I think "серологически" refers to "исследовано", not what follows, since a серологическое исследование is a test on serum. Also, I do not believe there is a widely used adjectival form of "pullorum" such as "pullorose/pullorotic", in all the examples I found it was simply "pullorum antigen".

"The whole blood test using the stained S. pullorum antigen on the farm detected antibody in just under 25% of the birds..."

e.g.

"...or the birds must be tested serologically negative for pullorum and fowl typhoid..."

So I would phrase it as:

"... must be tested serologically using a pullorum antigen"