viral infections Flashcards
Describe the key characteristics of viruses
- 20 nm - 1 um in size
- RNA or DNA
- No independent metabolism and no organelles
- Simple protein coat (and cell membrane from host cell)
Describe viral structure
Describe influenza structure
- Contain either RNA or DNA genome surrounded by protective viral encoded protein coat = capsid
- complete viral particle = virion
- main function of virion is to deliver its DNA/RNA genome into host cell to allow fenome to be expressed
- viral genome often associated within protein capsid formed of nucleoproteins, forms nucleocapsid
- in enveloped viruses nucleocapsid is surrounded by a lipid bilayer/ envelope derived from modified host cell membrane, studded with outer layer of virus envelope glycoproteins
- viral capsids function as a shell to protect viral genome from nucleases, and during infection attaches virion to specific receptors exposted on prospective host cell
- Influenza virus structure shown:
- influenza virion spherical in shape
- enveloped virus with lipid membrane taken from host cell
- inserted into the membrane are glycoplipids, called H spike or N spike as they are proteins linked to sugars – known as HA (hemagglutinin) and NA (neuraminidase), used to subtype influenza e.g. H1N1
What is the baltimore classification of viruses?
How many classes are there, describe each class (don’t worry too much about this)
- Baltimore classification of viruses is a classification system that groups viruses into families depending on their type of genome (DNA/ RNA, single stranded (ss), double stranded (ds) and their method of replication.
- 7 groups:
- 1 –> dsDNA viruses –> must enter host nucleus before replication, require host cell polymerases and host cell to be in S phase of cell cycle
- Examples: adenoviruses, herpesviruses, papillomaviridae
- 2–> ssDNA viruses –> most have circular genomes, form dsDNA in host nucleus.
- group 3 –> dsRNA virus –> replicates within host cytoplasm
- 4 and 5 –> singlge stranded RNA viruses –> replication happens in cytoplasm or nucleus
- group 4 –> positive sense RNA viruses, can be directly accessed by host ribosomes to immediately form proteins, reproduce in cytoplasm
- group 5–> negative sense RNA viruses, cannot be directly accessed by host ribosomes, need to be transcribed by viral polymerase into a “readable” form.
- Group 6 –> positive sense single stranded RNA viruses that replicate through DNA intermediate (retroviruses, use reverse transcriptaase to convert +ve sense RNA into DNA, then transcribed into RNA, spliced by integrase, then tranlsated into viral protein.
- Group 7 –> double stranded DNA virus that replicates through single stranded RNA intermediate (e.g. Hep B). Double stranded DNA genome serves as template for production of viral mRNA which then serves as template for viral reverse transcriptase.
- 1 –> dsDNA viruses –> must enter host nucleus before replication, require host cell polymerases and host cell to be in S phase of cell cycle
Describe the viral lifecycle
- Viruses follow several stages to infect host cells: attachment, penetration, uncoating, biosynthesis, maturation, and release.
- after binding host receptors using viral proteins, animal viruses enter through endocytosis or membrane fusion (penetration)
- many viruses are host specific and only infect a certain type of host cell within certain tissues
- once virus has penetrated host cell it becomes uncoated, releasing viral contents.
- during biosynthesis viral replication depends on the class of virus.
- ssDNA, host enzymes will synthesise a second strand complementary to genome strand, producing dsDNA. dsDNA now replicated, transcibed and translated.
- if viral genome RNA: +ssRNA can be translated directly to make viral proteins, it acts like cellular mRNA. If -ssRNA, it must be replicated into +ssRNA by viral RNA dependent RNA polymerase, brought in by the virus.
- alternative mechanism for +ssRNA viruses is that they carry reverse transcriptase within the capsid, synthesises complementary ssDNA, which is then made into dsDNA which integrates into the host genome. Intergrated viral genome = provirus and can remain in host to establish chronic infection.
- Once viral replication has occured, assembly of new virions occurs. Viral genome, enclosed within the capsid.
- Then release, viral particles are released, and can become enveloped in host cell lipid membrane, or become covered by viral envelope protein.
What allows viruses to be specific in the cell type that they infect?
