Cellular immune response of ducks to duck hepatitis B virus infection

Duck virus enteritis (duck plague) - a comprehensive update

Duck virus enteritis (DVE), also called duck plague, is one of the major contagious and fatal diseases of ducks, geese and swan. It is caused by duck enteritis virus (DEV)/Anatid herpesvirus-1 of the genus Mardivirus, family Herpesviridae, and subfamily Alpha-herpesvirinae. Of note, DVE has worldwide distribution, wherein migratory waterfowl plays a crucial role in its transmission within and between continents. Furthermore, horizontal and/ or vertical transmission plays a significant role in disease spread through oral-fecal discharges. Either of sexes from varying age groups of ducks is vulnerable to DVE. The disease is characterized by sudden death, vascular damage and subsequent internal hemorrhage, lesions in lymphoid organs, digestive mucosal eruptions, severe diarrhea and degenerative lesions in parenchymatous organs. Huge economic losses are connected with acute nature of the disease, increased morbidity and mortality (5%-100%), condemnations of carcasses, decreased egg production and hatchability. Although clinical manifestations and histopathology can provide preliminary diagnosis, the confirmatory diagnosis involves virus isolation and detection using serological and molecular tests. For prophylaxis, both live-attenuated and killed vaccines are being used in broiler and breeder ducks above 2 weeks of age. Since DEV is capable of becoming latent as well as shed intermittently, recombinant subunit and DNA vaccines either alone or in combination (polyvalent) are being targeted for its benign prevention. This review describes DEV, epidemiology, transmission, the disease (DVE), pathogenesis, and advances in diagnosis, vaccination and antiviral agents/therapies along with appropriate prevention and control strategies.

Viral load in 1-day-old ducklings acutely infected with duck hepatitis B virus by different doses and routes of inoculation

In order to define clearly the conditions leading to the outcome of acute duck hepatitis B virus (DHBV) infection, 1-day-old Pekin ducklings were infected with DHBV by different routes and given different doses of inoculum. Groups of 24 ducklings were inoculated either intravenously via the vena cruralis, or intraperitoneally with pooled serum containing either 1.6 x 10(7) or 1.6 x 10(4) DHBV genomes. One control duck from each group was inoculated with an equal volume of normal duck serum. A sensitive and reproducible real-time polymerase chain reaction assay based on TaqMan technology was developed for the detection and quantitation of DHBV DNA in the serum and liver. DHBAg was observed in the hepatocytes by immunohistochemistry. Histological changes in the liver tissue were also observed. The results demonstrate that ducklings at each time point and in all groups developed detectable viraemia. In each group, DHBV DNA in the liver was at a lower level than in serum and the peak DNA titre was found in serum earlier than in the liver. In the low-dose groups it was always at a lower level than in the high-dose groups. The DHBV replication levels appeared to be directly related to the number of DHBAg-positive hepatocytes. The variation trends of DHBAg-positive hepatocytes were similar in the high-dose groups. Histological changes were associated with liver viral DNA levels. We suggest that this dose and route of inoculation can be used as a model to study acute DHBV infections.

The effect of surgical immunomodulation on liver inflammation and clearance of DHBV infection

The key to developing a therapeutic vaccine for chronic hepadnavirus infection lies in the characteristics of the host-immune response which leads to clearance of acute infection. Groups of 28-day-old ducks which had been surgically bursectomized (n = 10) or thymectomized (n = 13) on the day of hatch or were untreated (n = 21) were inoculated with 10(9) viral genome equivalents (vge) DHBV, then bled twice a week, and euthanased 40 days later. Serum and liver were tested for DHBV DNA and total leukocytes and peripheral blood mononuclear cells (PBMCs) counted. Liver and spleen sections were either stained with hematoxylin and eosin, and graded for inflammation or stained with peroxidase-labeled anti-human CD3 antibody and examined for T lymphocyte distribution. PBMC counts were similar in all groups. DHBV infection combined with bursectomy increased significantly, while thymectomy decreased significantly the total leukocyte count. The spleen and liver bursectomy increased T lymphocyte number while B cells were decreased. Converse changes were observed in thymectomized ducks. Histological evidence of hepatitis was present in infected control and bursectomized ducks but not in the uninfected control or infected thymectomized ducks. In control animals, DHBV challenge caused viremia in 17 and persistent infection in 11 (56%). Fewer thymectomized ducks (3/13, 23%) and significantly more (100%) bursectomized ducks remained persistently infected (P < 0.001). Unexpectedly, bursectomy led to persistence of infection while clearance of infection occurred normally in thymectomized ducks despite decreased T lymphocyte numbers. This suggests that clearance requires T and B lymphocyte collaboration.

