Submitted to: Transboundary and Emerging Diseases
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/8/2010
Publication Date: 10/1/2010
Citation: Metwally, S., Mohamed, F., Faaberg, K., Burrage, T., Prarat, M., Moran, K., Bracht, A., Mayr, G., Berninger, M., Koster, L., To, T.L., Nguyen, V.L., Reising, M., Landgraf, J., Cox, L., Lubroth, J., Carrillo, C. 2010. Pathogenicity and molecular characterization of emerging porcine reproductive and respiratory syndrome virus in Vietnam in 2007. Transboundary and Emerging Diseases. 57(5):315-329. Interpretive Summary: Porcine reproductive and respiratory syndrome virus (PRRSV) is the foremost infectious disease of swine in the United States. Vietnam experienced swine disease outbreaks in 2007 that was similar to the "porcine high fever disease" that occurred in China during 2006, where PRRSV was thought to be the pathogen responsible for the often fatal disease. In order to better understand the causes of the disease in Vietnam, analysis of diagnostic samples from the disease outbreaks was undertaken by the Animal and Plant Health Inspection Service at the Plum Island Animal Disease Center. Porcine reproductive and respiratory syndrome virus (PRRSV), porcine circovirus type 2 (PCV-2), Escherichia coli and Streptococcus equi subspecies zooepidemicus were identified. The results showed that the Vietnamese PRRSV possesed a high identity to the reported Chinese strains, with a similar deletion in the replicase. Animal inoculation studies were conducted to determine the pathogenicity of the PRRSV isolate and to identify other possible agents present in the original specimens. Swine inoculated with isolated PRRSV alone showed clinical signs consistent with virulent U.S. PRRSV strains, and swine inoculated with tissue homogenates succumbed to overwhelming disease, indicative of sepsis. These results suggest that the cause of the swine deaths in Vietnam is a multifactorial syndrome, with PRRSV as a major factor.
Technical Abstract: In 2007, Vietnam experienced swine disease outbreaks causing clinical signs similar to the "porcine high fever disease" that occurred in China during 2006. Analysis of diagnostic samples from the disease outbreaks in Vietnam identified porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV-2). Additionally, Escherichia coli and Streptococcus equi subspecies zooepidemicus were cultured from lung and spleen and Streptococcus suis from one spleen sample. Further genetic characterization of the Vietnamese PRRSV isolates revealed that this virus belongs to the North American genotype (type 2) with a high nucleotide identity to the recently reported Chinese strains. Amino acid sequence in the nsp2 region revealed 95.7%-99.4% identity to Chinese strain HUN4, 68-69% identity to strain VR-2332, and 58-59% identity to strain MN184. Animal inoculation studies were conducted to determine the pathogenicity of PRRSV and to identify other possible agents present in the original specimens. Pigs inoculated with PRRSV alone and their contacts showed persistent fever, and two of five pigs developed cough, neurological signs and swollen joints. Necropsy examination showed mild to moderate bronchopneumonia, enlarged lymph nodes, fibrinous pericarditis and polyarthritis. PRRSV was re-isolated from blood and tissues of the inoculated and contact pigs. Pigs inoculated with tissue homogenates developed high fever, septicemia, and died acutely within 72 hours while their contact pigs remained normal throughout the experiment. Streptococcus equi subspecies zooepidemicus was cultured, and PRRSV was re-isolated only from the inoculated pigs. Results suggest that the cause of the swine deaths in Vietnam is a multifactorial syndrome, with PRRSV as a major factor. Additionally, it was observed that a partial deletion in the nsp2 gene did not appear to enhance virus pathogenicity in pigs.