Proceedings of the 10th International Colloquium on Paratuberculosis

#138
Estimating Prevalence <strong>of</strong> Paratuberculosis in sheep. A case study: <strong>the</strong> Viterbo
Province (Latium, Italy)
De Grossi L, Gelli A , Marabini S, Sezzi E
Istituto Zoopr<strong>of</strong>ilattico Sperimentale delle Regioni Lazio e della Toscana, Rome, Italy
ABSTRACT
The number <strong>of</strong> prevalence studies carried out among small ruminants is small and <strong>the</strong>re is
no studies that provided accurate and unbiased prevalence estimates.
The objective <strong>of</strong> <strong>the</strong> present study was to assess <strong>the</strong> ovine paratuberculosis
prevalence in Viterbo Province.
A total <strong>of</strong> 1500 individual blood samples belonging to 30 flocks, were tested with
commercial ELISA test (Pourquier).
The apparent prevalence on animal and herd level was calculated as <strong>the</strong> number <strong>of</strong>
test-positive animals among <strong>the</strong> total number <strong>of</strong> animals tested (AP), or test-positive herds
among <strong>the</strong> total number <strong>of</strong> herds tested (HAP). An estimate <strong>of</strong> <strong>the</strong> true prevalence (TP) was
calculated from <strong>the</strong> AP by correction via <strong>the</strong> Rogan-Gladen estimator.
INTRODUCTION
The prevalence <strong>of</strong> an infection is an important tool to know <strong>the</strong> diffusion <strong>of</strong> <strong>the</strong> infection itself
and to apply measures as eradication, control and surveillance. The number <strong>of</strong> prevalence
studies carried out among small ruminants is small and <strong>the</strong>re are no studies that provide
accurate and unbiased prevalence estimates.
The objectives <strong>of</strong> <strong>the</strong> present study were to assess <strong>the</strong> ovine paratuberculosis
prevalence in Viterbo Province. We met <strong>the</strong> minimum requirements <strong>of</strong> a prevalence study
(Nielsen and T<strong>of</strong>t, 2009) which are descriptions <strong>of</strong>: (a) target and study populations, including
<strong>the</strong> age distribution;(b) <strong>the</strong> test used, including test accuracy and protocol ; (c) study period;
(d) APs which are converted to TPs.
There is a distinction between true prevalence (<strong>the</strong> proportion <strong>of</strong> a population that is
actually infected) and apparent prevalence (<strong>the</strong> proportion <strong>of</strong> <strong>the</strong> population that tests
positive for <strong>the</strong> disease). Given point estimates for sensitivity (se), specificity (sp), and
apparent prevalence (AP), one may calculate true prevalence using <strong>the</strong> following expression:
True Prevalence = (AP+sp-1)/(se+sp-1).
MATERIALS AND METHODS
Fifty sheep flocks were randomly selected and were proportionally allocated according to <strong>the</strong>
5 sanitary districts <strong>of</strong> <strong>the</strong> province. A total <strong>of</strong> 1500 individual blood samples, 30 per flock,
were tested. A commercial ELISA test (Pourquier) was used for screening and confirmation.
Sera were treated according to <strong>the</strong> protocol provided by <strong>the</strong> manufacturer.
The Apparent Prevalence on animal and herd level was calculated as <strong>the</strong> number <strong>of</strong>
test-positive animals among <strong>the</strong> total number <strong>of</strong> animals tested (AP), or test-positive herds
among <strong>the</strong> total number <strong>of</strong> herds tested (HAP). An estimate <strong>of</strong> <strong>the</strong> true prevalence (TP) was
calculated from <strong>the</strong> AP by correction via <strong>the</strong> Rogan-Gladen estimator (Rogan and Gladen,
1978). Sensitivity and specificity <strong>of</strong> <strong>the</strong> test, used for calculation <strong>of</strong> true prevalence based on
apparent prevalences were, Se: 0.40 (40%) and Sp: 0.985 (98%)
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