Q fever - Causes of Q fever

Q fever is caused by Coxiella burnetii (C. burnetii) bacteria. 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.

However, 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 don't have any symptoms, so it can be difficult to tell if an animal is infected. However, the bacteria can cause an increase in miscarriages among infected cows or 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.

How Q fever is spread to humans

Q fever can be spread to humans in a number of ways. The c. burnetii bacteria are tough and can survive in the outside environment for up to six months. They can be 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 not treated with heat to kill off bacteria)

Although it is possible for a bite from an infected tick to lead to Q fever, there have not been any recorded cases of this in the UK.

Human to human transmission

It is possible, although rare, to catch Q fever from another person. Most experts believe Q fever can only be transmitted from one human to another through:

sexual intercourse
a pregnant woman passing on the infection to her unborn child

Long-term Q fever

It is not clear why some people go on to develop long-term (chronic) Q fever. Most cases of chronic Q fever develop in people with a pre-existing health condition that makes them more vulnerable to infections. These conditions include:

coronary heart disease or congenital heart disease, particularly disease affecting the heart's valves, such as narrowing of the valves (stenosis)
having previously had heart valve replacement surgery
kidney disease
cancers that affect the blood, such as lymphoma and leukaemia

People with a weakened immune system are more vulnerable to infection. A weakened immune system can either be caused by a health condition, such as an HIV infection, or a side effect of certain medical treatments, such as chemotherapy or long-term steroid use.