Pathogenic evaluation of a turkey coronavirus isolate (TCoV NC1743) in turkey poults for establishing a TCoV disease model

Turkey coronavirus (TCoV) can cause a highly contagious enteric disease in turkeys with severe economic losses in the global turkey industry. To date, no commercial vaccines are available for control of the disease. In the present study, we isolated a field strain (NC1743) of TCoV and evaluated its pathogenicity in specific-pathogen-free (SPF) turkey poults to establish a TCoV disease model. The results showed that the TCoV NC1743 isolate was pathogenic to turkey poults with a minimal infectious dose at 10⁶ EID₅₀/bird. About 50 % of one-day-old SPF turkeys infected with the virus's minimal infectious dose exhibited typical enteric disease signs and lesions from 6 days post-infection (dpi) to the end of the experiment (21 dpi). In contrast, fewer than 20 % of older turkeys (1- or 2-week-old) infected with the same amount of TCoV displayed enteric disease signs, which disappeared after 15-18 dpi. Although all infected turkeys, regardless of age, shed TCoV, the older turkeys shed less virus than the younger birds, and 50 % of the 2-week-old birds even cleared the virus at 21 dpi. Furthermore, the viral infection caused day-old turkeys more body-weight-gain reduction than older birds. The overall data demonstrated that the TCoV NC1743 isolate is a highly pathogenic strain and younger turkeys are more susceptible to TCoV infection than older birds. Thus, one-day-old turkeys infected with the minimal infectious dose of TCoV NC1743 could be used as a TCoV disease model to study the disease pathogenesis, and the TCoV NC1743 strain could be used as a challenge virus to evaluate a vaccine protective efficacy.

Comparison of 3'-end encoding regions of turkey coronavirus isolates from Indiana, North Carolina, and Minnesota with chicken infectious bronchitis coronavirus strains

Objective

To analyze the 3′-end structural protein-encoding region of turkey coronavirus (TCoV) isolates associated with outbreaks of acute enteritis in Indiana, North Carolina, or Minnesota.

Methods

Four isolates of TCoV were sequenced over the entire 3′-end structural protein-encoding region and compared phylogenetically along with the corresponding sequences of infectious bronchitis virus (IBV) strains.

Results

The sequence similarity between TCoV and IBV was lower than that among TCoV isolates or that among IBV strains. The variation of sequences between TCoV and IBV was mainly contributed by the S protein gene. The sequence similarity of S gene between TCoV and IBV was lower than that among TCoV isolates or that among IBV strains. The phylogenetic tree based on the S protein region was similar to that based on the entire 3′-end structural protein-encoding region with TCoV isolates and IBV strains grouped in two separate clusters. The phylogenetic tree based on other genes had a very different topology with TCoV isolates randomly forming groups with different IBV strains.

Conclusions

These results suggested that TCoV probably shared the same origin with that of IBV and acquired sequences of S gene for turkey intestine tropism during the process of evolution in a separate environment.

Complete sequences of 3' end coding region for structural protein genes of turkey coronavirus

Overlapping fragments of genomic RNA spanning 6963 nucleotides from 5′ end of spike (S) protein gene to 3′ end of nucleocapsid (N) protein gene of turkey coronavirus (TCoV) were amplified by reverse-transcription-polymerase chain reaction (RT-PCR). The primers were derived from the corresponding sequences of infectious bronchitis virus (IBV). The PCR products were cloned and sequenced and their nucleic acid structure and similarity to published sequences of other coronaviruses were analyzed. Sequencing and subsequent analysis revealed 9 open reading frames (ORFs) representing the entire S protein gene, tricistronic gene 3, membrane (M) protein gene, bicistronic gene 5, and N protein gene in the order of 5′–3′. The overall nucleic acid structures of these encoding regions of TCoV were very similar to the homologous regions of IBV. The consensus transcription-regulating sequence (TRS) of IBV, CT(T/G)AACAA, was highly conserved in TCoV genome at the levels of nucleotide sequence and location in regarding to the initiation codon of individual genes. Pair-wise comparison of gene 3, M gene, gene 5, or N gene sequences with their counterparts of IBV revealed high levels (82.1–92.0%) of similarity. Phylogenetic analysis based on the deduced amino acid sequences of S, M, or N protein demonstrated that TCoV was clustered within the same genomic lineage as the IBV strains while all the other mammalian coronaviruses were grouped into separate clusters corresponding to antigenic groups I or II. There were substantial differences of S protein sequence between TCoV and IBV with only 33.8–33.9% of similarity.

