OVERVIEW

What is Q fever?

Q fever is an acute or chronic infection caused by Coxiella burnetii (also known as Rickettsia burnetii or Coxiella burnetii), one of the most widespread zoonotic diseases globally^[1].

Coxiella burnetii is an obligate intracellular Gram-negative bacterium with strong resistance to physical and chemical factors, enabling it to survive for long periods in the external environment^[2].

Is Q fever highly prevalent?

Q fever is distributed worldwide and often occurs among farm workers who have frequent contact with animals and farm products.

In China, Q fever has been reported in over 20 provinces, autonomous regions, and municipalities, including Beijing, Hebei, and Inner Mongolia. Studies indicate that from 1989 to 2013, the prevalence rates were approximately 10% in humans, 15% in cattle, and 12% in goats, primarily in hilly, mountainous, and pastoral areas^[3][4].

Which department should I visit for Q fever?

In general hospitals, you can seek treatment at the Infectious Diseases Department.

SYMPTOMS

What is the cause of Q fever?

Q fever is caused by Coxiella burnetii, a pathogen primarily found in the feces, urine, and milk of poultry or farm animals such as sheep, cattle, and goats. Transmission routes include^[5]:

• Respiratory transmission: This is the primary route. The pathogen expelled from infected animals can form aerosols and enter the human body through the respiratory tract, causing infection.
• Contact transmission: The pathogen can enter through broken skin or mucous membranes. Occasionally, infection may occur through tick bites or when pathogens in tick feces enter via scratched wounds.
• Digestive transmission: The milk of infected animals often contains the pathogen, and consuming unpasteurized raw milk or water may lead to infection. However, digestive transmission has not been confirmed, and some researchers believe it results from inhaling aerosols formed by contaminated milk or water.

CAUSES

What are the common manifestations of Q fever?

The average incubation period after Q fever infection is 18–21 days, with acute and chronic stages.^[6][7]

• Acute Q fever

Symptomatic acute Q fever occurs in about half of infected individuals, with the most common symptoms including prolonged fever (>10 days), fatigue, chills, myalgia, and severe headache. Additionally, pneumonia is a key manifestation of acute Q fever, with respiratory symptoms such as dry cough and chest pain appearing 4–5 days after onset. Physical examination often reveals moist rales in the lungs, indicating pulmonary consolidation.

Compared to adults, children with acute Q fever generally experience milder illness, often presenting with flu-like symptoms such as fever, headache, and cough, as well as gastrointestinal symptoms like diarrhea, abdominal pain, vomiting, and anorexia. Moreover, children are more likely than adults to develop rashes. Reports indicate that up to 50% of pediatric patients with acute Q fever may exhibit rashes.

• Chronic Q fever

Chronic Q fever is rare, occurring in <5% of patients with acute infection, and may develop months, years, or even decades after the initial acute infection. Potential symptoms and signs include endocarditis, chronic hepatitis, chronic pulmonary infections, osteomyelitis, and osteoarthritis.

Among these, endocarditis is the primary manifestation of chronic Q fever, resembling subacute bacterial endocarditis caused by *Streptococcus viridans*. The aortic valve is most frequently affected, but vegetations may form on any valve. It may also be accompanied by clubbing of fingers, arterial emboli, hepatomegaly, splenomegaly, and purpura.

Without timely treatment, the fatality rate of Q fever can reach 1%, and it is even higher in patients with endocarditis.

DIAGNOSIS

How is Q fever diagnosed? What tests may be required?

Diagnosing Q fever based solely on clinical symptoms is challenging. A clinical diagnosis must combine epidemiological data, clinical manifestations, and laboratory tests.

1. **Epidemiology**: Whether the patient is from a Q fever-endemic area, has a history of contact with livestock or wild animals in such areas, has been bitten by ticks or other blood-sucking arthropods, or has pre-existing cardiovascular conditions like valvular heart disease or prosthetic heart valves.

