population) are established by comparing how well a test correlates with an established “gold standard” for the condition. The gold standard for JD is the identification of the etiologic agent, Map, in tissues that show characteristic histopathologic lesions. A test’s sensitivity and specificity must be determined, along with the prevalence of the condition, in order to calculate the positive predictive value of a positive test and the negative predictive value of a negative test.

Diagnostic test performance depends on the stage of disease. JD in dairy cattle is clinically categorized into four stages (see Table 2–2). In Stage 1, animals are infected, asymptomatic, and no organisms are detected in feces. In Stage 2, animals are asymptomatic, but organisms can be detected in feces. Stage 3 animals are symptomatic with weight loss and diarrhea. Stage 4 is advanced clinical disease, animals are symptomatic with signs of lethargy, emaciation and profuse diarrhea. Diagnostic tests will generally tend to perform better in individual animals in the later stages of the disease. (This may not be true for immunological tests, where anergy of either cell-mediated or antibody responses to Map has been noted in animals with a heavy bacterial burden.) At the herd level, tests will tend to perform better as the proportion of individuals in more advanced stages of disease increases.

For control programs, it is important to make this distinction between test performance at the individual animal level and test performance at the herd level. One valuable screening test for control programs is the enzyme-linked immunosorbent assay (ELISA) for antibodies against Map. This test has relatively low sensitivity at the individual animal level, but fairly good sensitivity at the herd level. It also has significant advantages over fecal culture for screening, which is important in large-scale control programs. These advantages include relatively low cost, simplicity, and rapid results (Tables 3–1, 3–2, 3–3).

Table 3–1. Detectability of Johne’s Disease at Varying Clinical Stages

Stage I

Stage II

Stages III, IV

Signs of disease

No

No

Yes

Fecal culture

No

Maybe

Yes

PCR

No

Maybe

Yes

Acid-fast bacilli

No

Maybe

Yes

IFN-γ

Maybe

Yes

Maybe

Serology

No

Maybe

Yes

Notes: PCR: polymerase chain reaction

IFN-γ: gamma interferon



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