Evaluation of the indirect fluorescent antibody test for diagnosis of Babesia gibsoni infections in dogs

I. Yamane From the Departments of Veterinary Microbiology and Immunology (Yamane, Thomford, Conrad) and Epidemiology and Preventive Medicine (Gardner), University of California, Davis, CA 95616, the USDA, Agricultural Research Service, Livestock and Poultry Science Institute, Zoonotic Diseases Laboratory, Bldg 1040, Beltsville Agricultural Research Center-East, Beltsville, MD 20705 (Dubey), and the Department of Microbiology, Pathology and Parasitology, North Carolina State University, Raleigh, NC 27606 (Levy).

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J. W. Thomford From the Departments of Veterinary Microbiology and Immunology (Yamane, Thomford, Conrad) and Epidemiology and Preventive Medicine (Gardner), University of California, Davis, CA 95616, the USDA, Agricultural Research Service, Livestock and Poultry Science Institute, Zoonotic Diseases Laboratory, Bldg 1040, Beltsville Agricultural Research Center-East, Beltsville, MD 20705 (Dubey), and the Department of Microbiology, Pathology and Parasitology, North Carolina State University, Raleigh, NC 27606 (Levy).

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I. A. Gardner From the Departments of Veterinary Microbiology and Immunology (Yamane, Thomford, Conrad) and Epidemiology and Preventive Medicine (Gardner), University of California, Davis, CA 95616, the USDA, Agricultural Research Service, Livestock and Poultry Science Institute, Zoonotic Diseases Laboratory, Bldg 1040, Beltsville Agricultural Research Center-East, Beltsville, MD 20705 (Dubey), and the Department of Microbiology, Pathology and Parasitology, North Carolina State University, Raleigh, NC 27606 (Levy).

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J. P. Dubey From the Departments of Veterinary Microbiology and Immunology (Yamane, Thomford, Conrad) and Epidemiology and Preventive Medicine (Gardner), University of California, Davis, CA 95616, the USDA, Agricultural Research Service, Livestock and Poultry Science Institute, Zoonotic Diseases Laboratory, Bldg 1040, Beltsville Agricultural Research Center-East, Beltsville, MD 20705 (Dubey), and the Department of Microbiology, Pathology and Parasitology, North Carolina State University, Raleigh, NC 27606 (Levy).

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M. Levy From the Departments of Veterinary Microbiology and Immunology (Yamane, Thomford, Conrad) and Epidemiology and Preventive Medicine (Gardner), University of California, Davis, CA 95616, the USDA, Agricultural Research Service, Livestock and Poultry Science Institute, Zoonotic Diseases Laboratory, Bldg 1040, Beltsville Agricultural Research Center-East, Beltsville, MD 20705 (Dubey), and the Department of Microbiology, Pathology and Parasitology, North Carolina State University, Raleigh, NC 27606 (Levy).

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P. A. Conrad From the Departments of Veterinary Microbiology and Immunology (Yamane, Thomford, Conrad) and Epidemiology and Preventive Medicine (Gardner), University of California, Davis, CA 95616, the USDA, Agricultural Research Service, Livestock and Poultry Science Institute, Zoonotic Diseases Laboratory, Bldg 1040, Beltsville Agricultural Research Center-East, Beltsville, MD 20705 (Dubey), and the Department of Microbiology, Pathology and Parasitology, North Carolina State University, Raleigh, NC 27606 (Levy).

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Summary

We determined the extent of the serologic cross-reactivity in the indirect fluorescent antibody (ifa) test for Babesia gibsoni, and the optimal cut-off titer for seropositivity in the test. The lowest titer to B gibsoni detected in a dog with naturally acquired clinical babesiosis was 1,280, and 7 of 12 dogs had titer between 10,240 and 20,480. Two experimentally infected normosplenic dogs developed high titer (40,960 to 81,920) to B gibsoni, and the same sera reacted in ifa tests for B canis (titer ≤ 640), Toxoplasma gondii (titer ≤ 2,560), and Neospora caninum (titer ≤ 10,240). Dogs that were experimentally infected with B canis and T gondii had titer ≤ 160 to B gibsoni. Dogs that were experimentally infected with N caninum had titer (80 to 10,240) to N caninum, but failed to have serologic reactivity to B gibsoni. Serologic titer of healthy dogs from Australia, a country where B gibsoni is not known to exist, was ≤ 160 to B gibsoni. On the basis of these findings, a cut-off titer of 320 was considered to be appropriate for serodiagnosis of B gibsoni in dogs with clinical signs of babesiosis. A more conservative titer of 1,280 was established as the cut-off titer for seroepidemiologic studies based on relative costs and benefits of false-positive results and failure to isolate B gibsoni parasites after splenectomy and immunosuppression from a clinically normal dog with B gibsoni titer of 640. Results of the study indicate the importance of establishing optimal cut-off titer for the B gibsoni ifa test that takes into consideration the purpose of the test, seroreactivity to antigenically related parasites, and factors that contribute to interlaboratory test variation.

Summary

We determined the extent of the serologic cross-reactivity in the indirect fluorescent antibody (ifa) test for Babesia gibsoni, and the optimal cut-off titer for seropositivity in the test. The lowest titer to B gibsoni detected in a dog with naturally acquired clinical babesiosis was 1,280, and 7 of 12 dogs had titer between 10,240 and 20,480. Two experimentally infected normosplenic dogs developed high titer (40,960 to 81,920) to B gibsoni, and the same sera reacted in ifa tests for B canis (titer ≤ 640), Toxoplasma gondii (titer ≤ 2,560), and Neospora caninum (titer ≤ 10,240). Dogs that were experimentally infected with B canis and T gondii had titer ≤ 160 to B gibsoni. Dogs that were experimentally infected with N caninum had titer (80 to 10,240) to N caninum, but failed to have serologic reactivity to B gibsoni. Serologic titer of healthy dogs from Australia, a country where B gibsoni is not known to exist, was ≤ 160 to B gibsoni. On the basis of these findings, a cut-off titer of 320 was considered to be appropriate for serodiagnosis of B gibsoni in dogs with clinical signs of babesiosis. A more conservative titer of 1,280 was established as the cut-off titer for seroepidemiologic studies based on relative costs and benefits of false-positive results and failure to isolate B gibsoni parasites after splenectomy and immunosuppression from a clinically normal dog with B gibsoni titer of 640. Results of the study indicate the importance of establishing optimal cut-off titer for the B gibsoni ifa test that takes into consideration the purpose of the test, seroreactivity to antigenically related parasites, and factors that contribute to interlaboratory test variation.

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