Chlamydia species can be transmitted from one animal species to another, such as birds-to-cattle, birds-to-horses, and sheep-to-cattle; and from animals-to-humans and humans-to-animals. Animals may be co-infected with more than one type of chlamydia; and chlamydia psittaci (bird chlamydia) can mix in co-infected animals, to generate new serovars. Animals and humans can develop the same chronic diseases—from the same immortal pathogens.

In 1982, chlamydia psittaci was isolated from the lungs of a horse with a fatal respiratory infection. A psittaci pathogen was introduced into a Shetland pony intrathecally, and the pony developed local and metastatic lesions in the lung and liver. Post-mortem, the pony showed evidence of generalized psittacosis infection, lesions of interstitial pneumonia, and focal hepatic necrosis; and chlamydia psittaci was isolated from lung tissues. In 1992, nasal and conjunctival swabs of three hundred horses showed fifteen (five percent), were positive for chlamydia psittacosis, without evidence of disease. Chlamydia psittacosis has been isolated from fecal samples in dogs that were exposed to avian/bird chlamydia; and has been found in cats, dogs, horses, and cattle.