10.2 Tuberculin skin test

Tuberculin skin test is the standard procedure recommended by the World Organization for Animal Health (OIE) for the diagnosis of bovine TB in live animals. This test measures the delayed type hypersensitivity response to tuberculin, referred to as purified protein derivative (PPD), injected intradermally. There are two variations of the test. The single intradermal test (SITT), which uses PPD from M. bovis only (PPD-B), and the comparative intradermal test (CITT), which uses PPD-B and PPD from Mycobacteria avium, (PPD-A). In the SITT, PPD-B is injected intradermally in the neck region. A positive test is indicated by a delayed hypersensitivity reaction (Figure 4). The skin thickness at injection site is measured with a pair of calipers before and 72 hours after injection. A relative change greater than 4 mm in skin thickness at the site is considered positive for M. bovis infection [28]. The CITT is designed to address the cross-reaction between M. bovis and the M. avium. PPD-B and PPD-A are injected side by side, around 12 cm apart, and skin swelling is measured after 72 hours. The test result is considered positive, if the relative difference in the increase of skin thickness at the site of PPD-B injection is 4 mm greater than that at the site of PPD-A injection [28]. The sensitivity and specificity of the CITT has been estimated at 81–85 and 80.0, and 99.9%,

## Figure 4.

A comparative intradermal tuberculin test in a cow showing a positive reaction. PPD-A was injected at site A and PPD-B at site B. ©2018. JKN Kuria.

respectively [58, 64]. The CTT has higher specificity than the SITT since it can distinguish animals infected with nontuberculous mycobacteria, specifically the MAC complex, which include M. avium subsp. paratuberculosis, the causative agent of Johne's disease. Other MOTTs species with ability to cross-react with M. bovis have, however, been isolated from tuberculous lesion in cattle and related wild species. The advantage of the CITT over the SITT is therefore limited [5–7].
