Viral infection and pathogenesis Flashcards
Pathogenicity, Virulence -> Definitions
- As soon as a germ can lead to a disease, it is classified as a pathogen.
- Im deutschen Sprachgebrauch ist es nicht korrekt, Viren als „hoch pathogen“ oder „schwach pathogen“ zu bezeichnen.
Richtig ist, von „hoch virulent“ oder „schwach virulent“ zu sprechen.
Endemic (Enzooty)
Disease, which occurs within a definied area for an indefinite time periode and without having the tendence to spread (low morbidity, spatial but no temporal limitation).
Example: rabies
Epidemic (Epizooty)
Massed occurance of a dangerous infectious disease with a temporal and spatial limitation while having a high index of manifestation (high morbidity, temporally and spatially limited).
Example: FMDV Great Britain 2001
Pandemic (Panzooty)
Spreading of a epidemic across whole countries or continents (widespread disease without spatial limitation in a certain time period).
Example: Swine Influenza 2009
Morbidity
= [number of sick animals x 100] / number of animals at risk in %
Mortality
= [number of animals which died from the virus x 100] / number of animals at risk
Lethality
= [number of deaths x 100] / number of animals with symptoms
Incidence
= number of new cases of illness within a certain time period
Prevalence
= [number of all existing infection cases x 100] / size of the population
Seroprevalence
= [number of antibody positive individuals („historical infection cases“) x100] / size of the population
Virus entry into the organism
-> Infection = Disease?
To infect the host the virus needs to
- either infect a cell on the body surface
- or break through the protecting body surface
Parenteral inoculation: wounds, insect bites, injection needles, bite injuries
Body surfaces involved in virus entry excretion
- Infection through body surfaces or parental inoculation
- conjunctiva
- respiratory tract
- alimentary tract
- urogenital tract
- anus
- arthropod
- capillary
- scratch, injury
- skin
Infection
1. External skin
- cornificated cells as an almost insurmountable barrier
Exceptions:
Small surface wounds
Infection of epithelial or neural cells:
Pox-, Papillomaviruses (Replication only in fully differentiated keratinocytes)
Example: human papillomavirus (HPV) - deeper wounds, scratches, insect bites, injection needles, sexual contact (access to bloodstream)
- iatrogenic (caused by physician) by nonsterile injection needles: HIV, HBV, CMV, Epstein-Barr Virus,
- arthropods as vector: Alpha-, Flavi-, Reovirus, Bunyavirus and others
- bite by virus carrier (e.g. dog/rabies virus)
Replication cycle of HPV in der Haut
Stratum corneum: High virus production and release of infectious viruses
Stratum granulosum: Few viral particles, many viral genomes, increased transcription of early and late genes
Stratum spinous: Few viral genomes, Transcription of genes E1, E2, E6 and E7
Stratum basale: Very few viral genomes, Minor transcription of genes E1 and E2
Disease induced by Human Papilloma Virus (HPV)
- Transmission by skin contact, sexually
- Chronic-persistent infection of basal cells
- Development of warts through cell proliferation, which usually regress spontaneously
- Benign lesions in the genital area (nether regions) can degenerate into malign lesions. Condylomas can disappear spontaneously or persist (20%)
- Integration of the virus genome leads to transformation, not replication; development into cervical carcinoma after a latency of 20 - 40 years (in 3-6%).
Disease through HPV
- Cytological diagnosis of cervical smear test; classification by degree of severity of the neoplasia
- In most cases neoplasias are precursors of invasive carcinoma
- Can break through basal membrane and metastasize
- Therapy, no specific therapy, surgical removal, Imiquimod, IFNa/b locally and systemically
HPV vaccination
Nobel prize in Medicine 2008
Rabies -> Infection by bite
Transmission to humans: Bite by infected dogs
India: about 50 000 deaths by rabies per year
Germany:
- one fatal case in 1996 and 2004, one in 2007
- officially free of rabies since 2008
Countermeasures:
Extensive vaccination
- of dogs
- of foxes (vaccination bait)
- Vaccination in humans
also therapeutic after infection - passive (anti rabies IgG)
+ active (inactivated vaccine)
Neurotropism
Virus travels after short replication phase in muscle cells via neurons into the CNS, where it is protected from antibodies
Rabies -> transport to the brain
MAY TAKE SEVERAL MONTHS
- Virus inoculated
- Viral replication in muscle
- Virion enters peripheral nervous system
- Replication in dorsal ganglion
- Replication ascent in spinal cord
- Infection of spinal cord, brain stem, cerebellum, and other brain structures
- Descending infection via nervous system to eyes, salivary glands, skin and other organs
Rabies
Symptoms:
* Diagnosis
- hallucinations
- fear of water
- convulsions und paralysis - coma, death
– Clinics: in case of suspicion - involve epidemiology!
– Direct virus detection in the case of suspected rabies (patient alive): IFA or RT-PCR
e.g. saliva sample, skin samples (only positive results are convincing!)
