The signs and symptoms of Q fever are extremely variable besides being very mild in the majority of clinical cases, which themselves make up only about 45% of the infected population. Most infected individuals are asymptomatic.
Q fever has both acute and chronic forms. The acute infection develops between 14 and 39 days days after exposure, most commonly around 20. There are no classical signs of Q fever, with three types of presentations being reported.
Symptoms resembling the flu are the most frequently encountered in Q fever. They consist of fever lasting over a week but resolving in three weeks or less. The fever spikes to 104°F, accompanied by tiredness, headache, muscle aches. The fever lasts longer in older patients and may relapse in about 28% of patients.
Atypical pneumonia, which is manifested as a very mild cough without sputum, but with fever, is very common in patients with C. burnetii infection. It accounts for roughly 4% of all cases of community-acquired pneumonia. In most cases there are no crepitations, rhonchi or other signs on auscultation, but acute breathlessness may occur in a few patients. Chest X-rays are typically non-diagnostic. The illness may last for between 10 and 90 days, but the mortality rate is 0.5-1.5%.
Hepatitis may be completely asymptomatic, being reflected only by abnormal liver function tests. In other patients it may present with tender hepatomegaly but no jaundice, while a third form presents as pyrexia of unknown origin, with liver biopsy showing the doughnut granulomas formed by fibrin rings.
Q fever may also present as a maculopapular rash in one of every ten patients, inflammation of the myocardium or pericardium which often ends in a fatal outcome, and intense headache, perhaps due to encephalitis or meningitis. Rare manifestations include anemia with hemolysis, inflammation of the peripheral nerves or the optic nerve, pancreatitis or gastroenteritis, and significant lymph node enlargement.
Chronic Q fever may be diagnosed when the titers of IgG remain elevated for over six months after the onset of the illness. It is seen in about 5% of infections but its development may be gradual. It is due to the proliferation of C. burnetii in the macrophages, and a very high antibody level. Endocarditis, arteritis, osteomyelitis or osteoarthritis, and hepatitis are all forms of chronic infection. Endocarditis is most frequent in those with preexisting heart valve disease or weakened immunity. It may be suspected in endocarditis with a negative blood culture. Delayed diagnosis is the rule.
Chronic Q fever may rarely present as lymphoma-like illness, interstitial fibrosis of the lung, pericardial effusion, or aneurysmal infection.
During pregnancy, Q fever may show resurgence because of the reactivation of dormant C. burnetii. This may lead to increased rates of fetal loss, preterm birth and low birth weight, along with inflammation of the placenta or lowering of the platelet count. The mother herself is usually asymptomatic. Both acute and chronic Q fever have been described during pregnancy.