Lack of a threshold of genetic resistance to Marek's disease and the incidence of humoral antibody

Abstract

On challenge exposure of genetically susceptible (Line 7) and genetically resistant (Line 6) chickens to graded doses of Marek's disease virus, Line 5 chickens were at least 10(6)-fold more resistant to clinical Marek's disease than Line 7 chickens. Hence, if there was a "threshold" of resistance of Line 6 chickens to the disease, it was not overcome by 10(6) times the dose of Marek's disease virus required to induce neoplastic response in Line 7. Approximately similar doses of virus were required to induce a precipitating and virus neutralizing antibody response in Lines 6 and 7, indicating that both types of chickens were equally susceptible to infection with Marek's disease virus. As opposed to the marked resistance of maternal antibody positive Line 6 chickens used above, maternal-antibody-negative chickens of this line were highly susceptible to clinical Marek's disease. Since the genetic homogeneity of maternal antibody positive and maternal antibody negative populations of Line 6 chickens was equivocal, the difference of Marek's disease susceptibility between the two populations could not with certainty be attributed to maternal antibody alone. Line 7 chickens that survived an infection with Marek's disease virus produced virus neutralizing antibody as did genetically resistant Line 6 chickens. In many susceptible chickens, the virus neutralizing antibody and nerve lesions coexisted. Agar gel precipitin, immunofluorescent, and virus neutralizing antibody tests on 50 sera from birds inoculated with Marek's disease virus indicated that virus neutralizing and immunofluorescent tests were more sensitive for detecting Marek's disease antibody than the agar gel precipitin test. Among the sera positive by all 3 antibody tests, titers of virus neutralizing and immunofluorescent antibody were higher than those of agar gel precipitin antibody.