CVI Accepts, published online ahead of print on 14 October 2009

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Clin. Vaccine Immunol. doi:10.1128/CVI.00244-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Detection of Asymptomatic Antigenemia in Pigs Infected by Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) using a Novel Capture Immunoassay with Monoclonal Antibodies Against the Nucleocapsid Protein of PRRSV

Jian-piao Cai, Ya-di Wang, Herman Tse, Hua Xiang, Kwok-yung Yuen^*, and Xiao-yan Che^*

Center for Clinical Laboratory, Zhujiang Hospital, Southern Medical University, Guangzhou, P. R. China; Department of Microbiology, The University of Hong Kong, Hong Kong SAR, P. R. China; Institute of Veterinary Medicine, Guangdong Academy of Agricultural Sciences, Guangzhou, P. R. China

^* To whom correspondence should be addressed. Email: kyyuen{at}hkucc.hku.hk. chexiaoyan{at}126.com.

Routine surveillance for porcine reproductive and respiratory syndrome virus (PRRSV) infections is crucial for epidemiological control of this disease. Antibody tests are widely used but cannot differentiate between vaccination and re-infection. We developed a PRRSV antigen capture enzyme-linked immunosorbent assay (ELISA) using well-characterized monoclonal antibodies (MAbs) raised against the nucleocapsid (N) protein of North American and European PRRSV. This antigen assay detected purified N protein from both genotypes at levels as low as 0.4 ng and 0.8 ng, respectively. The specificity and sensitivity of the N antigen assay were evaluated with ground lung tissues from 8 PRRSV infected swine and 16 healthy swine and culture supernatants from six PRRSV isolates as well as other swine viruses conformed by RT-PCR. Antigen assays were positive in all 8 infected tissues and 6 different PRRSV isolates, with no false positives among healthy tissues and other swine viruses (i.e., pseudorabies and foot and mouth disease viruses). A number of sera, field-collected from 466 vaccinated and asymptomatic pigs in Guangdong, China between 2008 and 2009, tested positive by the N antigen assay (12.45%), RT-PCR (15.02%), and by a commercial test for antibodies against PRRSV (78.97%). Of the 466 sera, 47 were positive by both antigen and RT-PCR tests, 11 by antigen test only, and 23 by RT-PCR only; the two assays had an overall agreement of 92.7%, indicating a significant percentage of active PRRSV in asymptomatic pigs despite previous immunization. These findings suggest the antigen assay is a valuable field tool for epidemiological control of PRRSV that can be used for rapid screening, particularly in asymptomatic animals.