Paratuberculosis is a
chronic, granulomatous enteric infection caused by Mycobacterium
avium subsp. paratuberculosis(MAP) in ruminants.
Eradication of MAP in cattle herds is complicated by lack of
diagnostic tests for early diagnosis of infected animals. Available
diagnostic methods include detection of MAP by cultivation,
cell-mediated immune responses by IFN-γ assays on blood
samples or antibodies (Ab) in milk and blood by ELISA.
The objective of the
present study was to evaluate the association between IFN-γ
test results in calves and antibody ELISA status in the adult
cows.
During a three year
study period, blood was repeatedly (3 to 4 times) sampled from
15-24 months old heifers in 18 Danish dairy cattle herds and
analysed the following day by a whole blood IFN-γ test
supplemented with IL-12. After calving, milk samples were analysed
for MAP Ab three times per year per animal. For the present
analysis, the result of the latest available ELISA test was used.
Faecal samples were cultured once per year from adult cattle.
Animals were retrospectively grouped by their faecal culture (FC)
status.
Animals were considered
FC-negative if negative in all samples. The IFN-γ test was
considered positive if IFN-γ ≥ 1000 pg/ml in PPDj
stimulated and IL-12 potentiated blood samples. The ELISA test
result was considered positive if
ODCorrected>0.3.
Preliminary analyses of
the results were carried out by FC-stratified Fishers'exact
tests of 2x2 contingency tables of ELISA results cross-tabulated by
IFN-γ results. No associations were found between early
IFN-γ results and later ELISA results in animals positive
(n=77) or negative (n=1180) by FC.
Of the 77 FC positive
animals, 13 animals were tested both IFN-γ and ELISA positive.
However, 31 heifers that had been tested IFN-γ positive were
not ELISA positive later on. Conversely, 11 cows that were ELISA
positive had been tested negative as heifers by the IFN-γ
test. A large part of the tested animals were FC negative. Of 1180
FC negative animals, 17 were both IFN-γ and ELISA positive.
Close to half of the FC negative animals, or 593 heifers, were
tested positive only by IFN-γ . A smaller part of the FC
negative animals, that is 50 cows, were only ELISA positive. In
fact, 57% of the FC positive and 52% of the FC negative animals had
been tested positive by the early IFN-γ test.
The lack of association
between tests documented here may partly be related to concerns
regarding the low specificity of the present IFN-γ test for
individual diagnosis of paratuberculosis in young calves. In
addition, it may partly be ascribed to the lack of a true gold
standard replacing FC. At present, all tests have limitations at
certain points during progression of MAP infection. Further
evaluation and optimisation of the IFN-γ test using new and
more specific antigens is necessary for diagnosis in young animals.
Consequently, an association between milk antibody and IFN-γ
may not be expected until a specific and sensitive IFN-γ test
has been developed, but the results still suggest that a
cell-mediated immune response is infrequently followed by a humoral
immune response.
