A DNA prime-protein boost vaccination strategy targeting turkey coronavirus spike protein fragment containing neutralizing epitope against infectious challenge.

The present study was undertaken to determine immune response and protection efficacy of a spike (S) protein fragment containing neutralizing epitopes (4F/4R) of turkey coronavirus (TCoV) by priming with DNA vaccine and boosting with the recombinant protein from the corresponding DNA vaccine gene segment. Turkeys were vaccinated by priming with either one dose (G1-750DP) or two doses (G3-750DDP) of 750µg DNA vaccine expressing 4F/4R S fragment and boosting with one dose of 200µg 4F/4R S fragment. One dose of 100µg DNA vaccine mixed with polyethyleneimine (PEI) and sodium hyaluronate (HA) followed by one dose of 750µg DNA vaccine and one dose of 200µg 4F/4R S fragment were given to the turkeys in group G2-100DPH. After infectious challenge by TCoV, clinical signs and TCoV detected by immunofluorescence antibody (IFA) assay were observed in less number of turkeys in vaccination groups than that in challenge control groups. TCoV viral RNA loads measured by quantitative real-time reverse transcription-PCR were lower in vaccinated turkeys than those in challenge control turkeys. The turkeys in G3-750DDP produced the highest level of TCoV S protein-specific antibody and virus neutralization (VN) titer. Comparing to the turkeys in G1-750DP, significantly less TCoV were detected by IFA in the turkeys in G2-100DPH receiving an extra dose of 100µg DNA mixed with PEI and HA. The results indicated that DNA-prime protein-boost DNA vaccination regimen targeting TCoV S fragment encompassing neutralizing epitopes induced humoral immune response and partially protected turkeys against infectious challenge by TCoV.

Infectious bursal disease and hemorrhagic enteritis.

Infectious bursal disease (IBD) of chickens and hemorrhagic enteritis (HE) of turkeys are caused by infectious bursal disease virus (IBDV) and hemorrhagic enteritis virus (HEV), respectively. Both diseases have common features, including an acute stage followed by immunosuppression, resulting in lowered resistance to a variety of infectious agents and poor response to commonly used vaccines. The IBDV and HEV infections are widespread in commercial chicken and turkey flocks, respectively. The acute stage of the disease, the immunosuppression that follows, and the widespread distribution of both diseases, are major factors contributing to the economic significance of both diseases. The mechanism of immunosuppression for both infections has similarities, both affect lymphocytes and macrophages, and both are lymphocidal. In this report, an overview of both diseases with emphasis on some of the recent findings will be presented. There has been greater research activity on IBD than on HE, reflecting the relative economic importance of the species affected and the recent changes in the antigenic make up and pathogenicity of the IBDV.

Response of specific-pathogen-free turkeys to vaccines derived from marble spleen disease virus and hemorrhagic enteritis virus.

Tissue-culture-propagated marble spleen disease virus (MSDV-TC) and two preparations of spleen homogenate (MSDV-SH and MSDV-SH-TC) were compared as anti-hemorrhagic enteritis virus (HEV) vaccines in specific-pathogen-free turkeys. Both types of vaccines spread horizontally among turkeys, induced anti-HEV antibodies, and protected turkeys against challenge with virulent HEV. Antibody development and horizontal spread of virus occurred earlier in turkeys given MSDV-SH or MSDV-SH-TC than in those given MSDV-TC. Virulent HEV was serially passed in MDTC-RP19 cells. The 30th passage virus (HEV-P30) was nonpathogenic for turkeys but was immunogenic. Turkeys exposed to HEV-P30 had viral antigen in the spleen, developed neutralizing antibodies, and resisted virulent HEV. The principal difference between MSDV-TC and HEV-P30 vaccines was that MSDV-TC caused well-defined splenomegaly in turkeys, whereas HEV-P30 protected turkeys without causing spleen enlargement.

Evaluation of cell culture propagated and in vivo propagated hemorrhagic enteritis vaccines in turkeys.

The immune response of turkeys to a liquid, was compared with a previously frozen, cell culture propagated hemorrhagic enteritis (HE) vaccine. The liquid cell culture propagated HE vaccine was able to induce 100% seroconversion in turkeys 4 weeks after being vaccinated at 3.5 weeks of age; however, the previously frozen cell culture propagated HE vaccine induced 80% seroconversion 4 weeks post vaccination (P < 0.05). The average seroconversion in turkey flocks administered the liquid cell culture propagated HE was 97% in comparison with 98.5% in flocks given the splenic vaccine (P > 0.05). The complete absence of HE antigens in spleens of birds 5 days after being challenged with the virulent HE virus (40,000 TCID50 per bird) at an age of 9.5 weeks, was used as a model for successful protection against HE disease. The HE antigens were absent from spleens of all challenged birds that were previously vaccinated by the liquid cell culture propagated HE vaccine or splenic vaccine.

Field vaccination against hemorrhagic enteritis of turkeys by a cell-culture live-virus vaccine.

A cell-culture-propagated (CC) live-virus hemorrhagic enteritis (HE) vaccine was evaluated for efficacy and safety in two field trials conducted in North Carolina (NC) and Minnesota (MN). At 4 or 5 1/2 weeks of age, 9,839 poults in NC and 15,857 poults in MN were vaccinated with a CC HE vaccine administered via the drinking water. A comparable number of poults were maintained as unvaccinated controls. Vaccinated and unvaccinated poults were compared for seroconversion, response to laboratory challenge with a virulent HE virus at 3 weeks postvaccination, livability, percentage graded A, and average weight at marketing. In both trials, vaccination with the CC HE vaccine resulted in immunity against HE as indicated by seroconversion and by resistance to HE lesions following laboratory challenge with virulent HE virus. Compared with unvaccinated groups, vaccinated groups had a significantly higher percentage of turkeys graded A in the NC trial and in two of three flocks in the MN trial (P less than 0.005). Further, in the NC trial, livability was significantly higher (P less than 0.005) in vaccinated turkeys than in unvaccinated turkeys. These data indicate that the CC HE vaccine is efficacious and safe to use in the field.

Role of splenectomy in prevention of hemorrhagic enteritis and death from hemorrhagic enteritis virus in turkeys.

Hemorrhagic diarrhea, gross hemorrhagic enteritis, and death caused by intravenous virus injection of hemorrhagic enteritis virus (HEV) were prevented in otherwise susceptible turkey poults by surgical splenectomy. The splenectomized poults produced anti-HEV antibodies, which indicated that splenectomy did not completely prevent replication of the virus. These results indicate that the spleen is necessary for the development of the intestinal lesions of this disease. The role of a toxic factor in this disease is discussed.

Field trials of an immunization procedure against hemorrhagic enteritis of turkeys.

The efficacy of oral vaccination for hemorrhagic enteritis of turkeys was assessed by comparing flocks raised on the same premises, under the same management, with and without vaccination. The immunizing virus, a strain of marble spleen disease virus of pheasants, was administered via the drinking water. Vaccinated and unvaccinated turkeys differed significantly in feed conversion rates and spleen weights after challenge.