Chlamydophila abortus is a species in Chlamydiae that causes abortion and fetal death in mammals, including humans. Chlamydophila abortus was previously classified as Chlamydia psittaci along with all Chlamydiae except Chlamydia trachomatis. This was based on a lack of evident glycogen production and on resistance to the antibiotic sulfadiazine. In 1999 C. psittaci and C. abortus were recognized as distinct species based on differences of pathogenicity and DNA-DNA hybridization.
C. abortus is endemic among ruminants and has been associated with abortion in a horse, a rabbit, guinea pigs, mice, pigs and humans. Infected females shed bacteria near the time of ovulation, so C. abortus is transmitted orally and sexually among mammals. All C. abortus strains were isolated or PCR-amplified from placenta or fetal organs after spontaneous abortion. C. abortus infection generally remains inapparent until an animal aborts late in gestation or gives birth to a weak or dead foetus.