Chlamydia is a widespread bacterial infection affecting millions globally, found in both human and animal populations. This ubiquitous presence sometimes sparks curiosity about potential links between species, particularly regarding whether human chlamydia might have originated from animals like koalas. Exploring the scientific understanding of this connection reveals the distinct evolutionary paths these bacteria have taken in different hosts.
Chlamydia in Humans and Koalas
While the term “chlamydia” is broadly used, the bacterial species infecting humans and koalas differ. In humans, Chlamydia trachomatis is the primary cause of sexually transmitted infections (STIs) and ocular infections, such as trachoma. Koalas are predominantly affected by Chlamydia pecorum, though Chlamydia pneumoniae is also found in koala populations.
These bacterial species exhibit biological differences, including their preferred host cells and the diseases they cause. C. trachomatis primarily targets human epithelial cells in the reproductive tract and eyes, while C. pecorum in koalas can infect multiple systems, including the eyes, urinary tract, and reproductive organs. Due to host specificity, direct transmission of human STI-causing C. trachomatis from koalas is not expected. While C. pneumoniae can infect both humans and koalas, it manifests as a respiratory pathogen in humans and is less pathogenic in koalas compared to C. pecorum.
Impact on Koala Populations
Chlamydia has serious effects on koala populations, contributing to their threatened status. The infection in koalas, primarily caused by C. pecorum, leads to health problems. Koalas often develop conjunctivitis, which can cause severe inflammation, discharge, and lead to blindness.
The bacteria also cause urinary tract infections, manifesting as a “wet bottom” from urine staining, and reproductive tract infections. These reproductive infections can result in cysts and scarring, leading to infertility in both male and female koalas. The widespread prevalence of chlamydia, ranging from 0% to nearly 90% in some wild koala populations, poses a conservation challenge.
Tracing Chlamydia’s Evolutionary Origins
Genetic and phylogenetic studies provide insights into the evolutionary history of chlamydia species. Evidence suggests that Chlamydia trachomatis, the primary cause of human STIs, evolved within the human lineage. This specialization means C. trachomatis did not jump directly from animals like koalas to become a widespread STI.
While zoonotic spillover, the transmission of diseases from animals to humans, is a known phenomenon, scientific evidence does not support koalas as the source of human STIs caused by C. trachomatis. Scientists use genomic sequencing to trace disease origins, revealing distinct evolutionary paths for different chlamydia species. Despite the shared name and its impact on koalas, sexually transmitted chlamydia in humans has a separate and long-standing evolutionary history with our species.