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  • Author or Editor: Willy Burgdorfer x
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SUMMARY

Indirect immunofluorescent antibody (ifa), latex agglutination (la), and enzyme immunoassay (eia) methods were compared for evaluation of the serum antibody responses of dogs experimentally and naturally exposed to spotted fever-group rickettsiae. Selected sera (obtained on days 1, 42, 53, 124, 145, 236, 255, 264, and 292) were examined from three 8-month-old female Beagles inoculated with Rickettsia rickettsii on days 34 and 250 of the study. A second group of dogs comprised three 8-month-old female Beagles inoculated with R montana on days 34 and 102. Subsequently, these dogs were inoculated with R rickettsii on day 250. Serum samples were obtained from the second group of dogs on days 1, 96, 103, 132, 180, 215, 292, and 494. A third group consisted of 21 naturally exposed dogs, from which sequentially obtained serum samples were available, and which had clinical signs compatible with Rocky Mountain spotted fever. Clinical signs of disease in dogs of the third group resolved after treatment with tetracycline (22 mg/kg of body weight, po, q 8 h) was instituted. At least 2 sequentially obtained serum samples from each dog were tested. In general, the first sample was obtained just prior to treatment and the convalescent serum samples were obtained at weekly or greater intervals thereafter.

For correlation and reactivity data, an ifa test for IgG/IgM (using heavy and light chains-specific conjugate) was used as the reference standard for comparison of results with those of the other tests.

For analyzing sensitivity and specificity data in naturally exposed dogs, the assumption was made that fourfold change in the ifa IgG/IgM titer for R rickettsii or titer ≥ 1,024 identified infected dogs. Threshold titer indicative of specific rickettsial antibody was determined for each test: ifa IgG/IgM, 64; ifa IgM, 8; la, 16; EIA IgG, 64; and EIA IgM, 64. Marked cross-reactivity of sera with R rickettsii, R rhipicephali, and R montana was apparent for the ifa method using these antigens; lesser cross-reactivity was observed for R bellii. Although the ifa test for IgM, using R rickettsii as antigen, had high specificity (88.2%), it had low sensitivity (21.7%), limiting its use as a sole diagnostic test. The la test had a higher sensitivity (58.5%) than did any of the IgM ifa tests. The la test results correlated best with IgM ifa test results, and the closest agreement was obtained when R bellii was used as antigen. Because of its relatively high sensitivity (58.5%) and specificity (83.6%), the la procedure may be a valuable screening test for diagnosis of Rocky Mountain spotted fever in dogs. The IgG and IgM EIA methods had high sensitivity- 90.6 and 83.1%, respectively. The IgM EIA performed by us had low specificity; it detected infection with R montana in inoculated dogs. Comparing results of all test methods in naturally exposed dogs, the IgM EIA results were positive for the greatest number of samples with corresponding negative results by the reference IgG/IgM ifa test. This low specificity may preclude its routine use as a diagnostic test. No single serologic method correctly identified all affected dogs. On the basis of comparison with results obtained for the other serologic procedures and antigen-detection methods in 1 dog, the IgG/IgM ifa test used as a reference test may not have detected some infected dogs. Combination of a test that detects predominantly IgG, with either the la or IgM ifa test, would help to identify most infected dogs.

Free access
in American Journal of Veterinary Research

SUMMARY

The kinetics of specific IgM and IgG antibody response was characterized in four 9-month-old Beagles after inoculation of 2 × 102 plaque-forming units (pfu) of Sheila Smith strain of Rickettsia rickettsii. Immunoglobulin M antibodies were first detected by indirect immunoflorescence on postinoculation (pi) day 9, peaked by pi day 20, and were no longer detectable by pi day 80. Immunoglobulin G antibodies became detectable between pi days 22 and 28, peaked by pi day 42, and decreased gradually through pi day 130. Subsequent challenges with R rickettsii on pi days 216 (2 × 102 pfu/dog) and 1,029 (5 × 104 tissue culture infective dose [tcid 50]/dog) resulted in slightly different serologic responses. The initial challenge exposure failed to increase the concentration of IgG antibodies and induced only low concentrations of IgM antibodies. After the second challenge inoculation, IgM antibodies were not detectable and the concentration IgG antibodies increased slightly. Clinical abnormalities and seroconversion were documented in control dogs following each challenge exposure.

Examination of acute and convalescent serum samples from 55 dogs in which Rocky Mountain spotted fever was suspected clinically suggested that sole evaluation of IgM antibodies in acute-phase serum would result in inaccurate diagnoses because of false-positive and -negative results. Use of a composite conjugate that detects IgM and IgG antibodies to R rickettsii appears to be satisfactory for diagnostic purposes; however, concurrent quantitation of IgM antibodies may facilitate serodiagnosis in a select group of dogs in which a four-fold increase in convalescent antibody titer is not detected by use of the composite conjugate.

With the exception of a dog with a serum antibody titer of 1:8,192, we were unable to detect IgM or IgG antibodies in csf samples from 9 dogs with experimentally and 3 dogs with naturally acquired infections.

Free access
in American Journal of Veterinary Research