Respiratory syncytial virus diagnosis

Development of nested PCR assays for detection of bovine respiratory syncytial virus in clinical samples

Two nested PCR assays were developed for the detection of bovine respiratory syncytial virus (BRSV). Primers were selected from the gene encoding the F fusion protein (PCR-F) and the gene encoding the G attachment protein (PCR-G). Biotinylated oligonucleotide probes, termed F and G, were selected for the hybridization of the respective PCR products. The sensitivities of the PCR-F and PCR-G assays were similar, both detecting 0.1 tissue culture infective dose of the virus. The PCR-F assay amplified all bovine strains and one human strain (RS32) tested. No cross-reactions were observed with nine heterologous respiratory viruses. PCR-F products of bovine and human RSV strains were discriminated by using endonuclease restriction enzyme ScaI, which specifically cleaved, products of BRSV. Oligonucleotide probe F was also specific for products of BRSV. The PCR-G assay detected all bovine strains and none of the human strains tested. A faint electrophoretic band was also observed with products of Sendai virus. However, probe G did not hybridize with this product, only with products of BRSV. Nasal swabs collected from cattle with no symptoms and cattle in the acute stage of respiratory disease were analyzed for BRSV by the immunofluorescence (IF) method and by the PCR-F and PCR-G assays. The virus was detected by the PCR assays in 31 of 35 (89%) samples tested. Only 23 samples (66%) were positive by the IF method, and these samples were also positive by both the PCR-F and PCR-G assays. The 31 samples detected as positive by PCR originated from cattle presenting clinical signs of acute respiratory disease; the four PCR-negative samples originated from clinically asymptomatic neighboring cattle. All sampled animals subsequently seroconverted and became reactive to BRSV. Thus, the detection of BRSV by PCR correlated with clinical observations and was considerably more sensitive (66 versus 89%) than IF. These results indicate that both nested PCR assays provide rapid and sensitive means for the detection of BRSV infection in cattle. Considering its higher specificity, the PCR-F assay can be recommended as the method of choice in the analysis of clinical specimens of BRSV.

Identifying bovine respiratory syncytial virus by reverse transcription-polymerase chain reaction and oligonucleotide hybridizations

An assay to identify tissue culture cells infected with bovine respiratory syncytial virus (BRSV) that utilizes reverse transcription (RT), the polymerase chain reaction (PCR), and a synthetic oligonucleotide hybridization probe has been developed. The RT-PCR assay uses a BRSV-specific negative-sense oligonucleotide primer to synthesize cDNA from a BRSV fusion protein mRNA template and another BRSV-specific oligonucleotide primer (positive sense) upstream from the negative-sense primer for PCR amplification. In the presence of mRNA templates of BRSV isolates originating from locations throughout the United States, the BRSV RT-PCR assay resulted in amplified products (381 bp) that were specific to BRSV, as demonstrated in hybridizations with a positive-sense oligonucleotide probe complementary to internal sequences and in sequence comparisons with the F protein of BRSV 391-2. In analysis of the BRSV RT-PCR assay with prototype strains of human RSV subgroups A and B, amplification of a similar 381-bp RT-PCR product was not evident, and no RT-PCR product hybridized with the internal probe. We conclude that the specific ability to amplify DNA sequences of BRSV F protein mRNA by RT-PCR and then to demonstrate the presence of the amplified product with a BRSV-specific oligonucleotide probe will greatly add to the speed, sensitivity, and specificity of BRSV diagnostics.

Immunity to human and bovine respiratory syncytial virus

Human and bovine respiratory syncytial viruses resemble each other closely. During annual winter outbreaks, they cause similar respiratory tract disease in infants and calves. The disease is most severe in children and calves between 1 and 3 months old, when maternal antibodies against the virus are usually present. Reinfections, which are common, are accompanied by progressively milder illnesses in children, but are symptomless in calves. Because maternal antibodies suppress serum and mucosal antibody responses of all isotypes, the development of a vaccine that is effective in young children and calves with high levels of maternal antibodies has been severely hampered. Although virus administered intranasally to young calves with maternal antibodies does not evoke antibody responses, it can prime these calves for a protective memory response upon reinfection. Protection appears to be associated with the capacity to mount a mucosal memory IgA response. There are several indications that one or more immunopathologic mechanisms contribute to the disease. An Arthus reaction (type III) may have a role in the pathogenesis, because activated complement may cause most of the pathologic lesions, including edema and emphysema in uninfected parts of the lung. Lungs from calves with severe or fatal disease have depositions of complement component C3 and a low histamine content. The most immunogenic and protective antigen of the virus is the fusion (F) glycoprotein, which evokes a strong antibody response and is a target for cytotoxic T cells. On the F protein, epitopes that induce neutralizing and non-neutralizing antibodies, both of which may enhance complement activation, were identified. Immunity to the F protein may have beneficial and harmful effects.