Q Fever in Ruminants
Q fever is a zoonotic bacterial infection associated primarily with parturient ruminants, although domestic animals such as cats and a variety of wild animals have also been associated with human infections. Q fever occurs more frequently in persons with occupational contact with high-risk species. Q fever has a highly variable clinical presentation in humans, ranging from a self-limiting influenza-like illness to pneumonia, hepatitis, and endocarditis. It is highly infectious, and a single organism can reportedly cause infection via the aerosol route in humans. Q fever is considered a potential agent of bioterrorism due to its high rate of infectivity, stability in the environment, and potential for aerosol dispersion.
- burnetii can infect many species of domesticated animals and wildlife; in many species, the infection appears to be asymptomatic. Its reservoirs may be only partially known. Sheep, goats and cattle seem to be the most common domesticated animal reservoirs. Wild rodents may be important reservoirs in some areas, and cats are suspected in urban outbreaks. C. burnetii has also been isolated from dogs, rabbits, horses, pigs, camels, buffalo, deer, pigeons, swallows, parrots, crows, geese and other mammals and birds. Antibodies have been found in coyotes, raccoons, opossums, badgers, jackrabbits, black bears, musk ox and other species. There are also reports of C. burnetii in fish and snakes fish
Goat, placenta. The intercotyledonary placenta is thickened, opaque, and multifocally covered by tan clumps of exudate. Margins of several cotyledons are tan (necrosis), and centers are mottled red-brown (congestion and exudation).
Infection in ruminants is usually subclinical but can cause anorexia and late abortion. Reports have implicated C burnetii as a cause of infertility and sporadic abortion with a necrotizing placentitis in ruminants. Experimental infection in cats causes transient fever, dullness, and anorexia lasting several days.
In domestic ruminants, gross lesions are nonspecific, and differential diagnosis should include infectious and noninfectious agents that cause abortion. Immunofluorescence test on paired sera taken ³2 wk apart can be used to detect recent infection; however, shedding of C burnetii may occur in the absence of a measurable serum antibody titer. Culture, immunohistochemical, and PCR tests may be used to identify the organism in tissues.
In domestic ruminants, gross lesions are nonspecific, and differential diagnosis should include infectious and noninfectious agents that cause abortion. Immunofluorescence test on paired sera taken 2 wk apart can be used to detect recent infection; however, shedding of C burnetii may occur in the absence of a measurable serum antibody titer. Culture, immunohistochemical, and PCR tests may be used to identify the organism in tissues.
- burnetii can be detected in vaginal discharges, the placenta, placental fluids and aborted fetuses (liver, lung or stomach contents), as well as milk, urine and feces. Organisms are not shed continuously in milk and colostrum. In the placenta, organisms can be identified in exudates or areas of inflammation with a modified ZiehlNeelsen, Gimenez, Stamp, Giemsa or modified Koster stain; C. burnetii is an acid-fast, pleomorphic, small coccoid or filamentous organism. This organism is not usually detected by Gram stains. The presence of organisms, together with serological tests and clinical findings may be adequate for a diagnosis at the flock or herd level. Bacterial identity can be confirmed by immunohistochemistry or capture ELISA. PCR techniques are also available in some laboratories. Fresh, frozen or paraffin-embedded samples of blood, milk,feces, vaginal exudates, placenta, fetal tissue and other tissues can be tested by PCR.
A number of serologic tests are available; the most commonly used assays include indirect immunofluorescence, ELISA and complement fixation. Serology may be more helpful in screening herds than in individual animals. Some animals do not seem to seroconvert, and others shed organisms before they develop antibodies. Animals can also remain seropositive for several years after an acute infection. Crossreactions have been seen between some strains of C. burnetii and Chlamydia in ELISA and immunoblot assays.
- burnetii can be isolated in cell cultures, embryonated chicken eggs or laboratory animals including mice and guinea pigs; however, isolation is dangerous to laboratory personnel and is rarely used for diagnosis.