- Specificity of the cells viruses infect is encoded by the types of proteins they express within the viral coat and the receptors expressed on the host cell
Define:
Acute infection
Latent infection
Chronic/ persistent infection
- Acute infection = primary infection usually followed by clearance and immunity
- Latent infection = primary infection followed by restricted viral gene expression (very little detectable virus, can be later reactivated e.g. chicken pox and shingles; reactivated as you get older, immunosuppressed or stressed)
- Chronic/ persistent infection = primary infection followed by persistent viral replication, continues to cause some damage
Describe viral pathogenesis
Access –> e.g. contact, respiratory, ingestion
Adherence –> express viral proteins
Invasion –> normally by binding to certain surface molecule, triggers endocytosis/ membrane fusion
Multiplication –> hijack cellular machinery
Evasion –> intracellular prevents detection
Resistance –> viruses evolve rapidly, often produce many errors which enable evolution into more effective virus, therefore good at resisting antimicrobial tx.
Damage –> damage to host cells either directly or indirectly
Transmission –> respiratory droplets, diarrhoea etc…
Give some common viral infections that cause the common cold
Rhinoviruses
Parainfluenza virus
Respiratory syncytial virus
Give some viral causes of encephalitis/ meningitis / myelitis
Encephalitis/ meningitis:
- Measles
- Arbovirus
- Rabies
- JC virus
- LCM virus (lymphcytic choriomeningitis)
myelitis:
Poliovirus
HTLV-1 (human T cell leukaemia virus)
Give some viral causes of skin infection
Varicella/herpes zoster virus
Human herpesvirus 6
Human papillomavirus
Smallpox
Rubella
measles
Coxsackie A virus
Common viral cuases of pneumonia?
Influenza virus A/B
parainfluenza
RSV
Adenovirus
SARS coronavirus
Give common viral causes of gastroenteritis
adenovirus
rotavirus
norovirus
astrovirus
coronavirus
Give common viral causes of STI
herpes simplex type 2
human papillomavirus
HIV
viral cause of parotitis?
mumps
viral cause of eye infection?
herpes simplex virus
adenovirus
cytomegalovirus
What is the infection shown?
What type of infection is it, where does it enter the body
Where does it tend to manifest?
Who does it tend to affect?
Is there a specific diagnostic test or tx for this?
hand, foot and mouth disease
Enterovirus that enters via the gut, manifests within the skin
Produces lesions on the skin and ulcers on the mouth
No diagnostic tests for this virus, relies on clinical experience.
Commonly affects children, once theyve got the infection it will pass and they wont be affected again.
What pathology is shown?
Measles with characteristic rash; initial sx of measles develop around 10 days after infection. Infection normally clears in 7-10 days.
Initial Sx include: cold like sx (runny nose, sneezing and cough), sore red eyes that may be sensitive to light, high temperature (fever), koplik spots on the inside of the cheeks.
Koplik spots = often manifest 2-3 days before measles rash itself, white lesions in buccal mucosa (charactersitic feature of measles).
A few days later red- brown blotchy rash appears, usually starting on head/ upper neck before spreading outwards to the rest of the body.
HPV –> how common is it?
What can it cause?
What areas does it affect?
What cancers are linked to HPV?
HPV virus –> very common group of viruses, can cause genital warts, warts of the skin or lead to cancer through abnormal changes to infected cells. There are over 100 types. Many types of HPV affect the mouth, throat or genital area.
example within the skin: veruca strain is benign, often the immune system able to deal with it itself.
Type that causes genital warts and cervical subtype. Precursor for cervical cancer, hence why screening looks for HPV virus.
Acetic acid staining is used to detect the virus on the cervix.
Cancers linked to HPV include: cervical, anal, penile, vulval, vaginal, some types of head and neck cancer.
Which cranial nerve can be affected by herpes zoster virus?
remember varicella zoster = primary infection, chickenpox
herpes zoster is reactivation of latent VSV within the dorsal root ganglia.
It can sit dormant within the trigeminal nerve and become recativated.
When it reactivates it causes vesicular lesions, can cause lesions across all three distributions of the trigeminal nerve –> if this occurs within the opthalmic division this can be very serious.
