Determining Herd Status

Current Johne’s tests have limitations in sensitivity and specificity, especially in early stages of infection. As a result, large sample sizes are needed to determine with confidence whether a herd is free from infection.

Example Testing Thresholds (for 95% Confidence):

  • ELISA
    • Assumptions: 94% sensitivity, 98% specificity, 6.5% within-herd prevalence
    • Minimum animals to test: 126
    • Expected false positives: Up to 5
    • Note: In herds with ≤125 head, all animals must be tested to achieve equivalent confidence.
  • PCR
    • Assumptions: 95% sensitivity, 99.5% specificity, 6.5% within-herd prevalence
    • Minimum animals to test: 68
    • Expected false positives: About 1
       

Pooled Fecal PCR for Herd-Level Status:

Pooled PCR can be a cost-effective alternative, especially in larger herds or when a low prevalence is suspected. However, pooling samples reduces sensitivity.

  • Pooling 5 fecal samples:
    • Sensitivity ~67% (McKenna, 2018)
    •  Sensitivity improves with more positives per pool:
      • 1 positive animal: ~62%
      • 2 positive animals: ~86%
      • 5 positive animals: ~99%
    • Recommendation: Pool samples from animals of similar age to increase the likelihood of capturing high shedders (Kalis, 2000).
  • Pooling 10 samples:
    • Newer evidence (Ly, 2021) shows comparable sensitivity to pools of 5, with significant cost savings (see figure).

 

Note: Pooling is a herd-level screening tool and is not suitable for certifying individual animals as negative.

 

Graph of Herd-sensitivities reported by Ly et al., 2021(results derived from Fig. 2), calculated for different pool sizes and low prevalence levels for herds of 50, 100, 300, and 500 cattle.

Figure 1. Herd-sensitivities reported by Ly et al., 2021(results derived from Fig. 2), calculated for different pool sizes and low prevalence levels for herds of 50, 100, 300, and 500 cattle.