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

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.

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

State	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017	Total
Alabama	0	0	2	49	37	16	10	8	24	18	0	3	5	62	9	2	9	19	60	333
Alaska	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Arizona	0	0	0	0	13	391	113	150	97	114	20	167	69	133	62	107	103	78	111	1,728
Arkansas	0	0	0	43	25	28	28	29	20	9	6	7	1	64	18	11	18	9	18	334
California	0	0	0	1	3	779	880	278	380	445	112	111	158	479	379	801	783	442	553	6,584
Colorado	0	0	0	14	2,947	291	106	345	576	71	103	81	7	131	322	118	101	149	68	5,430
Connecticut	0	1	6	17	17	1	6	9	4	8	0	11	9	21	4	6	10	1	3	134
Delaware	0	0	0	1	17	0	2	0	1	1	0	0	1	9	3	0	6	0	1	42
Dist. of Columbia	0	0	0	34	3	2	5	2	0	8	2	6	15	10	1	3	5	1	4	101
Florida	0	0	12	28	94	41	21	3	3	3	3	12	24	73	7	17	13	8	5	367
Georgia	0	0	6	44	50	21	20	8	50	8	4	13	22	99	10	13	15	6	48	437
Hawaii	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	0	0	0	1
Idaho	0	0	0	0	1	3	13	996	132	39	38	1	3	17	40	19	13	9	25	1,349
Illinois	0	0	0	884	54	60	252	215	101	20	5	61	34	290	117	44	77	154	90	2,458
Indiana	0	0	0	293	47	13	23	80	24	4	4	13	9	77	23	10	21	18	26	685
Iowa	0	0	0	54	147	23	37	37	30	6	5	9	9	31	44	15	14	37	12	510
Kansas	0	0	0	22	91	43	25	30	40	31	13	19	4	56	91	54	34	37	27	617
Kentucky	0	0	0	75	14	7	5	6	4	3	3	3	5	23	3	1	2	8	10	172
Louisiana	0	0	1	329	124	109	171	180	40	49	21	27	10	335	54	125	5	59	53	1,692
Maine	0	0	0	0	0	0	0	0	0	0	0	0	0	1	0	0	1	0	0	2
Maryland	0	0	6	36	73	16	5	11	10	14	1	23	19	47	16	6	45	6	6	340
Massachusetts	0	0	3	23	17	0	6	3	6	1	0	7	6	33	8	6	10	17	6	152
Michigan	0	0	0	614	19	16	62	55	17	17	1	29	34	202	36	1	18	43	40	1,204
Minnesota	0	0	0	48	148	34	45	65	101	10	4	8	2	70	79	21	9	83	30	757
Mississippi	0	0	0	192	87	51	70	183	136	65	53	8	52	247	45	43	38	43	63	1,376
Missouri	0	0	0	168	64	36	30	62	77	15	5	3	10	20	29	13	29	11	19	591
Montana	0	0	0	2	222	6	25	34	202	5	5	0	1	6	38	5	3	6	11	571
Nebraska	0	0	0	152	1,942	53	188	264	163	47	52	39	29	193	226	142	68	95	68	3,721
Nevada	0	0	0	0	2	44	31	124	12	16	12	2	16	9	11	3	7	16	67	372
New Hampshire	0	0	0	0	3	0	0	0	0	0	0	1	0	1	1	0	0	0	1	7
New Jersey	0	6	12	24	34	1	6	5	1	10	3	30	7	48	12	8	26	11	8	252
New Mexico	0	0	0	0	209	88	33	8	60	8	8	25	4	47	38	24	14	6	33	605
New York	62	14	15	82	71	10	38	24	22	46	7	128	44	107	32	26	57	22	51	858
North Carolina	0	0	0	2	24	3	4	1	8	3	0	0	2	7	3	0	4	2	8	71
North Dakota	0	0	0	17	617	20	86	137	369	37	1	9	4	89	125	23	23	85	62	1,704
Ohio	0	0	0	441	108	12	61	48	23	15	2	5	21	121	24	11	35	17	34	978
Oklahoma	0	0	0	21	79	22	31	48	107	9	10	1	1	191	89	18	89	35	42	793
Oregon	0	0	0	0	0	3	7	69	26	16	11	0	0	11	16	8	1	4	6	178
Pennsylvania	0	0	3	62	237	15	25	9	10	14	0	28	6	60	11	13	30	16	20	559
Puerto Rico	0	0	0	0	0	0	0	0	0	0	0	0	0	1	0	0	0	0	0	1
Rhode Island	0	0	0	1	7	0	1	0	1	1	0	0	1	4	1	0	0	2	2	21
South Carolina	0	0	0	1	6	2	5	1	5	1	3	1	0	29	7	3	0	8	18	90
South Dakota	0	0	0	37	1,039	51	229	113	208	39	21	20	2	203	149	57	40	152	73	2,433
Tennessee	0	0	0	56	26	14	18	22	11	19	9	4	18	33	24	16	8	7	30	315
Texas	0	0	0	202	720	176	195	354	260	64	115	89	27	1,868	183	379	275	370	135	5,412
Utah	0	0	0	0	1	11	52	158	70	26	2	2	3	5	7	2	8	13	62	422
Vermont	0	0	0	1	3	0	0	0	0	0	0	0	1	3	2	0	0	2	3	15
Virginia	0	0	0	29	26	5	1	5	5	1	5	5	9	30	6	7	21	8	13	176
Washington	0	0	0	0	0	0	0	3	0	3	38	2	0	4	1	12	24	9	13	109
West Virginia	0	0	0	3	2	0	0	1	0	1	0	0	2	10	1	0	0	1	1	22
Wisconsin	0	0	0	52	17	12	17	21	13	8	1	2	3	57	21	6	9	13	51	303
Wyoming	0	0	0	2	375	10	12	65	181	8	12	6	3	7	41	5	8	11	7	753
Total	62	21	66	4,156	9,862	2,539	3,000	4,269	3,630	1,356	720	1,021	712	5,674	2,469	2,205	2,175	2,149	2,097	48,183