Existence of gene 5 indicates close genomic relationship of Turkey coronavirus to infectious bronchitis virus

A segment of genomic RNA extending from the 3'-end of the membrane (M) protein gene to the 5'-end of the nucleocapsid (N) protein gene of Turkey coronavirus (TCV) was amplified by reverse transcription-polymerase chain reaction (RT-PCR). The primers were derived from the corresponding sequences of Infectious bronchitis virus (IBV). The PCR products were cloned and sequenced and their nucleic acid structure and similarity to the published sequences of IBV were analyzed. Gene 5 containing two overlapping open reading frames (ORFs), 5a and 5b, was localized between M and N genes of TCV. The overall nucleotide sequences of the amplified regions from TCV isolates shared 88.4% to 91.8% similarity to the corresponding region of IBV strains. The consensus transcription-associated sequence of IBV, CTTAACAA, was highly conserved in the TCV genome with regard to nucleotide sequence and location in terms of the initiation codons of the genes 5 and N. The similarities between the predicted amino acid sequences of ORFs 5a and 5b of TCV isolates and the homologous genes of IBV strains were 85.4% to 94.0%. The results indicate the existence of gene 5 in the genome of TCV and a close relatedness of the TCV gene 5 to the IBV gene 5 in location and nucleotide sequence.

Antigenic relationship of turkey coronavirus isolates from different geographic locations in the United States

The purpose of the present study was to examine the antigenicity of turkey coronavirus (TCV) isolates from various geographic areas with antibodies to different viruses. Seventeen isolates of TCV were recovered from intestinal samples submitted to Animal Disease Diagnostic Laboratory, Purdue University, from turkey farms located in different geographic areas. The prototype TCV Minnesota isolate (TCV-ATCC) was obtained from the American Type Culture Collection. Intestinal sections were prepared from turkey embryos infected with different TCV isolates and reacted with polyclonal or monoclonal antibodies to TCV, infectious bronchitis virus (IBV), bovine coronavirus (BCV), transmissible gastroenteritis virus (TGEV), reovirus, rotavirus, adenovirus, or enterovirus in immunofluorescent antibody staining. All 18 TCV isolates have the same antigenic reactivity pattern with the same panel of antibodies. Positive reactivity was seen with polyclonal antibodies to the TCV Indiana isolate, the TCV Virginia isolate, TCV-ATCC, and the IBV Massachusetts strain as well as monoclonal antibodies to the TCV North Carolina isolate or the membrane protein of IBV. Antibodies to BCV or TGEV were not reactive with any of the TCV isolates. Reactivity of antibodies to unrelated virus, rotavirus, reovirus, adenovirus, or enterovirus with different TCV isolates was all negative, except positive response was seen between enterovirus antibody and a TCV western North Carolina isolate, suggesting coinfection of turkeys with TCV and enterovirus in that particular case. The results indicated that the TCV isolates from these geographic locations in the U.S. shared close antigenicity and were antigenically related to IBV.

Characterization of turkey coronavirus from turkey poults with acute enteritis

The present study was to characterize turkey coronavirus associated with turkey poult enteritis and mortality. Intestinal contents or intestines from affected turkey poults and inoculated turkey embryos contained coronaviruses as revealed by electron microscopy or were positive for turkey coronavirus by immunofluorescent antibody assay. Sucrose density gradient ultracentrifugation of the virus-containing intestinal homogenate yielded two opalescent bands corresponding to the buoyant densities of 1.14-1.15 and 1.18-1.20 g/ml, respectively. Coronaviral particles from intestinal contents or the sucrose density gradient preparation were mainly spherical in shape and had envelope and central depression. They were surrounded by a fringe of regularly spaced petal-shaped projections attached to the particles by a short stalk. Purified viruses hemagglutinated rabbit erythrocytes with a titer of 16. Major protein bands of purified viruses analyzed by SDS-PAGE were located at 200, 100-110, 50-60, and 30-35 kDa. The patterns of protein bands were consistent with those of Minnesota or Quebec turkey coronavirus isolates. A 568 bp nucleotide fragment of turkey coronavirus spike protein gene was amplified from RNA of inoculated turkey embryo intestine or purified virus. Sequence analysis of the 568 bp PCR product revealed high degree of identity with the corresponding spike protein gene sequence of human and bovine coronaviruses. The results indicated that turkey coronavirus was associated with turkey poults with acute enteritis.

Antigenic and genomic relatedness of turkey-origin coronaviruses, bovine coronaviruses, and infectious bronchitis virus of chickens

In earlier studies in our laboratory, we found that bovine coronavirus (BCV) was pathogenic for 1-day-old turkey poults. This finding prompted us to study the antigenic and genomic relatedness of turkey origin coronaviruses (TOCVs) to BCV. A one-step reverse transcription (RT)-polymerase chain reaction (PCR) targeting a 730-base pair fragment of the nucleocapsid (N) gene of BCV and a nested PCR targeting a 407-base pair fragment of the N gene were used in an attempt to detect TOCV from North Carolina, Indiana, and a prototype turkey coronavirus (TCV) obtained from the American Type Culture Collection. Both the one-step RT-PCR and the nested PCR amplified cell culture-passaged isolates of calf diarrhea strains of BCV but none of the 15 tested TOCVs or transmissible gastroenteritis coronavirus of swine. TOCVs also did not cross-react in a BCV antigen-capture (AC) enzyme-linked immunosorbent assay (ELISA) system with monoclonal antibodies (MAbs) against N, spike glycoprotein, and hemagglutinin esterase glycoprotein proteins of BCV as coating antibodies. The same TOCVs could be detected with primers designed from the genome of infectious bronchitis virus (IBV) of chickens. These primers amplified a 1082-base pair region spanning portions of the membrane glycoprotein (M) and N protein genes of IBV and TCV. The TOCVs also cross-reacted in an AC-ELISA with MAbs against the M and subunit 2 of spike glycoprotein of IBV.

