4-month-old pigThis pig was submitted with a history of sudden death.

Gross Description:  

There was a generalized serofibrinous pleuritis and multiple widely distributed foci of fibrinous pneumonia in both lungs. Regional lymph nodes were increased in size and hemorrhagic. Small white foci surrounded by a hyperhemic zone were disseminated in the skin.

Histopathologic Description:

In the lung section submitted, there is a serofibrinous pneumonia with many necrotic leukocytes (Fig. 4-1, 4-2). These lesions were multifocal and generalized in both lungs. The necrotic leukocytes appear as round cells with pyknotic nuclei, and cells with a streaming of pale basophilic chromatin, the so called oat cells. Small coccobacilli (gram-negative) are present in the alveolar exudate, and few bacterial emboli are present in some sections. Several capillaries are thrombosed. There is a severe fibrinous pleuritis with necrotic leukocytes similar to those in the lung lesions. 

Morphologic Diagnosis:  

Severe acute fibrinoleukocytic pleuropneumonia with many oat cells and the presence of coccobacilli.

Lab Results:  

Actinobacillus suis was isolated from the pleura, lung, skin and other organs.


Actinobacillus suis

Contributor Comment:  

The skin lesions observed grossly were characterized by small dermal vessels thrombosed and/or occluded by bacterial emboli (small gram-negative coccobacilli). They were infiltrated and surrounded by inflammatory cells, mainly necrotic leukocytes similar to those in the lung. Small coccobacilli were also present in the inflammatory infiltrates.

The multifocal and widespread pneumonia, and the skin lesions observed in this pig are compatible with a septicemia caused by Actinobacillus suis. Clinical cases of A. suis occur more frequently in high-health-status herds (6). The most common manifestation of the infection is septicemia and sudden death in suckling and recently weaned pigs (6). A disease resembling pleuropneumonia caused by A. pleuropneumonia, and skin lesions similar to those caused by Erysipelothrix rhusiopathiae are reported in older pigs (6).

The pneumonic lesions caused by A. suis can have two patterns. One of them is a focal locally extensive fibrinohemorrhagic, fibrinoleukocytic and necrotizing pneumonia or pleuropneumonia affecting the middle or the caudal lung lobes, which may be unilateral or bilateral (2). These lesions are very similar to those caused by A. pleuropneumonia, and are probably originating from an airborne entry of the organism (2). The other pattern is a generalized multifocal pneumonia indicating hematogenous origin. This multifocal widespread pneumonia is a common finding in cases of A. suis septicemia. Other lesions observed in septicemic cases are petechial hemorrhages in serosa and other organs, multifocal necrosis and inflammation in the liver, spleen, kidney and skin, splenomegaly, serofibrinous pericarditis, pleuritis and peritonitis, polyarthritis, valvular endocarditis, and rhomboid skin lesions similar to those observed in cases of erysipelas (6).

The fibrinous pneumonia with many necrotic leukocytes appearing as oat cells is characteristic of A. pleuropneumonia and A. suis in swine (2). Different serotypes of A. pleuropneumonia produce RTX-toxins (ApxI, II and III) which are cytotoxic for the porcine neutrophils and macrophages (2, 4). Some strains of A. suis produce a RTX-toxin (Apx I) (6). Oat cells are also present in the fibrinous pneumonia caused by Mannheimia haemolytica in cattle, sheep and goat (2). All serotypes of M. haemolytica produce a leukotoxin being a member of the RTX family of bacterial toxins (2). These necrotic leukocytes appearing as oat cells are also present in the inflammatory lesions of other organs in cases of A. Suis septicemia.

JPC Diagnosis:  

Lung: Pneumonia, necrotizing, histiocytic and neutrophilic, multifocal, marked, with vasculitis, necrotic leukocytes (oat cells), fibrin, diffuse interstitial and alveolar edema, and numerous colonies of coccobacilli, pig, porcine.

Conference Comment:  

Actinobacillus suis is a gram negative, nonmotile, nonencapsulated aerobic and facultative anaerobic coccobacillus that is often an inhabitant of the tonsils and upper respiratory tract of pigs of any age and the vagina of clinically healthy sows.(6) A. suis can cause rhomboid skin lesions secondary to vasculitis, and this manifestation can be confused with erysipelas. Petechial to ecchymotic hemorrhages can occur in multiple organs to include the lung, kidney, heart, liver, spleen, and intestines. These lesions are often most pronounced in the lungs with a striking resemblance to those of pleuropneumonia. In sows, A. suis can cause metritis, meningitis, and abortion. Histologically, bacterial thromboemboli randomly scattered in the vasculature of the previously mentioned organs is suggestive of A. suis.(6)
Actinobacillus pleuropneumoniaPigsSerofibrinous pleuritis and necrotizing hemorrhagic pneumonia; caudodorsal distribution
Actinobacillus equuliHorsesCommon cause of suppurative embolic nephritis in foals
Actinobacillus lignieresiiCattleGlossitis and stomatitis in cattle (wooden tongue)
Actinobacillus seminisSheepCommon cause of bilateral epididymitis in rams


1. Brown CC, Baker DC, Barker IK: Alimentary system. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 2, pp. 96-97. Elsevier Limited, St. Louis, MO, 2007
2. Caswell JL, Williams KJ: Respiratory system. In: Jubb, Kennedy and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 2 pp. 587-589, 601-606. Elsevier Limited, St. Louis, MO 2007
3. Foster RA, Ladds PW: Male genital system. In: Jubb, Kennedy and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 3 pg. 590. Elsevier Limited, St. Louis, MO 2007
4. Gottschalk M, Taylor DJ : Actinobacillus pleuropneumonia. In: Diseases of Swine, ed. Straw BE, Zimmerman JJ, DAllaire S, Taylor DJ, 9th ed., pp. 563-576. Blackwell Publishing, 2006.
5. Maxie MG, Newman SJ: Urinary system. In: Jubb, Kennedy and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 2, pg. 480. Elsevier Limited, St. Louis, MO, 2007
6. Taylor DJ: Actinobacillus suis. In: Diseases of Swine, ed. Straw BE, Zimmerman JJ, DAllaire S, Taylor DJ, 9th ed., pp. 827-829. Blackwell Publishing, 2006.

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4-1 Lung, pig

4-2 Lung, pig

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