoid tissues during lymphocyte development of foetal bone marrow & thymus ➔ clonal deletion of virus-antigen-reactive lymphocytes, as if it was self-antigen ➔ calf becomes immunotolerant to virus
NB: non-cytopathic biotype is the one that causes persistent infection, only mild diarrhoea. When calf is infected with cytopathic biotype, it develops mucosal disease & dies.
What strategy do Herpesviruses deploy to evade host immune response to infection, and what are some examples?
Strategy: Latency - virus enters quiescent/dormant stage of development undergoing no replication
- Acute-phase stage of infection with replication and cell destruction can occur in one cell type but latency can occur in another cell type eg., acute phase in epithelial cells & latency in neurons or lymphocytes
- Recrudescence (reactivation) to become fulminant again ➔ further shedding ➔ clinical disease can recur ➔ triggered by stress, meds or lowered immunity
Example: Feline Herpesvirus-1 (FHV) - Cats often shed virus after pregnancy as recrudescence often occurs after parturition ➔ kittens infected ➔ kittens show signs of “cat flu” & develop latent infection ➔ persistently infected
Example: Equine herpesviruses (EHV-1, EHV-4) - Virus moves into circulation via cell-associated viraemia, ie., lymphocytes become Trojan horse for virus ➔ virus moves to sites of secondary replication eg., pregnant uterus or CNS ➔ abortion or neurological disease
Is Canine Parvovirus an example of a virus that has undergone antigenic drift or antigenic shift?
Small numbers of mutations in three amino-acid residues of Feline Panleukopenia Virus (FPLV) CAPSID GENE allowed it to infect dogs & spread within new host order.
Are influenza viruses examples of viruses that have undergone antigenic drift or shift to successfully evade host immune response and detection?
Virus acquires different H type from different species than original host as result of re-assortment of viral segments
Pigs can act as mixing vessels (receptor for avian+human influenza viruses)
H1N1 - Spanish flu: avian virus might have jumped to humans
H2N2 - Asian flu: reassortment of H1N1 with human strain & acquired 3 new genes
H3N2 - Hong Kong flu: reassortment of human & avian flu viruses
H1N1 - re-emerging: possibly released by lab
H5 & H7 can become highly pathogenic
16 “H” types carry specific haemagglutinin (HA) antigens ie., spike envelope proteins
9 “N” types carry specific neuraminidase (NA)
How can antigenic drift lead to a change in a virus’s species specificity and tropism?
An influenza virus that undergoes antigenic drift in the genes for its haemagluttinin spike proteins could cause it to bind differently to sialic acid on different host cell types and even different species’ cells.
If the sialic acid is long and thin in structure due to α-2,3-link with galactose, then the AVIAN influenza virus's HA will bind, but if the sialic acid is slightly wider because it’s formed an α-2,6 linkage with galactose, then this kinked shape is recognised by HUMAN influenza virus's HA. If the cleavage sites are changed, even by just two amino-acid residues, then the influenza viruses could bind instead to different species’ receptors.
Ie., Kinked α-2,6 linkage sialic acid receptors don’t usually bind to avian influenza virus but deep in our lungs humans have receptors for α-2,3-linkage that is recognised by H5N1 avian influenza.
What distinguishes an arbovirus from a regular virus?
An arbovirus is an arthropod-born virus that replicates in the arthropod vector as well as in the host.
A virus that doesn't replicate in the arthropod vector but merely uses it for mechanical transmission of infection is not a true arbovirus.
Leporipoxvirus, carried by fleas and mosqitoes, causes myxomatosis in rabbits and is an example of (more than one):
A. a pox virus that targets epithelial cells of the skin
B. a virus that replicates in the cytoplasm of host cells
C. a virus that replicates in the nucleus of host cells
D. an arbovirus
A & B
Leporivirus that causes myxomatosis in rabbits is a pox virus that targets epithelial cells of the skin & replicates in host-cell cytoplasm.
As a pox virus, it is very large & carries its own replication proteins, thus doesn't need to enter the nucleus to replicate. Also, it uses insects and mosquitoes only as mechanical vectors, but doesn't replicate in them, so it isn't an arbovirus.
Arboviruses fall into five virus families:
Give examples of important arbovirus genera for each family.
