Research Document - 2001/002

Sensitivity and Specificity of macroscopic criteria for diagnosing Hematodinium sp. infections in snow crabs

By D.M. Taylor, G.P. Pestal, J.M. Hoenig, J.D. Shields

Abstract

The sensitivity of a diagnostic test is the probability the test will give a positive result given that the animal has the disease. The specificity is the probability of a negative result given the animal is not diseased. We estimated the sensitivity and specificity of macroscopic examination of snow crabs for detecting infection with Hematodinium sp. by comparing the visual diagnosis with results from microscopic examination of hemolymph smears. Estimated sensitivity was 53% and estimated specificity was 100%. This indicates that a positive result from macroscopic examination is definitive, given that the observer is well trained, but that macroscopic examination will fail to detect many infections. Because the incidence (i.e., occurrence of new) of Hematodinium infections may follow a seasonal trend, and newly infected animals may not exhibit symptoms of the disease, these estimates pertain to the Fall, the time of year at which the observations were made. Macroscopic observations made a few months earlier in the year may well miss more infections due to a higher proportion of infected animals being asymptomatic. Lastly, we note an alarming increase in prevalence of BCD in snow crabs from Conception Bay, Newfoundland. The prevalence was highest in female crabs; a finding that may indicate increasing costs to the snow crab fishery.

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