By Chicago’s own mosquito-trap ledger, West Nile season is surging: 187 trap pools have tested positive so far in 2026, and the weekly counts have surged in the last month — 44 positives in the week ending August 1, 49 the week after. At this point in the season, week 32, only three of the file’s twenty seasons ran higher: 2012 (369 positives by then), 2016 (282), and last year (253). One boundary governs every number on this page: these are laboratory tests of trapped mosquito pools — the file records nothing about human cases, and neither does this page.
West Nile Virus (WNV) Mosquito Test Results, Chicago data portal · 44,675 test rows, seasons 2007–2026 · retrieved August 18, 2026, with reported weeks through August 15What a row is
The file records one row per test of a mosquito pool — a batch of mosquitoes collected at one of the city’s traps in one surveillance week — with the trap, the week, the pool’s mosquito count, and a result. Eighty-two traps report in the 2026 season; seasons past have run between 78 and 125. Across all twenty seasons the ledger holds 44,675 tests: 3,894 positive, 35,520 negative, and 5,261 rows — about one in nine — carrying no result value at all, disclosed here and left exactly as the file has them.
Twenty seasons of trap tests
- 2012437 positive pools
- 2016(the file’s peak)463
- 2022(the quietest season since 2011)80
- 2025389
- 2026(through the week ending Aug 15)187
Every season’s positive count, in text
2007 234; 2008 99; 2009 19; 2010 76; 2011 57; 2012 437; 2013 239; 2014 211; 2015 113; 2016 463; 2017 177; 2018 212; 2019 151; 2020 102; 2021 188; 2022 80; 2023 253; 2024 207; 2025 389; 2026 187.
The big seasons in the file are 2016 (463 positive pools), 2012 (437), and 2025 (389); the quietest are 2009 (19) and 2022 (80). Positive counts are reported here as counts, not rates, because the denominator moves: seasons differ in how many tests were run and in how many rows carry no result, so a season’s positives measure what its traps found, on that season’s testing effort.
The 2026 season, week by week
- Week ending June 13(the season’s first positives)2
- Week ending July 1823
- Week ending July 2540
- Week ending Aug 144
- Week ending Aug 8(the season’s peak week so far)49
- Week ending Aug 1522
The season’s shape so far: first positives in mid-June, single digits through early July, then the late-July surge — 23, 40, 44, and 49 positives in four consecutive weeks — with the week ending August 15 at 22. Rows for the week ending August 22 were already arriving at retrieval; that week is incomplete and excluded from the table above. Whether 2026’s surge has peaked, the file cannot yet say.
Where 2026 stands, on a matched window
Comparing a live season against finished ones is a trap of its own, so this page compares only through week 32 — 2026’s last complete reported week. On that matched window, 2026’s 187 positives stand fourth among the twenty seasons: 2012 had reached 369 by week 32, 2016 had reached 282, and 2025 had reached 253. Every other season had fewer by this point. Where the rest of 2026 lands depends on weeks the traps have not yet reported.
Known limits
A positive pool means West Nile virus was detected in a batch of trapped mosquitoes; it is not a human case, a risk score, or a forecast, and this page draws no line from pools to people. The traps sit where the city chose to put them, so trap results are not a map of the city and no geography is computed here. Testing effort varies season to season, and about one row in nine carries no result; both are why this page counts positives rather than computing positivity rates. The file updates in season; every figure is a snapshot of the August 18, 2026 retrieval.
- Source West Nile Virus (WNV) Mosquito Test Results (jqe8-8r6s, Chicago data portal), 44,675 rows retrieved August 18, 2026.
- Counting One row is one recorded pool test. Season and week figures use the file’s own season_year and week fields; results are counted by the file’s positive and negative values, with rows carrying neither counted and disclosed as blank. The matched-window comparison sums positives in weeks 1 through 32 of each season. The in-progress reporting week at retrieval is excluded from weekly figures. No rates are computed.
Computed by KCM Desk from records retrieved August 18, 2026. If you spot an error, corrections come first.