Paired sera should be obtained wherever possible:
(a) acute sample - as early as possible in the illness, and
(b) convalescent sample 2 - 3 weeks after onset.
Serology uses an indirect immunofluorescence assay. Because this disease is absent in N.Z. it is not known what titres should be considered significant for infection, therefore all positive titres should be discussed with the Virology service.
Tests performed in batches of two. Turnaround time 1 - 2 weeks.
Low levels of phase II IgG antibody may be considered non-specific.
Laboratory findings must be used in conjunction with both the setting and the clinical presentation before a diagnosis of acute or chronic Q Fever infection can be made.
Serologic responses are time dependent. Specimens obtained too early in the infection may not contain detectable antibody levels. If Q fever infection is suspected, testing should be repeated in 2-3 weeks time.
Coxiella burnetii which causes Q Fever is an obligate intracellular parasite with world-wide distribution. Important reservoirs include dairy cattle, sheep and goats. Recent evidence has also implicated rodents as reservoirs as well cats that feed on them. Infection in these animals is enzootic and nearly always inapparent. Humans are infected via inhalation of contaminated dust particles and aerosols, and via handling and ingestion of infected meat and milk.
Q Fever has an incubation of approximately 2-3 weeks. Acute symptoms are high fever (40°C peaking in 2-4 days then declining for 1-2 weeks) accompanied by malaise, anorexia, myalgia, weakness and intense headache. Liver damage with hepatosplenomegaly occurs, often leading to hepatic granulomas when treatment is delayed or diagnosis is missed. Q Fever may manifest as pneumonitis or bronchitis. Endocarditis is uncommon, following a protracted latent phase and requiring existing heart valve damage.
The presence of IgG to phase II antigen is indicative of acute disease. The presence of IgG antibody to phase I antigen is indicative of chronic disease or convalescence.
References
1. Focus Technologies Q Fever IgG Product Insert
2. 1997 Red Book. Report of the Committee on Infectious Diseases, 24th Edition. American Academy of Paediatrics
For further information contact the laboratory (contact via Lablink: 22000 or (09) 307-8995 or 0800 522 7587) ,or:
the Virology team
virology@adhb.govt.nz