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

State	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017	Total
Alabama	0	0	2	34	25	15	6	8	17	11	0	1	5	38	3	0	5	13	40	223
Alaska	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Arizona	0	0	0	0	7	215	52	68	50	62	12	107	49	87	50	80	67	57	98	1,061
Arkansas	0	0	0	32	23	17	13	24	13	7	6	6	1	44	16	9	16	8	15	250
California	0	0	0	1	2	291	305	81	154	292	67	72	110	297	237	561	585	335	401	3,791
Colorado	0	0	0	6	621	41	21	66	99	17	36	26	2	62	90	46	57	59	29	1,278
Connecticut	0	0	6	10	12	0	4	7	2	5	0	7	8	12	1	3	8	1	2	88
Delaware	0	0	0	0	12	0	1	0	1	0	0	0	1	2	3	0	0	0	0	20
Dist. of Columbia	0	0	0	14	3	1	3	0	0	4	2	3	10	8	0	1	3	1	1	54
Florida	0	0	12	28	61	33	10	3	3	3	2	9	20	52	5	12	12	6	4	275
Georgia	0	0	6	28	27	14	9	2	23	4	4	4	14	46	4	11	13	4	44	257
Hawaii	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Idaho	0	0	0	0	0	1	3	139	11	4	9	0	1	5	14	6	5	3	16	217
Illinois	0	0	0	553	30	29	137	127	63	12	5	45	22	187	86	36	51	98	72	1,553
Indiana	0	0	0	180	15	8	11	27	14	3	2	6	7	46	19	9	16	15	18	396
Iowa	0	0	0	27	81	13	14	22	12	3	0	5	5	11	24	5	4	16	10	252
Kansas	0	0	0	22	89	18	17	17	14	14	4	4	4	20	34	18	12	18	12	317
Kentucky	0	0	0	53	11	1	5	5	4	3	3	2	4	13	1	0	1	5	9	120
Louisiana	0	0	1	204	101	85	117	91	27	18	10	20	6	155	34	61	41	38	38	1,047
Maine	0	0	0	0	0	0	0	0	0	0	0	0	0	1	0	0	1	0	0	2
Maryland	0	0	6	22	49	10	4	10	6	6	0	17	10	25	11	6	31	6	5	224
Massachusetts	0	0	3	19	12	0	4	2	3	1	0	6	5	25	7	5	7	10	5	114
Michigan	0	0	0	557	14	13	54	43	16	11	1	25	32	141	24	1	16	42	32	1,022
Minnesota	0	0	0	16	48	13	18	31	44	2	1	4	1	34	31	6	3	38	13	303
Mississippi	0	0	0	162	34	31	39	89	50	22	31	3	31	103	27	26	25	27	46	746
Missouri	0	0	0	126	39	27	17	51	61	12	4	3	6	17	24	10	23	9	17	446
Montana	0	0	0	2	75	2	8	12	37	0	2	0	1	1	10	2	3	3	3	161
Nebraska	0	0	0	84	194	7	55	45	21	7	11	10	14	42	54	41	19	35	19	658
Nevada	0	0	0	0	2	25	14	34	2	9	7	0	12	5	8	3	4	13	31	169
New Hampshire	0	0	0	0	2	0	0	0	0	0	0	1	0	1	1	0	0	0	0	5
New Jersey	0	5	12	16	21	1	3	2	1	6	3	15	2	22	10	6	23	11	6	165
New Mexico	0	0	0	0	74	32	20	3	39	5	6	21	4	24	24	19	12	6	23	312
New York	59	14	13	68	57	7	30	16	16	32	6	89	28	61	18	19	42	20	45	640
North Carolina	0	0	0	2	16	3	2	1	4	2	0	0	2	7	3	0	4	2	8	56
North Dakota	0	0	0	3	94	2	12	20	49	2	0	2	1	39	64	12	10	24	20	354
Ohio	0	0	0	310	84	11	46	36	13	14	0	4	10	76	21	10	23	12	23	693
Oklahoma	0	0	0	16	56	16	17	27	59	4	8	1	1	103	60	9	49	21	34	481
Oregon	0	0	0	0	0	0	1	7	7	3	1	0	0	0	7	7	0	2	3	38
Pennsylvania	0	0	3	42	145	10	14	8	5	12	0	19	5	33	6	11	17	12	15	357
Puerto Rico	0	0	0	0	0	0	0	0	0	0	0	0	0	1	0	0	0	0	0	1
Rhode Island	0	0	0	1	5	0	1	0	0	1	0	0	1	2	1	0	0	2	1	15
South Carolina	0	0	0	1	3	0	5	1	3	0	3	1	0	20	3	3	0	6	16	65
South Dakota	0	0	0	17	151	6	36	38	48	11	6	4	0	62	57	12	11	35	27	521
Tennessee	0	0	0	47	21	13	15	16	5	12	4	2	16	19	17	12	5	3	22	229
Texas	0	0	0	202	431	119	128	233	170	40	93	77	20	844	113	253	196	252	87	3,258
Utah	0	0	0	0	0	6	21	56	28	6	1	1	1	3	4	1	5	7	39	179
Vermont	0	0	0	0	0	0	0	0	0	0	0	0	1	1	1	0	0	2	2	7
Virginia	0	0	0	16	19	5	0	0	3	0	5	4	8	20	6	5	13	6	12	122
Washington	0	0	0	0	0	0	0	0	0	2	26	1	0	4	0	7	8	8	10	66
West Virginia	0	0	0	3	1	0	0	1	0	1	0	0	2	5	1	0	0	0	1	15
Wisconsin	0	0	0	22	7	5	11	11	7	4	1	0	2	44	17	3	6	10	47	197
Wyoming	0	0	0	0	92	2	6	15	23	0	4	2	1	3	16	0	3	8	4	179
Total	59	19	64	2,946	2,866	1,148	1,309	1,495	1,227	689	386	629	486	2,873	1,267	1,347	1,455	1,309	1,425	22,999

