Chlamydia is a household term, used to describe the sexually transmitted bacterial infection, caused by Chlamydia trachomatis.

However, C. trachomatis is only one in a family of chlamydial strains, which can cause a number of illnesses in humans, and in animals too.

There are three chlamydial (or chlamydia-derived) species of bacteria known to affect humans, which this page will discuss, including:

  1. Chlamydia trachomatis
  2. Chlamydophila pneumoniae
  3. Chlamydophila psittaci

However, there are also several others which tend to only affect animals, including:

  • Chlamydia muridarum (larger rodents, such as mice and hamsters)
  • Chlamydia suis (pigs)
  • Chlamydophila abortus (mammals)
  • Chlamydophila caviae (guinea pigs)
  • Chlamydophila felis (cats)
  • Chlamydophila pecorum (larger mammals, such as those used in livestock)

Chlamydia trachomatis

This is the strain responsible for chlamydia infections in humans. There are around 200,000 cases of chlamydia reported in the UK on an annual basis, making it the most commonly diagnosed STI.

It can affect several parts of the body, including the genitals, anus, eyes and throat, and cause a variety of symptoms. However, in 80 percent of female cases and 50 percent of male cases, it will cause no noticeable signs, making it an easy infection to miss.

Where they do occur, symptoms may include:

  • in genital cases: urinary pain, discharge from the penis or vagina, abdominal pain, testicular pain, or intermenstrual bleeding;
  • in rectal cases: pain and discharge from the anus;
  • or in ocular cases: redness and discomfort in the eyes (conjunctivitis).

Where the infection is not treated, it can spread and cause complications and further illness, including pelvic inflammatory disease.

Chlamydophila pneumoniae

This strain can cause pneumonia in humans, which is an illness characterised by inflammation in the lungs. This might cause symptoms such as:

  • coughing 
  • breathing problems
  • fever
  • increased heart rate
  • or chest pain

Cases which are not severe can be treated with antibiotics, however more serious cases will usually require hospital admission. It can take around six months for someone to fully recover from a serious bout of pneumonia.

As a respiratory illness, pneumonia is typically acquired by breathing in germs. Like other airborne infections, these can transmitted through touch, coughing or sneezing.

Pneumonia can be caused by several different types of bacteria, as well as viral and fungal infections; so it’s difficult to determine how many cases of are caused by chlamydophila pneumoniae alone. However, some estimates tend to range between six and 25 percent.

This strain of bacteria can also cause bronchitis, and studies have also linked infection with chlamydia pneumoniae with an increased risk of lung cancer, and atherosclerosis. The presence of this bacteria can also increase the severity of asthma symptoms.

Chlamydophila psittaci

This species of chlamydia bacteria can cause a condition called psittacosis (also known as parrot fever), and is typically carried and transmitted by birds. In humans, psittacosis presents symptoms which are close to those of influenza (headache, fever, and muscular pain) and can lead to pneumonia. Psittacosis is thought to have been first observed in the late 1800s, however the bacteria was not properly identified until the 1960s.

The condition is not common. Public Health England reports that, in England and Wales, there are only between 25 and 50 instances confirmed on an annual basis. It is typically passed on through contaminated bird feathers or droppings; therefore, individuals at higher risk of contracting the infection are those who are commonly exposed to birds.

As well as birds and humans, Chlamydia psittaci can also cause infections in mammals, and reportedly in some reptiles.

Page last reviewed:  Tuesday, Apr 25 2017