Influence of vaccination with avirulent herpesvirus on subsequent infection of chickens with virulent Marek's disease herpesvirus.

Vaccination of chickens with turkey herpesvirus (HVT) or attenuated Marek's disease herpesvirus (aMDHV) blocked infection with virulent MDHV (VMDHV) for approximately 5 weeks after contact exposure. However, there was no apparent blockage of infection when challenge virus was administered intraabdominally (IA). Evidence for infection with VMDHV was based on viral isolation by in vivo assay or by detecting precipitins to "A" antigen associated with virulent virus. The HVT stimulated production of neutralizing antibody against VMDHV in a high percentage of chickens, whereas the aMDHV was a comparatively poor inducer of such antibody. Despite this difference, both of the vaccinal viruses conferred protection against development of Marek's disease.     

Effects of strain of chickens and vaccination with turkey herpesvirus on Marek's disease and lymphoid leukosis in breeding stocks.

1. A total of 3236 females from eight meat-type strains, half of which were vaccinated for Marek's disease (MD), and 11,193 Leghorn females from ten strains, all vaccinated for MD, were adventitiously exposed to MD and lymphoid leukosis (LL) viruses and observed to 392 and 497 d of age, respectively. 2. In the meat-type birds, vaccination reduced total mortality from 43-4% to 27-1% and mortality due to MD from 16-4% to 5-4% but did not affect mortality and LL (2-9% and 3-4%). 3. In the vaccinated Leghorns total mortality was 11%, including 2-1% from MD and 1-2% from LL. 4. Significant differences between strains of chickens were found in total mortality, as well as in MD and LL mortality. 5. Strain by vaccination interaction was observed in total rearing and adult mortality, as well as in the MD mortality of adult meat-type females. 6. Leghorn strains with higher rate of egg production and meat-type strains with lower growth rate to have better viability.     

Marek's disease vaccination, with turkey herpesvirus, and enrofloxacin modulate the activities of hepatic microsomal enzymes in broiler chickens

Chickens were vaccinated against Marek's disease intramuscularly at one day of age. Enrofloxacin was given ad libitum in the drinking water at concentrations of 50, 100 and 250 mg/L from 8 days to 13 days of age when the animals were killed and the activities of cytochrome P-450 enzymes in the liver were measured. Vaccinated non-treated chickens served as a positive control. A negative control group was neither vaccinated nor treated. Vaccination decreased the activity of aniline hydroxylase and ethylmorphine N-demethylase in the positive control group. Subsequent application of enrofloxacin in the lowest concentration (50 mg/L) decreased, while that given at the highest level (250 mg/L) significantly increased the activity of the same microsomal enzymes. Relative liver weights and concentrations of proteins in 9000 x g supernatant were not affected by vaccination or treatment.     

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     

Effect of acyclovir on the replication of turkey herpesvirus and Marek's disease virus

The toxicity of acyclovir for chick embryo fibroblasts and its effect on the replication of turkey herpesvirus (strain FC 126) and Marek's disease virus (strain HPRS 16) multiplied on fibroblast culture was studied. The influence of using acyclovir on the development of the tumour process in birds infected with a virulent Marek's disease virus was also determined. Acyclovir used in doses below 12.5 micrograms ml-1 proved to be nontoxic for chick embryo fibroblast culture. It inhibited in vitro replication of turkey herpesvirus and Marek's disease virus. It was also shown to diminish the development of tumours in birds infected with Marek's disease virus.     

In situ production of interferon in tissues of chickens exposed as embryos to turkey herpesvirus and Marek's disease virus

Chicken eggs at embryonation day (ED) 18 or newly hatched chicks were inoculated with turkey herpesvirus (HVT), Marek's disease virus (MDV), or virus-free diluent and, at intervals after inoculation, tissue homogenates of virus-exposed and virus-free chickens or chicken embryos were examined for interferon (IFN) activity. Homogenates of lung, thymus and spleen specimens from chickens given HVT at ED 18 had IFN activity. Activity of IFN in the lungs was studied further. Homogenates of lung specimens from chickens exposed to HVT at hatching also had IFN activity, although the concentration of IFN was lower than that in chickens given HVT at ED 18. The pathogenic isolates of MDV (JM-MDV), but not the attenuated (Md11/75C-MDV) or nonpathogenic (SB1-MDV) isolates, inoculated at ED 18 also induced high lung IFN activity. Exposure to a combination of HVT and SB1-MDV induced IFN activity comparable with that in chickens given HVT alone. The IFN activity in homogenates of lung specimens from virus-exposed chickens was species specific and heat and pH stable, but was destroyed by trypsin treatment. Occasionally, low IFN activity also was detected in homogenates of tissue specimens from virus-free chickens or chicken embryos. This IFN activity could have been produced constitutively or may have been induced by substances (inducers) in the environment.     

