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Mosquito-borne ZoonosesCMED/EPI 526

Spring Quarter 2008

Outline

• WNV Outbreak – NYC• Cases• Descriptive Epidemiology – time,

place• Vectors-Hosts• Descriptive Epidemiology – person• Viral persistence• Surveillance• Prevention

Outbreak investigation• Background

– Aug 99 - Chief of infectious diseases atFlushing Hospital calls the Dept of Health inNYC regarding increase in encephalitiscases.

– Thinks it is botulism

Sample collection

• Samples collected from the hospital andsent to the Diagnostic lab and CDC

• Results - St. Louis Encephalitis (9/3)

• Sept 99 - NYChealth dept hearsabout large numbersof dead birds

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Animal investigation

• Mid-June 99- BaysideVeterinary Clinic findscrows with nervoussystem disorders, treatsand releases survivors

• Aug 99 - Many reports ofdead crows - samplessent to NYS Dept ofEnvironmentalConservation

Continued

• 8/16 - ProMED posting of crows beingpoisoned in NYC

• 8/23 - Birds sent to NVSL (Ames, IA)• 8/25 - Bird deaths at Bronx Zoo

– Veterinary pathologist notes similar lesionsin exotic zoo birds and crows

Chilean Flamingo

Crow

Is there a connection?

• 9/21 - Virus isolatedfrom crow and mosquitosame virus isolated fromhumans

• 9/25 - News breaks ofWest Nile Virus

Closing remarks on NYCoutbreak

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Year EEE WEE SLE CE2000 3 0 2 1122001 9 0 79 1342002 9 0 28 1672003 15 0 43 1132004 6 0 12 1122005 21 0 7 732006 8 0 7 64

Cases of Mosquito-borne Zoonoses by YearUnited States, 2000-2006

Surveillance for West Nile Virus, United States, 1999-2006

11,8984,1061,1214,2682006

11,3865,2661,1432,942422005

8,3717,3311,3862,47041*2004

7,84711,5975,1459,86246*20036,03315,74512,0384,15644*20029187,3387316626*20015154,305632111*200016?256241999MosquitoesBirdsHorsesHumansStatesYear

Humans & Horses – casesBirds – Dead birds positive for virusMosquitoes – mosquito pools positive for virus*Includes D.C.

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Spokane

San JuanIsland

Pierce

King

Snohomish

Skagit

Whatcom

Clallam

Jefferson

GraysHarbor

Thurston

Kitsap

Clark

Cowlitz

Lewis

Skamania

Wahkiakum

Mason

Yakima 6 WEE; 1 SLE

Chelan

Okanogan FerryStevens

PendOreille

Lincoln

Kittitas Whitman

Klickitat

BentonWallaWalla

ColumbiaGarfield

Asotin

Franklin

Adams

Grant

Douglas

Pacif ic

Reported Cases of Mosquito-borne Disease, WA1970-1982

Western Equine Encephalitis N=10St. Louis Encephalitis N= 2

Host and environmental factors to be considered in transmission of viruses by arthropod vectors

Vector/reservoir host factors:• Contact with critical host• Population indices of vector and hosts• Diurnal feeding habits of the vector• Susceptibility of and ability of the virus to replicate in the host (Intrinsic

incubation period – point from inoculation of virus by mosquito to point whenthere is sufficient titer of virus in blood to infect other mosquitoes)

• Host feeding preferences and longevity (influenced by season)

Vector/environmental factors:• Precipitation – increased (flooding) and decreased (drought)• Temperature• Vector competence (ability of vector to become infected with and transmit

virus). (Extrinsic incubation period – point from ingestion of blood meal topoint when salivary glands have sufficient titer of virus to enable efficienttransmission by mosquito) (influenced by temperature)

• Proximity of vectors/reservoirs to human populations (range of mosquitoes)

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Mosquitoes and Birds

• 284 different Birds– Corvids (crows, jays, magpies) are over

represented– House sparrow– Clinically found dead

• 59 types of Mosquitoes– Culex most often identified– C. pipiens most often

Vectors and reservoir hosts of arbovirusesVectors

Disease Reservoir Reservoir hosts HumansEEE Birds Culiseta melanura Aedes sollicitans

Aedes vexans

WEE Birds Culex tarsalis Culex tarsalis

SLE Rural Birds Culex tarsalis Culex tarsalis

Culex pipiens Culex pipiens

Urban Birds Culex pipiens Culex pipiensCulex nigripalpis Culex nigripalpisC. quinquefasciatus C. quinquefasciatus

WNV Birds Culex pipiens Culex pipiensCulex tarsalis Culex tarsalisC. quinquefasciatus C. quinquefasciatus

Culex restuans

CE Rodents Aedes triseriatus Aedes triseriatus

Pathogenicity of arboviruses in horses andhumans

Disease Horses HumansEEE +++ ++WEE ++ +WNV +++ ++SLE silent +CE silent ++

+=mild, ++=moderate, +++=marked

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Strategies of viral persistence innature

• Chronic infection of vertebrate hosts (birds)• Reintroduction of the virus by migratory birds• Persistence in hibernating adult mosquitoes• Vertical transmission in mosquitoes• Alternate arthropod vectors (ticks)

• Monitor weather conditions – temperature,humidity, precipitation

• Wild-bird population:• Moribund/death birds – test for virus• Healthy birds – test for antibodies

• Sentinel chicken flocks – test for antibodies• Mosquito collections – test for virus• Horses – encephalitis

Surveillance Activities

Strategies to prevent infectionswith arboviruses

Environment:Reduce habitats of mosquito productionScreen windows/doors

Mosquitoes:Larvicide applicationsAerial spraying (adulticide)Mosquito fish

Humans:Mosquito-avoidance behaviorsRestrict outdoor activity at dawn and duskWear long-length clothing when outdoorsUse mosquito repellant