Specific real-time reverse transcription-polymerase chain reaction for detection and quantitation of turkey coronavirus RNA in tissues and feces from turkeys infected with turkey coronavirus.

Turkey coronavirus (TCoV) infection causes acute atrophic enteritis in the turkey poults, leading to significant economic loss in the U.S. turkey industry. Rapid detection, differentiation, and quantitation of TCoV are critical to the diagnosis and control of the disease. A specific one-step real-time reverse transcription-polymerase chain reaction (RRT-PCR) assay for detection and quantitation of TCoV in the turkey tissues was developed using a dual-labeled fluorescent probe. The fluorogenic probe labeled with a reporter dye (FAM, 6-carboxytetramethylrhodamin) and a quencher dye (AbsoluteQuencher) was designed to bind to a 186 base-pair fragment flanked by the two PCR primers targeting the 3' end of spike gene of TCoV. The assay was performed on different avian viruses and bacteria to determine the specificity as well as serial dilutions of TCoV for the sensitivity. Three animal trials were conducted to further validate the assay. Ten-day-old turkey poults were inoculated orally with 100 EID(50) of TCoV. Intestinal tissues (duodenum, jejunum, ileum, cecum), feces from the cloacal swabs, or feces from the floor were collected at 12 h, 1, 2, 3, 5, 7, and/or 14 days post-inoculation (DPI). RNA was extracted from each sample and subjected to the RRT-PCR. The designed primers and probe were specific for TCoV. Other non-TCoV avian viruses and bacteria were not amplified by RRT-PCR. The assay was highly sensitive and could quantitate between 10(2) and 10(10) copies/microl of viral genome. The viral RNA in the intestine segments reached the highest level, 6x10(15) copies/microl, in the jejunum at 5 DPI. Eighty-four intestine segments assayed by the developed RRT-PCR and immunofluorescence antibody assay (IFA) revealed that there were 6 segments negative for TCoV by both assays, 45 positive for TCoV by IFA, and 77 positive for TCoV by RRT-PCR. Turkey coronavirus was detected in the feces from the cloacal swabs or floor 1-14 DPI; however, the viral RNA load varied among different turkey poults at different intervals from different trials. The highest amount of viral RNA, 2.8x10(10) copies/microl, in the feces was the one from the cloacal swab collected at 1 DPI. The average amount of TCoV RNA in the cloacal fecal samples was 10 times higher than that in the fecal droppings on the floor. Taken together, the results indicated that the developed RRT-PCR assay is rapid, sensitive, and specific for detection, differentiation, and quantitation of TCoV in the turkey tissues and should be helpful in monitoring the progression of TCoV induced acute enteritis in the turkey flocks.

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.

The effect of immunosuppression on protective immunity of turkey poults against infection with turkey coronavirus.

The objective of the present study was to evaluate the protective effect of humoral and cellular immunities on turkeys infected with turkey coronavirus (TCV). Two trials were conducted with two separate hatches of turkey poults. Turkey's were experimentally immunosuppressed with cyclosporin A (CsA) or cyclophosphamide (CY) and infected with TCV. Prior to infection, treatment with CsA selectively suppressed T cell activity as revealed by 2-3 fold decreased (p < 0.1) lymphocyte proliferation responses to a T cell mitogen, concanavalin A (Con A). Treatment with CY mainly induced B cell deficiency as indicated by significant reductions (p < 0.05) in antibody responses to sheep erythrocytes 7 days after injection. Body weight gain of turkeys treated with CY was significantly lower (p < 0.05) than that of untreated turkeys at 9 days post-infection (PI). Turkeys treated with CY had 1-2 fold higher immunofluorescent antibody assay (IFA) scores for TCV antigens (p < 0.05) in the intestine than untreated turkeys at 9 or 14 days PI. These results suggested that humoral immunity against TCV infection may be important in turkeys.

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.