Effects of modified-live infectious bronchitis virus vaccines on the head-associated lymphoid tissue (HALT).

Specific-pathogen-free (SPF) leghorn chicks were inoculated with different modified-live infectious bronchitis virus (IBV) vaccines to determine if the vaccines interfered with immune competence of the head region. A total of 16 vaccines were evaluated comprising nine Massachusetts, three Arkansas, two Holland, one Florida, and one combination vaccine (containing both Connecticut and Massachusetts). Chicks were vaccinated when they were 4 weeks, 2 weeks, or 1 day of age. When all chickens were 4 weeks 3 days of age, their glands of Harder (GH) were assayed for the ability to respond to antigenic stimulation. Tissues from chicks given GH-depressing and non-GH-depressing vaccines were also collected and scored for histological changes in the head-associated lymphoid tissue (HALT) sites and the trachea. All 16 vaccines depressed the GH response to antigenic stimulation when given to 4-week-old chicks. Six of these vaccines (two Massachusetts, two Arkansas, and two Holland) also depressed the GH response when given to 2-week-old chicks, and one, an Arkansas vaccine, depressed the GH response when given to 1-day-old chicks. The main histological changes associated with the vaccines were increases in lymphocyte populations in the nasal mucosa, eyelid, and, for some, the lacrimal gland and the GH. In addition, lymphoid follicles were increased in the eyelid, to a lesser degree in the GH, and occasionally in the trachea. No relationship was found between histologic changes and vaccine-induced suppression of the GH response.