Virus persistence and transmission Flashcards
a) Viral diseases that enter through the respiratory tract and remain local (4)
b) Viral diseases that enter through the respiratory tract and spread systemically (5)
a) i) common cold - rhinovirus ii) influenza - influenza virus iii) chest infection - respiratory syncytial virus iv) chest infection - SARS-CoV-2
b) i) measles ii) mumps iii) rubella iv) FMDV v) VZV
a) Enteric viral diseases (4)
b) Viral diseases of the nervous system (3)
c) Hepatic viral diseases (2)
a) i) rotavirus ii) norovirus (calicivirus) iii) adenovirus (enteric types) iv) poliovirus (mild enteritis)
b) i) rabies (hydrophobia, rabies virus) ii) herpes encephalitis (HSV) iii) poliomyelitis (poliovirus)
c) i) acute hepatitis (HAV, HBV, HCV) ii) chronic hepatitis (HBV, HCV) →leads to hepatocellular carcinoma
Haemorrhagic virus fevers (vector and disease caused)
a) Bunyavirus
b) Flavivirus
c) Arenavirus
d) Filovirus
a) Mosquito/tick/rodent - Rift valley fever, Crimean-Congo haemorrhagic fever, Hantavirus fever
b) Mosquito - Dengue fever, yellow fever
c) Rodent - Lassa fever
d) Bat - Ebola haemorrhagic fever, Marburg haemorrhagic fever
a) Local infections
b) Systemic infections
c) Systemic infection example - Ectromelia virus
a) Virus replicates at portal of entry and doesn’t spread, shed from same site. Acute infections, short incubation time, often short duration. eg common cold, influenza, gastroenteritis (rotavirus, poliovirus), common warts (HPV 6 & 11)
b) Spreads from portal of entry (via blood, lymphatics or nerves). Longer incubation period before disease (multiple rounds of replication). More severe. Dependent on CD8+ T cell for clearance. eg smallpox, chickenpox, measles, FMD
c) Ectromelia virus (Mousepox). Portal of entry - respiratory tract epithelium, 1st replication here → draining lymph node, 2nd replication → bloodstream, 3rd replication (primary viraemia) → spleen, liver, vascular endothelium 4th and 5th replication (amplification stage) → bloodstream, 6th replication (secondary viraemia) → lungs and skin, 7th to 11th replication, lesions caused, portal of exit (skin-to-skin contact, coughed)
a) Factors limiting viral spread (2)
b) Strategies for virus survival (plus graph showing acute, chronic (persistent), and latent viruses) with examples
a) i) temperature - rhinoviruses grow at 32°C (temp of upper resp tract epithelium), but grow poorly at 37°C (the lower resp tract). ii) Basolateral versus apical: Vesicular stomatitis virus (basolateral, buds internally, where viral glycoproteins are trafficked to → systemic infection). Influenza (apical, buds externally → local infection)
b) (see graph)
Acute infection - defined duration, cleared from the body, so only one chance to transmit. eg variola virus, rotavirus, rhinovirus, RMDV, HBV, influenza virus, measles virus, rubella virus
Persistent (chronic) infection - virus always produced, so transmission always possible. eg HBV, HCV, retroviruses (HIV)
Latent infection - can be dormant for yeard/decades, transmot to new hosts. eg all herpes viruses (HCMV, EBV, HSV, VZV, KSHV)
Features of herpes virus latency
Some cells are permissive for replication, others are non-permissive. In non-permissive cells, the genes required for virus replication are not expressed, so latency ensues (virus infection in quiescent state). Simulation of non-permissive cells may allow replication (reactivation), so cells become permissive (the state of the cell determines whether it will be latent or lytic). eg HCMV - latent in myeloid progenitor, but lytic in macrophages, fibroblasts, endothelial and epithelial cells
Sites of herpes virus replication (permissive site) or latency (latent site)
a) HSV, VZV
b) HCMV
c) EBV
a) Permissive - mucosal or cutaneous epithelium. Latent - sensory neurons
b) Permissive - tissue macrophage, glandular epithelium. Latent - pre-monocyte
c) Permissive - activated B cell, oral epithelium. Latent - memory B cell
Portals of exit
a) Contaminated blood products (3)
b) Insect bites (1)
c) Through skin (7)
d) Alimentary canal (4)
e) Respiratory tract (6)
f) Saliva (5)
g) Genital tract (5)
h) Breast milk (1)
i) Placenta (3)
a) HBV, HCV, HIV
b) Yellow fever virus
c) HPV (6 & 11), HSV, VZV, measles virus, variola virus, monkeypox virus
d) Norovirus, rotavirus, hepatitis A virus, poliovirus
e) SARS-CoV-2, rhinovirus, influenza virus, measles virus, VZV, variola virus
f) HSV type 1, HCMV, EBV, mumps virus, rabies virus
g) HSV type 2, HIV, HPV (16 & 18), HBV (during birth), monkeypox virus
h) HCMV
i) HCMV, rubella virus, Zika virus
a) Factors affecting viral transmission
b) Factors affecting virion stability
a) Virion stability, duration of shedding, concentration of virus, availability of susceptible hosts
b) Enveloped viruses have low stability - i) the envelope is fragile ii) disruption of the membrane causes loss of infectivity iii) only spread by close contact. eg measles, influenza, HSV, VZV, SARS-CoV-2
Non-enveloped viruses have high stability
Faecal-oral route (poor hygiene/water) - poliovirus and hepatitis A virus, poliovirus is stable for long periods in water
Spread over long distances - eg FMDV
a) Factors affecting the duration of shedding
b) Factors affecting virus concentration
c) Factors affecting availability to susceptible hosts
a) i) Acute infection - short period of shedding ii) Persistent infections - chronic have continual shedding, latent have repeated shedding
b) i) Acute infection - shed at high concentrations (eg influenza, rotaviruses in watery diarrhoea) ii) Persistent infection - often lower concentrations compensated by the longer duration of shedding (eg herpesviruses)
c) i) Population size - more susceptible hosts mean easier transmission and virus survival, but below a threshold of hosts, virus will die out (eg measles, could have been kept below that threshold, but reduction in vaccine uptake meant enough susceptible hosts) ii) Availability of multiple host species - if the virus can infect multiple species it can survive more easily, or if there is an animal/insect reservoir (eg rabies virus, yellow fever virus)
a) Vertical trasnmission
b) Multiple transmission routes - HCMV
a) i) Congenital infection - across the placenta, infects foetus in utero (eg HCMV, rubella, Zika) ii) Perinatal - during birth or from breast milk (eg HSV - genital herpes in birth canal, HCMV - birth canal or breast milk, HBV - blood contact during birth) iii) Germ line transmission - endogenoes retroviruses, xenografts
b) Congenitally, perinatally, oral contact, sexual transmission, blood transfusion, organ transplantation. Regardless of the portal of entry, the virus is systemix and replicates at multiple portals of exit
