Volume 11, Number 2—February 2005
Research
Sporadic Cryptosporidiosis Decline after Membrane Filtration of Public Water Supplies, England, 1996–2002
Stella Goh*, Mark Reacher†
, David P. Casemore‡, Neville Q. Verlander§, André Charlett§, Rachel M. Chalmers¶, Margaret Knowles#, Anthony Pennington*, Joy Williams*, Keith Osborn**, and Sarah Richards††
, David P. Casemore‡, Neville Q. Verlander§, André Charlett§, Rachel M. Chalmers¶, Margaret Knowles#, Anthony Pennington*, Joy Williams*, Keith Osborn**, and Sarah Richards††
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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