The results of the blood tests confirmed that the patient did in fact have Q fever. So what is Q fever? That’s a very interesting query… okay, sorry, I’ve been told time and again not to use the word “interesting” to describe anything microbiological but I couldn’t resist!
Q fever is the name given to infection caused by the bacterium Coxiella burnetii; the “Q” really does stand for “Query” as the cause of the fever in the original 1935 outbreak in abattoir workers in Australia was unknown. Q fever occurs all over the world with a few exceptions, apparently there is no Q fever in New Zealand, possibly because New Zealand has no native mammals and therefore did not originally have any animals to act as hosts for the ticks which are the main reservoir of C. burnetii.
- Animal farmers and abattoir workers
- People who live downwind of animal farms where contaminated manure, straw or dust blow across their houses and gardens
- People who live in endemic countries e.g. French Guiana
The clinical presentation of Q fever following an incubation period of about 20 days is very variable. Up to 50% of patients are asymptomatic. The most common acute presentations of Q fever are:
- Self-limited flu-like illness – high fever (up to 40oC), tiredness, headache and muscle pains are common and typically last for 1-3 weeks
- Pneumonia – usually mild with a non-productive cough and fever lasting anywhere from 10 days to 3 months, mortality is approximately 1%
- Hepatitis – raised liver enzymes, hepatomegaly and fever
- Endocarditis – this is thought to be an autoimmune complication of acute infection in about 1% of patients and occurs in the presence of antiphospholipid antibodies which can be detected in the patients’ blood
- Rash – occurs in 10% of patients and can be maculopapular or purpuric
- Pericarditis and myocarditis – this rare complication occurs in less than 1% of patients with acute Q fever but has a high mortality (25%)
- Meningitis or encephalitis – occurs in less than 1% of patients
Symptomatic infection is more likely to occur in adults, with adults more likely to have pneumonia or endocarditis and children hepatitis. Oddly, the clinical presentation also tends to vary on where in the world infection was acquired; in Canada pneumonia is more common whereas in France and Spain hepatitis is the usual presentation.
Chronic infections occur in up to 5% of patients following both symptomatic and asymptomatic acute infection with Q fever. Chronic infections are more likely to occur in those with underlying immunosuppression, pregnancy, pre-existing cardiovascular disease or prosthetic joints. The bacterium lives and continues to replicate within the hosts macrophages (which renders them unable to kill the bacterium, as it would be self-destructive) resulting in a sustained bacteraemia. There are also high levels of autoantibodies and these cause further damage to tissues.
The most common chronic infections are:
- Endocarditis – usually associated with low-grade fever, tiredness, weight loss, and night sweats. It occurs between 2 months and 2 years after the original infection and is more common in the presence of a pre-existing heart valve abnormality
- Vascular infection – presents with fever, abdominal pain and weight loss. There is usually a pre-existing aneurysm or vascular graft
- Bone and joint infection – presents with fever and joint pain in the same way as any other joint infection and can occur in both native and prosthetic joints
How do you diagnose Q fever?
C. burnetii is a bit of a nuisance when it comes to diagnosis as it doesn’t grow in routine blood cultures or on agar; it can only survive inside other cells (a bit like a virus). In order to make a diagnosis you have to look for the body’s immune response to infection by detecting antibodies against C. burnetii.
There are 2 main types of antibody against C. burnetii, phase I and phase II. Phase I antibodies are produced when the bacterium is highly infectious and capable of infecting humans whereas phase II is produced when the bacterium has undergone antigenic shift into a non-infectious form (it has changed the antigens it is expressing rather than changing its genetics as with Influenza). The different phases of antibody can be used to differentiate acute and chronic infection but be warned the results are counter intuitive. Phase I DOES NOT go with acute infection and phase II with chronic infection as you would imagine! It feels wrong but it is the way it is!!
Okay, so you’ve made the diagnosis, how do you treat your patient?
The treatment of Q fever depends on whether it is acute or chronic and can take up to 2 years in order to completely eradicate the bacterium. In adults the choices are usually:
So the patient underwent echocardiography which fortunately showed no pre-existing heart disease. He continued on Doxycycline for 14 days and made a full recovery. Failure to diagnose and treat the initial acute infection can lead to the development of chronic Q fever. The public health team were able to confirm that there had been no other cases relating to the same farm but a general reminder was sent to all the local hospitals asking them to be vigilant for further cases.