A simple in vitro differentiation between turkey herpesvirus and Marek's disease virus

The growth and plaque formation by turkey herpesvirus (HVT) amd Marek's disease herpesvirus (MDHV) were examined in QT35 cells, a continuous fibroblast cell line derived from chemically induced tumors of Japanese quail. HVT grew and formed plaques consistently in QT35 cells when inoculated with cell-culture-propagated virus or peripheral mononuclear leukocytes (PML) from chickens that had been inoculated with HVT. Both oncogenic and nononcogenic strains of MDHV, however, failed to grow and induced neither plaques nor cytopathic effects in QT35 cells, whether inoculated with cell-culture-grown virus or heavily infected PML. When PML from chickens infected with both HVT and MDHV were assayed, only HVT plaques had developed, despite the presence in the inocula of high levels of MDHV with less HVT. The QT35 cell line provides a simple in vitro system for differentiating between HVT and MDHV and for selective isolation and identification of HVT from chickens infected with both HVT and MDHV.     

Replication of turkey herpesvirus and Marek's disease virus in chick embryo skin organ culture.

Turkey herpesvirus (HVT) and an attenuated Marek's disease virus (MDV) replicated in organ cultures of chick embryo skin as assessed by immunofluorescence and/or electron microscopy. HVT-specific immunofluorescent antigen was detected in the feather follicle epithelium (FFE) and in the surface layer of the skin epidermis. Electron microscopy of infected explants revealed herpes-type cytopathology. Immature particles of both viruses appeared first in the nucleus. Oval or horseshoe-shaped non-enveloped particles of HVT and enveloped virions of MDV were seen in the cytoplasm of some transitional cells. The difference in the ability of HVT and MDV to form an envelope was believed to account for the difference in their transmissibility in chickens. The results indicated that HVT replicated in the FFE and in the epidermis of the skin. However, attempts to localise the site(s) of MDV replication by electron microscopy were unsuccessful.     

Comparative pathogenesis studies with oncogenic and nononcogenic Marek's disease viruses and turkey herpesvirus.

An apparently nononcogenic Marek's disease virus (SB-1) and turkey herpesvirus could be readily isolated from spleen, bursa of Fabricius, thymus, and peripheral blood lymphocytes of chickens beginning 4 to 6 days after inoculation, but unlike infections with two isolates of oncogenic Marek's disease virus (JM-10 and CU-2), virus replication in these cells was rare, and necrosis in the organs was essentially absent. Splenic enlargement was observed regularly during the first 4 to 11 days after inoculation, and Marek's disease tumor-associated surface antigen was observed on splenic and other lymphocytes in the four viral inoculation groups. Cellular cytotoxicity of splenic lymphocytes was demonstrated in vitro with cultured Marek's disease tumor cells (MSB-1 lymphoblastoid cell line) as the target in a chromium-release assay. The four viral infections induced sensitized lymphocytes.     

A comparison of the efficacy against Marek's disease of cell-free and cell-associated turkey herpesvirus vaccine.

One-day-old White Leghorn and broiler chicks with maternal antibody to turkey herpesvirus (HVT) were vaccinated with 300 or 1,000 plaque-forming units (PFU) of cell-free or cell-associated HVT vaccine and challenged with virulent Marek's disease virus (MDV) by contact exposure. Broiler chicks receiving 300 PFU of cell-associated HVT had a 3.3% incidence of MD lesions, whereas only 2.0% of those receiving 1,000 PFU had macroscopic lesions. Broiler chicks vaccinated with 300 PFU of cell-free vaccine had 6.8% gross lesions, and 0.67% of the birds receiving 1,000 PFU had MD lesions. Unvaccinated broiler chickens had a 28.3% incidence of MD lesions. Unvaccinated White Leghorn chickens had a 48.9% incidence of macroscopic lesions, whereas 5.4% of the birds receiving 300 PFU of cell-associated HVT had gross lesions, and 8.3% of the birds vaccinated with 1,000 PFU had lesions. In contrast, 6.7% of the chicks vaccinated with 300 PFU of cell-free HVT had MD lesions, and only 4.0% of those receiving 1,000 PFU of cell-free HVT had macroscopic lesions.     

Effect of aflatoxin B1 on the efficacy of turkey herpesvirus vaccine against Marek's disease

The effect of feeding aflatoxin B1 (AFB1) (0.5 ppm) was studied in young chicks. The frequency and the severity of gross and microscopic lesions of Marek's disease were significantly higher in those birds which had been vaccinated with turkey herpesvirus (HVI) and birds challenged with Marek's disease virus which had been given AFB1 in the feed than in those given normal feed. The protective efficacy of HVT vaccine, as judged on the basis of gross and histopathological lesions, was 86.1 and 77.3 per cent in normally fed birds in comparison to 37.6 and 8 per cent in AFB1 fed birds.     

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.