Q Fever: The Disease

A bacterial zoonosis

Q fever, or coxiellosis, is a bacterial zoonosis caused by Coxiella burnetii.
It affects humans, ruminants (cows, sheep, goats, etc.), carnivores (dogs, cats), rodents, and birds. It is present on every continent and island, with the exception of New Zealand.
Rarely severe at onset, the disease can, in about 1% of cases, become chronic with serious cardiovascular complications (endocarditis, aneurysmal infections).

Q fever is classified as an occupational disease under both the general and agricultural social security systems.

In this context, Santé publique France’s missions are:

• Surveillance of the disease through the National Reference Center for Rickettsia, Bartonella, and Coxiella, to track trends and detect unusual events and clusters of cases

• Conducting investigations in the event of an alert or an outbreak

Modes of transmission

Most mammals and birds can be infected with Coxiella burnetii and serve as a source of the bacteria for humans. However, ruminants are most often the source of human infections. Ruminants can shed the bacteria in feces, vaginal secretions, or milk.
Once released into the environment, the bacteria can survive for many weeks, even under harsh conditions such as cold temperatures or acidic soil, and cause new human infections. It can also be carried by the wind up to 30 km away.
Direct contact with animals is therefore not essential to contract Q fever, and the disease can thus affect populations beyond those working in the livestock industry.

Humans can become infected:

• Most often through the respiratory tract, by inhaling the bacteria carried by air or wind along with dust,

• or through direct contact with infected animals (or animal products, such as in the case of a miscarriage, or with contaminated surfaces).

Other modes of transmission have been suggested but are not considered truly effective:

• Infection through consumption of raw milk from infected animals. Two cases of asymptomatic seroconversion were reported in the 1950s in the U.S., among several dozen people who voluntarily exposed themselves repeatedly to contaminated milk. Thus, the risk of infection through consumption of raw milk is considered virtually zero by French and European health authorities.

• Transmission via ticks has been suggested but not proven.

Prevention

Prevention of Q fever relies primarily on controlling the disease in animals and preventing the spread of the bacteria into the environment from infected livestock farms.

A vaccine for humans exists, produced in Australia and available under an ATU in France. Its use remains complicated due to the need for serological testing prior to vaccination. In practice, it is not used in France.

Chemoprophylaxis following exposure to the bacterium is not recommended.

Diagnosis

Q fever affects people of all ages, but it is most commonly reported in those between 30 and 70 years old.
The incubation period is 2 to 6 weeks, and the infection is asymptomatic in 60% of infected individuals.

Q fever in humans can present in an acute or chronic form.

The acute form can present in various ways:

• A nonspecific infectious syndrome with high fever (up to 40°C), muscle, and joint pain. This is the most common form. It generally resolves within a few days to a few weeks, even without treatment.

• Pneumonia that is generally mild but can last several weeks.

• Biological hepatitis without jaundice and often without gastrointestinal symptoms (no vomiting or diarrhea). Recovery usually occurs within 2 to 3 weeks.

• More rarely, pericarditis, myocarditis, or nervous system involvement (meningoencephalitis).

In children, fever lasts for a shorter duration than in adults (7 to 10 days) and gastrointestinal symptoms (diarrhea, vomiting, loss of appetite) are more common (up to 80% of cases). A rash is present in 50% of cases. Respiratory symptoms (pneumonia) are milder than in adults.

The chronic form occurs in approximately 1% of cases of acute Q fever. Manifestations are primarily rare but serious infections: infections of cardiac valve prostheses, infections of aneurysms, and endocarditis.

Acute Q fever is potentially more severe in certain population groups (risk of chronic disease, particularly endocarditis), especially in:

• Pregnant women at risk of premature birth or miscarriage, but not at risk of fetal malformation

• People with heart valve damage or vascular lesions (aneurysms) who have a higher risk of chronic infection than other patients.

The diagnosis of Q fever can be confirmed biologically by:

• Serological tests that detect antibodies against the bacterium. These tests distinguish between acute infections (by measuring so-called phase II antibodies) and chronic infections (by measuring so-called phase I antibodies). Interpreting the results can sometimes be difficult and may require several successive samples.

• Gene amplification

• Culturing samples from vascular lesions or endocarditis lesions to isolate the bacterium.

In contrast, culturing blood samples is most often negative.

Treatment based on antibiotics

Treatment for Q fever relies on antibiotic therapy, tailored by the physician based on the clinical presentation and any risk factors for severity or progression to chronicity. The HCSP recommendations provide guidance for the management and follow-up of the most complex cases.