A screening sampling plan to detect Mycobacterium avium subsp. paratuberculosis-positive dairy herds

Mycobacterium avium subsp. paratuberculosis (MAP) is the etiological agent of paratuberculosis, a chronic contagious bacterial disease primarily affecting dairy cattle. Paratuberculosis represents a dual problem for the milk production chain: in addition to economic losses to affected herds, MAP may have zoonotic potential. Infected herds must be identified to implement programs designed to reduce the incidence of disease within and between herds and prevent MAP entering the food chain. The objective of this study was to evaluate the sensitivity and specificity of a screening sampling 1 plan (SSP) to detect MAP-positive dairy herds by repetitive analysis of bulk tank milk (BTM) samples by enzyme-linked immunosorbent assay (ELISA) and in-line milk filter (IMLF) samples by polymerase chain reaction (PCR). BTM and ILMF were collected twice from 569 dairy herds in Southern Italy. Additionally, 12312 individual milk samples were collected, 9509 from 102 SSP-positive herds (SSP MAP-positive) and 2507 from 21 randomly selected SSP-negative herds (SSP MAP-negative). The SSP MAP-positive herds were 102 (21.3%;95% CI = 18.0 – 24.9); the within-herd apparent prevalence (AP) ranged between 0.00% to 22.73% (mean 6.07%); a significant difference in within-herd AP was shown between SPP MAP-positive herds and SSP MAP-negative herds. A highly significant association was shown between the median AP herd status (> 5%) and the positivity to at least one ILMF or BTM sample. The SSP detected a minimum of 56.25% of low AP herds (AP < 2.00%) up to a maximum of 100% of herds with a within-herd AP > 8.00%. Overall the SSP detected 85.57% of herds in which at least one individual milk sample was positive to ELISA. The proposed screening sampling plan proved to be a cheap and useful tool to detect MAP-positive herds with a higher risk of infection diffusion and milk contamination. Although the SSP cannot be used for MAP-free certification of herds, it could be useful to prioritize appropriate control measures aimed at reducing the prevalence of infection in dairy herds and milk contamination.