Q fever is caused by bacteria known as coxiella burnetii (or c. burnetii). The bacteria are usually spread to animals by infected ticks. Samples of c. burnetii have been found in almost all types of animals, including reptiles and fish.
The most commonly affected animals, and those that pose the biggest risk to humans, are:
- sheep
- cows
- goats
- domesticated pets, such as dogs, cats and guinea pigs
Most animals with Q fever do not experience any symptoms, so it can be difficult to tell if an animal is infected. However, the bacteria can cause an increase in miscarriages among an infected herd of cows or flock of sheep.
The bacteria can be released by an infected animal through its:
- milk
- blood
- urine
- stools (faeces)
- fluids and products produced during birth, such as the afterbirth (placenta)
Birth by-products pose the greatest risk because they contain a high number of c. burnetii bacteria. For example, it is estimated that one gram of infected placenta contains enough bacteria to infect 100 million guinea pigs with Q fever.
How Q fever is spread to humans
Q fever can be spread to humans in a number of different ways. The c. burnetii bacteria are tough and can survive in the outside environment for up to six months. They can spread through:
- breathing in small particles released by infected animals, particularly when animals are being slaughtered or giving birth
- breathing in small particles from contaminated soil, dust, hay, straw bedding or other substances
- infected particles coming into contact with a cut on your skin or the white part of your eyes
- drinking unpasteurised milk from an infected animal (unpasteurised milk is milk that has not been treated with heat to kill off bacteria)
It is possible that a bite from an infected tick could lead to Q fever, but there have not been any recorded cases of this in the UK.
Human to human transmission
It is possible, but very rare, to catch Q fever from another person.
Most experts believe that Q fever can only be transmitted from one human to another through:
- sexual intercourse
- by a pregnant woman passing on the infection to her unborn child
Chronic Q fever
It is unclear why some people go on to develop the long-term (chronic) form of Q fever. Most cases of chronic Q fever develop in people with a pre-existing health condition, which makes them more vulnerable to infections. These health conditions include:
Vulnerability to infection is higher in people who have a weakened immune system, caused either by a health condition, such as HIV or AIDS, or as a side effect of certain medical treatments, such as chemotherapy or long-term steroid use.