– Direct virus detection in the case of suspected rabies (dead patient) Autopsy: tissue pieces from the brain – hippocampus, cerebellum, pons
* Negri-bodies (alone not sufficient!)
* Immunofluorescence (IFA), detection of rabies antigen in the brain
* Virus isolation
* REITEN-PCR
A Purkinje cell in the cerebellum: The eosinophilic inclusion bodies in the cytoplasm, called Negri bodies, contain viral particles
Negri bodies (NB)
Histopathological changes associated with rabies encephalitis:
Hematoxylin and Eosin stain
Negri bodies: Cellular inclusions in
- pyramidal cells of Ammon’s horn
- Purkinje cells of the cerebellum
- medulla and various other ganglia
- contain TLR3 (central part) and rabies nucleocapsid protein N (periphery)
- TLR3−/− mice show better survival rates and less viral replication
Toll-like Receptor 3 (TLR3) Plays a Major Role in the Formation of Rabies Virus Negri Bodies
Human neurons produce TLR3, a protein involved in early host defence mechanisms and the modulation of neuronal survival.
In this study, we showed that rabies virus exploits TLR3 function to store viral proteins and viral genomic material in particular areas of the cell where virus multiplication occurs. We found that, during the course of infection, large (1–3 μm) spherical inclusions were formed within the region around the nucleus. These inclusions were composed of an inner core of aggregated TLR3 surrounded by a coat of viral proteins and genomic material. These inclusions were revealed to be the previously described
Negri Bodies (NBs).
In absence of TLR3, NBs were no longer formed and virus multiplication rate decreased. Mice deficient in TLR3 were more resistant to rabies and had lower levels of infection in their brains. This study shows how neurotropic viruses, such as rabies virus, hijack normal functions of neuronal proteins and use cell compartmentalisation to promote
viral multiplication.
Direct immunofluorescence
Fluorochrom (FITC) + Akanti N -> Adsorption -> Penetration -> Uncoating -> Transkription -> Translation -> Replikation -> Morphogenese -> Freisetzung
Iatrogenic rabies transmission
- woman dies months after trip to India; serves as organ donor (cornea, kidneys, liver, pancreas): rabies infection had not been detected
- 3 of 6 organ recipients came down with rabies and died
USA: 6-jähriger Junge stirbt an Tollwut
Meldung vom: 16.01.2018
A 6-year-old boy has died of rabies in Florida. The boy was scratched by a sick bat that his father had found and kept. The boy then washed his hands, but a rabies vaccine was not administered - so there was no immediate treatment after exposure.
About a week after being scratched by the bat, the 6-year-old was admitted to the emergency room with hallucinations and convulsions. As he was already showing symptoms of rabies, his survival was virtually impossible at this point.
A subsequent rabies vaccination was no longer possible, so the doctors in charge applied an experimental treatment called the Milwaukee Protocol New Scientist as well as Cambridge.org - but without success. The boy died on Sunday in Orlando Hospital as a result of his rabies infection.
Wild animals account for the vast majority of rabies cases in the United States; in 2015, more than 5,500 rabid animals were identified in the US. Only one or two people die of rabies in the United States each year. Prior to this case in Florida, the last person to die from rabies in the US was a 65-year-old woman from Virginia who was bitten by a dog.
Vector transmission Arthropde borne (Arbo)
Tick-borne viruses
e.g. – pathogen
of Tick Borne Encephalitis Virus (TBEV)
- transmitted by ticks
- first appearance in the Vienna Woods
- many lumberman with paralysis
- full protection through vaccination
West Nile Virus (Genus Flavivirus)
Infection
2. Respiratory tract
Epithelial cells of the respiratory tract
- target cells of many viruses: important site of infection
Countermeasures:
- translocation of mucous layer by ciliated epithelium
- only particles < 5μm can pass to the alveoli (thus big drops can not pass)
- control by alveolar-macrophages
Aerosols
- within and beyond 1 meter
- can float in air for hours
- can be inhaled
- < 5 µm
- 5-100 µm
Droplets
- Can travel less than 1 meter
- Fall to the ground in under 5 seconds
- Cannot be inhaled
- > 100 µm
Infections of oropharynx and respiratory tract
- Rubella Virus
- Rhinovirus
- Coronavirus
- Parainfluenza virus
- Respiratory syncytial virus
- Influenza virus
- Adenovirus
- Herpes Simplex Virus
- Epstein Barr virus
Examples of virus infections via the respiratory tract
LOCALISED INFECTION OF THE UPPER RESPIRATORY TRACT
Rhinovirus, Coxsackievirus (Picornaviridae) Arenaviruses, Hantaanvirus (Buyaviridae) Parainfluenza virus (types 1-4) and RSV (Mononegavirales); Coronaviridae Influenza A and B virus (Orthomyxoviridae) Adenoviruses (types 1-7, 14, 21)
LOCALISED OF THE LOWER RESPIRATORY TRACT
RSV, Parainfluenzavirus (types 1-3), Influenza A und B viruses, Adenoviruses (types 1-7, 14, 21) Infectious bovine rhinotracheitis virus (Herpesviridae)
ENTRY OVER THE RESPIRATORY TRACT WITH SUBSEQUENT SYSTEMIC SPREADING
FMDV, Rubellavirus, Arenaviridae, Hantavirus, Mumps virus, Measles virus, Varizella Zoster Virus, Pox viruses
Infection of the respiratory tract by influenza viruses
Influenza hemagglutinin (HA) binds to terminal silica acid residues as receptor
Highly virulent avian influenza virus (H5N1): sialic acid alpha 2-3 Gal
human influenza viruses (H1N1) or (H3N2): silica acid alpha 2-6Gal
Transfer to humans has not occurred so far, since human to human transmission is inefficient
Receptor abundances as molecular basis?