Nucleocapsid protein gene sequence analysis reveals close genomic relationship between turkey coronavirus and avian infectious bronchitis virus

Antibodies to infectious bronchitis virus (IBV) cross-react with turkey coronavirus (TCV) in immunofluorescence assay (IFA) indicating that IBV and TCV may share an amino acid sequence similarity. To determine its extent, the gene encoding the nucleocapsid (N) protein of TCV was amplified by reverse transcription-PCR (RT-PCR) from RNA purified from intestines of embryos of turkeys infected with various TCV isolates and from allantoic fluid of chicken embryos infected with IBV M41 strain, the obtained N genes were cloned, sequenced and compared with known sequences of N genes of five IBV strains. The primers for amplification were designed from the genome of IBV PCR products were obtained only from two of eight TCV isolates tested. It was found that the two TCV isolates were identical with five IBV strains by 90.1-94.1% at the N gene level. It was also observed that the N gene of eight TCV isolates originating from various regions of the USA could not be amplified by the primers designed from the N gene of bovine coronavirus (BCV).

Antigenic characterization of a turkey coronavirus identified in poult enteritis- and mortality syndrome-affected turkeys

A turkey coronavirus (TCV [NC95]) was characterized by antigenic comparison with other avian and mammalian coronaviruses using immunofluorescence (FA) and immunoperoxidase (IP) procedures. Based on FA and IP procedures, TCV (NC95) was determined to be antigenically indistinguishable from turkey enteric (bluecomb) coronavirus (TECV). In addition, TCV (NC95) and TECV were found to be closely related to infectious bronchitis virus (IBV); a one-way antigenic relationship was demonstrated. Polyclonal antibodies specific for TECV and IBV reacted strongly against TCV (NC95), as determined by FA procedures. Monoclonal antibodies (MAbs) specific for IBV matrix protein (MAb 919) reacted strongly against TCV (NC95) and TECV as determined by FA and IP procedures; an IBV peplomer protein-specific MAb (MAb 94) did not recognize the two viruses. These studies suggest an identification of TCV (NC95) as a strain of TECV, and provide evidence of a close antigenic relationship between these viruses and IBV.

Genomic relationship between turkey and bovine enteric coronaviruses identified by hybridization with BCV or TCV specific cDNA probes

Genomic relationships between turkey and bovine coronavirus (TCV and BCV), which are currently placed in distinct antigenic groups, were demonstrated by hybridization using specific cDNA probes. BCV-specific recombinant plasmid probes p 52, p 27, and p 247, holding inserts derived from (probably nonstructural) genes, and plasmids pN 17 and pN 9 holding the N and M gene, respectively, permitted the detection of isolates of both BCV and TCV with similar sensitivities. Similarly, probing supernatants of cell cultures infected with several isolates of TCV, using probes pN 17 and pM 78, respectively holding the N gene of BCV and TCV, resulted in equally intense detection signals. Only a slight detection of MHV-3, which is antigenically related to BCV, was observed, whereas the probes did not allow the detection of IBV, TGEV, and HCV-229E, which are placed in antigenic groups separate from those of BCV and TCV. Detection of TCV was improved by hybridization with BCV-specific single-stranded (ss) probes holding sequences of the N and M genes and synthesized by the polymerase chain reaction. Diagnosis of TCV in 134 clinical samples by hybridization was better with PCR-produced ss BCV-specific probes than with ds PCR-produced probes or a combination of six recombinant plasmid probes holding non-overlapping BCV-specific cDNA sequences. Detection signals were absent when probing clinical samples with 32P-labelled pUC-DNA.

Characterization of group II avian adenoviruses with a panel of monoclonal antibodies

The interaction between a panel of ten monoclonal antibodies and hemorrhagic enteritis virus, a group II avian adenovirus, was determined. The monoclonal antibodies reacted with all nine isolates of group II avian adenoviruses, but not with any of five types of group I avian adenoviruses. All ten monoclonal antibodies recognized antigenic determinants on the hexon protein of hemorrhagic enteritis virus when analyzed by immunoprecipitation and immunoblotting. They reacted only with the native hexon protein and not with protein denatured by sodium dodecyl sulfate or guanidine-HCl/urea treatment combined with reduction and carboxymethylation. Based on the results of competitive binding assays, the panel of monoclonal antibodies could be subdivided into two groups, which recognized different antigenic domains of the hemorrhagic enteritis virus hexon protein. The monoclonal antibodies in group 1 neutralized hemorrhagic enteritis virus infectivity while the monoclonal antibodies of group 2 did not. Group 1 consisted of eight monoclonal antibodies which could be further subdivided into subgroups 1A, 1B, 1C and 1D. The subdivision of the monoclonal antibodies was based on the degree of blocking in the competitive binding assays and differences in their ability to induce enhancement. In general, the monoclonal antibodies had a higher avidity for the virulent isolate of hemorrhagic enteritis virus than for the avirulent hemorrhagic enteritis virus isolate.