West Nile virus most often occurs in Iowa’s western counties

0
413
Average annual incidence of West Nile virus neuroinvasive disease reported to CDC by county, 1999-2017

A recently published study from Iowa State University medical entomologists found transmission of West Nile virus most often occurs in Iowa’s western counties, where the data also found the heaviest concentrations of the mosquito species most often identified as the vector of disease transmission.

The study, published this week in the academic journal Scientific Reports, drew on 15 years of data collected by mosquito surveillance efforts led by the ISU Medical Entomology Laboratory.

The research found the species Culex tarsalis, which is most often associated with transmission of West Nile, is more abundant in Iowa’s western-most counties.

These findings correlate with human cases of West Nile virus, which also occur at higher rates in western Iowa than in other parts of the state, said Ryan Smith, assistant professor of entomology and director of the medical entomology laboratory.

The study also examined the intensity of West Nile virus infection in humans and mosquitoes in Iowa over the last 15 years, which peaks in early September.

An ongoing concern

Smith said the study should reinforce for Iowans that West Nile remains a real concern in the state, with transmission of the disease occurring every year.

“This is the most comprehensive study of West Nile virus transmission in the state since the virus was first introduced here,” Smith said.

“It’s a reminder to Iowans that this virus is here and probably isn’t going away anytime soon.”

West Nile virus is the leading cause of mosquito-borne disease in the United States, according to the Centers for Disease Control. Infected mosquitos transmit the disease to humans through their bite, and one in five infected humans develop symptoms, such as a fever. About one in 150 cases become serious and potentially fatal, according to the CDC.

The number of human cases in Iowa fluctuates every year , Smith said.

While this study is an important step in understanding when and where West Nile virus is transmitted, researchers are still working to determine the factors that influence yearly transmission rates, he said.

The study analyzed data collected by the mosquito surveillance program led by the ISU Medical Entomology Laboratory between 2002 and 2016.

The laboratory conducts yearly surveillance of mosquito populations, using a network of traps across the state.

Each trap is regularly emptied and the mosquitoes inside are catalogued.

Some are tested for the presence of the virus.

Smith said nuisance species, such as Aedes vexans, are the most common mosquitoes in Iowa.

But the species most likely to carry the virus are Culex pipiens and Culex tarsalis.

These species are most active around dusk and dawn during the summer and early fall months. Smith said the lab’s research showed Culex tarsalis tends to feed on humans more often than other Culex species, making it a prime suspect in West Nile transmission.

Counties along Missouri River at higher risk

The study shows counties in western Iowa, particularly those along the Missouri River, are at a higher risk of West Nile virus infections due to the increased abundance of Culex tarsalis populations in this region of the state.

The findings mirror similar surveillance studies in Nebraska and South Dakota, which also implicate Culex tarsalis in the majority of West Nile virus cases.

Smith said Iowa is positioned along a transition zone for North American mosquito populations, with Culex tarsalis more abundant in Nebraska, South Dakota and western Iowa and becoming less abundant in central and eastern Iowa.

“The research gives a strong indication that this particular mosquito species, Culex tarsalis, is likely most often responsible for the West Nile transmission in the state,” he said.

“However, other regions of the state are still at risk of transmission from other Culex species.”

Smith recommended Iowans use insect repellent with DEET if they intend to be outside for an extended period of time during the summer months.

He also recommended Iowans stay indoors at dusk and dawn, when Culex mosquitos are most likely to feed.


West Nile virus disease cases reported to CDC by state of residence, 1999-2017

State1999200020012002200320042005200620072008200920102011201220132014201520162017Total
Alabama0024937161082418035629291960333
Alaska00000000000000000000
Arizona00001339111315097114201676913362107103781111,728
Arkansas000432528282920967164181118918334
California000137798802783804451121111584793798017834425536,584
Colorado000142,94729110634557671103817131322118101149685,430
Connecticut016171716948011921461013134
Delaware0001170201100193060142
Dist. of Columbia0003432520826151013514101
Florida00122894412133331224737171385367
Georgia0064450212085084132299101315648437
Hawaii00000000000000010001
Idaho00001313996132393813174019139251,349
Illinois000884546025221510120561342901174477154902,458
Indiana000293471323802444139772310211826685
Iowa00054147233737306599314415143712510
Kansas0002291432530403113194569154343727617
Kentucky00075147564333523312810172
Louisiana001329124109171180404921271033554125559531,692
Maine00000000000001001002
Maryland006367316511101412319471664566340
Massachusetts003231706361076338610176152
Michigan000614191662551717129342023611843401,204
Minnesota000481483445651011048270792198330757
Mississippi000192875170183136655385224745433843631,376
Missouri0001686436306277155310202913291119591
Montana000222262534202550163853611571
Nebraska0001521,94253188264163475239291932261426895683,721
Nevada000024431124121612216911371667372
New Hampshire00003000000101100017
New Jersey0612243416511033074812826118252
New Mexico000020988338608825447382414633605
New York621415827110382422467128441073226572251858
North Carolina0002243418300273042871
North Dakota0001761720861373693719489125232385621,704
Ohio000441108126148231525211212411351734978
Oklahoma0002179223148107910111918918893542793
Oregon0000037692616110011168146178
Pennsylvania003622371525910140286601113301620559
Puerto Rico00000000000001000001
Rhode Island000170101100141002221
South Carolina00016251513102973081890
South Dakota000371,0395122911320839212022031495740152732,433
Tennessee0005626141822111994183324168730315
Texas0002027201761953542606411589271,8681833792753701355,412
Utah000011152158702622357281362422
Vermont00013000000 0132002315
Virginia0002926515515593067 21813176
Washington00000003033820411224913109
West Virginia0003200101002101001122
Wisconsin00052171217211381235721691351303
Wyoming00023751012651818126374158117753
Total6221664,1569,8622,5393,0004,2693,6301,3567201,0217125,6742,4692,2052,1752,1492,09748,183

