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Volume 11, Number 2—February 2005

Research

Sporadic Cryptosporidiosis Decline after Membrane Filtration of Public Water Supplies, England, 1996–2002

Stella Goh*, Mark Reacher†Comments to Author , David P. Casemore‡, Neville Q. Verlander§, André Charlett§, Rachel M. Chalmers¶, Margaret Knowles#, Anthony Pennington*, Joy Williams*, Keith Osborn**, and Sarah Richards††
Author affiliations: *Carlisle and District Primary Care Trust, Carlisle, United Kingdom; †Health Protection Agency, Cambridge, United Kingdom; ‡University of Wales, Aberystwyth, Ceredigion, Wales, United Kingdom; §Health Protection Agency Centre for Infections, Colindale, London, United Kingdom; ¶Singleton Hospital, Swansea, Wales, United Kingdom; #Cumberland Infirmary, Carlisle, United Kingdom; **United Utilities, Great Sankey, Warrington, United Kingdom; ††West Cumberland Hospital, Whitehaven, United Kingdom

Main Article

Table 1

Exclusions and recruitment of case-patients and controls

Exclusion criteria n (%)
Before membrane filtration,
March 1, 1996–February 29, 2000 After membrane filtration,
March 1, 2000–August 31, 2002 Before and after membrane filtration,
March 1, 1996–August 31, 2002
Case-patients
Refused to participate 1 (0.5) 0 (0) 1 (0.4)
Could not complete adequate interview 1 (0.5) 1 (2.4) 2 (0.8)
Did not respond to letters or phone calls 2 (1.0) 0 (0) 2 (0.8)
Did not meet study case definition
No history of diarrhea 1 (0.5) 1 (2.4) 2 (0.8)
Mixed enteric infection 1 (0.5) 0 1 (0.4)
Secondary case 36 (17.4) 10 (23.8) 46 (18.5)
Travel outside UK in 14 days before onset 8 (3.9) 8 (19.0) 16 (6.4)
Visitor to study area 1 (0.5) 0 (0) 1 (0.4)
Residence outside study area 1 (0.5) 0 (0) 1 (0.4)
Case-patient or household member previously interviewed as case or control 2 (1.0) 0 (0) 2 (0.8)
Potential case-patients approached 207 (100) 42 (100) 249 (100)
Potential cases excluded 54 (26.1) 20 (47.6) 74 (29.7)
Total case-patients enrolled 153 (73.9) 22 (52.4) 175 (70.3)
Controls
Refused or unavailable for interview
Refused to participate 23 (3.0) 12 (7.9) 35 (3.8)
Unavailable at requested interview times 125 (17.1) 23 (15.2) 148 (15.9)
Said interview times were not convenient 35 (4.5) 3 (2.0) 38 (4.1)
Address not found 3 (0.4) 0 (0) 3 (0.3)
Did not meet study control definition
History of diarrhea 46 (5.9) 6 (4.0) 52 (5.6)
Travel outside UK in 14 days before interview 8 (1.0) 4 (2.6) 12 (1.3)
Not resident in study area in 14 days before interview 3 (0.4) 1 (0.7) 4 (0.4)
Moved from study area 27 (3.5) 7 (4.6) 34 (3.7)
Residence outside study area 2 (0.3) 0 (0) 2 (0.2)
Control or household member already interviewed as a case or control 7 (0.9) 1 (0.7) 8 (0.9)
Not enrolled for administrative reasons or reason not recorded
Interview cancelled; 3 controls already enrolled for associated case 19 (2.4) 25 (16.6) 44 (4.7)
Interview cancelled; potential control found to be in wrong age group 9 (1.2) 0 (0) 9 (1.0)
Reason for exclusion not recorded 3 (0.4) 0 (0) 3 (0.3)
Potential controls approached 778 (100) 151 (100) 929 (100)
Potential controls excluded 310 (39.8) 82 (54.3) 392 (42.2)
Total controls enrolled 468 (60.2) 69 (45.7) 537 (57.8)

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