Studies of turkey herpesvirus viremia in two strains of vaccinated chickens strain difference and effects of vaccine dose.

Turkey herpesvirus (HVT) viremia was studied at 3-week intervals through 21 weeks of age in individual chickens of experimental (WSU-VS) and commercial strains (C-WL) of White Leghorns vaccinated with graded doses of HVT. HVT viremia was consistently detectable in all WSU-VS birds through 21 weeks postvaccination (PV) regardless of vaccine dose employed, whereas the viremia could not be detected in some of the C-WL birds 9 to 15 weeks PV and thereafter. C-WL birds that lost detectable viremia remained so up to 21 weeks PV, or returned to low levels of viremia which was followed in some birds again by no detectable viremia. Viremia titers were significantly lower in the C-WL than in the WSU-VS birds despite the same vaccination. Both strains of chickens exhibited significantly lower viremia titers when vaccinated with 240 plaqueforming units (PFU) than with 1,480 or 6,600 PFU of HVT, and other C-WL birds lost detectable viremia earlier and more frequently with smaller vaccine doses. This observation indicated a possible genetic difference between the two strains of chickens in susceptibility and viremic response to HVT as well as dose-dependence of HVT viremia.