Paratuberculosis: Serial testing of qPCR-positive faecal pools by individual faecal qPCR assay

AHEAD

ANIMAL HEALTH

Thomas

Marian

Maarten F. Weber Royal GD, Deventer, the Netherlands

Objectives

To quantify the proportion of positive pooled sample that results in ≥1 positive result at subsequent individual faecal sampling of the cattle included in the pooled sample

Materials and methods

Retrospective analysis of results of subsequent individual faecal qPCR assays of cattle in positive pools submitted in 2014-2022. Data analysis was restricted to pools consisting of samples from 5 cattle for which individual faecal samples were submitted within 90 days after submission of the positive pooled sample.

Results

Data of 1015 positive pooled faecal samples from 208 herds were retrieved, included pooled samples from 2 (n= 14 pooled samples), 3 (n = 15), 4 (n = 40) or 5 (n = 946) cattle. For 759 of the latter 946 pooled samples, the Ct value of the pooled sample could be retrieved and valid results of subsequent individual faecal samples of each of the 5 cattle were available. For 513 of these 759 pooled samples (68%), subsequently at least one individual sample was positive as well. This proportion depended on the Ct value of the positive pool. The proportion of positive pooled sample results in ≥1 positive result of Ct values between 22 to 33, 33 to 38, 38 to 39 and 39 to 40 was 93%, 83%, 67% and 46%, respectively.

Conclusions

Not every positive pooled sample results in ≥1 positive result at subsequent individual faecal sampling of cattle included in the pooled sample

Take home message: Prior to sampling, veterinarians and farmers should be aware that serial testing of pooled faecal samples of cattle followed by testing of newly collected individual samples of cattle in positive pools may result in negative individual test results.

Dijkstra,

Aalberts,

0 100 200 300 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Number of samples Ct value pooled faecal sample Test result of individual samples Ct <33 33 ≤ Ct < 38 38 ≤ Ct < 39 39 ≤ Ct <40 No positive individual samples 6 (7%) 39 (17%) 58 (33%) 143 (54%) At least one positive individual sample 83 (93%) 191 (83%) 117 (67%) 122 (46%) Total 89 230 175 265