Diagnosis of Chronic Q Fever Infections
The isolation of the organism is not practicable in the majority of laboratories and is less certain and more time-consuming than serology. CFT is the most widely used test but IF and ELISA techniques are also used. In acute Q fever, phase II antibodies are always present whereas phase I antibodies are usually transient and of low titre. In chronic Q fever, phase I antibodies are always present and are often very high. Phase II antibodies are usually present also, often at a considerably higher titre than phase I antibodies. A single serum is usually sufficient to produce the diagnosis; rising titres are rarely found in chronic infections. The differences in antibody responses between acute and chronic infections may be because the phase II antigen is more superficial, or perhaps because in chronic infections, organisms persist in phase I. There has been much discussion about diagnostic levels of phase I antibody in chronic Q fever. Serological tests differ in sensitivity from laboratory to laboratory but most authors accept a phase I CF titre of 1:200 or more as diagnostic of chronic infection.
The prognosis of Q fever endocarditis is highly variable. There had been many reports of fatal cases. Some patients die within a few months despite appropriate antibiotic treatment. However, most patients respond satisfactorily to antibiotic therapy but treatment needs to be kept up for a long period or even for life. Tetracyclines are the mainstay of treatment, either alone or in combination with other antibiotics. There has been controversy over the duration of antibiotic therapy; some authors suggest that treatment should be continued indefinitely, while others suggest that treatment should be for periods of at least 12 months, or until there is clinical evidence of resolution of endocarditis or the phase I CF antibodies have fallen below 200. Each patient should be treated individually. Patients should be warned that antibiotic treatment should continue for at east 2 years or more. Surgical replacement valves is indicated where the valves are severely damaged. However, numerous cases of infection of prosthetic valves have been recorded.
The majority of cases of acute Q fever do not require follow-up. They make a rapid clinical recovery with or often without tetracycline treatment. Some of the more severe cases require more careful consideration especially if complications such as myocarditis, hepatitis, encephalitis or haemolysis are seen. In these cases, as well serological testing for phase II antibodies, tests should be carried out for phase I antibodies. Rarely, the antibody persists for months without any signs of chronic infection. To date, little is known about the development of chronic Q fever following the acute attack, and it is not possible to postulate any predisposing factors which may lead to the development of chronic disease. However, f a patient suffering from a valvular abnormality of the heart develops Q fever, this is clearly an indication for thorough and probably prolonged antibiotic therapy and follow-up.