A practical approach to calculate sample size for herd prevalence surveys.

When designing a herd-level prevalence study that will use an imperfect diagnostic test, it is necessary to consider the test sensitivity and specificity. A new approach was developed to take into account the imperfections of the test. We present an adapted formula that, when combined with an existing piece of software, allows improved planning. Bovine paratuberculosis is included as an example infection because it originally stimulated the work. Examples are provided of the trade-off between the benefit (low number of herds) and the disadvantage (large number of animals per herd and exclusion of small herds) that are associated with achieving high herd-level sensitivity and specificity. We demonstrate the bias in the estimate of prevalence and the underestimate of the confidence range that would arise if we did not account for test sensitivity and specificity.

[1]  M. Collins,et al.  Economic decision analysis model of a paratuberculosis test and cull program. , 1991, Journal of the American Veterinary Medical Association.

[2]  S. Wells,et al.  Herd-level risk factors for infection with Mycobacterium paratuberculosis in US dairies and association between familiarity of the herd manager with the disease or prior diagnosis of the disease in that herd and use of preventive measures. , 2000, Journal of the American Veterinary Medical Association.

[3]  A. Busato,et al.  [The prevalence of paratuberculosis in the Plateau de Diesse region]. , 1995, Schweizer Archiv fur Tierheilkunde.

[4]  B. Gladen,et al.  Estimating prevalence from the results of a screening test. , 1978, American journal of epidemiology.

[5]  Y T Gröhn,et al.  Farm factors associated with the presence of Mycobacterium paratuberculosis infection in dairy herds on the New York State Paratuberculosis Control Program. , 1997, Preventive veterinary medicine.

[6]  D. Jordan,et al.  Aggregate testing for the evaluation of Johne's disease herd status. , 1996, Australian veterinary journal.

[7]  S. Wells,et al.  Herd-level economic losses associated with Johne's disease on US dairy operations. , 1999, Preventive veterinary medicine.

[8]  S. Jones,et al.  Development and evaluation of a rapid absorbed enzyme immunoassay test for the diagnosis of Johne's disease in cattle. , 1991, Australian veterinary journal.

[9]  O. L. Davies,et al.  Statistical Methods. 6th Edition. , 1968 .

[10]  W J Goodger,et al.  Herd prevalence and geographic distribution of, and risk factors for, bovine paratuberculosis in Wisconsin. , 1994, Journal of the American Veterinary Medical Association.

[11]  R W Sweeney,et al.  ELISA and fecal culture for paratuberculosis (Johne's disease): sensitivity and specificity of each method. , 2000, Veterinary microbiology.

[12]  S A McEwen,et al.  Herd-level test performance based on uncertain estimates of individual test performance, individual true prevalence and herd true prevalence. , 1998, Preventive veterinary medicine.

[13]  R M Cannon,et al.  Sense and sensitivity--designing surveys based on an imperfect test. , 2001, Preventive veterinary medicine.

[14]  Martin Hugh-Jones,et al.  Veterinary epidemiology: 2nd edition, Michael Thrusfield, Blackwell Science, Oxford, 1995, xiii + 479pp, £55.00/$84.95, ISBN 0-632-04036-X , 1996 .

[15]  J. Miller,et al.  Serologic evidence of paratuberculosis in Louisiana beef cattle herds as detected by ELISA , 1991 .

[16]  A R Cameron,et al.  A new probability formula for surveys to substantiate freedom from disease. , 1998, Preventive veterinary medicine.

[17]  G. W. Snedecor.  Statistical Methods , 1964 .

[18]  R W Sweeney,et al.  Evaluation of a Commercial Enzyme-Linked Immunosorbent Assay for the Diagnosis of Paratuberculosis in Dairy Cattle , 1995, Journal of veterinary diagnostic investigation : official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc.

[19]  M. Reichel,et al.  Comparison of serological tests and faecal culture for the detection of Mycobacterium avium subsp. paratuberculosis infection in cattle and analysis of the antigens involved. , 1999, Veterinary microbiology.