2. **Clinical manifestations**: Symptoms such as fever, fatigue, headache, and myalgia, often accompanied by interstitial pneumonia or liver dysfunction.

3. **Laboratory tests**^[5]:

• **Routine tests**: Most acute Q fever patients have normal blood cell counts, though about 30% may show elevated white blood cells. Chronic Q fever patients often exhibit significantly increased erythrocyte sedimentation rate (ESR). Mild proteinuria may occur during fever, and microscopic hematuria may be observed in Q fever endocarditis patients.
• **Serological tests**: Highly specific, including indirect immunofluorescence assay (IFA) and enzyme-linked immunosorbent assay (ELISA) to detect specific antibodies (Phase II and Phase I antibodies). Two to three serum samples may be required during the course of the illness.
• **Molecular testing**: Polymerase chain reaction (PCR) and DNA probe techniques can detect pathogen-specific DNA in samples, offering high specificity and sensitivity.
• **Pathogen isolation**: Blood samples (2–3 mL) collected during the febrile phase can be inoculated into animals for isolation, but this should only be performed in specialized research laboratories to prevent infection spread. Routine blood and sputum cultures are typically negative.
• **Other tests**: Liver function may show mild abnormalities, and liver biopsy may reveal diffuse granulomatous changes. Electrocardiograms may show T-wave or ST-segment abnormalities. Echocardiography may detect vegetations in cases of Q fever endocarditis.

TREATMENT

How is Q fever treated?

The treatment for acute Q fever remains oral doxycycline, with an initial dose of 200mg, followed by 100mg twice daily until symptoms improve for at least 5 days, with a total treatment duration of at least 7 days. Generally, 2–3 weeks of treatment are required.

Chronic Q fever endocarditis patients require prolonged treatment, ranging from several months to years or even lifelong, typically for at least 18 months. The current recommendation is doxycycline 100mg twice daily and hydroxychloroquine 200mg every 8 hours orally. Clinical symptoms, erythrocyte sedimentation rate, blood cell count, and antibody titers should be monitored to determine when to discontinue treatment. Generally, if heart valve damage occurs, surgical valve replacement is necessary [6].

If Q fever occurs during pregnancy, treatment with co-trimoxazole (trimethoprim/sulfamethoxazole) is required to reduce the risk of the mother developing chronic Q fever and prevent adverse pregnancy outcomes. After delivery, serological monitoring at 3, 6, 12, 18, and 24 months can be performed to check for Q fever recurrence or progression to chronic Q fever^[7].

DIET & LIFESTYLE

Is Q fever highly contagious? Can it spread between humans?

Coxiella burnetii can widely infect humans and animals through aerosols, and the population is generally susceptible to it, which is why the U.S. counterterrorism organization has listed it as one of the biological warfare agents [1].

The main sources of human Q fever infection are infected livestock such as sheep, cattle, and goats. Among animals, the pathogen is transmitted through ticks as vectors and can be passed down through eggs. After infection, animals may carry the pathogen in their milk, urine, and feces for extended periods. Human-to-human transmission is extremely rare.

PREVENTION

Can Q fever be prevented? How to prevent it?

Q fever can be prevented. For individuals, the main measures include protective actions such as wearing masks and practicing proper hand hygiene. In some countries, vaccination is also available. For institutions, common prevention methods include^[5][6]:

(1) Managing the source of infection:

• In non-endemic areas, strengthen quarantine measures for imported animals to prevent the introduction of asymptomatic or carrier animals.
• In endemic areas, identify and isolate infected animals promptly through clinical observation and serological screening.
• Milk or other products from infected animals must undergo strict sterilization before use, such as pasteurization for drinking milk.

(2) Cutting off transmission routes: Enhance disinfection in livestock facilities, improve ventilation systems, and emphasize personal protection.

(3) Protecting susceptible populations: For example, vaccination is available in Australia, recommended for workers in slaughterhouses, dairy farms, forestry, ranches, wool mills, farms, and other high-risk groups (e.g., pregnant women, individuals with heart valve disease).