Diagnostic testing History: Heifer #13
Heifer #13 was experimentally infected with M. paratuberculosis by feeding it a high dose (100 million bacterial cells) in its evening bottle of milk on each of three nights. Every 4 weeks it was tested for Johne's disease using BACTEC fecal culture, the ELISA for serum antibodies, and the gamma interferon test.

Blue arrows under the time axis show when the cow was fecal culture-positive. This cow was only found to be shedding M. paratuberculosis bacteria in her feces twice before the other tests became positive. Shortly before this heifer had her first calf as a 3-year-old, she became consistently fecal culture-positive. After calving she developed diarrhea. This pattern of fecal shedding shortly before onset of clinical signs is thought to be typical for most cases of paratuberculosis, although it may not happen until the second or third calving.

The red line indicates the gamma interferon response. The interferon response was biphasic (two peaks) and started shortly before serum antibodies were produced. Cow #13 was euthanized due to poor body condition and persistent diarrhea 2 months after calving. The interferon response declined to zero shortly before euthanasia.

The yellow line shows the rapid rise of antibodies in serum that occurred about 12 months before the cow died. This cow had a rapid rise and then relatively stable level of serum antibodies. Interestingly, at the time of calving her serum antibody level to M. paratuberculosis declined sharply. At every test interval, however, the serum antibody assay was classified as positive. Generally, tests for serum antibodies become positive late in the course of paratuberculosis and a rapid rise or high levels of serum antibodies indicates a bad prognosis: the animal will soon have clinical signs of Johne's disease.

The pattern of diagnostic tests seen in heifer #13 illustrates an important principle about diagnostic testing for paratuberculosis: even though an animal is infected, there is a long period during which the infection is not detectable by any diagnostic test currently available. This period or phase of M. paratuberculosis infection is referred to by several terms: eclipse phase, silent infection phase, or prepatent infection stage. This demonstrates that, at least in cattle, tests must be repeated over a period of two or more years to get confidence that negative test results indicate the animal truly is not infected.