The Order Chlamydiales comprises a diverse group of obligate intracellular Gram-negative bacterial pathogens that are widely distributed throughout the world, causing a wide-range of infections and disease in animals and humans.
The most extensively studied family group Chlamydiaceae consists of the single genus Chlamydia which encompasses all nine recognised species: including those that infect humans, causing sexually-transmitted infections (Chlamydia trachomatis) and pneumonia (Chlamydia pneumonia); as well as those infecting birds, causing psittacosis/ornithosis (Chlamydia psittaci); and infecting ruminants and pigs, resulting in reproductive and respiratory disorders, as well as causing enteritis, polyarthritis, encephalomyelitis, conjunctivitis, endometritis and mastitis (Chlamydia suis, Chlamydia pecorum and Chlamydia abortus). Several of these species, particularly C. psittaci and C. abortus, are known to be transmissible from animals to humans, where they cause significant zoonotic infections.
Other family groups (Simkaniaceae, Parachlamydiaceae, Rhabdochlamydiaceae, Waddliaceae, Criblamydiaceae) comprise an increasingly important and growing number of organisms often referred to as “Chlamydia-like” or “environmental chlamydiae”. While the role of many of these organisms in disease pathogenesis has yet to be defined, several novel species have emerged as putative ruminant abortifacient agents, as well as having a role in human miscarriage and pneumonia (Waddlia chondrophila and Parachlamydia acanthamoeba). Rhabdochlamydia has been associated with community-acquired pneumonia in humans, and very recently we have published evidence of the first association with bovine abortion.
Two other family groups (Piscichlamydiaceae and Clavichlamydiaceae) have been associated with infections in fish causing epitheliocystis.
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