Mannheimia haemolytica-associated fibrinonecrotizing abomasitis in lambs

Abstract

Mannheimia haemolytica-associated abomasitis has been clinically described as a cause of sudden death in lambs, but it is poorly characterized. We describe the pathological features of a severe fibrinonecrotizing abomasitis in 3 lambs that died suddenly. All 3 abomasums had a thickened submucosa due to edema and necrotic areas delimited by bands of degenerate neutrophils with slender nuclei (oat cells) and angiocentric distributions. The overlying mucosa was congested. Myriads of gram-negative coccobacilli were observed within the oat cell bands. M. haemolytica was isolated from the abomasum in all 3 animals and was serotyped as A2 in one of them. Pericarditis and pleuritis were observed in 2 of the lambs. Clostridium spp. were isolated in 1 lamb and detected by immunohistochemistry in the 3 animals, suggesting clostridial co-infection. M. haemolytica should be considered among the differential diagnoses of necrotizing abomasitis in lambs.

Keywords: Mannheimia haemolytica; PCR; abomasitis; fibrinonecrotizing; immunohistochemistry; lambs.

Figure 1.

Fibrinonecrotizing abomasitis, abomasum, lamb. (a) A macroscopic view of a transverse section reveals severe thickening of the wall with submucosal edema and congested submucosal and subserosal vessels. Intact mucosa is deep red. Case 1. (b) Subgrossly, the abomasal submucosa contains multiple areas of edema, necrosis, and fibrin delimited by serpiginous bands of inflammatory cells parallel to the muscularis mucosa and internal tunica muscularis and, occasionally, to the serosa (sequestra, asterisks); gas bubbles are also embedded within these areas. The mucosa remains intact, and the submucosal vessels are congested with occasional thrombi (arrowhead). Case 2. Hematoxylin and eosin (HE). (c) The submucosa exhibits marked edema and vasculitis, and the inflammatory rim of the sequestrum adjacent to the muscle layers infiltrates muscle fibers of the internal tunica muscularis. The non-ulcerated mucosa is congested at its base with an overlying band of ischemic necrosis on the surface. Case 3. HE. (d) Two congested submucosal vessels. One vessel (asterisk) is associated with bands of karyorrhectic debris and colonies of coccobacilli that partially infiltrate the wall. Intramural colonies replace the wall of the other vessel (arrowhead). Case 1. HE. Inset: colonies of coccobacilli are gram-negative. Case 1. Gram stain. (e) Lymphatics are distended by an accumulation of degenerate neutrophils admixed with colonies of coccobacilli in the lumen (asterisk). The colonies and degenerate neutrophils infiltrate the wall (arrowheads). Adjacent venules are intact. Case 1. HE.

Figure 2.

Fibrinonecrotizing abomasitis, abomasum, lamb. (a) Submucosal inflammatory band composed of a palisade of oat cells intermixed with colonies of coccobacilli. Case 1. HE. (b) Bacterial colonies are gram-negative. Case 3. Gram stain. (c) Immunohistochemistry with Mannheimia haemolytica OmpA^PH278 antibody revealed immunolabelled colonies of coccobacilli within the bands of inflammation. Case 1. M. haemolytica OmpA^PH278 immunohistochemistry. Inset: Higher magnification of the immunolabeled organisms.