Animals get chlamydia through direct contact with infected animals, most often during mating, birth, or close physical proximity. Unlike the single species that infects humans, there are several distinct types of chlamydia bacteria, each adapted to different animal hosts. Birds, koalas, cats, livestock, and pigs all carry their own versions, and the way each species picks up the infection depends on its biology and behavior.
Different Chlamydia Species for Different Animals
Chlamydia isn’t one disease. It’s a family of closely related bacteria, and each member tends to target specific hosts. Koalas and cattle carry C. pecorum. Cats carry C. felis. Birds carry C. psittaci. Sheep and goats are primarily affected by C. abortus. Pigs carry C. suis. These bacteria cause overlapping symptoms across species (eye infections, reproductive problems, respiratory illness), but they spread through different routes depending on how the host animals live and interact.
How Koalas Get Infected
Koalas are the most well-known animal carriers, and their infection rates are staggering. Some populations in New South Wales and Queensland have infection rates as high as 94%. A 2024 study in Victoria’s Greater Geelong region found 66.7% of koalas tested positive.
Koalas contract chlamydia through sexual contact, direct physical contact, and exposure to infected urine. There’s also a uniquely koala route: pap feeding. Mother koalas produce a special fecal substance that joeys eat directly from the mother’s body to acquire gut bacteria needed to digest eucalyptus leaves. If the mother is infected, she passes chlamydia to her joey during this process. This combination of sexual and maternal transmission makes chlamydia extremely difficult to control in koala populations, and it’s a major driver of population decline. The infection causes blindness, urinary tract scarring, kidney damage, and irreversible infertility in both males and females.
How Birds Spread Chlamydia
Birds carry C. psittaci, the species responsible for a respiratory illness called psittacosis. Infected birds shed the bacteria in their droppings and respiratory secretions. Other birds (and sometimes humans) become infected by inhaling dust contaminated with dried droppings. The bacteria can also spread through direct contact, such as bites or beak-to-beak interaction.
Psittacosis was originally linked to pet parrots, but it occurs across a wide range of bird species. Chickens, ducks, geese, pigeons, and wild birds all carry it. The infection can circulate through flocks quickly because birds in close quarters share contaminated air and surfaces. City pigeons are a notable reservoir, carrying the bacteria in urban environments where their droppings accumulate.
How Cats Contract It
Cats pick up C. felis through direct, close contact with an infected cat. The bacteria is shed in eye discharge, respiratory secretions, and rectally, and infected cats can remain contagious for up to two months. The bacterium doesn’t survive well outside a host, so environmental contamination is less of a concern than nose-to-nose or face-to-face contact between cats.
Kittens under one year old are the most commonly infected. Shelters and multi-cat households see the highest rates because cats are kept in close proximity. The primary symptom is conjunctivitis, a red, swollen, watery eye infection that can affect one or both eyes. Some cats also develop upper respiratory symptoms like sneezing and nasal discharge.
Transmission in Livestock
Sheep, goats, and cattle primarily deal with C. abortus, which causes a condition called enzootic abortion. The main transmission route is oronasal: animals ingest or inhale the bacteria from contaminated material. The highest-risk moment is during and after an infected animal gives birth or miscarries. The placenta, birth fluids, and contaminated bedding are loaded with bacteria, and other animals in the flock become infected by ingesting or breathing in this material.
Pasture contaminated by abortion products is another route. Animals grazing on grass where infected birth material was deposited can pick up the bacteria days later. In pigs, C. suis spreads through similar close-contact and environmental routes, and the bacteria has been shown to persist in barn dust, though its ability to remain infectious outside a host appears to be short-lived. One study tracking C. suis in dust found that while bacterial DNA remained detectable for over 40 weeks, the actual living bacteria couldn’t be recovered after the first week.
What Chlamydia Does to Animals
The symptoms are remarkably consistent across species, even though different bacteria are involved. Eye infections are nearly universal: koalas develop severe conjunctivitis that can progress to blindness, cats get swollen and weeping eyes, and birds develop respiratory and ocular symptoms. Guinea pigs, which carry their own species called C. caviae, also develop conjunctivitis and urogenital infections.
Reproductive damage is the other major consequence. In koalas, the infection scars the reproductive tract, causing cysts on the ovaries, inflammation of the uterus, and permanent infertility. In sheep and goats, chlamydia is one of the leading infectious causes of late-term abortion, with the hallmark being severe inflammation and tissue death in the placenta. Pigs can experience respiratory disease, joint inflammation, and reproductive failure including abortion and loss of milk production.
Many infected animals show no symptoms at all. Asymptomatic carriers are a major reason chlamydia persists in animal populations: they appear healthy while continuing to shed bacteria to others.
Can Animal Chlamydia Spread to Humans?
Some animal chlamydia species do cross into humans, though this is relatively uncommon. The biggest risk comes from birds. Up to 62% of human psittacosis cases can be traced to documented contact with infected birds or their droppings. People who work in poultry processing or keep pet birds face the highest exposure. The infection causes pneumonia-like respiratory illness.
C. abortus from sheep and goats poses a serious but rarer threat. Pregnant women who have contact with infected sheep during lambing season can develop life-threatening systemic infections and pregnancy loss. This is primarily an occupational hazard for farmers and veterinary workers.
Cat chlamydia can occasionally jump to humans, causing chronic eye infections through direct contact with an infected cat’s ocular secretions. The human version of chlamydia, C. trachomatis, is essentially exclusive to people and does not circulate in animal populations.