Horizontal transmission of turkey herpesvirus to chickens. 5. Airborne transmission between chickens

Replication kinetics of turkey herpesvirus in lymphoid organs and feather follicle epithelium in chickens

Marek’s disease virus (MDV) is an oncogenic alphaherpesvirus that causes immunosuppression, T-cell lymphomas, and neuropathic disease in infected chickens. To protect chickens from MDV infection, an avirulent live vaccine of turkey herpesvirus (HVT) has been successfully used for chickens worldwide. Similar to MDV for natural infection in both chickens and turkeys, HVT also infects lung in the early stage of infection and then lymphocytes from lymphoid organs. Virus replication requires cell-to-cell contact for spreading and semi-productive lytic replication in T and B cells. Then, cell-free infectious virions matured in the feather follicle epithelium (FFE) are released and spread through the feather from infected turkeys or chickens. To understand the lifecycle of HVT in inoculated chickens via the subcutaneous route, we investigate the replication kinetics and tissue organ tropism of HVT in chickens by a subcutaneous inoculation which is a major route of MDV vaccination. We show that the progeny virus matured in lymphocytes from the thymus, spleen, and lung as early as 2 days post-infection (dpi) and bursa of Fabricius at 4 dpi, whereas viral maturation in the FFE was observed at 6 dpi. Furthermore, semi-quantitative reverse transcription-PCR experiments to measure viral mRNA expression levels revealed that the higher expression levels of the late genes were associated with viral maturation in the FFE. These data that tropism and replication kinetics of HVT could be similar to those of MDV through the intake pathway of natural infection from respiratory tracts.

Pathogen transmission from vaccinated hosts can cause dose-dependent reduction in virulence

Many livestock and human vaccines are leaky because they block symptoms but do not prevent infection or onward transmission. This leakiness is concerning because it increases vaccination coverage required to prevent disease spread and can promote evolution of increased pathogen virulence. Despite leakiness, vaccination may reduce pathogen load, affecting disease transmission dynamics. However, the impacts on post-transmission disease development and infectiousness in contact individuals are unknown. Here, we use transmission experiments involving Marek disease virus (MDV) in chickens to show that vaccination with a leaky vaccine substantially reduces viral load in both vaccinated individuals and unvaccinated contact individuals they infect. Consequently, contact birds are less likely to develop disease symptoms or die, show less severe symptoms, and shed less infectious virus themselves, when infected by vaccinated birds. These results highlight that even partial vaccination with a leaky vaccine can have unforeseen positive consequences in controlling the spread and symptoms of disease.