Identification of T-cell epitopes associated with immunity within the surface protein of duck hepatitis B virus

Duck hepatitis is a convenient model of hepatitis B virus (HBV) infection, but the lack of immunological reagents hampers investigation of pathogenesis and vaccine development. The aim of this study was to define T-cell epitopes in the surface peptide recognized by vaccinated immune birds. Blastogenesis assays were used to test the proliferative response of spleen mononuclear cells to synthetic peptides spanning the pre-S/S region in 22 naïve and 13 immunized and challenged immune ducks. Roughly > or = 50% of the immune ducks responded to five immunodominant peptides eliciting a statistically greater proliferative response than in naïve birds. Fewer ducks responded to an additional six peptides. No statistically significant difference could be shown for the response to 11 peptides between the immune ducks and the naïve ducks. There was no clustering of the immunodominant peptides which were located throughout the surface antigen at sites of major swings in hydrophobicity. A number of peptides which induce lymphoblastogenesis in vaccinated immune ducks have been identified. Their role in spontaneous recovery from duck hepatitis B infection merits investigation.

Effect of dietary T-2 fusariotoxin concentrations on the health and production of white Pekin duck broilers

The effects of different dietary levels of T-2 toxin on production, biological, immunological, and pathological parameters of growing white Pekin ducks were studied to establish the "no effect" dietary concentration of, and "no effect" exposure time to, pure T-2 toxin. Day-old white Pekin ducks were randomly allotted to nine groups of 10 ducks each. One group served as a control, and no mycotoxin was added to its feed. The feeds of the experimental groups were supplemented with 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 3.0, and 4.0 mg purified T-2 toxin/kg, respectively, from Day 1 until Day 49 of the experiment. Dermatotoxic oral lesions developed in most experimental ducks within 2 d after the start of feeding T-2 toxin-contaminated feeds. The gradual disappearance of macroscopic signs indicated the development of tolerance in groups treated with the lower T-2 toxin content. No repair was found in the 3 and 4 mg/kg groups. Dietary concentrations of T-2 toxin below 0.4 mg/kg had no effect on the average weekly weight gain in the first 6 wk, but a severe decrease was found in the last week of the experiment. The 0.6 mg/kg dietary T-2 toxin had no effect on weight gain in the first 3 wk. At Week 4 and later, the weekly weight gain was significantly reduced, and the final live weight of this group was also significantly lower than that of the control. Dietary T-2 concentrations of 1 mg/kg and greater uniformly depressed growth rate. Only the 3 and 4 mg/kg groups refused feed during the first week. From Week 3 on, the feed intakes of the 0.6 to 4 mg/kg groups were usually less than that of the control group, indicating feed refusal. Serum and plasma chemical values and hematological parameters failed to show dose-dependent effects. The blastogenic response of lymphocytes to nonspecific and specific mitogens was distinctly impaired by the T-2 toxin at all levels in the feed. In the 3 and 4 mg/kg groups, the histological examination revealed lymphocyte depletion in the spleen and bursa of Fabricius.

Comparison of the kinetics of the specific cellular immune response to duck hepatitis B virus in infected and immune ducks

The kinetics of the cell mediated immune response by ducks acutely and chronically infected with, or immune to infection by duck hepatitis B virus (DHBV) was determined. This was measured by an antigen specific blastogenesis assay to duck hepatitis B surface antigen (DHBsAg) and duck hepatitis B core antigen (DHBcAg) using peripheral blood mononuclear cells (PBMC). The three outcomes of acute infection by DHBV were either clearance from both serum and liver, clearance from serum but not liver, or the development of persistent viraemia. Acutely infected ducks that failed to clear the infection also failed to develop a significant cellular immune response to both antigens. Ducks with chronic infection acquired as neonates or as the result of the failure to clear acute infection had an increasing cellular immune response over time. Two groups of immune ducks were examined. These were either ducks that had become immune following infection or that had been vaccinated. Both groups of ducks demonstrated significant cellular responses following challenge with DHBV irrespective of the level of their responses before challenge. However, there was a reduction in the response of their PBMC over a 4-week-period postchallenge. The range of cellular immune responses to DHBV antigens observed in this study has a number of counterparts in hepatitis B infection of humans. Coupled with the defined clinical outcomes that can be established in the duck/DHBV model, further study of the cellular immune response to DHBV is warranted.