Effect of diluting Marek's disease vaccines on the outcomes of Marek's disease virus infection when challenged with highly virulent Marek's disease viruses

Dilution of Marek's disease (MD) vaccines is a common practice in the field to reduce the cost associated with vaccination. In this study we have evaluated the effect of diluting MD vaccines on the protection against MD, vaccine and challenge MD virus (MDV) kinetics, and body weight when challenged with strains Md5 (very virulent MDV) and 648A (very virulent plus MDV) by contact at day of age. The following four vaccination protocols were evaluated in meat-type chickens: turkey herpesvirus (HVT) at manufacturer-recommended full dose; HVT diluted 1:10; HVT + SB-1 at the manufacturer-recommended full dose; and HVT + SB-1 diluted 1:10 for HVT and 1:5 for SB-1. Vaccine was administered at hatch subcutaneously. One-day-old chickens were placed in floor pens and housed together with ten 15-day-old chickens that had been previously inoculated with 500 PFU of either Md5 or 648A MDV strains. Chickens were individually identified with wing bands, and for each chicken samples of feather pulp and blood were collected at 1, 3, and 8 wk posthatch. Body weights were recorded at 8 wk for every chicken. Viral DNA load of wild-type MDV, SB-1, and HVT were evaluated by real time-PCR. Our results showed that dilution of MD vaccines can lead to reduced MD protection, reduced relative body weights, reduced vaccine DNA during the first 3 wk, and increased MDV DNA load. The detrimental effect of vaccine dilution was more evident in females than in males and was more evident when the challenge virus was 648A. However, lower relative body weights and higher MDV DNA load could be detected in chickens challenged with strain Md5, even in the absence of obvious differences in protection.     

Cyclophosphamide-induced amelioration of Marek's disease in Marek's disease-susceptible chickens.

Bursa- and thymus-dependent functions were examined in Marek's disease (MD)-susceptible normal chickens and in chickens treated with 5 and 16 mg of cyclophosphamide (CY) at the time of hatching. Chickens not exposed to Marek's disease virus (MDV) and treated with CY temporarily lost mitogenic response to concanavalin A but regained full response after 5 weeks. Bursa-dependent functions, such as presence of germinal centers in spleen and cecal tonsils, morphologic features of bursa, and sheep red blood cell antibody response were completely lost in chickens treated with 16 mg of CY and only partly retained in chickens treated with 5 mg of CY. In chickens exposed to MDV, the degree of thymus-dependent spleen cell mitogenic response was directly related to frequency and severity of MD. Chickens treated with 16 mg of CY had a mild mitogenic depression and low frequency and severity of MD lesions, whereas those treated with 5 mg of CY and those not treated had marked mitogenic depression and high frequency and severity of MD. Suppressions of bursa- and thymus-dependent functions by MDV alone were also evident when comparing MDV-exposed and nonexposed chickens. The results also indicate that presence of small, residual amounts of humoral factor(s) may enhance MDV oncogenesis.     

Marek's disease vaccination of chickens

Interpretive summaries of interesting articles on veterinary pharmacology are provided by the Pharmacology Chapter of the Australian College of Veterinary Scientsts     

Immune responses against Marek's disease virus

It is more than a century since Marek's disease (MD) was first reported in chickens and since then there have been concerted efforts to better understand this disease, its causative agent and various approaches for control of this disease. Recently, there have been several outbreaks of the disease in various regions, due to the evolving nature of MD virus (MDV), which necessitates the implementation of improved prophylactic approaches. It is therefore essential to better understand the interactions between chickens and the virus. The chicken immune system is directly involved in controlling the entry and the spread of the virus. It employs two distinct but interrelated mechanisms to tackle viral invasion. Innate defense mechanisms comprise secretion of soluble factors as well as cells such as macrophages and natural killer cells as the first line of defense. These innate responses provide the adaptive arm of the immune system including antibody- and cell-mediated immune responses to be tailored more specifically against MDV. In addition to the immune system, genetic and epigenetic mechanisms contribute to the outcome of MDV infection in chickens. This review discusses our current understanding of immune responses elicited against MDV and genetic factors that contribute to the nature of the response.     

