1. Factors Influencing United States
West Nile Virus Prevalence in the
United States
By: Matthew Weik
GEOG 510: Seminar in Physical Geography:
Date: April 1, 2014

2.  -West Nile as a Virus
 -Impacts of WNV
 -Species Responsible for WNV
 -Factors Effecting WNV
 -WNV Outbreaks
 -Conclusion
Outline

3. The West Nile Virion
 Genus:Flavivirus ; Family: Flaviviridea
 E dimers interact with pits on cell membrane
 Inserts Genomic RNA
 Transcribed within ER
 Modified in Golgi-body
 Then assembled and exits cell
 Replication begins very slowly and rapidly picks up

4.  Human:
 31,365 illnesses and 1,250 deaths
 West Nile Fever
 Months of depression, altered moods, headaches, and fatigue
 West Nile Neurological Disease
 Meningitis, encephalitis, and acute flaccid paralysis (leading to incomplete recovery)
 Economic:
 Average of $40,000+, is the cost per an individual suffering from WNV
 CDC given $2.7 million to aid states afflicted worse in 2000
 Later increased to $20 million over following years
 Avian:
 American Crow (Corvus brachyrhunchos) has nearly 100% mortality from WNV
 Population had trend of decline starting 1999, with a dramatic drop by 2004
 Decreased abundance occurred where forest was less than 36% and urban was greater than 11%
Impacts of West Nile Virus

7.  Mosquito (vector): Culex spp.
 Avian (host):
 American Crows
 American Robins
 House Finches
Species Responsible for WNV
Prevalence

8.  Cx. Pipiens and Cx. Restuans
 Pipiens: commonly known as northern house mosquito
 Preference for larval habitat is Urban: storm drains and catch-
drainage basins
 Negatively impacted by large rain events
 Restuans: commonly known as white-spotted mosquito
 Also prefers more urban than rural larval habitat; known to makes
use of untreated swimming pools, and other foul water areas.
 Cx. Tarsalis
 Prevalent among in western U.S., and North Great Plains
 Preference for rural larval habitat: irrigated fields, puddles in tire
tracks, etc…
 Positively impacted by large rain events; increasing larval habitat
 Cx. Quinquefasciatus
 Primary WNV vector in southern U.S.
 Seems to prefer urban larval habitat
 The effect of land cover, temperature, and humidity on its
population are lacking
Culex spp.

9.  American Crows
 Competent vector, but high mortality rates
effect abundance in long term.
 Poor hosts for sustaining WNV in an area
 Close relationship with human infection cases:
 41% of human population lived within clusters
of infected crows; however 75% of human WNV
cases came from within these clusters.
 American Robins and House Finches
 Wide range and large population: resulting
from fragmented forests, intensive agriculture,
and increased urbanization
 Easily became primary reservoir hosts of WNV
infections in the U.S.
Aves

10.  Climate
 Land Use
 Hydrogeographic Area
 Anthropogenic Factors
Factors Effecting WNV

11.  In urban areas around Chicago, Illinois, temperature and precipitation accounted
for 79% variation of WNV infections in mosquitoes
 Thought to alter prevalence of vector borne disease three ways:
 Distribution and abundance of mosquitoes through reproduction, development and
survival
 Blood seeking activity of mosquitoes
 Influencing the rate of viral amplification within mosquitoes
 Temperature has a direct impact
 Increasing temperatures speed up and increase abundance of both WNV virions and
mosquitoes
 Precipitation has a complex interaction
 Larval populations are altered differently, depending on vector species and location,
through rain events.
Climate

12.  WNV amplification within mosquitoes:
 Lowest threshold temperature is 14.3 degrees Celsius
 Disease activity occurs mainly in summer months
 Larval development
 High temperature extremes cause sudden death; moderately high temperatures cause thermal wounding
 Low temperatures cause changes in cell integrity, morphogenesis of tissue, reproduction, and the sex ratio of
adult mosquitoes
 Temperature range for larval development: 19-31 degrees C
 19 degrees C produces larger mosquitoes, in high numbers
 25 degrees C was highest optimal temperature: producing smaller sized mosquitoes in less numbers
 31 degrees C produced the smallest mosquitoes and saw a increased mortality amongst immature stages
 Gonotrophic cycle of adult mosquitoes
 Feeding and egg laying cycle of female mosquitoes
 Increased temperatures shorten time between females taking blood meals and when they oviposit over water sources
 Therefore increases rate at which cycles are repeated
Temperature (Climate)

