Likely the new ones were resistant to what they brought in and yours were not.
That's the reason for quarantine as far from your birds as possible, for as long as possible. Then prior to introducing them, take one of your sacrificial birds and house it together or very close to the new birds for days to weeks.
I wanted to add that when sending a chicken (or any other animal) for necropsy, include a writeup of the bird's history - age, species, breed, feed, illnesses and especially any medications given and when. Giving antibiotics prior to sending for necropsy can skew the results of tests.
Also, tell FedEx that the box contains medical samples, not a dead bird.
That's the reason for quarantine as far from your birds as possible, for as long as possible. Then prior to introducing them, take one of your sacrificial birds and house it together or very close to the new birds for days to weeks.
I wanted to add that when sending a chicken (or any other animal) for necropsy, include a writeup of the bird's history - age, species, breed, feed, illnesses and especially any medications given and when. Giving antibiotics prior to sending for necropsy can skew the results of tests.
Also, tell FedEx that the box contains medical samples, not a dead bird.
