The genus Chlamydia comprises a diverse group of obligate intracellular bacteria that infect a wide range of hosts, from humans to various animal species. These pathogens must live inside host cells to reproduce. While Chlamydia trachomatis causes human sexually transmitted infections, it rarely affects animals. When discussing animals carrying Chlamydia, the focus is usually on different species within the same bacterial family that cause significant diseases in wildlife and livestock. Some of these animal-specific bacteria can transmit to humans, representing a major concern due to their zoonotic potential.
Common Animal Reservoirs of Chlamydia
Many animals harbor Chlamydia species, serving as reservoirs that cause widespread disease in animal populations and occasionally transmit to humans. Birds are well-known carriers, frequently housing Chlamydia psittaci. This bacterium causes avian chlamydiosis, affecting over 460 species of birds, including parrots, pigeons, and commercial poultry.
Marsupials in Australia face a severe threat from Chlamydia pecorum, a major pathogen in koala populations. The infection is highly prevalent in many koala habitats, often leading to debilitating and fatal disease. C. pecorum is also found in livestock globally, suggesting potential cross-host transmission.
Domestic livestock are substantial reservoirs for multiple Chlamydia species, causing considerable economic impact in agriculture. Sheep, goats, and cattle commonly carry C. abortus and C. pecorum. C. abortus is a primary cause of infectious abortion in small ruminants. In livestock, C. pecorum can cause issues including arthritis, conjunctivitis, and reproductive tract infections.
How Animal Chlamydia Spreads to People
Transmission of animal Chlamydia to humans is a zoonotic event. The primary mechanism of spread is the inhalation of contaminated aerosols. This occurs most often with C. psittaci when people breathe in dust particles containing the bacteria, which are shed in the dried feces, urine, or respiratory secretions of infected birds.
The resulting human disease, psittacosis or “parrot fever,” typically presents as an atypical pneumonia. Transmission risk is higher for individuals with occupational contact with birds, such as pet store employees, poultry workers, and veterinarians. Direct contact with an infected bird’s plumage or beak can also transmit the bacterium, though this is rare.
A different zoonotic risk exists with livestock infected by C. abortus, especially during lambing season. Infected ewes and goats shed millions of bacteria in their placenta, birth fluids, and uterine discharges after birth or abortion. Humans assisting with lambing or veterinary procedures can acquire the infection by inhaling aerosolized bacteria from these materials. Exposure to abortion products poses a specific danger, especially to pregnant women.
Consequences of Chlamydial Infection in Wildlife and Domestic Animals
Chlamydial infections inflict severe consequences on animal health, affecting reproduction and species survival. In koalas, C. pecorum infection is a major conservation crisis, causing painful and debilitating symptoms. The bacteria target the ocular and urogenital systems, leading to severe conjunctivitis and often permanent blindness.
The infection also causes severe urinary tract disease and reproductive tract damage, frequently resulting in infertility in both male and female koalas. This reproductive failure significantly contributes to the decline of koala populations across Australia. Controlling chlamydiosis is a central focus of koala conservation efforts.
In domestic animals, Chlamydia abortus is a major concern for sheep and goat farmers, primarily causing Enzootic Abortion of Ewes (EAE). The infection typically leads to abortion or the birth of stillborn or very weak lambs during late pregnancy. This results in substantial economic losses for the agricultural industry due to reduced flock productivity.
Birds infected with C. psittaci exhibit a range of symptoms, from asymptomatic carriage to severe systemic illness. Clinical signs include lethargy, respiratory distress, diarrhea, and ocular and nasal discharge. The disease can cause high mortality rates in poultry and pet birds, leading to significant losses for commercial farms.