Introduction: Prevalence of Paratuberculosis in Cattle

Paratuberculosis is widely distributed through­out the world in many domesticated and wild ruminant species. Because of the long incuba­tion period of paratuberculosis in cattle (be­tween 2 and 7 years), the majority of infected cattle (around 95%) are considered to be in the subclinical stages of the disease, with less than 5% of infected cattle displaying clinical signs of illness.

Studies have been conducted in the United States as well as in other countries to estimate MAP prevalence in cattle. Faecal cultures, en­vironmental cultures and antibody-detection methods have been the most common diagnos­tic methods used to determine the prevalence of infection. Estimates of MAP prevalence are com­plicated by several factors including the low sen­sitivity of available diagnostic tests for subclinical animals, infected cattle in the subclinical phase may be culled for reduced fertility or production and that cattle in the clinical phase of the dis­ease are usually culled before a final diagnosis of MAP infection is made (Fecteau, 2018). In the US National Animal Health Monitoring System's (NAHMS) Dairy 2007 study, the apparent herd­level prevalence of MAP was 70.4% (National Animal Health Monitoring System Dairy, 2007). A follow-up analysis of the 2007 NAHMS data, using a Bayesian method, which adjusts for sen­sitivity and specificity, determined that the true US dairy herd-level prevalence of MAP was actu­ally 91.1% (Lombard et al., 2013). Within herds, the most recent estimate was 02 7.3% of cattle, with a mean of 5.5% (National Animal Health Monitoring System, 2002). High within-herd prevalence is usually associated with higher MAP faecal shedding amongst positive animals, MAP faecal shedding at a younger age, shorter incu­bation period and higher numbers of animals in the clinical phase of the disease (Lombard, 2011). Compared with dairy cattle, the herd prevalence of MAP in beef cattle is lower, with one study estimating MAP prevalence at 7.9% of US herds (Lombard, 2011). This difference in prevalence between beef and dairy is worldwide, and could be explained by differences in breed, environmen­tal conditions, feeding practices and calf-rearing practices (Roussel, 2011).

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