[5] The most frequent clinical sign following B. suis infection is abortion in pregnant females, reduced milk production, and infertility.

[6][7] Swine also develop orchitis (swelling of the testicles), lameness (movement disability), hind limb paralysis, or spondylitis (inflammation in joints).

[13] This structural characteristic allows for B. suis to interact with lipid rafts on the surface of macrophages to be internalized, and the formed lipid-rich phagosome is able to avoid fusion with lysosomes through this endocytic pathway.

[13] Once inside macrophages, B. suis is able to endure the rapid acidification in the phagosome to pH 4.0–4.5[16] by expressing metabolism genes mainly for amino acid synthesis.

[15] The acidic pH is actually essential for replication of the bacteria by inducing major virulence genes of the virB operon[17] and the synthesis of DnaK chaperones.

Summary: Because B. suis is facultative and intracellular, and is able to adapt to environmental conditions in macrophages, treatment failure and relapse rates are high.