TABLE 2.
Properties of virulent and avirulent avian influenza A viruses
| Property | Avirulent strains | Virulent strains |
|---|---|---|
| HA subtype | H1–H15 | H5, H7 |
| Replication | Respiratory and/or intestinal tracts | Most organs |
| Disease | Inapparent or mild: respiratory signs, depression, diarrhea, decreased egg production | systemic or “fowl plague” symptoms: depression; listlessness; ruffled feathers, swelling around the eyes; cessation of egg laying; respiratory signs; excessive lacrimation; sinusitis; edema of the head, face, neck and legs; cyanosis of unfeathered skin, particularly the combs and wattles; diarrhea; nervous system disorders |
| Mortality | Low | High (up to 100%) |
| Plaque formation in cell culture without exogenous protease | No | Yes |
| HA cleavage by intracellular proteases | No | Yes |
| HA cleavage site sequence | Single arginine | Multiple basic amino acids |
| Endoproteases responsible for HA cleavagea | Factor Xa-like protease (in ovo); plasmin (?), protease Clara (?), trypsin (?), bacterial protease (?) (in vivo) | Intracellular proteases expressed in most tissues (subtilisin-like proteases such as furin and PC6) |
a
Proteases that can proteolytically activate the avirulent virus HAs can do so also for the virulent virus HAs.