Histological changes in lungs of calves exposed to an aerosol of Pasteurella haemolytica.

An experiment was designed to study the interaction of Pasteurella haemolytica with an attenuated bovine herpesvirus 1 in calves. Low titre of the virus culture used for aerosol exposure failed to produce measurable interaction. However, the experiment provided the first opportunity to study the light-microscopic changes in lungs of calves (n = 3) to a low-dose exposure (5-min aerosol) of P. haemolytica A1 from a fresh 5-h log-phase culture. The histopathological study was confined to tissue exposed to only P. haemolytica. A limited macroscopic pneumonia was produced in ventral parts of cranial lobes. Four days after exposure, a typical reaction featured four zones. Zone 1a at the centre with acute inflammatory processes and necrosis of phagocytic cells was surrounded by a broad band of compacted, largely necrotic macrophages and polymorphonuclear leucocytes (PMNL) in alveoli of zone 1b. Necrosis was confined to zone 1. Zone 2a frequently occupied the remainder of the lobule with irregular distribution of congestion, oedema with a fibrinous component, and infiltration by numerous PMNL, macrophages and other mononuclear inflammatory cells. The narrow zone 2b was located between zones 1b and 2a and had oedema with a fibrinous component, numerous fibrocytes, few inflammatory cells and empty capillaries. It is suggested that zone 2 served to isolate zone 1 by surrounding it with nonfunctional tissue. The pathogenicity of P. haemolytica is discussed for uncompromised lungs and lungs compromised by virulent BHV1 infection.