13.  Abundance of mosquitoes is limited by the amount of precipitation an area receives
 Tied to Culex spp., their geographic ranges, and land uses they are inhabiting
 Cx. pipiens, primary vector above 36 degrees latitude and east of Mississippi River, have their
typical larval habitat (urban catch basins) flushed out during heavy rain events
 Cx. tarsalis, primary rural vector west of the Mississippi River, heavy rainfall is beneficial; supplying
females with larval habitats for ovipositing
 Cx. nigripaplpus (specific WNV in Florida) showed increase WNV amplification after early season
drought, combined with subsequent wetting and decreased water table depth
 Drought conditions and diminished precipitation have an effect on human infection cases
 Within semi-permanent wetlands increased larval breeding sites, which contain fewer predators
of mosquitoes and competitors with larvae for organic material
 In Northeast passerine birds, the preferred blood meal of Culex mosquitoes, move farther out of
range in search for water sources
 Forces infected mosquitoes to feed on alternative, or accidental, hosts: humans, equines, cows, and
other mammals
Precipitation (Climate)

16.  On a county-wide scale human infections of WNV persist in areas of increased urbanization and
irrigated agricultural land
 Northern Great Plains showed a strong relationship between orchard acreage at both landscape
and local scales, while irrigated land was only at the landscape scale
 Henrico County, Virginia:
 Study by Deichmeister et al, 2010, assessed ability of urban, suburban, and rural to produce WNV
infected mosquitoes
 Looked at area of canopy cover, building footprint, and drainage basins as predictors of vector
abundance
 Urban: high density residential, commercial, and industrial presence
 Produced a greater proportion of infected mosquitoes, as predicted being both located in eastern U.S.
and containing primarily urban species of Culex mosquitoes
 Canopy cover and building footprint were poor predictors, while storm/water drainage basins was a
good predictor
 Suburban: low density residential, higher proportion of public/ open lots, and few dispersed
commercial/industrial areas
 Rural was not mentioned further
Land Use

17.  Defined as fixed water features within a landscape
 Very important at micro-scale
 Walsh et al, 2012, looked at relationship between hydrogeographic area and WNV abundance in the
state of New York
 Modeled that one square kilometer increase in hydrogeographic area lead to 2% decrease in risk of
WNV;
 Found to be significant independently of both temperature and precipitation
 Primary vectors for this region, Cx. pipiens and Cx. ruetans, prefers urban areas.
 Ponds and lakes, largest contributor to hydrogeographic area, are typically located in rural areas.
 Relationship has only been observed for New York region, and cannot be assumed for other parts of
U.S.
 However the opposite effect has also been found:
 Decreases in water table levels are thought to fragment habitat for larval mosquitos; providing closer
congregations between adult blood feeding mosquitoes and competent birds hosts.
Hydrogeographic Area

18.  Things humans do to increase infections and outbreaks, such as changes in outdoor activity and exposure to
mosquito populations
 Hot summer seasons:
 Changes in dress increase skin exposure to blood feeding mosquitoes
 Outdoor work and recreational activities become put off until cooler evening temperatures; which are host-seeking
time for mosquitoes
 Regions with infrequent warm spells:
 Lack of air conditioning forces residents to leave windows open, allowing individuals indoors to be exposed to
mosquitoes
 1999, the invading year of WNV, was a hot year in New York:
 Many residents were sleeping outside, letting water gather for gardens and plants, and allowing swimming pools to
lay still
 Leaving perfect conditions for blood feeding females and sustaining larval habitats
 Possibly made it easier for WNV outbreak to reach the level it did.
 Increased television viewing and use of air conditioning helps to limit WNV exposure by keeping people indoors
and away from mosquitoes
Anthropogenic Factors