Hypothesis
Mucosal epithelial cells of the upper and lower respiratory tract carry different sugar residues structures on their surface
- Bad reputation of H5N1 in upper respiratory tract (1918 virus very efficient)
- Inefficient infection and release
New primary culture experiments: Hypothesis wrong?
Sala 2-3Gal/Sala 2-6Gal distinction is not sufficient :
Sugar topology is crucial as well
Infection
3. Oropharynx and gastrointestinal tract
- oral virus uptake
- infection of mucosa in mouth and pharynx
- tonsils
-> Adenovirus, Pestiviruses - gastric and intestinal area: only when stable against acid pH!
- Enteroviruses (e.g. Hepatitis A)
- Calicivirus (Noroviruses)
- Reoviruses
->uncoated
coated:
- enteral Coronaviruses: protection by casein/associated proteins
Infection of the gastrointestinal tract
Intestinal mucosa is an effective barrier
- different pH-values
- mucus
- phagocytes und antibodies
- GALT “gut assoz. lymphoid tissue“
- e.g. Peyer‘s Patches
Lymph follicles in mucosa have specialized epithelia on luminal side, which contains M-(membranous epithelium) cells
M-cells take up antigens and present them to lymphocytes which are located beneath:
Trans-cytosis = Transfer through M-cell membranes without degradation
Infection of the gastrointestinal tract
-> Virus entry through M-cells
- Entry via M-cells followed by further spreading: Reovirus Poliovirus: only trans-cytosis – no replication in M-cells?
- Infection and replication in M-cells without further spreading Destruction of M-cells leads to inflammation/diarrhea:
Virus of transmissible gastroenteritis TGEV (corona virus of pigs) Rotavirus
Routes of viral infection (Entry)
DIGESTIVE TRACT
Local replication: Coronavirus, Rotavirus
Systemic replication: Enterovirus, Reovirus, Adenovirus
UROGENITAL SYSTEM
Local replication: Papillomavirus HPV (carcinoma)
Systemic replication: HIV-1, HBV, HSV
EYE
Local replication (normal case): Coxsackievirus, Echovirus, Adenovirus
Systemic replication (rarely): Enterovirus 70, HSV
Host specificity and tissue tropism
Definition:
Host – Species, race or age (infection, replication, excretion) which serves viral reproduction
Viruses with minor host specificity
- Alphaviruses: Hosts are birds, arthropods, mammals
- Genus Flavivirus: Hosts are birds, arthropods, mammals
- Rabies virus
Viruses with high host specificity
- Herpesviruses (exception Pseudorabies virus)
- Papillomaviruses
- Retroviruses
- Hepatitis Viruses: HCV, HBV
Viral tropism
Preferred locations of replication in the body
- Affinity of viruses to certain organs / cell types z.B. dermatotropic, epithelotropic, pneumotropic, hepatotropic, neurotropic, lymphotropic
Viruses with distinct tropism
Hepatotropic: Humans: hepatitis pathogens HAV, HBV, HCV + Rabbit: Caliciv. Rabbit Hämorrhagic Disease Virus (RHDV)
Neurotropic: Rabies virus; FSME virus; Herpes viruses (EBV, HSV, VZV)
Pneumotropic: Influenza virus
Epithelotropic: Papilloma viruses
Dermatotropic: Parapoxvirus: Molluscum contagiosum V., “Dellwarzenvirus”
Lymphotropic: Human T-cell-lymphotropic virus type I, HTLV I, Eppstein Barr Virus
Viruses without broad tropism
Pathogens causing systemic diseases
- Morbillivirus: distemper virus (“Staupevirus” in dogs, „Seehundsterben“)
- Filoviruses, Adenoviruses, Pestiviruses, Poliovirus, VSV
Host specificity and tissue tropic are determined by
- Receptor
- Cellular factors of viral replication
- Cellular factors of virus maturation
-> Combinations possible
Receptor
A receptor is a structure on the cell surface, to which a ligand can bind specifically (ligand = virus)
The cell surface molecules vary between different hosts or even between different tissue.
