Pathogenesis and Diagnosis of Viral Infection Flashcards
1
Q
Entry of Virus
A
- Must overcome innate defences to enter body: physical & immunological
- Different viruses adopt different strategies & have different portals of entry
- Although a single virus may use several entry routes, which may depend on host species
- E.g. Influenza infection in birds occurs via faecal/oral route & by respiratory route in people
2
Q
Skin
A
- In general, skin is an effective barrier unless breached by abrasions or bites (rabies)
- Parapox virus can enter via the skin–> Orf infections in goats & sheep- zoontotic
- Papillomaviruses–> warts
3
Q
Respiratory
A
- Defences: mucociliary escalator, resident commensal bacteria in upper respiratory tract, Sneezing & coughing - Innate immunity (e.g. alveolar macrophages, complement, cytokines, Natural Killer cells)
- Transmission: Aerosolized droplets expelled or contact with saliva from infected individual
- Examples: Influenza, FMDV, Rhinovirus, Adenovirus
4
Q
Alimentary
A
- Defences: low pH in stomach, bile, enzymes & commensal microbiota
- Examples: Norovirus, Classical Swine Fever
- Prion (scrapie/BSE in contaminated food)
- Feline Infectious Peritonitis (FIP)- (a coronavirus which is resistant to trypsin & low pH and can bypass gastric environment)
- Rotavirus (adapted so when proteolytic enzymes cleave it activated)
5
Q
Blood
A
- Transmission:
- Via hematophagous insects- Bluetongue, Schmallenberg & West Nile Virus
- Via iatrogenic (result of medical error) transfer- Hepatitis B Virus, HIV
6
Q
Transplacental
A
- Viraemic pregnant animals can transmit virus to fetus in utero/ via colostrum/ perinatally through secretions/ transplacentally (vertical transmission)
- Special case that avoids environmental shedding & transmission
- Advantages: Infects fetus before Immune System is developed, therefore sees antigen as self –> don’t mount immune defence against it –> ill thrifty/ poor doers
- Examples: Pestiviruses (BVD Virus & Classical Swine Fever) & PRRS
7
Q
Local Infections
A
Replication occurs in epithelium at initial infection site. Some cell-cell spread occurs but virus doesn’t disseminated–> other tissues
- Usually acute (short incubation period & short duration)
- Site of shedding = site of entry
- Examples: para/influenza infections, rotaviral/ coronal infections, some pox & papilloma
8
Q
Systemic Infections
A
Local replication followed by spread to other sites (dissemination via lymph/ blood/ nerves) –> secondary sites of replication
- Allows entry & exit routes to differ (E.g. FMDV (foot and mouth)- enters via oropharynx route but can be shed in milk etc)
- More severe pathology generally (longer incubation period)
- Greater involvement of adaptive immune response
9
Q
Spread in Lymph
(systemic spread)
A
- E.g. can breach basement membrane–> sub epithelial & migrate within phagocytic Leukocytes–> lymphatic system
- Or an disseminate from alimentary tract via M cells –> MALT
10
Q
Hematogenous Spread
(systemic Spread)
A
- Viremia:
-Primary viremia- (presence of virus in blood) spread from site of entry target tissue
-Secondary viremia- may occur following replication in secondary sites ↑ viral titres & dissemination
- Virus may be:
- Free in serum (FMDV)
- Or ‘cell associated’- carried in macrophages (canine distemper)/ lymphoycytes (FIV)
11
Q
Spread via Nerves
(systemic)
A
Rabies- Introduced via bite
- Primary replication in muscle cells–> motor nerves to reach CNS–> Further replication in spinal cord & brain–> enters salivary gland for onwards transmission
- Rabies gets past the BBB but not Ab’s
12
Q
Shedding
(systemic)
A
- Critical step for transmission to a new host
- Generally a virus reaches highest titre in organ tissues from which its shed
- Often, the sites of entry & exit are the same BUT not always, e.g. some systemic infections
13
Q
Viral Tropism
A
- Cellular tropism- preference of some viruses for particular cells- HIV has preference for RBCs.
- Tissue tropism- preference particular tissue- Influenza generally affects lung tissue.