1. Flaviviridae - Flavivirus
2. Bunyaviridae - Bunyavirus
3. Reoviridae - Orbivirus
4. Asfarviridae - Asfivirus
5. Togaviridae - Alphavirus
Give examples of diseases caused by arboviruses in the virus family Flaviviridae & genus Flavivirus.
Most arbovirus diseases that affect humans are caused Flaviviridae flavivirus arboviruses.
Positive-sense single-stranded RNA viruses, enveloped.
West Nile Virus
What is West Nile Virus?
An arbovirus in the Flaviviridae family, genus Flavivirus.
Epidemiology: Endemic in Africa, Asia, Australia, Middle East, Europe and US
Vectors: Mosquitoes mainly of genus Culex
Hosts: Mosquitos & Birds
(Humans, horses, bats, chipmunks, skunks, squirrels, rabbits, llamas are dead-end hosts that can carry low viraemia only ie., not infectious)
Inoculation by mosquito → viraemia → mosquito vector takes blood meal from infected bird → infects other birds or mammals → targets cells of CNS in mammals
Clinical: Neurotropic in mammalian hosts
Case fatality of horses with clinical signs ~33% but high proportion of infected animals show no clinical signs
Humans: <1% cases result in serous illness (encephalitis, meningitis, death)
Control: No commercial vaccine though DNA vaccine recently produced; Mosquito control
What is Louping Ill?
A tick-born arbovirus of the Flaviviridae family & Flavivirus genus that affects sheep in upland UK.
Vector: Ixodes ricinus (sheep tick)
Hosts: Mainly sheep, but also red grouse
- Cattle, horses, deer & man are dead-end hosts that end up with only low-level viraemia, subclinical not infectious. In rare cases causes severe neurological symptoms eg. meningoencephalitis
Transmission: Tick takes blood meal in sheep ➔ virus replicates in lymphoid tissue ➔ transient viraemia of 1-5 days ➔ immune system eliminates from non-neuronal tissue but virus attacks CNS cells
Sheep - encephalomyelitis, ataxia, leaping gait, paralysis, but not all develop clinical disease;
Red grouse - high mortality, up to 70%
Control: Tick-control through acaricide treatment of domestic sheep 3x a year and regular vaccination against virus 2x in first year (young lambs borne to vaccinated ewes are protected by MDA) if possible/affordable.
Why could Louping Ill, an tick-borne arbovirus disease that affects upland sheep that can be prevented with vaccine, become an increasing problem in the UK?
The flavirus is present on hills and moors & prevalent in spring, but they could increase in numbers with climate change, which could extend the springtime period and/or expand the region of infection from hills to other areas.
Sheep can also develop immunity to the 3x-yearly acaricide treatment and vaccine.
Give examples of livestock diseases caused arboviruses that fall in the Reoviridae family, Orbivirus genus.
Blue Tongue (Blue Tongue Virus, BTV)
African Horse Sickness (AHSV)
What is Blue Tongue Virus? How is it transmitted & controlled?
BTV is an arbovirus of the Reoviridae family & Orbivirus genus. It affects sheep & cattle, but sheep have much more visible clinical signs.
Epidemiology: Endemic in warmer climates but from 1998-2004 there were outbreaks of diff. serotypes; emergency vaccination in 2008 brought it under control but we're seeing more now.
Vector: Culicoides spp biting midges
Hosts: Culicoides midges, domestic sheep / cattle / wild ruminants
Transmission: Midges inoculate hosts with BTV ➔ replicates in monocytes, macrophages, lymphocytes, endothelial cells in lymphoid tissue, skin, lungs & elsewhere ➔ prolonged but transient viraemia ➔ damage to endothelial cells / blood vessels in target tissues ➔ ulcers & haemorrhage in oral cavity, upper GIT, pulmonary artery, oedema
Sheep - focally extensive reddening & ulceration of lower lip; vascular leakage (oedema) & haemmorhages around head & face; extensive erosion & shedding of mucosa on tongue; fever; necrosis of skeletal & cardiac muscle; erosion & ulceration of oral cavity, oesophagus & forestomachs; congestion; bloody nasal discharge
Cattle - similar to sheep but in general show much less sign of disease
Control: Attentuated virus vaccines (OVP); Inactivated virus vaccines are safest but drawbacks include multiple doses & limited number of serotypes avail.
