The most frequent route of infection is via the gastrointestinal tract but infection may also occur through the respiratory tract, genital tract, conjunctiva, and skin. Contaminated secretions include blood, urine, feces, and milk of infected animals. Animals that have aborted will shed large amounts of bacteria and contaminate their environment.

Although transmission of B. abortus via natural breeding does not occur very often, artificial insemination with contaminated semen can transmit the infection. Embryo transfer poses a potential risk if the embryoes are stored in bovine serum albumin that is harboring B. abortus.