Avian pneumoviruses and emergence of a new type in the United States of America.

Avian pneumovirus (APV) primarily causes an upper respiratory disease recognized as turkey rhinotracheitis (TRT) or swollen head syndrome (SHS) in chickens. The virus was first isolated in South Africa during the early 1970s and has subsequently been reported in Europe, Asia and South America. In February 1997, a serologically distinct APV isolate was officially reported in the USA following an outbreak of TRT during the previous year. This was the first report of these virus types in the USA; they were previously considered exotic to the USA and Canada. The predicted matrix (M) proteins of European APV type A and B isolates share 89% identity in their amino acid sequence. However, the predicted M protein of APV/CO is only 78% similar to the APV type A and 77% similar to the APV type B protein sequence. The predicted amino acid sequence of the US APV isolate's fusion (F) protein has 72% sequence identity to the F protein of APV type A and 71% sequence identity to the F protein of type B. This compares with the 83% sequence identity between the predicted amino acid sequences of the F proteins of APV types A and B. The lack of sequence heterogeneity among the US APV isolates over 2 years suggests that these viruses have maintained a relatively stable population since the first outbreak of TRT. Phylogenetic analysis of the M and F proteins, together with the serological uniqueness of the US APV isolates, supports their classification as a new APV, designated type C.