Author
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Witter, Richard |
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Gimeno, Isabel |
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Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/27/2006 Publication Date: 9/1/2006 Citation: Witter, R.L., Gimeno, I.M. 2006. Susceptibility of adult chickens, with and without prior vaccination, to challenge with Marek's disease virus. Avian Diseases. 50(3):354-365. Interpretive Summary: Marek’s disease (MD), a virus-induced cancer-like disease of chickens, is considered a major disease problem in commercial poultry. Vaccination has dramatically reduced the incidence of the disease, but some adult chicken flocks continue to experience problems. However, little is known about the susceptibility of older chickens to the disease. The objective of this research was to determine whether old chickens would develop the disease if infected as adults. We have determined that old chickens are susceptible provided they have no immunity from prior exposure or vaccination and that old chickens from vaccinated flocks are resistant to new infections. This important information will help scientists in academia and industry understand the reasons why MD occurs in adult chicken flocks and eventually lead to better control of the disease. Technical Abstract: Marek's disease (MD) outbreaks may occur in previously healthy adult layer or breeder flocks. However, it is not clear whether such outbreaks can be solely attributed to recent challenge with highly virulent strains of MD virus (MDV) or whether other factors contribute. To clarify this issue, adult White Leghorn chickens of laboratory strains or commercial crosses with or without prior vaccination or MDV exposure were challenged at 18-102 weeks of age with highly virulent MDVs and lesion responses were measured. Horizontal transmission was studied in one trial. Challenge with highly virulent MDV strains of adult chickens, which were free from prior MDV vaccination or exposure, induced a high rate of transient paralysis and tumors, and horizontal spread of virus was detected. The magnitude of the response was similar to that induced in chickens challenged at 3 wks of age. In contrast, challenge of adult chickens, which had been vaccinated or MDV-exposed early in life, generally failed to induce transient paralysis or tumors, although some birds showed limited virological evidence of infection and transmission of the virus to contacts. The MD responses were influenced significantly by the virulence of the challenge virus strain, and to a lesser extent by virus dose and route of exposure. Histopathological examination of lesions induced in adult chickens revealed differences compared to lesions in younger birds. The low susceptibility of previously vaccinated and exposed groups after 18 wks of age suggests that late outbreaks of MD in commercial flocks are not likely due to recent challenge alone and that additional factors may be involved. |
