Q fever is a disease caused by Coxiella burnetii, a type of bacteria that is found around the world. Animals are carriers of the disease and can spread it to humans. Infection of humans usually occurs by breathing in organisms from air that contains barnyard dust contaminated by dried placental material, birth fluids, and excreta of infected herd animals.
Other modes of transmission to humans, including tick bites and human to human transmission, are rare.
In 1999, Q fever became a notifiable disease in the United States but reporting is not required in many other countries. Because the disease is underreported, scientists cannot reliably assess how many cases of Q fever have actually occurred worldwide. The name Q fever comes from an epidemic of fever that occured in Queensland, Australia, where several workers in a meat processing plant became ill. The Q stands for "query" since people did not know what was causing the disease.
Only about one-half of all people infected with C. burnetii show signs of illness. Most acute cases of Q fever begin with sudden onset of one or more of the following: high fevers (up to 104-105° F), severe headache, confusion, sore throat, chills, sweats, non-productive cough, nausea, vomiting, diarrhea, abdominal pain, and chest pain. Fever usually lasts for one to two weeks. Weight loss can occur and persist for some time. Thirty to fifty percent of patients with a symptomatic infection will develop pneumonia. Additionally, a majority of patients have abnormal results on liver function tests and some will develop hepatitis. In general, most patients will recover to good health within several months without any treatment. Only 1%-2% of people with acute Q fever die of the disease.
Chronic Q fever, an infection that lasts for more than 6 months is uncommon but is a much more serious disease. Patients who have had acute Q fever may develop the chronic form as soon as 1 year or as long as 20 years after initial infection. A serious complication of chronic Q fever is an infection of the heart valves called endocarditis. It generally involves the aortic heart valves, less commonly the mitral valve. Most people who develop chronic Q fever have pre-existing valvular heart disease or have a history of vascular graft. Transplant recipients, patients with cancer, and those with chronic kidney disease are also at risk of developing chronic Q fever. As many as 65% of people with chronic Q fever may die of the disease.
The incubation period (the period of time between exposure to a disease and the devlopment of symptoms) for Q fever varies depending on the number of organisms that initially infect the patient. Infection with greater numbers of organisms will result in shorter incubation periods. Most people become ill within 2-3 weeks after exposure. Those who recover fully from infection may possess lifelong immunity against re-infection.
Cattle, sheep, and goats are the primary reservoirs or carriers of C. burnetii. Infection has been noted in a wide variety of other animals, including other species of livestock and in domesticated pets. Coxiella burnetii does not usually cause clinical disease in these animals, although abortion in goats and sheep has been linked to C. burnetii infection. Organisms are excreted in milk, urine, and feces of infected animals. Most importantly, during birthing the organisms are shed in high numbers within the amniotic fluids and the placenta. The organisms are resistant to heat, drying, and many common disinfectants. These features enable the bacteria to survive for long periods in the environment. Infection of humans usually occurs by inhalation of these organisms from air that contains airborne barnyard dust contaminated by dried placental material, birth fluids, and excreta of infected herd animals. Humans are often very susceptible to the disease, and very few organisms may be required to cause infection.
Because the signs and symptoms of Q fever are not specific to this disease, it is difficult to make an accurate diagnosis without appropriate laboratory testing. Results from some types of routine laboratory tests in the appropriate clinical and epidemiologic settings may suggest a diagnosis of Q fever. For example, a platelet count may be suggestive because persons with Q fever may show a transient thrombocytopenia or decrease in platelet number. To determine if the organism is actually present, it is necessary to do additional testing. These tests involve checking for antibodies to the bacteria or using special stains that allow the bacteria to become visible immunohistochemical staining.
Doxycycline is the treatment of choice for acute Q fever. Antibiotic treatment is most effective when initiated within the first three days of illness. A dose of 100 mg of doxycycline taken orally twice daily for 15-21 days is a frequently prescribed therapy. Quinolone antibiotics have demonstrated good in vitro activity against C. burnetii and may be considered by the physician. Therapy should be started again if the disease relapses.
Chronic Q fever endocarditis is much more difficult to treat effectively and often requires the use of multiple drugs. Two different treatment protocols have been evaluated: 1) doxycycline in combination with quinolones for at least 4 years and 2) doxycycline in combination with hydroxychloroquine for 1.5 to 3 years. The second therapy leads to fewer relapses, but requires routine eye exams to detect accumulation of chloroquine. Surgery to remove damaged valves may be required for some cases of C. burnetii endocarditis.
In the United States, Q fever outbreaks have resulted mainly from work exposure involving veterinarians, meat processing plant workers, sheep and dairy workers, livestock farmers, and researchers at facilities housing sheep. Prevention and control efforts are directed primarily toward these groups and environments.
The following measures are used in the prevention and control of Q fever:
- Educating the public on sources of infection.
- Appropriately disposing of placenta, birth products, fetal membranes, and aborted fetuses at facilities housing sheep and goats.
- Restricting access to barns and laboratories used in housing potentially infected animals.
- Using only pasteurized milk and milk products.
- Using appropriate procedures for bagging, disinfecting, and washing of laboratory clothing.
- Vaccinating (where possible) individuals engaged in research with pregnant sheep or live C. burnetii.
- Quarantining imported animals.
- Ensuring that holding facilities for sheep are located away from populated areas. Animals should be routinely tested for antibodies to C. burnetii, and measures should be implemented to prevent airflow to other occupied areas.
- Counseling persons at highest risk for developing chronic Q fever, especially persons with pre-existing cardiac valvular disease or individuals with vascular grafts.
A vaccine for Q fever has been developed and has successfully protected humans in occupational settings in Australia. However, this vaccine is not commercially available in the United States. Persons wishing to be vaccinated should first have a skin test to determine a history of previous exposure. Individuals who have previously been exposed to C. burnetii should not receive the vaccine because severe reactions, localized to the area of the injected vaccine, may occur. A vaccine for use in animals has also been developed, but it is not available in the United States.
How is Q Fever Spread
Q fever can get into the air easily. Humans then breathe in the infected air. It can also be spread by drinking infected milk or being sprayed by infected blood. An infected animal can spread the disease while giving birth since the air can become full of particles that contain the bacteria.
Coxiella burnetii is a highly infectious agent that is rather resistant to heat and drying. It can become airborne and inhaled by humans. A single C. burnetii organism may cause disease in a susceptible person. This agent could be developed for use in biological warfare and is considered a potential terrorist threat.