Example Poliovirus
- CD155 is poliovirus receptor (immunoglobuline super family)
- transgenic mice which carry the human PVR develop poliomyelitis
- animal model for a human viral disease
Only the receptor determines host specificity
(exception! Infection ≠ replication≠ disease) (infectable ≠ permissive)
Factors for viral nucleic acid replication
Hepatitis B Virus: liver specific enhancers
JC Polyomavirus: enhancer only functional in oligodendrocytes
Human Papillomavirus type 11: only in fully differentiated keratinocytes
-> cellular transcription- factors are essential for viral promoter
Host proteins as part of the viral genome replication machinery: if expression is species or tissue specific, determinants for viral replication
Factors for viral maturation
e.g. proteases, which are needed for the maturation of viral proteins hemagglutinin from influenza virus is usually only cleaved in respiratory epithelium; as a result the virus is restricted to this tissue.
Most Influenza viruses are non-virulent for poultry and cause locally restricted viral infections of the intestine
-> Classical avian flu
- Pathogen: highly virulent influenza A strains
- In contrast to “normal“ Influenza virus which replicates in the gut the virulent form displays a systemic infection
Highly virulent avian flu virus:
Cleavage site in HA mutated (now poly-basic); thus HA can be cleaved after synthesis by the cellular protease Furin in the Golgi
- released viruses are infectious without further activation steps
Systemic infection: spleen, liver, lung, kidney, CNS lethal
Human influenza A virus -> Hemaglutinin (HA)
Trimeric receptor and fusion protein
HA-cleavage is required for infectivity. The fusion peptide gets exposed by cleavage.
Cleavage mostly extracellular: Infection is locally restricted since protease only occurs locally (previously: Clara cells in bronchiolar epithelia secrete tryptase Clara)
Novel results: Membrane bound cellular proteases TMPRSS2 and HAT in airway epithelium activate influenza A virus
Laboratory: exogenic addition of trypsin to activate virus!
Rarely: ubiquitious HA-cleavage after mutation of cleavage site ubiquitous production of infectious viral progeny systemic spread possible
Result: highly virulent virus mutants
HA-cleavage decides between local or systemic virus spread / low or high virulence
Mechanisms of viral spread in the body
- Primary infection of a single cell; afterwards spreading of newly formed viruses in a virus specific manner
- Local spreading along epithelial surfaces (mucosae) starting from the site of primary infection e.g. influenza: lung epithelium
- Spread to neighboring cells or systemic over body fluids
- Polarity of cells (epithelium) is often crucial for their susceptibility and for virus spread
Epithelial cells
-> Polarity; two different surfaces (apical, basolateral)
apical
– towards the environment
basolateral
– towards the inside of the body
Test ?!
-> Trans well System
Cell culture in a membrane coated chamber; selective release of viruses into the media above or under the cells?
Apical release
Measles Virus (Morbillivirus/Paramyxoviridae):
- Matrix protein accumulates at the apical membrane (transport signal)
- Co-transport of coat protein (only at the basolateral membrane)
Result: Selective budding at the apical membrane of the lung epithelium release into the airflow, virus spreading
Apical: West Nil Virus, SARS Coronavirus, Influenza Virus
Basolateral release
VSV
- Matrix protein and glycoprotein both have tropism for the basolateral membrane (signals in cytoplasmatic tail)
- Virus budding selectively on the basolateral membrane
Basolateral: Marburg Virus, Crimean-Congo hemorrhagic fever virus, VSV, HSV
Reovirus preferentially infects the basolateral surface and is released from the apical surface of polarized human respiratory epithelial cells.
Subepithelial invasion and lymphogenic control
After basolateral release (or damage of the epithelium) viruses reach
- the lymphatic system
- by drainage or through infected tissue macrophages the local lymph nodes
- where antigen presentation (decomposed virus inside macrophages) or virus replication takes place
Productive infection of macrophages
Retroviruses, Staupevirus (Morbillivirus); b- and g-Herpesviruses Measles virus
Systemic virus spread by lymph or macrophages
Virus spread via bloodstream: viremia
- efficient and fast virus spreading via the bloodstream
Primary viremia:
- direct virus infection of the host
- without clinical symptoms
Secondary viremia:
- after virus replication in infected organs a huge number of viruses is released
- mostly accompanied by clinical symptoms (fever etc.)
