After reading some articles from The Poultry Site and The Merck Manual, it seems that the disease (gumboro) is common all over the world, and that it is spread by carrier birds who have not been vaccinated. Your vet in Nigeria should know how common the disease is. So, yes, if the disease is confirmed in your chickens, it is either coming from the breeder birds, or somewhere else along the way. Transporting the birds could also be a possibility.
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