**3. Discovery of chlamydial organisms from past to present**

Chlamydial organisms, first described by Halberstaedter and von Prowazek in 1907, were isolated from human trachoma. During isolation, conjunctival cells were injected into orangutan cells, and intracellular inclusion bodies were identified in this way [11, 12]. These organisms were classified as protozoa, not bacteria, and given the name Chlamydozoa. The Greek word "chlamys" means a cloak, referring to the red elementary bodies (EBs) embedded in a blue matrix in bacteria [13].

Psittacosis, a zoonotic disease, was first described by Ritter in 1879. In 1985, Harris and Williams identified respiratory disease in people who had contact with tropical pet birds, which was found to be psittacosis [13]. The disease was named Psittokos after an outbreak in parrots in 1892. The etiologic agent was isolated in 1930, and research was

carried out on lovebirds. In the same year, *Chlamydia*, the etiologic agent of lymphogranuloma venereum (LGV) in humans, was isolated, and the two agents, which had similar characteristics, were known as psittacosis-LGV group viruses [5, 13].

Until the 1930s, *Chlamydia psittaci* was known to cause disease only in exotic and psittacine birds. With the reporting of new cases, it was discovered that the agent could also cause disease in other birds, including fulmar petrels, domestic pigeons, and ducks. The term ornithosis was used to describe the disease that developed in non-psittacine birds. By the 1950s, the zoonotic importance of *Chlamydia* was further understood, as outbreaks of human psittacosis-related ornithosis were seen in people in contact with ducks and turkeys [14].

The first notable case in farm animals occurred in 1936 when researcher Greig observed reproductive disorders and abortions in a flock of sheep, which he called enzootic abortion of ewes. He argued that the disease was caused by nutritional disorders, as he could not identify the etiologic agent. However, in the 1950s, it was proven by Stamp et al. that the etiologic agent of this disease was the psittacosis-LGV group organism. In addition, a respiratory disease seen in cats was also associated with this group of organisms in later years [13].

After all these developments, it was proven that *Chlamydia* was not a virus. They had a cell wall containing RNA and DNA, their cell wall structure was similar to Gram-negative bacteria, their reproductive cycle was different from viruses, and they had ribosomes sensitive to antibiotics. These characteristics classify them as part of the prokaryotic family [13].
