Here are a couple of articles that describe the isolation of APV.
Links provided for each article.
A modified enzyme-linked immunosorbent assay for the detection of avian pneumovirus antibodies
Journal of Veterinary Diagnostic Investigation, Vol 12, Issue 4, 381-384
S Chiang, AM Dar, SM Goyal, MA Sheikh, JC Pedersen, B Panigrahy, D Senne, DA Halvorson, KV Nagaraja, and V Kapur
Isolation of avian pneumovirus from an outbreak of respiratory illness in Minnesota turkeys
Journal of Veterinary Diagnostic Investigation, Vol 12, Issue 2, 166-168
SM Goyal, SJ Chiang, AM Dar, KV Nagaraja, DP Shaw, DA Halvorson, and V Kapur