Sorry for all of your losses. What have been all of the symptoms of the chickens as they became sick? Have you seen gasping to breathe, crackles or wheezes when breathing, mucus from the nostrils or beak, head shaking, bubbles or foam in eyes, swollen faces or eyelids, diarrhea, or discolored or purple spotted combs? The vet you have contacted should tell you if the birds need to be put down.
If you lose them all, I would disinfect your coop and facilities and let it sit for a few months before getting any new birds. If this is one of those bad viruses, it can kill many birds in your area (Nigeria.) Are there hatcheries where you can one day get day old chicks? That is the best way to start again with healthy chickens. Newcastles disease or avian influenza could be the culprit, but without testing it is hard to know. But the vet should have a couple of the bodies tested. When one of those diseases affect an area, they usually control it by killing all chickens, and other fowl in the area to limit it’s spread.