Example: Pathogenesis of mouse pox (Ektromelia)
- Skin: Invasion, Multiplication
- Regional lymph node: Multiplication
- Bloddstream: Primary viremia
- Spleen and liver: Multiplication, Necrosis
- Bloodstream: Secondary viremia
- Skin: Focal infection, Multiplication
In the blood a virus can exist
- non associated: Parvoviruses, Enteroviruses, Flaviviruses (genus)
- adsorbed to cells, erythrocytes, platelets: African swine fiver virus exists only in adsorbed form
- intracellular, inside lymphocytes, monocytes: Retroviruses, Staupevirus, b- und g-Herpes viruses, Pestiviruses
Often in combination: virus exists for example inside blood cells and in the serum
Special relevance of macrophages
- these phagocytes are present in all body compartments
- early contact to pathogens
Vascular endothelium
- Barrier between bloodstream and tissue
- Crossing of viruses over thin vessel walls (capillaries, venules)
-> passive
-> cell-associated (e.g. to lymphocytes, makrophages)
-> after infection of endothelial cells (measles virus, CMV, parvoviruses)
Invasion of the CNS
CNS: privileged tissue
-> specially protected against the transfer of pathogens
1. Transfer from the blood to the CNS (meninges/chorioid plexus): Encephalitis, Meningitis, Meningoencephalitis
Measles virus, Mumps virus; TBEV (FSME)
2. Infection of peripheral neurons (centripetal spreading)
Migration of viruses to the CNS via axonal transport (up to 10 mm/h)
Neural spreading:
- Poliovirus (central, but rare)
- rabies virus (central, always)
- Yellow fever virus (central)
- Herpes simplex 1, 2 (peripheral)
Via olfactoral Neurons:
- Herpes simplex Virus
- Human Coronavirus OC43/SARS CoV-2
Herpes viruses and rabies virus with integrated GFP hence used as a TRACER for neuronal connections
Viral spreading via neurons
4 Steps:
1. Entry
2. Transport to the cell body
3. Replication
4. Transsynaptical spreading
Neural spreading:
- Poliovirus (central, but rare)
- rabies virus (central, always)
- Yellow fever virus (central)
- Herpes simplex 1, 2 (peripheral)
Distinction:
Measles virus and Mumps virus enter the CNS via the blood
Invasion of the fetus by diaplacental infection
Special relevance in ruminants and swine
- Important point: architecture of placenta
- in pigs, ruminants and horses: Placenta epitheliochrialis
- As a result are the blood circulations of mother and fetus broadly separated!
Examples for diaplacental viral infections:
- Cow: Virus of bovine Diarrhea/Mucosal Disease (BVDV)
- Pig: Parvovirus, PRRSV, Virus of classical swine fever
- Horse: Equine Herpes virus-1 (EHV-1)
- Ruminants: Schmallenberg Virus (Orthobunyavirus related to Akabane virus)
Humans: Rubella virus, Lassa Virus, Zika Virus …
Virus excretion
Requirement for the maintenance of the infection chain.
Local infection (e.g. airway infection): Virus excretion often via the same way as invasion, only in reverse direction.
Generalized (systemic) infection: virus excreted from one, but as well from multiple sites, e.g. all body openings
- Skin: Secrete (MKSV, VSV), scab (MKS Virus, Poxviruses), direct contact (nose-/ genital mucosa, Herpes virus)
- Airway secrete: coughing, sneezing (aerosol); (Influenza virus, SARS virus)
- Salivary: Rabies
- Feces: Diarrhea pathogen (Corona-, Rota-, Enteroviruses)
- Genital secrete: Sperm (HIV), amniotic fluid (Parvoviruses)
- Urine: Kidney infection leads to virus excretion in the urine (Hanta Virus, FMDV)
- Milk: Virus spreading from the mammary gland leads to infection of offspring (Lentiviruses, z. B. HIV, West-Nile Virus; TBEV)
- Blood, organs: Arthropods!
Feeding of slaughterhouse- and kitchen-waste (BSE, CSFV, FMDV)
Determinants of virulence and host resistance
Pathogenicity, Virulence, Host resistance
Pathogenicity
Feature of a microorganism to cling to the host, replicate inside a host species and thereby cause an infectious disease.
Pathogenicity always refers to a pathogen-host-system in a given environment and describes the basic qualities of a pathogen species; strain variations do not play a role for the principal classification of a pathogen.
Virulence
Quantitative statement about the grade of disease-causing features („toxicity“, aggressivity) of a certain virus strain or a virus isolate (this applies as well for other pathogens). The virulence within a pathogen species might vary substantially (avirulent → slightly virulent → moderate virulent → highly virulent).
Virulence is quantifiable: e.g. 50% of experimental animals die (lethal dose LD50) after infection with Ektromelia virus (mice pox)
LD50:
- slightly virulent: 10^6 virions
- moderate virulent: 5 x 10^3 virions
- highly virulent: 5 virions
Another example: Classical Swine Fever Virus
Causes of differences in virulence
- Virulence of a pathogen is for the main part determined by genetics
- Changes in virulence are based on mutation(s)
– point mutation
– insertion, deletion
– recombination
– reassortment
Host resistance
Term, which summarizes especially the unspecific defence against pathogens.
Wirtsresistenz lässt sich im deutschen Sprachgebrauch in Empfänglichkeit und Empfindlichkeit aufgliedern.
Empfänglichkeit (susceptibility)
Grundsätzliche Eigenschaft des Wirtsorganismus (Stamm, Ordnung, Spezies) durch ein Pathogen besiedelt (infiziert) zu werden (z. B. Säugetiere sind nicht empfänglich für die meisten Pflanzenviren).
