Scientists have sequenced the genome of the bacterium that causes Q fever, an illness that leaves a person incapacitated for several weeks with a debilitating headache and fever. The pathogen, Coxiella burnetii, is a potential bioweapon because it takes the form of an aerosol and causes flu-like symptoms. Thus, health officials might not initially recognize the pathogen as the cause of an outbreak.
  C. burnetti within vacuole of host cell. Courtesy Robert Henizeni |
“Q fever gives you an incredibly severe and debilitating headache,” says Rekha Seshadri, of The Institute for Genomic Research (TIGR) of Rockville, Maryland, where the sequencing was done. “For two weeks you’re wiped out.”
The sequencing revealed that C. burnetii is in the process of streamlining its genome. This genome reduction is typical of parasitic bacteria that exist symbiotically with their hosts: As they become increasingly dependent on their hosts to fulfill their biological needs, parasitic bacteria eliminate genes they no longer need.
The bacterium has been difficult to study because it cannot be cultured on artificial media, and it grows slowly. The microbe replicates about every ten hours (compared to a half hour for E. coli), which means it can take two weeks to grow enough for some experiments.
“This organism is so difficult to work with that only a small community of researchers studies it,” says John F. Heidelberg of TIGR. “Who wants to spend their career waiting around for an organism to grow?”
Now, he adds, researchers can use the genome sequence to explore hypotheses about the organism’s biology and also to identify proteins that might make new vaccine targets. (A vaccine has been developed by the military in Australia but is not available for civilian use.)
Historically, farm workers have been most at risk for the disease. The bacterium lives in cattle, sheep, and goats, among other animals, though it does not seem to cause them disease. People become infected through contact with the urine, blood, or feces of infected animals.
Another way to become infected is by inhaling particles carrying the bacterium, such as barnyard dust. The pathogen is shed in the amniotic fluid and the placenta of infected animals, and it can linger in the environment after the birthing. The microbe can withstand heat and is resistant to many common disinfectants.
The disease was called “Query” fever when it was first described in 1935. The report said a “mysterious febrile illness” had broken out among meat packing workers in Brisbane, Australia. The bacterium is found everywhere, and during World War II, thousands of soldiers in Europe became ill with the disease.
The US Centers for Disease Control in Atlanta has C. burnetii on its “B” list of potential bioweapons. It is highly infectious—a single organism may be sufficient to cause Q fever. This may be one reason the US military investigated the possibility of using the bacterium to debilitate enemy soldiers in the 1950s and 1960s.
There are two types of Q fever, acute and chronic. Most people get the acute form, which responds to drugs; symptoms last several weeks. About one percent of infected people have chronic Q fever, which is more difficult to treat and may require being on antibiotics for several years.
The sequenced strain, called Nine Mile, was discovered and isolated in the mid-1930s by Herald Cox and Gordon Davis at Rocky Mountain Laboratories in Hamilton, Montana. Current members of that facility, part of the US National Institute of Allergy and Infectious Diseases, participated in the genome sequencing.
The US Defense Advanced Research Projects Agency helped fund the project.
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