Arboviruses Flashcards
General characteristics - families of viruses, vector-virus associations
500 individual species with 6 families e.g. flaviviridae (japanese encephalitis, yellow fever, dengue)
Arthropod-borne virus:
- single virus transmitted by a few vectors; single vector transmits several different viruses
- biological, not mechanical transmission (specific virus-vector combinations)
e.g. Mosquitoes - dengue, zika, yellow fever, chikungunya
Ticks - tick-borne encephalitis, crimean-congo haemorrhagic fever
Mites - sandfly fever
Different amplifying hosts in virus life cycle
Human as amplifying host
- high asymptomatic ratio (virus requires humans to replicate and sustain transmission)
- transmission cycle not involving other species
- relies on high human and vector population density
- VECTOR CONTROL
Animal as amplifying host
- animal usually asymptomatic
- ANIMAL CONTROL
—> humans as accidental/dead end host (not for sustaining normal virus life cycle)
==> severe and fatal outcomes (human not needed for virus survival)
Factors affecting geographical distribution of viruses/disease
Vector population Climatology Animal reservoir Urbanisation Human travel --> movement of vector population = new people susceptible and infected --> outbreak risk
e.g. Japanese encephalitis common in SE Asia, St Louis encephalitis common in America
Clinical presentations and associated viruses
Non-specific febrile illness + rash, arthralgia, headache
e. g. Dengue, Chikungunya, Zika
- -> short incubation 3-7 days
- -> biphasic fever due to immune-mediated factors
- -> pain: frontal headache, orbital, back, bone, small joints, muscle
- -> skin rash (maculopapular, non-vesicular, pruritic)
- -> dyscrasia: WBC and Plt low
- -> usually self limiting
Encephalitis
e. g. Jpn encephalitis, West Nile, St Louis
- -> PAN-encephalitis; vomiting, confusion, general seizures
- -> high mortality
Hepatitis e.g. yellow fever
Haemorrhage (can be fatal)
e. g. Lassa, Dengue complication
- -> petechiae, epistaxis, mucosal bleeding, intracranial bleeding
Shock (can be fatal)
e. g. Dengue complication
- -> extravasation of protein into interstitial spaces
- -> associated (but not due to) blood loss
Management of arbovirus infection
Diagnosis: clinical + serology & CSF for IgM and PCR
Ribavirin
- useful for broad spectrum of arboviruses e.g. Lassa
- major side effect: haemolysis
Vaccine only available for:
- yellow fever, Jpn encephalitis, tick-borne encephalitis, dengue
Dengue fever - significance, geographical distribution, virus serotypes, transmission vector characteristics
Most important arbovirus human pathogen
Life threatening diseases: haemorrhagic fever, shock syndrome
Most prevalent in hot, warm and rainy countries (mosquitoes)
Locally: HK cases mostly imported; sporadic local cases detected in recent yrs (september mostly)
Dengue virus (flavivirus, flaviviridae)
- enveloped ssRNA
- 4 serotypes DEN-1/2/3/4 –> no cross protection between serotypes
Transmission:
- Vectors: Aedes aegypti (principal)/Aedes albopictus (secondary; more common in HK)
- -> domesticated mosquitoes - rest indoors; lay eggs in clean water in early morning/late afternoon; nervous feeders
- -> need to take several blood meals during each reproductive cycle (enough blood to lay eggs)
- human as AMPLIFYING HOST
Dengue fever: clinical course
1-2% symptomatic either:
- Non-specific febrile illness and rash (50%)
- Dengue fever syndrome –> with or without haemorrhage
- Dengue haemorrhagic fever (rare) –> no shock or dengue shock syndrome
Dengue fever: classical dengue fever
Adolescents and Adults Biphasic fever + 2 of the following: - frontal headache - retro-orbital pain - myalgia - arthralgia - rash - mild haemorrhagic manifestations e.g. petechiae
(others: increase liver enzymes, decrease Plt; nausea, vomiting)
Acute phase 3-7 days
Dengue fever: dengue haemorrhagic fever
Usually children <15 yrs old
Febrile phase: biphasic fever (initially like classic DF)
Appears to improve with normal/subnormal temperature but then sudden deterioration
High fever, haemorrhagic phenomena, hepatomegaly, circulatory failure –> dengue shock syndrome
Plasma leakage with increase Hct, serous effusion and hypoproteinaemia
Pathogenesis of Dengue haemorrhagic fever and dengue shock syndrome
Primary infection
- Ab produced are homotype neutralising e.g Type 1 Ab against Type virus
and
- heterotype-cross reactive, non neutralising e.g. Type 2/3/4 Ab
- CD4 T cells are create homotype specific memory and heterotype cross reactive memory as well
- CD8 T cells are homotype specific cytotoxicity
Secondary infection with heterotype
- cross-reactive Ab from first infection binds to virus BUT NOT NEUTRALISING
- Ag-Ab complex facilitates entry of virus into macrophage –> replication and disruption of macrophage
==> CYTOKINE STORM (TNF, IL1/2/6, C3a, C5a, histamine)
==> plasma leakage, coagulopathy, shock
(100-fold increase in viraemia)
Dengue fever: prevention
Vaccine
- has to protect all 4 serotypes or else artificially creating “1st infection” and increasing risk of DHF or DSS in secondary infection
- current vaccine only limited efficacy - only for countries with high prevalence
Vector control
- surveillance e.g. Larvitrap
- improve water supply and storage
- solid waste management
- chemical/biological control (e.g. repellent like DEET)
Japanese encephalitis - geographical areas, amplifying hosts, vectors, role of humans in life cycle, clinical features
SE Asia, India, southern Russia
Uncommon in HK
Amplifying host: pigs, water birds
Vectors: mosquitoes (Culex tritaeniorhynchus)
Humans as ACCIDENTAL HOST (insufficient viraemia to infect mosquitoes)
Clinical features:
- mostly asymptomatic
- 4-14 days incubation
- PRODROMAL phase (2-3 days): headache, fever, respiratory symptoms, nausea
- ACUTE phase (3-4 days): pan-encephalitis, high fever, convulsions, confusion, coma
- SUBACUTE phase (7-10 days): decrease CNS severity
- CONVALESCENCE (4-7 wks): weakness, incoordination, weight loss
30% fatality, 50% permanent neurological handicap
Japanese encephalitis - management
No effective antiviral available
Live/inactivated vaccine:
- universal immunisation in some countries
- last dose >10 days before departure for travellers
Vaccination for pigs in some countries