Empfindlichkeit (sensitivity)
Quantitative Eigenschaft eines empfänglichen Wirtsorganismus, die das Ausmaß der Erkrankung nach Infektion beschreibt.
Causes of host resistance
Susceptibility:
Genetically determined
- receptors
- factors of target cells
Sensitivity:
Physiologically determined
- age (often especially prone: very young or old individuals)
- habits, nutritional condition
- hormonal status
- immunosuppression, stress
- multiple infections
The absence of essential factors leads to resistance in potential hosts
Absence of essential factors leads to resistance
Poliovirus
Mice have no a receptor for the human Poliovirus resistant
HIV
Humans with a 32-basepair deletion in the chemokine receptor gene (CCR 5 D32) are resistant to HIV variants which use this co-receptor (does not help against infection with variants that use CXCR4)
Noroviruses
- Blood group antigen (“Histoblutgruppenantigen HBGA”) is Norovirus receptor
- FUT2-gene (alpha-1,2-Fucosyltransferase): susceptibility allele
- homozygous-negative individuals (5-20% in Scandinavia) resistant to e.g. Norwalk Virus infection (many but not all noroviruses)
Resistance through host factors
-> Resistance to viruses through cellular restriction factors
How did they find them?
Different strains of mice differ in their permissiveness
Different cell in cell culture differ in their permissiveness
Restriction by crossbreeding / cell fusion transferable to a permissive system
Dominant restriction factor
Collaborative Cross
Derivation of the CC. A unique funnel breeding scheme will be used to derive each CC strain. This breeding approach is designed to randomize the genetic makeup of each inbred line. Each of the eight parental strains occupies each A-H position an equal number of times.
The evolution of innate immunity
There has been evolutionary progression from RNA-based immunity in plant and invertebrate cells to protein-based immunity in vertebrate cells. In RNA-basded immunity, incoming viral RNA is processed by Dicer into small RNAs that directly target the virus through RNAi mediated by the RNA-induced silencing complex (RISC). In protein-based-immunity, incoming viral RNA is recognized by PRRs that signal to activate interferon expression, which then triggers the expression of many ISGs to inhibit viral replication. Some of the ISGs encode intrinsic antiviral factors constitutively present in certain cell types and can block viral replication immediately and directly.
CPE and diseases
Cytopathic effect (CPE) of a virus in a organism as the reason for a disease. CPE in vitro and a disease in vivo do not have to correlate.
Many cytopathogenic viruses do not lead to a lethal disease in vivo (enteroviruses).
Non cytopathogenic viruses yet can be lethal (classical swine fever).
Depending on the affected organ, the damage on its tissue leads to symptoms of varying intensity:
Skeletal muscle: HCMV (smoth muscles)
Heart muscle: Parvovirus in dogs, Coxsackie-viruses (myocarditis)
Mechanisms of disease development (pathogenesis)
Pathogenesis:
A acute virus infections
B persiting virus infections
What causes the disease?
1. Direct damaging of tissues and organs
Special case: Damaging of the immune system (e.g. HIV)
2. Immuno-pathological viral diseases (indirect)
Examples for the direct damaging with damaging of the organ function
Corona-, Rota- : Damaging of the intestinal epithelium
Alpha-, Herpes-: Damaging of neurons
Cardio-: Degeneration of the myocardium
Influenza-: Damaging of airway mucosa
Ebola-: Damaging of the endothelium
Family Reoviridae, genus Rotavirus
- Group A (B,C) enteritis of humans
- Replication in the epithelium of the small intestine
destroyed Microvilli: Rotavirus infected enterocyte
Microvilli: non infected enterocyte
Rotavirus
Massive effect of a few infected cells on many non-infected neighbouring („bystander“) cells leads to life-threatening diarrhea
Infection/NSP4 only required in infected cell; ADP signalling from infected cells and IP3 signalling from bystander cells causes calcium waves finally leading to serotonin stimulation of the enteric nervous system.
Knowledge of mechanism may allow pharmacological intervention! (in signalling pathway, not anti-viral
After rotavirus infection, nonstructural protein 4 (NSP4) induces intracellular Ca2+ waves that mediate the release of adenosine diphosphate (ADP). ADP activates purinergic receptors on bystander cells, which leads to diarrhea. ADP also activates phospholipase C and release of inositol triphosphate (IP3), which amplifies intracellular and intercellular Ca2+ waves.
Viral immune suppression benefits secondary infections
Diseases can arise indirect e.g. by facilitation of facultative pathogenic germs:
e.g. Influenza virus
supports Streptococcus pneumoniae,
Lung inflammation often has not viral but bacterial source
(Viral/bacterial co-infection often reason of factors diseases; virus supports bacterium, but in part reverse as well!)
Special case: Damaging of immune system
Damaging of the immune system occurs, if cells of the immune system are targets for the virus.
Elimination of single lymphocyte populations can lead to an immune suppression.