What is African Horse Sickness Virus (AHSV) & how is it controlled?
AHSV is a mosquito-borne arbovirus of the family Reoviridae & genus Orbivirus, which affects equids.
Epidemiology: Endemic to central tropical regions of Africa; Seasonal (late summer/autumn) and an epizootic cyclical incidence, associated with drought followed by heavy rain; Northern expansion of vector field to Mediterranean threatens Europe
Vector: Culicoides spp. biting midges (C. imicola mainly)
Hosts: Equids - horses, zebra, donkeys, mules
Transmission: Mosquito inoculation with AHSV ➔ replicates in blood cells 7-14 days ➔ Cell-associated viraemia (RBCs, WBCs) 4-8 days, up to 21 days in horses ➔ vasculitis, ↑ vascular permeability
Cardiac form: swelling of head, neck, chest; mild fever, swelling above eye
Pulmonary form: most severe with pulmonary oedema & 95% mortality
Mortality rate in horses is 70-95%, mules around 50%, and donkeys around 10%; infection in zebra and African donkeys is subclinical
Control: vector control, live-attenuated vaccine
What are some examples of arbovirus diseases that fall into the family Bunyaviridae?
Schmallenberg (genus Bunyavirus)
Rift Valley Fever (genus Phlebovirus)
Crimea-Congo Haemhorragic Fever (genus Nairovirus)
We mainly need to know about Schmallenberg.
What is Schmallenberg virus and how is it controlled?
Schmallenberg virus is an arbovirus in the family Bunyaviridae, genus Bunyavirus. It affects sheep and cattle in Europe and the UK.
Vector: Culicoides spp biting midges
Transmission: Midge inoculates host with SBV → infects immune cells → inflammation, viraemia (4-6 days) → targets neurons / CNS
Clinical: arthrogryposis in neonate sheep & calves especially
Predominant CNS lesions or conditions were cerebellar and cerebral hypoplasia, hydranencephaly, porencephaly, hydrocephalus, and micromyelia
What is African Swine Fever Virus, and how is it controlled?
ASFV is a tick-borne arbovirus of the Asfarviridae family, genus Asfivirus.
Don't mix this up with African Horse Sickness Virus, which is an Orbivirus in the Reoviridae family.
Epidemiology: Enzootic in most countries of sub-Saharan Africa; eradicated in Europe except for Sardinia
Vector: Soft ticks, genus Ornithodoros
Hosts: Domestic pigs
Transmission: Soft tick takes blood meal & inoculates pig with ASFV ➔ virus replicates in perinuclear region of cytoplasm of macrophages, monocytes (doesn’t need nucleus machinery) ➔ incubation 3-15 days ➔ viraemia ➔ haemorrhage
Clinical: Acute - Lethal haemorrhagic disease
Reddening (white pigs) – tips of ears, tail, distal extremities, ventral chest, abdomen
Anorexia, listlessness, cyanosis, incoordination within 24–48 hours before death, ↑ pulse & respiratory rate,V&D
Death within 6–13 days, up to 20 days
Lots of haemhorrages everywhere
Survivors become carriers
In domestic swine, mortality ~ 100%
Control: No vaccine or treatment
- import controls & careful waste disposal in free countries
- limit pig movementto avoid contact with ticks
- rapid slaughter & disinfection during outbreaks
Give an example of disease caused by arboviruses in family Togoviridae.
Equine encephalitis (genus Alphavirus)
We didn't really go over this in the course but it was highlighted by Noad as something we need to know.
What is the difference between African Swine Fever Virus and African Horse Sickness Virus?
Both are arboviruses.
African Swine Fever Virus is in the family Asfarviridae, genus Asfivirus. Vector is the soft tick (genus Ornithodoros), host is the pig. Clinically it causes haemhorragic disease and death or carrier state. The virus replicates in the cytoplasm of host cells, in the pernuclear area, not the nucleus. Mainly sub-Saharan, eradicated in Europe except for Sardinia.