19.  Initial outbreak of WNV in 1999 managed to overwinter and become endemic within a matter of
years
 Unexpected and unprecedented
 In 2003 an outbreak occurred, but in Northern Great Plains region
 Attributed to the immunological inexperience of birds and humans with WNV in its expanding range
 2007, outbreaks effected Northern Great Plains and spread cases west of the Mississippi River
 Massive increases in larval habitat were examined with satellite imagery in abandoned and
unmaintained swimming pools left behind from the 2006 economic crisis in Bakersfield, California
 2012, second largest number of cases reported (over 5,000)
 Epicenter was Dallas, Texas
 Occurred after four years of extremely low number of cases across U.S.
 Two possible factors contributing to major outbreaks of WNV
 The previous year’s precipitation
 Avian flock immunity
WNV Outbreaks

20.  Drought is suggested to have larger impact on predators and
competitors than on mosquitoes themselves
 Allows mosquito abundance human infection cases to increase
the subsequent year
 Possibility of larval habitats from growing season and
overwintering mosquitoes initiate outbreaks in eastern U.S.
 Positive correlations between previous year’s precipitation and
WNV incidences empirically support this idea
Previous Year Precipitation

21.  Developed Immunity and natural turnover rate drive the ability for WNV to establish a reservoir among avian species
 American Robins and House Finches do not see same mortality rates as American Crows
 Surviving an acute infection allows host to be immune
 The WNV virion is still present and still able to infect blood feeding mosquitoes
 Viral RNA has been detected in kidneys and spleen tissue in immune birds; found to persist in body for up to 8 months
 Drove the initial 3 year cycle that WNV initially showed:
 A hidden invasion year
 Large amplification, up to outbreak levels, in year 2
 Subsidence in third year when flock immunity has been accrued by host populations
 After first several outbreaks flock immunity increased, while crow populations decreased, allowing for a 4 to 5 year period of low incidences
 Immunity can be passed to children from previously infected birds
 Decreasing the rate of new infections and new immunities building against WNV
 Natural turnover rate of bird species encourages large outbreaks
 The birds that have accrued immunity themselves or from parents die of natural causes
 Diluted flock immunity levels within the host populations of bird hosts allowed WNV to have major resurgence, causing the 2012 outbreak.
 Larvicides and adulticides have different effect on flock immunity levels
 Los Angeles, CA, chooses public education and enhanced larvicides (chemicals used to kill larval in water habitats)
 Allows flock immunity of avian host to increase, preventing viral amplification long term
 Sacramento, CA, chooses aerial adulticides (chemicals used to kill adult mosquitoes through the air)
 Interrupts transmission between adult mosquitoes and hosts all together, flock immunity drops, and virus amplification is possible in later years
Avian Flock Immunity

22.  West Nile virus has complex interactions between climate, land use, hydrogeographic area, and anthropogenic factors
 WNV abundance has caused over 35,000 human cases of disease since 1999
 Increased temperatures increase amplification and replication of viral RNA, development of larvae into mosquitoes, and rate of
gonotrophic cycle of female mosquitoes
 Increased precipitation increases amplification in eastern U.S., unless heavy rainfall is involved, while low levels of precipitation increase
amplification in western and rural northern regions; where heavy rainfalls are found to be beneficial
 Urban and rural areas are dominated by different host vectors
 Cx. pipiens and Cx. ruetans prefer urban, water basin drainage, areas
 Cx. tarsalis prefer irrigated agricultural, especially orchards, rural areas
 Hydrogeographic area was seen as decreasing WNV abundance, but only in the specific northeastern region, New York.
 Anthropogenic factors: wearing less clothing, waiting till evening for outside activities, and keeping windows open where air conditioning
is not commonplace leads to enhanced human infection cases
 Previous year’s precipitation and Avian flock immunity levels are thought to be major factors contributing to WNV outbreaks, and explain
the gap between 2007 and 2012 outbreaks
 WNV is an arboviral nuisance, which is predicted to stay endemic to the U.S., possibly forever. Requiring constant surveillance to fully
prevent and understand outbreaks
Conclusion

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