The Virus
West nile virus (WNV) is a neurotropic, arthropod-borne virus (arbovirus), within the family Flaviviridae. Flaviviridae are distributed worldwide, consisting of about 75 species in 9 serogroups, many members of which cause disease in humans and animals. Other important human Flaviviridae pathogens include yellow fever, dengue, Zika, chikungunya, St. Louis encephalitis, Japanese encephalitis, Murray Valley encephalitis, tick-borne encephalitis, Kyasanur Forest disease, and Omsk hemorrhagic fever viruses.
The WNVs are spherical particles with a membrane envelope and dense core. Replication is not fully understood. The virus is deposited by mosquito into the skin where it contacts Langerhans cells, whose job is to acquire antigens and transport them to lymph nodes. Langerhans cells present the WNV antigens to T-lymphocytes. The initial replication of WNV is believed to take place in the regional lymph nodes. "WNV then spreads systemically to visceral organs, such as the kidney and spleen, where a second round of replication takes place, presumably in epithelium cells and macrophages, respectively (Rios 2006). The typical incubation period is usually 2 to 6 days but ranges from 2 to 14 days. This period can be longer in people with certain medical conditions that affect the immune system.
Neuroinvasive disease occurs when WNV crosses the blood brain barrier (BBB). Crossing the BBB appears to be related to the level and duration of viremia (Rios 2006). Neuroinvasive disease results from viral infection of central nervous system structures; meninges (meningitis), brain tissue (encephalitis), anterior horn of the spinal cord (West Nile poliomyelitis)
West Nile viral disease is a nationally notifiable arthropod-borne virus (arboviral) disease. Cases are reported to ArboNET. by state and local health departments. ArboNet. maintains data on arthropod-borne virus infections among blood donors, veterinary disease cases, mosquitoes, dead birds, and sentinel animals. "Approximately 85% of arboviral disease cases occurred during April–September. Weather, zoonotic host and vector abundance, and human behavior are all factors that can influence when and where outbreaks occur (MMWR)."
| Total cases of domestic arboviral disease reported to the CDC for 2020 |
Characteristic |
Virus type,* no. (%) of cases |
|
West Nile
(n = 731) |
La Crosse
(n = 88) |
Powassan
(n = 21) |
St. Louis encephalitis
(n = 16) |
Eastern equine encephalitis
(n = 13) |
Jamestown Canyon
(n = 13) |
Age group, yrs |
|
|
|
|
|
|
<18 |
16 (2) |
83 (94) |
3 (14) |
0 (—) |
3 (23) |
0 (—) |
18–59 |
302 (41) |
3 (3) |
6 (29) |
5 (31) |
2 (15) |
6 (46) |
≥60 |
413 (56) |
2 (2) |
12 (57) |
11 (69) |
8 (62) |
7 (54) |
Sex |
|
|
|
|
|
|
Male |
459 (63) |
52 (59) |
14 (67) |
12 (75) |
7 (54) |
10 (77) |
Female |
272 (37) |
36 (41) |
7 (33) |
4 (25) |
6 (46) |
3 (23) |
Period of illness onset |
|
|
|
|
|
|
Jan–Mar |
5 (1) |
1 (1) |
1 (5) |
1 (6) |
0 (—) |
0 (—) |
Apr–Jun |
38 (5) |
7 (8) |
10 (48) |
2 (13) |
0 (—) |
7 (54) |
Jul–Sep |
607 (83) |
76 (86) |
4 (19) |
10 (63) |
12 (92) |
6 (46) |
Oct–Dec |
81 (11) |
4 (5) |
6 (29) |
3 (19) |
1 (8) |
0 (—) |
Clinical syndrome |
|
|
|
|
|
|
Nonneuroinvasive |
172 (24) |
4 (5) |
1 (5) |
2 (13) |
0 (—) |
3 (23) |
Neuroinvasive |
559 (76) |
84 (95) |
20 (95) |
14 (88) |
13 (100) |
10 (77) |
Encephalitis† |
343 (61) |
66 (79) |
13 (65) |
8 (57) |
12 (92) |
9 (90) |
Meningitis† |
150 (27) |
16 (19) |
5 (25) |
3 (21) |
1 (8) |
0 (—) |
AFP†,§ |
26 (5) |
0 (—) |
1 (5) |
0 (—) |
0 (—) |
0 (—) |
Unspecified† |
40 (7) |
2 (2) |
1 (5) |
3 (21) |
0 (—) |
1 (10) |
Outcome |
|
|
|
|
|
|
Hospitalization |
583 (80) |
83 (94) |
17 (81) |
15 (94) |
13 (100) |
10 (77) |
Death |
66 (9) |
0 (—) |
1 (5) |
3 (19) |
4 (31) |
0 (—) |
| ArboNET is a passive surveillance system that leads to underestimation of actual disease prevalence. For a case to be captured by the system, a patient must seek care, the clinician must request appropriate diagnostic tests, and results must be reported to public health authorities. An estimated 30–70 nonneuroinvasive cases occur for every neuroinvasive WNV disease case reported (10). On the basis of the number of neuroinvasive WNV disease cases reported for 2020, from 16,770 to 39,130 nonneuroinvasive WNV disease cases are estimated to have occurred; only 172 (≤1%) were reported. Second, because ArboNET does not require information about clinical signs, symptoms, or laboratory findings, clinical syndrome of certain cases might be misclassified. |
| Source: Soto RA, Hughes ML, Staples JE, Lindsey NP. West Nile Virus and Other Domestic Nationally Notifiable Arboviral Diseases — United States, 2020. MMWR Morb Mortal Wkly Rep 2022;71:628–632. DOI: http://dx.doi.org/10.15585/mmwr.mm7118a3external icon |
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The Flaviridae family of viruses is responsible for a number of highly pathogenic diseases in humans.
References
Colpitts T, Conwaya MJ, Montgomery RR, Fikriga E. (2012) West Nile Virus: Biology, Transmission, and Human Infection. Clin. Microbiol. Rev. October 2012 vol. 25 no. 4 635-648
Krow-Lucal E, Lindsey N.P., Lehman J, Fischer M, Staples J.E. (2015) West Nile Virus and Other Nationally Notifiable Arboviral Diseases — United States, 2015 MMWR Weekly / January 20, 2017 / 66(02);51–55. Retrieved 11/9/17 https://www.cdc.gov/mmwr/volumes/66/wr/mm6602a3.htm
Rios M, Zhang MJ, Grinev A, Srinivasan K, Daniel S, Wood O, Hewlett IK, Dayton AI (2006). Monocytes-macrophages are a potential target in human infection with West Nile virus through blood transfusion. Transfusion. 46:659–667
Soto RA, Hughes ML, Staples JE, Lindsey Np (2022). West Nile Virus and Other Domestic Nationally Notifiable Arboviral Diseases — United States, 2020. MMWR Morb Mortal Wkly Rep;71:628–632. DOI: http://dx.doi.org/10.15585/mmwr.mm7118a3external icon
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