Letters to the editors

Abstract

Abstract Extract Madam:- Since a number of animal diseases such as brucellosis and tuberculosis in cattle, and hydatids in sheep are now reaching a low prevalence (P: percentage of diseased animals in a population), it is perhaps appropriate to remind readers that prevalence has a marked influence on the accuracy with which a diagnostic test can identify diseased animals. Whereas an ideal test will give positive results with all diseased animals and negative results with all disease-free animals, the ideal is rarely attainable and most tests return some false positive and false negative results. Many published diagnostic tests(2) Hamel, K.L. and Ris, D.R. 1982. The use of a cathodic antigen in the immunoelectro-phoretic serodiagnosis of Echinococcus granulosus in sheep. Vet. Immunol. Immunopathol., 3: 419–425. [Google Scholar] (4) Ris, D.R. 1974. The complement fixation test for the diagnosis of Brucella ovis infection in sheep. N.Z. vet. J., 22: 143–146. [Taylor & Francis Online] , [Google Scholar] (5) Worthington, R.W., Weddell, W. and Penrose, M.E. 1984. A comparison of three serological tests for the diagnosis of B. ovis infection in rams. N.Z. vet. J., 32: 58–60. [Taylor & Francis Online], [Web of Science ®] , [Google Scholar] have a sensitivity (Se: percentage of diseased animals with a positive result) and a specificity (Sp: percentage of disease-free animals with a negative result) of between 90 and 100%, and perform well when the prevalence of a particular disease is high. But, when it is low, as is the case when a disease is nearly eradicated, or in a group of animals selected for export, the same tests have a much lower accuracy in the detection of diseased animals. (3) Moffat, J. 1985. Grasping the nettle on Tb in deer. N.Z. J. Agrie., 150(7): 4–7. [Google Scholar] It is not the tests that have changed, but the conditions under which they are used.