Methylmercury: effect on oncogenic and nononcogenic viruses in mice.

Mice fed methylmercury chloride at dosages of 1 or 10 ppm for 84 days had significantly higher mortality rates when inoculated with encephalomyocarditis virus (nononcogenic) than did nonmethylmercury-treated mice. However, methylmercury fed to mice which were inoculated with Rauscher leukemia virus (oncogenic) did not alter the course of neoplasia. These results demonstrate that prolonged exposure to subclinical concentrations of methylmercury increased susceptibility of a host to a nononcogenic, but not to an oncogenic, virus.     

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.     

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.     

Differentiation of oncogenic and nononcogenic strains of Marek's disease virus type 1 by using polymerase chain reaction DNA amplification.

Differentiation of oncogenic and nononcogenic strains of Marek's disease virus type 1 (MDV1) was attempted by polymerase chain reaction (PCR) using the primers chosen from the sequence within the long inverted repeats of MDV1 DNA. PCR of the DNAs extracted from oncogenic-strain-infected cells and Marek's disease tumor cell lines produced a major product containing two or three copies of 132-base-pair (bp) repeat units, whereas PCRs of the DNAs extracted from nononcogenic-strain-infected cells yielded amplified products with various sizes corresponding to the number of 132-bp repeat units. The primers chosen from the glycoprotein A genes of MDV1 and herpesvirus of turkeys also were used for determination of their serotype specificity. The PCR procedure was found to be a simple and sensitive procedure for identification of MDV1 and herpesvirus of turkeys and for estimation of oncogenicity of MDV1.     

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.     

Dose-dependent inhibition of virus rescue from lymphocytes latently infected with turkey herpesvirus or Marek's disease virus

The number of plaque-forming units (PFU) of turkey herpesvirus (HVT) isolated per 10(6) latently infected splenic lymphocytes was determined by co-cultivation on permissive monolayer cultures in 35-mm-diameter Petri dishes. Doses of 1 x 10(6) spleen cells or less per culture gave uniform dose-related titers, whereas doses of 8 x 10(6) cells often yielded less than 1-2% of the expected number of PFU. Intermediate doses gave proportionally reduced virus yields. This dose-dependent inhibition was observed with spleen cells from birds within a week after infection and became more marked with time. A similar phenomenon occurred with a non-oncogenic Marek's disease virus (MDV) isolate (SB-1) but not with oncogenic MDV isolates (CU-2, JM-10, GA-5), except in genetically resistant birds. High numbers of uninfected spleen cells mixed with low numbers of HVT-infected cells during assay reduced titers only slightly. Immunosuppression by combined neonatal thymectomy and cyclophosphamide treatment before HVT infection prevented the inhibition, but embryonal bursectomy had no effect.     

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.     

Trials with Marek's disease vaccines prepared from a turkey herpes virus and an attenuated Marek's disease virus.

In field trials involving over 224,000 fowls in 11 different commercial flocks, three vaccines were used, namely a freeze-dried vaccine prepared from a turkey herpes virus, a cell-associated virus vaccine prepared from the same isolate and a cell-associated vaccine prepared from a strain of Marek's disease virus isolated from a fowl. The mortality from Marek's disease was reduced by 80 per cent to 95 per cent in birds vaccinated with the freeze-dried vaccine. Cell associated vaccines gave slightly less protection.     

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.     

Influence of turkey herpesvirus vaccination on the B-haplotype effect on Marek's disease resistance in 15.B-congenic chickens.

Eight recently developed 15.B congenic lines of chickens were tested for Marek's disease (MD) resistance by intra-abdominal injection of cell-associated preparations of MD virus of a virulent strain (JM), a very virulent strain (Md5), or Md5 after vaccination with turkey herpesvirus (HVT) strain FC126. Chickens of the 15.N congenic line (B15B21 or B21B21) were very resistant to JM-induced MD, in contrast to chickens homozygous for the B-haplotypes 2, 5, 12, 13, 15, or 19. After Md5 infection, more than 88% of the chickens in all of the congenic lines developed MD. However, when chickens were vaccinated with HVT before being inoculated with Md5, the B5 and B12 homozygotes were more resistant to MD than were the B2, B13, or B19 homozygotes, and B15 and B21 homozygotes had intermediate resistance. B5B5 and B2B5 F2 chicks inoculated with HVT and Md5 had a lower prevalence of MD than B2B2 sibs. These results demonstrate that a protocol involving HVT vaccination of chicks followed by infection with very virulent MD virus will allow the detection of B-haplotypes determining MD resistance, some of which are not detectable in unvaccinated chicks challenged with virulent MD.