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

Neuroinvasive disease	Non-neuroinvasive disease	Total
Year	Cases No.	Deaths No. (%)	Cases No.	Deaths No. (%)	Cases No.	Deaths No. (%)
1999	59	7  (12)	3	0  (0)	62	7  (11)
2000	19	2  (11)	2	0  (0)	21	2  (10)
2001	64	10  (16)	2	0  (0)	66	10  (15)
2002	2,946	276  (9)	1,210	8  (1)	4,156	284  (7)
2003	2,866	232  (8)	6,996	32  (<1)	9,862	264  (3)
2004	1,148	94  (8)	1,391	6  (<1)	2,539	100  (4)
2005	1,309	104  (8)	1,691	15  (1)	3,000	119  (4)
2006	1,495	162  (11)	2,774	15  (1)	4,269	177 (4)
2007	1,227	117  (10)	2,403	7  (<1)	3,630	124  (3)
2008	689	41  (6)	667	3  (<1)	1,356	44  (3)
2009	386	32  (8)	334	0  (0)	720	32  (4)
2010	629	54  (9)	392	3  (1)	1,021	57  (6)
2011	486	42  (9)	226	1  (<1)	712	43  (6)
2012	2,873	270  (9)	2,801	16  (1)	5,674	286  (5)
2013	1,267	111  (9)	1,202	8  (<1)	2,469	119  (5)
2014	1,347	87  (6)	858	10  (1)	2,205	97  (4)
2015	1,455	142  (10)	720	4  (<1)	2,175	146  (7)
2016	1,309	105  (8)	840	1  (<1)	2,149	106  (5)
2017	1,425	146  (10)	672	0  (0)	2,097	146  (7)
Total	22,999	2,034  (9)	25,184	129  (<1)	48,183	2,163  (4)

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

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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