Example:
HIV, FIV: Infection of T-helper-cells
Pestiviruses (CSFV!): Leucopenia (Destruction of B-cells)
Parvoviruses: Leucopenia (Feline Panleucopenia, parvovirosis dog)
PDV (Staupe-virus): Leucopenia
Measles virus: Leucopenia/poor IFgamma production
Immune suppression supports other diseases (AIDS as the classical example), which is triggered by e.g. Herpes-, Papova-, Adenoviruses as well as microbial pathogens (esp. facultative pathogenic germs)
Virus induced immune response damages host
-> Disease can develop because of the reaction of the host immune system
Immune pathology
e.g. Hepatitis A virus, Hepatitis B virus, Hepatitis C virus
-> Damage of liver cell by immune reaction against viral antigen or by “Cytokine storm”?
Is Influenza virus 1918 a example for this?
What can we learn from Reconstructing the extinct 1918 pandemic Influenza Virus?
- untypical strong damaging of lung; edema, hemorrhages
- killer in mice
- high and long-lasting cytokine- and chemokine production
„ Cytokine storm“ as reason for high mortality?
Parallels between H5N1 and 1918 influenza virus
- untypical strong damage of the lung
- high and long-lasting cytokine- and chemokine production
- high mortality (avian strain in humans up to 60%)
„Cytokine storm“ as reason of high mortality?
Comparative analyses possible due to cloned 1918 virus
Macaques show after infection with 1918 virus - Very high mortality
- low b-interferon response
- very long lasting cytokine production
- Depletion of lymphocytes
Does deregulated immune response damage the host?
Inhibition of cytokine response does not protect from lethal infection with H5N1 (in mice)
- Mice with knockout of TNFa, IL-6, CC chemokine ligand 2 die of H5N1 infection
- Glucocorticoid treatment also is not protective
Cytokine storm in H5N1 irrelevant?
Highly virulent avian H5N1 influenza viruses
- HA
- HA-cleavage: H5 has efficient Furin cleavage site
- HA-receptor binding
Salalpha2-3Gal (avian)/Salalpha2-6Gal (human) distinction is not sufficient: Topology of the sugar is important as well
2 amino acid exchanges can be sufficient to provoke a change in specificity - PB2
- polymerase subunit; especially important for repl. in humans is aa 627 (Lys instead of Glu); found in human H5N1 isolates of 2005
- 1918 PA, PB1, PB2 and N for high repl.-efficiency in „lower resp. tract“ - NS1
- Interferon antagonist
- NS1 of H5N1 correlates with high titers of TNFa, IFNb, and other cyto- and chemokines - NA
- Affects efficiency of receptor binding and HA cleavage
Until now, no efficient human to human spread
Different viral proteins are determing the host tropism
Next influenza pandemic WILL happen, but extent and source of the causing virus is absolutely unclear!
Model of pathogenesis
- Local infections (locally limited, no general systemic infection); most often in skin and mucosa
– Respiratory viral diseases
– Enteral viral diseases
– Viral diseases of skin, eye … - General infections (whole body is affected)
– Example: Measles, MKS, Panleukopenia, Parvovirosis
Persisting infections
Persisting infections are temporally unlimited (in contrast to acute infections).
Steady or sporadic production of virions or persistence of the viral genome. Difference???
Relevance for epidemiology: Virus reservoir, source for new infections
Classification on basis of virus replication in the course of the infection:
1. Chronical infections
2. Latent infections
3. „slow infections“
Viral persistence: Mechanism of immune evasion
Classification of viral infections:
- basing on detection of infectious virus
1 Acute Infections
2 Persistent infections: „long persisting“ infections
types of persistence:
- Chronic infection (e.g. hepatitis C virus, HCV)
- slow infection (e.g. lentiviruses, HI virus; measles virus/SSPE) - transforming infections (e.g. DNA viruses, onco-retroviruses)
- Latency (herpesviruses; alpha-herpesviruses, e.g. HSV and varicella zoster virus in neurons)
Development of infections
- Acute infection e.g. influenza virus, rhinovirus
- Chronic infection e.g. hepatitis C virus, Persistence of BVDV special case without immune response
- Latent infection e.g. herpes simplex virus (HSV)
- Slow infection e.g. HIV, Measles „SSPE“
Common features of persisting infections
FOR FIRST LATENT INFECTIONS
After the primary, acute infection it comes to the persistence of the viral genome
During latency the infectious virus cannot be detected
The viral nucleic acid is present as a episome (=extrachromosomal DNA-molecule). Periodical reactivation leads to the production of visions (with or with our clinical symptoms).
Latency is a general phenomenon for herpesviruses (e.g. BHV-1, PRV, EHV-1, EHV-4, FeHV, cytomegaloviruses)
Alpha-Herpesviruses: Persistence in neurons
FOR SECOND CHRONIC INFECTIONS
Permanent virus replication. Infectious virus always detectable.
