Chlamydia felis (formerly Chlamydophila felis and before that Chlamydia psittaci var. felis) is a Gram-negative, obligate intracellular bacterial pathogen that infects cats. It is endemic among domestic cats worldwide, primarily causing inflammation of feline conjunctiva, rhinitis and respiratory problems. C. felis can be recovered from the stomach and reproductive tract. Zoonotic infection of humans with C. felis has been reported. Strains FP Pring and FP Cello have an extrachromosomal plasmid, whereas the FP Baker strain does not. FP Cello produces lethal disease in mice, whereas the FP Baker does not. An attenuated FP Baker strain, and an attenuated 905 strain, are used as live vaccines for cats. Taxonomy and Phylogeny The genus Chlamydia contains the species C. trachomatis, C. psittaci, C. abortus, C. felis, C. muridarum, C. suis, C. caviae, C. pecorum, and C. pneumoniae. However, there is some dispute that some of these species, including C. felis, should be classified in a separate genus, Chlamydophila, but this has not been widely accepted. C. felis is closely related to C. pneumoniae (causes pneumonia in humans), C. trachomatis (causes Chlamydia in humans), and C. muridarum (causes pneumonia in mice). C. pneumoniae shares 879 orthologs, genes that come from a common ancestor, with C. felis while C. trachomatis and C. muridarium both share 841 orthologs with C.felis. All Chlamydia species are Gram-negative, obligate intracellular pathogens with two distinct life stages (see Characterization) that are able to infect a wide range of mammals and birds around the world. Discovery James A. Baker published the finding of Chlamydia felis (known as Chlamydophila felis at the time) in 1942, but did not classify the organism. The organism was not formally classified until 1999. Baker started researching C. felis due to the number of atypical pneumonia cases observed in cats and later discovered that the atypical human pneumonia cases coincided with feline cases. Cats infected with this atypical pneumonia were recognized by their symptoms of sneezing, coughing, and ocular and nasal discharge. The disease was characterized by its highly infectious nature and long infection time. To determine what the causative agent was, Baker made a suspension of the infected cat lungs and used the suspension to infect mice via their nasal passage. The infected specimens died 2-5 days after initial infection. After autopsy of the deceased specimens, Baker confirmed that they died from the same disease from the condition of the lungs of the mice. He