Comparison of immunoperoxidase plaque staining and neutralizing tests for canine distemper virus

The use of immunoperoxidase plaque staining (IP) in the antibody test for canine distemper virus (CDV) and its diagnostic value were investigated. Antibody titres by the IP test and neutralizing test (NT) against CDV were compared in 114 specific pathogen-free (SPF) dogs, including 84 CDV-vaccinated and 30 unimmunized dogs. All of the unimmunized dogs, which had negative NT titres, also showed negative IP titres, and all of the CDV-vaccinated dogs, which had positive NT titres, showed positive IP titres. Furthermore, the IP titres showed similar changes with time after vaccination to the NT titres in five CDV-vaccinated SPF dogs. These findings indicate that the IP test has similar specificity to the NT. Next, the IP and NT titres were compared in 190 clinically suspected cases of canine distemper (CD), including 107 dogs with neurological signs and 83 dogs without neurological signs, and also in 50 clinically healthy dogs. Among the suspected cases of CD, of 46 dogs with neurological signs and 29 dogs without neurological signs, and which had negative NT titres, 33 (72%) and 19 (66%), respectively, showed positive IP titres. Many suspected cases of CD showed a high IP titre despite a low NT titre. In contrast, among healthy dogs, none of 9 NT-negative dogs showed positive IP titres. The difference in the IP and NT titres (log10) [reciprocal of IP titre] - log10) [reciprocal of NT titre]) was measured to investigate the agreement between IP test and NT. The differences were 1.488 +/- 0.867, 1.245 +/- 0.833, 0.595 +/- 0.351 and 0.594 +/- 0.372, respectively, in the suspected cases of CD with neurological signs, the suspected cases of CD without neurological signs, the healthy dogs and the SPF dogs. The differences in the suspected cases of CD were significantly high compared to those for the SPF dogs (p < 0.001). Furthermore, in 8 dogs diagnosed as cases of CD from their paired sera, whose initial sera showed a markedly low NT titre compared to the IP titre, 5 dogs without neurological signs developed neurological signs later. These findings suggest that the IP test can detect CD during its early phases and in dogs with low antibody levels, undetectable by NT, and so would be useful for serological diagnosis.