- Host tropism- narrow= affect 1 species, but some can affect a number- ebola (bats & people)
Determined by:
- Susceptibility: Appropriate cell surface receptors for entry (susceptibility)
- Permissivity: Requirements of virus for differentially expressed intracellular gene products to complete the infection
- Accessibility
- Overcoming host immune defences
14
Q
Mechanisms leading to Pathology
A
- Viral fitness is determined by survival & transmission: not advantageous to disable the host
- Where viruses & hosts have co-evolved, often see minimal disease E.g. myxomatosis –> ↓ virulence
15
Q
Viral Pathology
(DIRECT EFFECTS)
A
1. Cytopathic effects (CPE)- damage to host cell caused by virus. Changes usually visible by light microscopy- use a plaque assay. Severity of disease not always linked to CPE seen in cell culture - Effects of tissue damage / inflammation depends on organ affected (may have large reserve e.g. Liver/ tolerate damage better)
Mechanisms underlying CPE:
- ‘Host cell shut-off’
- Lysosomal damage
- Cell membrane abnormalities (lose morphology)
- Syncytia formation (E.g. measles, CDV, herpesvirus, EIA virus)
- Formation of ‘inclusion bodies’ (E.g. negri bodies in rabies virus infections)
2. Cytocidal effects (cell death)- Via (i) cell lysis (ii) apoptosis (iii) deadly CPE
3. Cell transformation (i.e. uncontrolled growth). Can cause neoplasia-
- Retroviruses (via insertional mutagenesis where provirus inserts into host DNA/ oncogene expression)
- Papillomaviruses (Bovine–> warts, Human–> cervical cancer)
- Herpesviruses
16
Q
Virus Indirect Effects
A
Immune Mediated Pathology
- Immune cytolysis of infected cells= essential to recovery, BUT–> damage & clinical signs often consequence
of immune response (e.g. cytokines are endogenous pyrogens)
- Immunopathology may be due to Type I-IV hypersensitivity reactions (CTLs, CD4 cells, Antibody), e.g.
- CTLs (cytotoxic T- lymphocytes): Myocarditis caused by coxsackievirus B infection of mice requires the presence of CTLs
- CD4+ T cells: induce cytokines & activate effector cells. Recruited neutrophils and mononuclear cells release proteolytic enzymes, free radicals & cytokines
- Antibody (Type III): Dengue virus, Feline Infectious Peritonitis virus
17
Q
Iceberg Concept
A
- The Iceberg Concept- the majority of viral infections pass unnoticed
- Disease is determined by: host, viral & environmental factors
18
Q
Host Factors
(disease)
A
- Age
- Gender
- Nutritional status
- Level of productivity (e.g. high-yielding dairy cows)
- Species/ breed resistance or susceptibility – Immunity (from vaccination/ previous infections)
- Geographical origin (e.g. if imported may be naïve to local diseases)
- Physiological stress (e.g. weaning/ pregnancy/ concurrent disease/ movement of animals)
25
Q
Koch’s Postulates
A
Published in 1980, 4 criteria to establish causative relationship between microbe and specific disease:
- The microbe must be found in organisms afflicted by the disease, but not in healthy organisms
- The organism must be isolated from the disease host and grown in culture
- The cultured microbe should cause disease when introduced into a healthy organism
- The same organism must be re-isolated from the experimentally infected host
31
Q
Electron Microscopy
A
Negative Staining
- Uses heavy metal stains that don’t bind to sample but stain the background
- Stain is non-permeable for electrons & appears dark on image
- Virus= permeable & appears light in colour
- Can Identify virus based on its morphology: Type of capsid symmetry & whether it is naked/ enveloped
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 requires purification of sample
32
Q
Presence of Viral Genome: PCR
A
- Polymerase chain reaction (PCR)
- Based on ability of Taq polymerase to amplify DNA
- Use short oligonucleotide primers, specific for virus sequence
- Generation of amplicon of predicted size indicates presence of virus nucleic acid in sample.
- DNA vs RNA viruses - detection of RNA viruses
- PCR template = DNA or cDNA
- RNA might need to be converted–> cDNA via Reverse Transcription-PCR
Advantages:
- Very sensitive, can use very small amounts of starting material
- Does not require live virus
- Specific as primers are designed for target DNA
- Real time PCR- allows quantification, uses fluorescent technology- quicker as no gel stage
Disadvantages:
- Very sensitive, danger of contamination (can–> false positives)
34
Q
Antigen ELISA
(Immunoassays)
A
(biological fluids)- aka a Sandwich ELISA
- Plates coated with antibody specific to virus
- Virus in sample binds to antibody
- Second virus-specific, enzyme-labelled antibody is added
- Enzyme acts on substrate to generate a colour change
43
Q
Antibody ELISA
A
- Plate is coated with antigen (whole virus/ purified virus protein)
- Antibody in serum sample binds to antigen
- Second antibody which recognises IgG is added
- Second antibody carries an enzyme label
- Enzyme converts colour substrate and colour change is detected
