The genus chlamydia includes several species that can cause sexually transmitted infections (STIs), pneumonia and other respiratory infections, eye infections, and more illnesses.
When you hear the word chlamydia, you probably think of an STI. But chlamydia is actually a group of bacteria that can affect humans and other animals.
The four main types are chlamydia trachomatis, chlamydia pneumoniae, chlamydia psittaci, and chlamydia pecorum.
Even though they all belong to the same family, each bacterium behaves differently in terms of which animals are affected, how they spread, and what kind of illness they cause.
Antibiotics can successfully clear most chlamydia infections, regardless of the species involved.
| Species | Transmission | Primary exposure | Primary illness |
|---|---|---|---|
| C. trachomatis | Among humans | Anal or genital secretions | STIs and eye infections |
| C. pneumoniae | Among humans | Airbone respiratory droplets | Respiratory infections |
| C. psittaci | Birds to humans | Airborne fecal particles | Psittacosis |
| C. pecorum | Among non-human animals | Direct contact | Eye infections, blindness, arthritis, pregnancy loss, and infertility |
C. trachomatis is best-known for causing the STI chlamydia. It’s primarily transmitted through genital-genital, genital-anal, oral-genital, and oral-anal contact without a condom or other barrier method.
It can also be transmitted to a newborn during vaginal delivery and lead to blindness. Untreated chlamydia can lead to chronic pelvic pain, pelvic inflammatory disease, ectopic pregnancy, and infertility.
Chlamydia and other STIs are often asymptomatic. The only way to know your STI status is to get tested.
C. trachomatis can cause trachoma, a rare eye infection that can lead to blindness if left untreated. Trachoma is usually transmitted through direct or indirect contact with eye and nose secretions.
Trachoma symptoms include discharge, swollen eyelids, and sensitivity to light.
C. pneumoniae can infect the lungs and airways. It causes a variety of respiratory illnesses, including sore throat, bronchitis, and pneumonia.
It’s considered an atypical cause of pneumonia because it doesn’t show up on standard tests right away and might not respond as quickly to common treatments.
C. pneumonia is primarily transmitted through respiratory droplets released into the air when coughing or sneezing. That’s why it’s so important to cover your mouth or sneeze into your elbow to help contain any germs.
It’s most common among school-age children and adults 65 and older who live in close or crowded environments.
Staying up to date on the pneumonia vaccine can help reduce your risk of other strains of pneumonia. Wearing a mask or respirator can significantly reduce your risk of exposure and associated illness.
C. psittaci usually lives in birds, not people. However, it can spread to humans through airborne fecal particles, and when it does, it can make you very sick.
People who work with birds — including pet shop employees, poultry workers, and zookeepers — are at an increased risk. Wearing a mask or respirator can help prevent exposure to aerosolized or dried fecal matter.
In humans, it often starts like the flu but can quickly turn into psittacosis, a serious lung infection also known as “parrot fever.”
In birds, the infection may be asymptomatic or cause respiratory distress, diarrhea, or even death.
C. pecorum mainly affects livestock and wildlife. For example, it can cause conjunctivitis, polyarthritis, and genitourinary infections in sheep, cattle, and goats.
It poses a significant threat to koalas. In severe cases, eye and genitourinary infections can lead to blindness and infertility.
C. pecorum is not considered a major risk to humans, although scientists are still studying whether it could potentially be transmitted to people.
Even though these four bacteria are all in the chlamydia family, they affect very different parts of the body, spread in different ways, and target different hosts.
If you work with animals, are sexually active, or are exposed to people with respiratory illnesses, understanding these differences helps you protect your health and the health of those around you.