Approximately 15 billion chickens are produced each year, so clearly it will take some considerable time, effort, and expense to achieve this goal. For example, approximately sixteen different layer breeds have significant market share worldwide. Each is a three- or four-way cross. Thus, roughly 64 transgenic pure-lines would be required if one wished to reestablish the status quo. Chickens can be bred remarkably quickly. Adult layer stocks produce about 1.8 viable female chicks per week, with an expected viability of more than 90 percent. Replacing the 3 to 4 billion layers would take just 2 to 3 years once the transgenic lines had been made.
Transgene expression using lentivectors appears to be very stable, and it is arguable that if transgene expression persists over two generations (as has already been demonstrated in chickens) (McGrew et al., 2004), it is unlikely that it is ever going to suffer from transgene silencing—but time will tell. The selection of resistant mutant viruses is another distinct possibility. By using several independent inhibitory strategies it is hoped that the virus will be unable to overcome the blocks to its replication, thus ensuring the long-term effectiveness of the approach. This is a prerequisite if it is to be worthwhile to move toward the large-scale production of transgenic birds.
Because the transgenes are integrated at random in the chicken genome, there is the possibility of deleterious effects in some birds, depending on the location of the integration site. The availability of the chicken genome sequence will make analysis of integration sites quite straightforward, and facilitate the elimination of transgenic birds with the most obviously undesirable gene disruptions. The great advantage of the transgenic approach is that the single desired trait (resistance to influenza virus) can be inserted directly into commercial breeds, obviating the need to introduce the traits by cross-breeding with its associated problems of co-introducing undesirable traits. Once the founder transgenic birds have been fully characterized and shown to be healthy, the birds would be bred normally and subsequent generations would not require repeated genetic modification.
The strategy used to deliver the transgenes uses a highly efficient and stable delivery vector. The vector has been modified extensively to make it