Often: permanent virus secretion (source of infection!) (e.g. human: HCV, HBV; cattle: BVDV; Inapparent FMD virus infection)
Immunological reactions against virus infections
-> Periods after infection until activation
- Interferone system/ innate immune system -> hours
- Cellular immune response - T-killer cells (CD8)
- T-helper cells (CD4)
- Macrophages
- „natural“ killer cells (NK)
-> days - Antibodies
- Neutralizing Ab are extremely important for the protection against reinfection
- Main determinant for success in vaccination
- does not allways protect against reinfection by the same pathogens 113
e.g. influenza virus, hepatitis C virus, HI virus
-> ca 1-2 weeks
Viral infections
Acute (lytic) infection:
- IFN-system block; taking over host metabolism;
- Cell lysis often after only a few hours (e.g. many picornaviruses)
Latent infection:
- No providing of a target; in latency no virus in circulation (herpesviruses)
- Immune privileged tissues: tissues in which specific viruses are not eliminated e.g. HSV in neurons (CD8+-killer cells quite inefficient against inf. cells without MHC cl I expression); manipulation of the cell
Persisting chronic infection:
- IFN-system block (always!)
- Inducing immune tolerance, e.g. pestiviruses: virus of the bovine viral diarrhea (BVDV), no adaptive immune response!
- Immune evasion (e.g. HCV, HIV, HCMV)
- permanent evasion from B- and T-cell response (e.g. HCV, HIV) - Prevention of antigen presentation via MHC-class I and
MHC-class II (e.g. HCMV)
- Destruction of immune system/-cells (e.g. HIV; Cytomegalieviruses)
- Strategy: HCV shows very high antigenic variability
Estimated number of formed viruses/per patient and day:
ca. 1011-1012
Variance per genome copy: between 1 and 10 exchanges
- footrace: Antigenic main species is attacked via clonal propagation of fitting B- and T-cells
- minor representatives of quasispecies are not eliminated and spread
Infection of a population
Epidemiology:
Science of the nature of diseases (in a population) (originally: scientific study on the outbreak of infectious diseases)
Terms in medicine
Duden:
„Wissenschaft von der Entstehung, Verbreitung, Bekämpfung und den sozialen Folgen von Epidemien, zeittypischen Massenerkrankungen und Zivilisationsschäden“
Epizootiology: Terminveterinarysciences
Epidemiology of animal diseases
Communication of viral diseases
I. Horizontal communication
II. Vertical communication
Infection of a population
-> Horizontal infection
- Transmission by direct physical contact (skin-, nose-, genital-contact)
- Transmission by indirect contact (feeding facility, stable floor, clothes, needles, [iatrogen] etc.)
- Transmission by an agent: food, drinking water (feeding with slaughterhouse waste (KSP, Aujezsky), concentrate („Kraftfutter“; BSE).
- Transmission via air: „droplet infection“ (e.g. Influenza-, Rhinovirus), dust infection.
- Special form: Transmission by live vectors: biological (cyclic) transmission, with and without replication of the pathogen e. g. Arthropods as intermediate host: ticks (TBE), gnats (yellow fever), flies1,18 fleas
Infection of a population
-> Vertical transmission
Communication of virus by integrated DNA copies in germ cells (retroviruses)
Diaplacental transmission during pregnancy:
(e.g. many herpesviruses (e.g. mare miscarriage („Stutenabort“), Aujeszky‘al disease) swine fever virus, parvovirus (swine)
- Rubella virus (80% in the first third of pregnancy); CMV, HIV-1 (ca. 10-20%)
Perinatal infection:
Infection during passage through the birth canal (e.g. HIV-1, Herpes simplex virus; HCV, HBV)
In a wider sense: Transmission by mother‘s milk e. g. HIV, HTLV
Definition: in temporal and factual context of birth
Infection of a population -> Types of epidemiological studies
Aim of studies:
Determination of the current status as well as the development of statistical predictions of disease development via features of the pathogen, population density, transmission pathway etc.
„Gapless “ monitoring of a population.
Basis: „Tierseuchengesetz“, disclosure duty and notification requirement for certain infectious diseases (e.g. measles, rabies).
System only works for diseases with obvious clinical signs. Legal obligation for all professionals (farmer, verterinarian, meat inspector).
Examples: CSFV, rabies
Spot tests within a population: (surveillance)
e. g. blood samples from fatstock (animals in slaughterhouse) to measure the prevalence of specific diseases; antigen or antibody detection
Seroepidemiology:
Seroconversion gives indirect evidence for the presence of pathogens in the
population; important method for epidemiology
Advantage: Antibodies can be detected for a long time period. Detection is easily possible (e.g. ELISA). Antibody detection plays an important role in vaccination programs with marked vaccines (serological differentiation of an immune reaction against the vaccine or the wildtype virus respectively, e.g. the Aujeszky‘s disease (gI- virus) and the classical swine fever split vaccine.
Molecular epidemiology:
Modern methods: can serve for typing of pathogens, to resolve the evolution and origin of a pathogen (e.g. HIV, Influenza virus …).
