The bacterium that causes Q fever — an infection usually transmitted to humans by barnyard animals — can trigger anti-neutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis (AAV), a recent case report suggests.
Titled “Coxiella burnetii endocarditis as a possible cause of ANCA-associated vasculitis,” the report was published in Rheumatology.
The causes of autoimmune diseases like AAV are largely unknown. One hypothesis is that infections can trigger these diseases. The underlying logic essentially is that the immune system becomes activated to fight off infectious invaders, and in the course of this activation, the body’s own healthy cells are “caught in the crossfire” and targeted by the immune system.
Although this idea is impossible to prove in an ethical experiment with people, looking in detail at the sequence of events preceding individual cases of AAV can provide clues as to the underlying cause.
This case report details the diagnosis of a 73-year-old man who went to the hospital after experiencing symptoms of fever, joint and muscle pain, and lack of energy for three months.
Blood tests revealed the presence of antibodies against Coxiella burnetii, the bacterial species that causes Q fever, originally known as query fever — a disease that is typically mild and causes flu-like symptoms. Originally identified as “query” by a doctor in Australia because of the inexplicable nature of the disease, the pathogen also was isolated from ticks in Montana. Sheep, goats, and cattle are now the main source of infection for humans, research shows.
Although this bacteria itself was not detected in the patient’s blood, echocardiography (ultrasound of the heart) showed some aortic insufficiency, or “leaking” of the heart valves, that had not been present in prior examinations.
From these data, “the diagnosis of possible C. burnetii persistent endocarditis was established,” the researchers said. Endocarditis is an infection of the endocardium, which is the inner lining of the heart chambers and heart valves.
Accordingly, the patient was started on a course of antibiotics.
The man’s fever resolved within two weeks of starting this treatment, but joint and muscle pain persisted. During that time, elevated levels of protein were found in the man’s urine, indicative of kidney damage.
Subsequent blood tests revealed the presence of ANCA against proteinase 3 (PR3), an established cause of AAV. The results of a kidney biopsy were indicative of inflammatory damage also associated with the disease, “confirming the diagnosis of ANCA-associated vasculitis with articular and renal involvement.”
The patient then was started on a course of immune system-suppressing therapies: the corticosteroid prednisone and the chemotherapy medicine cyclophosphamide as induction therapy, followed by Rituxan (rituximab) as maintenance therapy. These treatments were given in combination with the antibiotic course to treat the Q fever.
With this combination of treatments, all of the patient’s symptoms resolved: joint and muscle pain eased, and kidney function was stabilized.
“This case emphasizes the importance of searching for an autoimmune disease in patients with Q fever in case of persistence of symptoms after the first weeks of antibiotic therapy,” the researchers said in the case report.
Interestingly, ANCAs have been previously reported in people with Q fever. But this is the first instance of ANCA vasculitis, specifically, developing following an infection with Coxiella burnetii.
It is, of course, impossible to definitively prove that the bacterial infection directly caused the autoimmune disease in this patient, the researchers said. However, the team believes this to be the most likely explanation.
“In our patient, the temporal sequence of events strongly suggested a causal relationship between Q fever and ANCA-associated vasculitis,” the investigators said. They added that their findings suggest that “C. burnetii should be considered as a potential trigger for ANCA-associated vasculitis.”