For my point of view the lentogenic NDV(vaccinal or field strains) is the predisposing factor for the spreading of LPAI, through increasing the inflammation and proteases in in all organs. Proteases are needed for the cleavage of HA and virus entry. So, NDV infection or vaccination enhances the LPAI multiplication and shedding which lead to outbreak. All this will occur only if the LPAI already in the farm. In conclusion lentogenic NDV infection will lead to rapid spread and multiplication of LPAI.
It may be attributed to the stimulation of cellular components (macrophage, lymphocytes,.. etc) as well as cytokines production as response to LPAIV infection in level higher than that of response to NDV vaccine only which act as an immune potentiation to brids leading to high NDV HI titer.