Arboviruses
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- Created by: fionnualamaire94
- Created on: 29-11-16 17:36
Arbovirus general info
- extrinsic incubation period (EIP) - time taken from virus ingestion to subsequent transmissability
- arbovirus - arthropod-borne virus - virus transmitted by arthropods (mosquitos, ticks, biting flies)
- Cycle of infection:
- insect bites infected vertebrate host with sufficiently high viraemia to infect vector
- virus replicates within insect
- replication usually causes no harm to the insect vector and the virus disseminates thorugh its tissues and eventually reaches a location that can enable its onward transmission
- once titre of virus in salivary glands has reached sufficient level can be passed onto new vertebrate host when insect feeds again
- alternative route of vector infection is from transovarial transfer to subsequent generations
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Properties of arthropod vectors
- must be present in the environment where virus is circulating
- must feed upon host species that carries the virus
- must be susceptible to infection by bite-sized dose of virus
- must be able to replicate the virus in its salivary glands to levels that can be transmitted to new host
- particular arboviruses are often associated with a restricted range of insect vector hosts
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Arbovirus life cycles
- simplest cycle involves a primary vertebrate host species and a primary vector species -virus is transmitted in a continuous cycle between these hosts
- more promiscuous vectors may cross-feed on and infect other host species causing disease these secondary hosts may produce sufficient viraemia to contribute to cycle or may be dead-end hosts
- e.g. West Nile fever virus (WNFV) circulates in sylvatic cycle between passerine birds and Culex mosquitos
- Culex pipiens will cross-feed on humans and horses and cause disease
- both humans and horses are dead end hosts for WNFV
- e.g. Bluetongue virus (BTV) replicates in sheep and cows and is transmitted by Culicoides midges
- sheep get clinical bluetongue - cattle are usually inapparently infected
- infected cattle act as amplifier hosts - important maintenance reservoir
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Seasonality
- arbovirus transmission is dependent on the vector and consequently is sucject to vector population size and behaviour
- when vector numbers are low, trasmission rates fall
- vector population size is seasonal, being determined by rainfall and temperature
- temperature is also a critical parameter for virus replication
- lower the temp the lower the rate of replication - longer EIP
- at temperatures below 12C virus replication may cease altogether - eg in BTV
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Overwintering
- in temperate climates winter poses restriction on arbovirus distribution
- vector numbers/activity may be absent for parts of year - vector free period
- either side of the vector free period vector numbers/activity may be substantial but temperature conditions may reduce virus replication below threshold for transmission - transmission free period
- if duration of these periods exceeds time that virus can maintain itself in infected vertebrate host the virus will be eliminated before transmission cycle can restart
- in order for an arbovirus to persist in a temperate region it must have a mechanism for overwintering:
- transovarial transmission to subsequent generations of vector
- vector hibernation - migration to warmer areas eg barns
- longer duration/persistent infection of reservoir host
- transplacental transmission
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Control of arboviruses (general principles)
- control can be achieved by several approaches:
- identifying and isolating/eliminating infected hosts
- movement restrictions - except possibly in transmission free periods
- vaccination - to reduce susceptible host population size below level required for sustained transmission
- housing animals during peak activity periods
- insecticidal dripping
- reduce vector nubers by destroying habitats and using insceticides
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Impact of Climate Change
- global warming - will shorten overwintering periods, extend vector-active seasons, increase vector activity at night, increase vector numbers
- virus replication will increase - shortening EIP
- rapid growth of vector results in 'leaky guts' circumventing mesenteron escape barrier - allows previously non-vector species to become competent vectors and potentially increasing geographical and ecological range of virus
- climate instability may change rainfall patters
- more severe weather may blow vectors large distances enabling colonisation of new areas
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Bluetongue Virus
- transmitted by Culicoides midges
- C. imicola - classically
- C. obsoletus
- C. pulicaris
- C. dewulfi
- infects all known ruminant species
- principally a disease of sheep - usually causes inapparent infection in cattle and goats
- meat and dairy products pose no transmission hazard
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Geographic distribution of Bluetongue
- widely distributed over Africa, Asia, and the Americas
- present in Australia in cattle, has not yet caused disease in sheep
- periodically occured in Europe but did not usually persist
- increasingly common outbreaks in Italy, Yugoslavia, Turkey, Bulgaria, Greece and Corsica - outbreak in N. Europe in 2006 - UK 2007 - now eradicated
- expansion of C. imicola territory enabled BTV to establish itself in S and E Europe
- blown across from N Africa and overland via Turkey, Yugoslavia and Bulgaria
- disease spread beyond C. imicola's range as C. pulicaris and obsoletus are competent vectors and have more northerly range - baton effect
- route to N Europe unknown
- blown from N Europe to UK - aerial plankton
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Control of Bluetongue
- largely eliminated from Europe by widespread and effective vaccination policy
- policy applied to BTV8 outbreak in UK - v. successful
- protection zone covered whole of UK
- strongly encouraged but voluntary vaccination programme (33p-39p per ml dose - 1 for sheep 2 for cows)
- UK part of larger 'restricted zone' - much of continental Europe too - movement permitted within zone
- UK later declared lower risk zone - restrict importation from other zones - keep vaccinating
- vectors - removal of habitat and use of insecticides
- last confirmed case - November 2008
- UK declared BTV free since July 2011 - vaccination no longer permitted
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Overwintering of BTV
- no evidence of transovarial transmission
- infectious virus is detectable in blood for 5 weeks in sheep and 8 weeks in cattle - too short for effective over-wintering
- persistent infections of cattle are probably important for over-wintering
- possibly latent infection in gamma-delta T cells that harbour the virus over winter and are recruited to bite sites and re-express virus at the start of the next season
- virus detectable in blood by PCR for up to 14 weeks in sheep and 20 weeks in cattle
- transplacental feature is rare for wild strains but known for vaccine strains
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West Nile Fever Virus
- circulates in passerine birds
- transmitted by Culex pipiens mosquitoes
- cross-feeding transmission to horses and humans
- can cause severe neurological infection - esp in horses
- rapidly lethal to corvids - act as heralds of the arrival of WNFV to a new location
- viraemia in birds is usually short-lived
- virus overwinters by:
- transovarial transmission
- vector hibernation
- long-lived vectors eg ticks
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Geography/Control WNFV
- pre-1999 WNFV was absent from USA
- introduction through unknown means led to extremely rapid spread across entire continental USA
- large number of competent vectors
- virus adapted and expanded its range of vectors
- control - now endemic in USA
- controlled in horses in USA
- vaccination
- exposure avoidance
- insect repellents
- WNFV infects birds in UK - no evidence of disease in horses or humans in UK
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