Chlamydia pathogens can be transmitted from one animal species to another, including birds-to-cattle, birds-to-horses, birds-to-pigs, cattle-to-sheep, sheep-to-cattle, and infected animal-to-human. Animals can be co-infected with more than one type of chlamydia; and psittacosis is capable of recombination, in co-infected animals, and emerging as a new species,

Today, the major reservoir for chlamydia pneumonia and trachoma is humans. Chlamydia pneumonia is thought to have originated in birds and ducks; and, today is found in humans, cattle, sheep, and other domestic and wild animals. Birds are the major reservoir for psittacosis, and different species carry species-specific serovars. Psittacosis has been identified in humans, birds, cats, dogs, horses and pigs; and environments contaminated with bird feces.