African Horse Sickness Virus is in the family Reoviridae, genus Orbivirus. Vector is biting midges (Culicoides, esp. C. imicola), host is equids. Clinically there's the milder cardiac form that causes swelling about the head, neck & face; and the more severe pulmonary form, which results in pulmonary oedema & death in about a majority of infected horses. The virus causes cell-associated viraemia in WBCs, & significantly increases vascular permeability.
Explain the prion theory that accounts for the transmissible spongiform encephalopathies.
The “prion” theory, also known as the “protein only” theory, proposes that a normal cellular protein, named PrPc, is converted spontaneously into or comes into contact with a pre-existing abnormal isoform, PrPSc, which goes on to convert other normal isoforms into self-aggregates, fibrils and plaques.
These accumulations of PrPSc interfere with normal neurological function, resulting in disease of the CNS.
NORMAL: The normal PrPc protein is a cellular protein that is highly expressed on neurones.
PrPc is a continuously produced and degraded, ie., a “recycling protein”, with a half-life of ~ 24 hours. It is also protease sensitive, which means it can be degraded by proteases.
Secondary and tertiary structure of normal PrPc contain many α-helices, which don’t easily form compact aggregates, plaques or amyloid fibrils, as in tertiary form they form right-angles to each other.
ABNORMAL: The disease isoform, PrPSc, like the normal isoform, are highly expressed in neurones, so when they aggregate, they interfere with neuronal function, specifically synaptic function.
PrPSc is also relatively resistant to protease, making it difficult to degrade using low levels of protease.
Secondary & tertiary structure of PrPSc contain a lot of β-sheets, which are basically parallel structures that easily aggregate to form amyloid fibrils and plaques.
The isoform also facilitates conversion of the normal protein into the diseased form, and is transmissible through ingestion of the diseased protein.
Other misfolding-protein neurodegenerative diseases include:
Alzheimers (amyloid plaques in the brain)
Dementia with Lewy bodies (α-synuclein)
Describe the pathogenesis of prion disease, aka Scrapie.
Transmission of scrapie occurs via the alimentary tract. Dissemination of prions into the environment (soil) can occur from infectious placenta or amniotic fluid and possibly environmental contamination by saliva or excrement.
The protein enters animal, mainly sheep, through intestines or cuts in the skin. It’s frequently transmitted in family lines, so some form of maternal transmission may occur at a pre- or postnatal stage.
Prions stimulate cytokine production from astrocytes (astrocytosis) & microglia (brain macrophages) but there is NO infiltration of neutrophils, T-cells or B cells ➔ no adaptive immune response ➔ no IgG antibodies (so can’t detect by antibody ELISA unless in PrP knockout mice) ➔ replication requires active immune system ➔ replicates within follicular dendritic cells ➔ travels from GALT (Peyer’s patches) & gut to damage CNS / neuronal synapses, which is irreversible
Explain the pathogenesis of Bovine Spongiform Encephalopathy (BSE)
Cattle became exposed to the abnormal prions through the feeding of ruminant-derived protein within feedstuffs such as meat-and-bone meal (MBM). The details of pathogenesis are unknown, but studies indicate that after oral exposure the agent replicates in the Peyer’s patches of the ileum followed by migration, via peripheral nerves, to the CNS.
No evidence of horizontal transmission or environmental factor.
Describe the mechanisms of detection of prion diseases (scrapie & BSE).
Prion disease is now more often detected by 96-microwell ELISA method.
Prion disease is also detected by Western Blot identification.
Describe the Western Blot method of detecting BSE.
1/ Run an extract of cells or tissue suspected of BSE infection through gel electrophoresis, which separates out proteins by 3D structure or length of their polypeptides, using molecular weight or electric charge.
2/ Gel-separated proteins then transferred onto a membrane, usually nitrocellulose.
3/ Proteins on membrane stained with antibodies specific to the target protein.
There are three forms of the PrPc molecule: it is the same PrP protein glycosylated three different ways. Antibody can detect the differences, although it’s difficult to produce an antibody that selectively binds to PrPSc.
Antibody to PrP can be produced by:
- Immunising an animal with PrP from a different species.
- Add a T-cell epitope to PrP protein so it will be recognised by T-cell as foreign
- Immunise PrP-knockout mice (mice with no PrP proteins) with PrP to make antibody
4/ Split the sample into two and protease-digest one sample. The one that’s left over because it’s resistant is the infective PrPSc protein.