Diagnosis

Several diagnostic methods are available for the detection of MAP infection in cattle, and each have their relative merits and applica­tions. They are classically divided into two cat­egories: (i) immune-based tests that detect the animal's immune response to MAP, and (ii) or­ganism detection tests that detect MAP organ­isms or MAP bacterial DNA.

Fig. 11.2. Photograph of ileum manifesting lesions typical of paratuberculosis. To the left, the intestinal wall is thickened due to infiltration by leucocytes and epithelioid cells. To the right, there is mesenteric oedema, with a dilated mesenteric lymphatic channel, shown by the arrow. (Reproduced with the kind permission of Michael Collins.)

Fig. 11.3. Advanced paratuberculosis in Holstein (a) and Guernsey (b) cows. Signs of disease include weight loss, seen as marked reduction of muscle mass and visible skeletal bones such as ribs, pelvis and shoulder girdle. (Reproduced with the kind permission of Michael Collins.)

11.4.1 Testing strategies

Diagnosis of infection in cattle depends in part on whether diagnostic efforts are aimed at de­tecting infection in an individual animal or a herd. A consensus statement regarding testing recommendations for US cattle was published in 2006 (Collins et al., 2006). The main purpose of the report was to provide simple, practical and cost-effective recommendations for cattle herds. Those recommendations were summa­rized and adapted by Collins in a review chapter on the diagnosis of paratuberculosis (Collins,

2011).

2012).

11.4.2 Diagnostic testing in a known- infected herd

In a known-positive herd, the main goal is to decrease environmental MAP load and there­fore decrease the risk of exposure to suscepti­ble animals. Determination of individual cow's MAP infection status will facilitate cow and calf management around calving (i.e. segregation of calving areas, colostrum management), and culling of high-positive cows. In commercial herds with high prevalence (>5%), individual ELISA on milk or serum is recommended, keep­ing in mind to interpret results quantitatively, based on S/P ratios (Wells et al., 2002a). In low- prevalence herds (for a 1000- cow herd with high prevalence (van Schaik et al., 2003). In another study that assessed fae­cal pooling strategies in low-prevalence herds using a simulation analysis, the weighted mean herd probabilities of detecting a positive herd ranged between 52 and 99.3%, with the low­est probability for pools of 10 with one positive cow in the herd and the highest probability for pools of 5 with five positive cows in the herd (McKenna et al., 2018).

11.4.4 Diagnostic testing for eradication

The choice of test is dictated by herd prevalence and by how quickly and aggressively a producer wants to eradicate MAP from his herd. In gen­eral, the fastest approaches are also the most expensive to the producer. For example, whole­herd individual faecal culture or PCR followed by culling of positive animals has a better chance of quickly eradicating MAP from a seedstock herd but may be costly, while a less expensive (and less aggressive) approach might involve pooled fae­cal culture or serum/milk ELISA.

11.4.5 Diagnostic testing for individual animals

Diagnostic testing of an individual or group of animals that are not showing signs of paratu­berculosis (pre-purchase screening) is difficult, especially in animals that are J., Steadham, E., Haynes, J., Bailey, T.

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