Print only version Cdc-pdf[PDF – 2 pages]T

West Nile virus neuroinvasive disease cases reported to CDC by state of residence, 1999-2017

State1999200020012002200320042005200620072008200920102011201220132014201520162017Total
Alabama002342515681711015383051340223
Alaska00000000000000000000
Arizona000072155268506212107498750806757981,061
Arkansas00032231713241376614416916815250
California000122913058115429267721102972375615853354013,791
Colorado00066214121669917362626290465759291,278
Connecticut006101204725078121381288
Delaware0000120101000123000020
Dist. of Columbia00014313004231080131154
Florida0012286133103332920525121264275
Georgia0062827149223444144641113444257
Hawaii00000000000000000000
Idaho000001313911490151465316217
Illinois000553302913712763125452218786365198721,553
Indiana000180158112714326746199161518396
Iowa00027811314221230551124541610252
Kansas00022891817171414444203418121812317
Kentucky0005311155433241310159120
Louisiana001204101851179127181020615534614138381,047
Maine00000000000001001002
Maryland0062249104106601710251163165224
Massachusetts00319120423106525757105114
Michigan000557141354431611125321412411642321,022
Minnesota00016481318314421413431633813303
Mississippi000162343139895022313311032726252746746
Missouri00012639271751611243617241023917446
Montana00027528123702011102333161
Nebraska0008419475545217111014425441193519658
Nevada0000225143429701258341331169
New Hampshire00002000000101100005
New Jersey051216211321631522210623116165
New Mexico00007432203395621424241912623312
New York591413685773016163268928611819422045640
North Carolina0002163214200273042856
North Dakota00039421220492021396412102420354
Ohio0003108411463613140410762110231223693
Oklahoma0001656161727594811103609492134481
Oregon000000177310007702338
Pennsylvania0034214510148512019533611171215357
Puerto Rico00000000000001000001
Rhode Island000150100100121002115
South Carolina00013051303102033061665
South Dakota00017151636384811640625712113527521
Tennessee000472113151651242161917125322229
Texas00020243111912823317040937720844113253196252873,258
Utah00000621562861113415739179
Vermont00000000000011100227
Virginia000161950030548206513612122
Washington00000000022610407881066
West Virginia000310010100251000115
Wisconsin00022751111741024417361047197
Wyoming00009226152304213160384179
Total5919642,9462,8661,1481,3091,4951,2276893866294862,8731,2671,3471,4551,3091,42522,999

Print only version Cdc-pdf[PDF – 2 pages]T

West Nile virus disease cases and deaths reported to CDC by year and clinical presentation, 1999-2017

Neuroinvasive diseaseNon-neuroinvasive diseaseTotal
YearCases
No.
Deaths
No. (%)
Cases
No.
Deaths
No. (%)
Cases
No.
Deaths
No. (%)
1999597  (12)30  (0)627  (11)
2000192  (11)20  (0)212  (10)
 2001 64 10  (16) 2 0  (0) 66 10  (15)
 2002 2,946276  (9) 1,210 8  (1) 4,156 284  (7)
20032,866232  (8)6,99632  (<1)9,862264  (3)
20041,14894  (8)1,3916  (<1)2,539100  (4)
20051,309104  (8)1,69115  (1)3,000119  (4)
20061,495162  (11)2,77415  (1)4,269177 (4)
20071,227117  (10)2,4037  (<1)3,630124  (3)
200868941  (6)6673  (<1)1,35644  (3)
200938632  (8)3340  (0)72032  (4)
201062954  (9)3923  (1)1,02157  (6)
201148642  (9)2261  (<1)71243  (6)
 2012 2,873270  (9) 2,801 16  (1) 5,674286  (5)
 2013 1,267111  (9) 1,202 8  (<1) 2,469119  (5)
 2014 1,34787  (6) 858 10  (1) 2,20597  (4)
 2015 1,455142  (10) 720 4  (<1) 2,175146  (7)
2016 1,309105  (8) 840 1  (<1) 2,149106  (5)
20171,425146  (10)6720  (0)2,097146  (7)
Total 22,9992,034  (9)  25,184 129  (<1) 48,1832,163  (4) 

Source: ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention


More information: Brendan M. Dunphy et al, Long-term surveillance defines spatial and temporal patterns implicating Culex tarsalis as the primary vector of West Nile virus, Scientific Reports (2019). DOI: 10.1038/s41598-019-43246-y
Journal information: Scientific Reports
Provided by Iowa State University

LEAVE A REPLY

Please enter your comment!
Please enter your name here

Questo sito usa Akismet per ridurre lo spam. Scopri come i tuoi dati vengono elaborati.