Bovine
paratuberculosis (PTB) is widely diagnosed by ELISAs. Nevertheless,
in spite of the pre-adsorption step with M
.phlei, which
intends to minimize cross-reactions with environmental mycobateria,
lack of specificity may happen, due to the shared antigens of its
agent, Mycobacterium avium paratuberculosis, with M.
bovis. A herd
that was proved to be TB and PTB free by serology, intradermal
testing and bacterial culturing three months before the study was
selected. For this experiment, 63 animals were divided in four
groups. Group A was tested with PPDbov only, group B with PPDbov
and PPDav, group C only with PPDav and group D with PPD diluents,
as control. Blood samples of each animal were collected moments
before PPD inoculation and after 3, 15, 30, 60 and 90 days. All
sera were tested for PTB by an accredited "in-house"
ELISA-PPA and 36 also selected to be confirmed by a commercial
ELISA (Pourquier). Three (4.76%) animals were reactive to
ELISA-PPA, one (1.58%) from group A and two (3.17%) from group B.
Considering only animals tested by recommended tests, i.e. single
or comparative intradermal testing, reactive animals represented
8.82% of the herd. Most samples became reactive between the
30th and 60th days and two animals remained reactive until the
90th day after ITT. Although not reactive, an evident increase
in S/P values along the experiment was observed in other 29 cows,
7/17 from Group A (41.2%), 10/17 from Group B (58.8%) and 12/ 17
(70.5%) from Group C. In the commercial ELISA, the three reactive
animals confirmed to be reactive. From the other 29 animals, two
were reactive and 22 confirmed the phenomena. The two used
PTB-ELISAs were highly correlated (k=0.78). We
demonstrate that intradermal tuberculin tests may temporarily (up
to 90 days) interfere in the immune status of the animal and
determine false-positive reactions in ELISAs used for the
serodiagnostic of paratuberculosis. Therefore, in order to avoid
such occurrence, cattle should not be bled for PTB serodiagnostic
for a period of at least 90 days after tuberculin
testing.
