Gumboro Disease or commonly called Infectious Bursal Disease (IBD) is an acute highly contagious viral disease of young chickens. The bursa of Fabricius is the primary organ in the development of the immune system of chickens. This rounded sac-like structure is just above the cloaca of chickens. This structure will continually develop until it reaches 4-5 months and then it will start to regress or shrink.
Incidence of IBD is now considered to be very high. Young birds are highly susceptible.... between 3 to 6 weeks of age.
The best time to administer Gumboro vaccine is at 2-3 weeks of age (in high-risk areas) followed at 16-18 weeks of age (as booster shot).
It is important to immunize your Broodstocks to confer parental immunity to their progeny. Such maternal antibodies will protect the chick for 1-3 weeks.
The chicks were not affected...simply because they are still confined in a limited area. Improper handling of the vaccine might have triggered the outbreak.. I hope, the swelling of the face, is just a simple case of Coryza (bacterial) because it can easily be treated with an antibiotic (capsule or injectable), given for 4-5 consecutive days.(mercury drug). What worries me ay yung panlolobo ng katawan because that is another complication. It would be best to instruct your VET to bring the suspected bird to a Diagnostic Lab for proper evaluation. Please request to include Sensitivity Testing.
RULE: Rear your chicks isolated from older birds because mixed rearing multiplies the chances of disease infection.