Page 867 - Veterinary Immunology, 10th Edition

screening tests. These include the Brucella-buffered antigen tests, in
VetBooks.ir which killed, stained Brucella organisms are suspended in an acid

buffer (pH 3.6). The dye used, either the red dye rose-bengal or a
mixture of crystal violet and brilliant green, enables the test to be

easily read. At this low pH, nonspecific agglutination by IgM
antibodies is eliminated. The Brucella-buffered plate agglutination
test has a specificity of as high as 99% and a sensitivity of 95%. The
efficient and widespread use of these tests has eliminated bovine

brucellosis from so many countries.
S. enterica Pullorum infection in poultry can be diagnosed by a
slide agglutination test, in which killed bacteria stained with
gentian violet are mixed with whole chicken blood. Agglutination is

readily seen if antibodies are present. Leptospirosis is diagnosed by
a microscopic agglutination test, in which mixtures of living
organisms and test serum are examined under the microscope for
agglutination. This technique preferentially detects IgM antibodies

and is thus an excellent test for detecting recent outbreaks as well as
for distinguishing between infected and vaccinated animals.
It is not mandatory that serum be used as the source of antibody
for diagnostic tests. The presence of antibodies in body fluids other

than serum, such as milk whey, vaginal mucus, or nasal washings,
may be of more significance, especially if the infection is of a local
or superficial nature. One such example is the milk ring test used to
detect the presence of antibodies to B. abortus in milk (Fig. 26.8).

Fresh milk is shaken with bacteria stained with hematoxylin or
triphenyl tetrazolium and is allowed to stand. If antibodies,
especially IgM or IgA, are present, the bacteria will clump and
adhere to the fat globules of the milk and rise to the surface with

the cream. If antibodies are absent, the stained bacteria will remain
dispersed in the milk, and the cream, on rising, will remain white.

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