When we use the term chlamydia to denote a common sexually transmitted bacterial infection, we are actually talking about one very specific strain of bacteria called Chlamydia trachomatis. This bacterium is just one of many different species from the Chlamydia genus, a family of microbial organisms that can cause a number of illnesses and health complications in both humans and animals.

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As of now, the scientists have identified three species of bacteria belonging to the Chlamydia genus that have been known to affect human beings. These are:

  • Chlamydia trachomatis
  • Chlamydophila pneumoniae
  • Chlamydophila psittaci

In addition to this, numerous other species that affect only animals are known to modern science. We can briefly present them in the following manner:

Bacterial species

Usual hosts

Chlamydia muridarum

Rodents; mice and hamsters

Chlamydia suis


Chlamydophila abortus


Chlamydophila caviae

Guinea pigs

Chlamydophila felis


Chlamydophila pecorum

Larger mammals, livestock

Chlamydia trachomatis

Chlamydia trachomatis is the strain of the bacteria that is responsible for chlamydia infection in humans. With more than 200.000 new cases of this bacterial infection being diagnosed annually within the UK alone, this condition is the most commonly diagnosed STI. Although usually present in the genitals, it can also infect anus, eyes or throat, causing an array of uncomfortable symptoms. Despite this, in the majority of those affected, chlamydia will remain completely asymptomatic – in roughly 50% of male cases and up to 80% of female cases.

If they do occur, the usual symptoms will include:

  • Painful urination
  • Genital or rectal discharge
  • Abdominal pain (in women)
  • Pain in the testicles
  • Bleeding between periods

In cases where chlamydia infects the eyes, typical symptoms of conjunctivitis will occur, including redness, swelling and discomfort in and around the eyes.

If left untreated, chlamydia can cause several complications, including pelvic inflammatory disease, inflammation of the testicles and infertility.

Chlamydia pneumoniae

Another strain that can cause health issues in humans, Chlamydia pneumoniae can cause inflammation of the lungs and symptoms such as:

  • Cough
  • Breathing problems
  • Fever
  • Increased heart rate
  • Chest pain

Most cases of infection with this strain of chlamydia can be easily treated with antibiotics, but if the illness progresses to a more severe stage, hospitalisation might be required. It can take as long as six months for one to recover from a serious case of C.pneumonieae infection.

As this is a respiratory illness, it is most often caused by breathing in the germs. And just like other airborne infections, C.pneumonieae can be easily transmitted through coughing, sneezing or coming into contact with the affected person.

Pneumonia can be caused by several different types of bacteria, fungi and viruses, so determining how widespread C.pneumonieae actually is based on this data alone might be a rather complicated task. However, some estimates place the prevalence of this condition to somewhere between 6% and 25% of all pneumonia cases.

At the same time, C.pneumonieae can also cause bronchitis and increase the risk of other more serious conditions such as atherosclerosis, exacerbation of asthma symptoms and even lung cancer.

Chlamydophila psittaci

C.psittaci is a strain of bacteria from the Chlamydia genus that is known to cause health condition known as psittacosis (colloquially called parrot fever) – a bacterial infection that will cause symptoms similar to those of influenza in those affected. Usually transmitted by birds, psittacosis has been discovered as a distinct illness in the late 1880s, but the bacteria causing it wasn’t identified until 1960s.

However, this condition is relatively rare, with somewhere between 25 and 50 reported cases annually within the United Kingdom. It is typically passed on through contaminated bird feathers or droppings, so it can be easily and successfully avoided by simply staying away from the birds and avoiding contact with them.

This condition affects both birds and mammals, and the state of the art research into the topic seems to suggest that certain species of reptiles might also be affected.