Published on 22/03/2015 by admin
Filed under Pediatrics
Last modified 22/03/2015
This article have been viewed 1002 times
Chapter 224 Q Fever (Coxiella burnetii)
Megan E. Reller, J. Stephen Dumler
Q fever (for query fever, the name given following an outbreak of febrile illness in an abattoir in Queensland, Australia) is rarely reported in children but is probably underdiagnosed. Symptomatic patients can have acute or chronic disease.
Although previously classified within the order Rickettsiales, Coxiella burnetii (the causative agent of Q fever) is genetically distinct from the genera Rickettsia, Orientia, Ehrlichia, and Anaplasma. Hence, based on small subunit rRNA gene sequencing, it has been reassigned to the order Legionellales, family Coxiellaceae. C. burnetii is highly infectious for both humans and animals; even a single organism can cause infection. The agent has been nationally notifiable since 1999 and is listed as a Category B agent of bioterrorism by the Centers for Disease Control and Prevention. Unlike Rickettsia, the organism can enter a sporogenic differentiation cycle, which renders it highly resistant to chemical and physical treatments.
C. burnetii resides intracellularly within macrophages. The organism undergoes a lipopolysaccharide phase variation similar to that described for smooth and rough strains of Enterobacteriaceae. Unlike Ehrlichia, Anaplasma, and Chlamydia, C. burnetii that express phase I lipopolysaccharide can survive and proliferate within acidified phagosomes to form aggregates of >100 bacteria. In contrast, organisms that express phase II lipopolysaccharide are killed in the phagolysosome.
The disease is reported worldwide, except in New Zealand. Although seroepidemiologic studies suggest that infection occurs just as often in children as in adults, children less often present with clinical disease than do adults. Approximately 60% of infections are asymptomatic, and only 5% of symptomatic patients require hospitalization. Seroprevalence surveys show that 6-70% of children in endemic European and African communities have evidence of past infection, and in France the overall incidence of Q fever is estimated to be 50 cases per 100,000 persons. Cases in Africa are likely misdiagnosed as malaria. Reported cases of Q fever in the USA have increased by 6.5-fold from 26 cases in 2001 to 171 cases in 2007, which might reflect an increase in incidence, increased reporting after September 11, 2001, improved diagnostic tools, or a combination of factors. Reported cases in Asia and Australia have also increased. Most infections in children are identified during the lamb birthing season in Europe (January through June), following farm visits, or after exposure to placentas of dogs, cats, and rabbits. The largest community outbreak ever described occurred in the southeastern part of the Netherlands in 2008 and was associated with intensive goat farming. More than 20% of cases of clinically recognized acute or chronic Q fever occur in immunosuppressed hosts or in persons with prosthetic valves or damaged native valves or vessels. Although infections are recognized more often in men than in women, reported cases in boys and girls are equal.
In contrast to other rickettsial infections, humans usually acquire C. burnetii by inhaling infectious aerosols (e.g., contaminated barnyard dust) or ingesting (and likely aspirating) contaminated foods. Ticks are rarely implicated. Cattle, sheep, and goats are the primary reservoirs, but infection in other livestock and domestic pets has also been described. Organisms are excreted in milk, urine, and feces of infected animals, but especially in amniotic fluids and the placenta. An increase in incidence has been associated with the seasonal mistral winds in France that coincide with lamb birthing season and with consumption of cheese among children in Greece. In Nova Scotia and Maine, exposure to newborn animals, especially kittens, has been associated with small outbreaks of Q fever in families. Exposure to domestic ruminants is the major risk in Europe and Australia, although many urban dwellers in France also acquire Q fever without such an exposure. Clinical Q fever during pregnancy can result from primary infection or reactivation of latent infection and has been associated with miscarriage, intrauterine growth retardation, and premature births. Obstetricians and other related health care workers are at risk for acquiring infection because of the quantity of C. burnetii sequestered in the placenta.
The pathology of Q fever depends on the mode of transmission, route of dissemination, specific tissues involved, and course of the infection. When acquired via inhalation, a mild interstitial lymphocytic pneumonitis and macrophage- and organism-rich intra-alveolar exudates are often seen. When the liver is involved, a mild to moderate lymphocytic lobular hepatitis may be seen. Inflammatory pseudotumors can develop in the pulmonary parenchyma or other tissues. Classic fibrin-ring (“doughnut”) granulomas, generally associated with acute, self-limited infections, occasionally are identified in liver, bone marrow, meninges, and other organs. Typically, infected tissues are also infiltrated by lymphocytes and histiocytes.
Recovery from symptomatic or asymptomatic acute infection can result in persistent subclinical infection and may be maintained by dysregulated cytokine responses. The persistence of C. burnetii in tissue macrophages at sites of pre-existing tissue damage elicits low-grade chronic inflammation and, depending on the site of involvement, can result in irreversible cardiac valve damage, persistent vascular injury, or osteomyelitis. Endocarditis of native or prosthetic valves is characterized by infiltrates of macrophages and lymphocytes in necrotic fibrinous valvular vegetations and an absence of granulomas.
Only about 40-50% of people infected with C. burnetii develop symptoms. Two forms of symptomatic disease occur. Acute Q fever
Nelson Textbook of Pediatrics Expert Consult