鸡马立克病研究进展

鸡马立克病是由马立克病病毒引起的一种淋巴细胞增生性传染病,通常以外周神经和包括虹膜和皮肤在内的其他各种器官和组织的单核细胞浸润为特征.目前,仍然严重威胁着养禽业的发展,疫苗虽然可以预防马立克病的发生,但免疫失败时有发生,常常导致本病的局部暴发.论文对该病的病原、流行病学、临床症状、病理变化、发病机理、诊断及防控等方面进行了综述.     

A standard vaccine against Marek's disease

HVT-7, the standard vaccine against MD, was prepared from HVT strain FC126 grown in fibroblast cultures from SPF embryos. Ampoules of freeze dried material were prepared from a cell free suspension of virus in a solution of SPGA. The vaccine is stored at −20°C.The primary purpose of the standard vaccine was to control the variation encountered in the assay of virus content of HVT vaccines. The virus content of the standard vaccine was determined under a range of assay conditions, and the method in current use was shown to be satisfactory.A mean value for the virus content of the standard vaccine was determined using a constant assay method, by titrating 27 ampoules over a period of time. A further series of assays performed after one year's storage showed there to be no significant loss of titre.When several ampoules were titrated at the same time, some vial to vial variation was detected, but this was less than normal assay to assay variation.During routine determinations of the virus content of commercial HVT vaccines, an assay of the standard vaccine was carried out simultaneously to determine whether the assay conditions were acceptable. Assays where the value for the virus content of standard vaccine fell outside the expected range were considered invalid.The stability of the standard vaccine after reconstitution in SPGA was considered satisfactory: when reconstituted in phosphate-buffered saline, the rate of decrease in virus content was significantly greater. The vaccine could therefore be used as a control preparation in stability tests.Preliminary investigations showed that the behaviour of the standard vaccine in vivo was similar to that of satisfactory commercial vaccines, so that the preparation may also be of value in tests of vaccines for their ability to produce viraemia and confer protection.     

Immune mechanisms in Marek's disease.

Resistance to progressive tumor development in MD is either naturally inherited or can be induced by vaccination with apathogenic or attenuated MDV or with HVT. Studies on the effects of immunosuppression on resistance have shown that natural and vaccine induced resistance may be mediated through immune responses. Cell-mediated immune responses rather than humoral responses appear to be of principal importance. The antigen(s) against which protective cell-mediated immunity is elicited are not yet clearly delineated. Both virus-related and tumor antigens may be involved. Progress in the understanding of cell-mediated immunity in MD has been slow because of lack of reproducible in vitro tests to measure this response in infected chickens. The development of lymphoblastoid cell lines from MD lymphomas, however, has enabled the development of an in vitro cytotoxicity test. In this test, which utilizes MSB-1 cells as the target cells, a specific cell-mediated immune response, presumably against the tumor antigen, MATSA, was detected in chickens infected with MDV. Further studies using similar in vitro tests will facilitate a better understanding of the role cell-mediated immune responses might play in development of MD.     

Immune complex-mediated glomerulopathy in Marek's disease

Chickens infected with Marek's disease (MD) virus developed immune complex (IC)-mediated glomerulopathy. Fluorescent antibody staining technique using antichicken globulin and antichicken complement was used to demonstrate IC in the kidney glomeruli. During the initial stages of MDV infection, IC deposits were seen on the glomerular basement membrane, but subsequently the entire glomerulus was involved. Mesangial cells also had IC deposits. Chicken complement was demonstrated in the glomeruli which had IC deposits. The number of glomeruli with IC deposition was higher in tumor-bearing birds than in non-tumor-bearing birds. Histologically, kidney lesion were characterized by thickening of basement membrane and proliferation of mesangial cells. It is suggested that IC-mediated glomerulopathy might be one of the major causes of death in MD.