Cow #11 (actually a heifer since it never had a calf) was experimentally infected with M.
paratuberculosis by feeding it a low dose (one 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
sporadically shedding M. paratuberculosis bacteria in her feces. Fecal culture
was the first test to detect that the animal was infected. This is not always the case.
For unknown reasons, possibly bad luck or possibly genetics of the cow, the infection
progressed very rapidly.
The red line indicates the gamma interferon response. The interferon response was biphasic
(two peaks) and started before serum antibodies were produced. Shortly before cow #11
died, the interferon response declined to zero.
The yellow line shows the rapid rise of antibodies in serum that occurred about 6 months
before the cow died. 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 cow #11 illustrates an important principle about
diagnostic testing for paratuberculosis: even though an animal is infected, at any given
point in time any single diagnostic test can easily miss detection of the infection in the
animal. Only by use of multiple tests over time can confidence in negative test results be
obtained.
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