Arbo

Question 0

Definition of arbovirus

Answer 0

= "Arthropod-borne" - usu mosquito
Defined by epidemiology, not taxonomy
Most important are toga or flavi
 - not all toga (rubella) or flavi (Hep C) are arbo
No direct human-human transmission

Small, enveloped, icosahedral, +RNA

All human are encephalitis!

• equine (East and West)
• St Louis
• West Nile
• California

Question 1

Phenotypic mixing

Answer 1

Coinfection -> virion with capsid A and genome B
Disappears after one generation (genome is unchanged)

vs antigenic shift - combination of genomes via reassortment of strands

Question 2

Arbovirus life cycle

Answer 2

Intrinsic incubation = infection -> viremia -> 1 week -> symptoms
Extrinsic incubation = 2 weeks -> infectious bites or transovarian to progeny mosquitoes

Question 3

Equine encephalitis (Eastern and Western)

Answer 3

Reservoir in birds -> mosquito population ->
Humans and horses die from encephalitis (destruction of CNS)
- both “dead end” hosts - not enough viremia to reinfect mosquitoes
- children most susceptible
Both togaviruses

Question 4

St Louis, West Nile

Answer 4

Flaviviruses
Encephalitis - elderly most susceptible
Humans = “dead end” hosts, bird reservoir

St Louis - endemic to US
West Nile - landed in US (mosquito in suitcase) -> now most common
Japanese - get a vaccine if you’re travelling

Question 5

Dengue fever

Answer 5

Flavivirus -> fever, severe h/a and joint pain - not fatal
- humans are not “dead end”

Dengue hemorrhagic

• second infection with different antigenic subtype (1-4)
• antibodies cross-react -> macrophages -> massive cytokine response -> bleeding -> death

Question 6

Yellow fever

Answer 6

Flavivirus
Replicates in endothelium -> viremia -> liver -> jaundice, nausea -> death (high mortality)
- incubation 7 days
Humans not “dead end” - risk for reintroduction by travellers (ex SE US)
Live attenuated vaccine 17D

Question 7

Maternal-child transmission

Answer 7

Perinatal = maternal blood/fluids
- resembles postnatal/horizontal disease
- Hep B, HIV, HSV
Transplacental - small particles can cross -> congenital abnormalities
- parvo B19, rubella, CMV, LCMV
- defenses = maternal IgG, fetal Ig and interferon after 4 months

Question 8

Parvovirus B19

Answer 8

Unenveloped, icosahedral, linear ssDNA
Respiratory -14 days> asymptomatic, “slapped cheek rash”, transient arthritis
- requires dividing cell -> bone marrow -> disrupts RBC production -> transient aplastic anemia if preexisting problem with production or destruction (Sickle Cell)
- treat with IGIg

Transplacental -> always fatal!
- via hydrops fetalis if trimesters 1,2

Question 9

Rubella

Answer 9

Togavirus but NOT arbo
Respiratory -18 days> viremia -> rash, transient arthritis
Shed for 1 week before and after rash, can be subclinical

Congenital = BAD
Live attenuated vaccine

Question 10

Congenital rubella syndrome

Answer 10

• > cataracts, heart defects (PDA), deafness, retardation
• infected cells in these areas grow more slowly?
• highest risk in first trimester
• fetus makes IgM -> still produces virions -> IgG after birth

Question 11

Rubella vaccine

Answer 11

Attenuated live, part of MMR
Children -> effective herd immunity (less infectious than measles)

Risks:
- unvaccinated children
- immigrant children and women
(can use vaccine in women -> wait 2 months before pregnancy to avoid congenital)