History

In October 2015, an 11-month-old female Katahdin lamb was submitted for postmortem examination. This lamb was part of a flock of 36 sheep the owner purchased as a group in southern Indiana about 1 month prior to the onset of illness. Of the 36 sheep, 7 had died. Some of the sheep were noted to have ptyalism and diarrhea before quickly progressing to death. The lamb submitted for examination was found dead without noticeable clinical signs.

Gross Findings

Grossly, the mucous membranes and meninges were congested, and lymph nodes of the lamb's head and neck were congested and enlarged. Petechiae to ecchymoses were present in various tissues, including the thoracic vertebral fascia, adventitia over the base of the pulmonary artery, epicardium and endocardium of the heart, and serosal surface of the uterus (Figure 1). The lungs were diffusely red, wet, and heavy with multifocal hemorrhages in the pulmonary parenchyma. There was abundant foam in the trachea and smaller airways, suggestive of pulmonary edema. Coalescing ulcers on the dorsal surface of the caudal portion of the tongue were evident. The cecal mucosa was thickened and had multifocal hemorrhages. The colon contained whipworms (Trichuris spp).

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Photographs obtained during postmortem examination of an 11-month-old female Katahdin lamb that was found dead without premonitory signs. A—In a view of the opened thorax, there are multifocal to coalescing areas of hemorrhage in the lungs and ecchymoses (arrows) in the fascia of the thoracic vertebrae. B—Notice the petechiae (circle) on the adventitia of the pulmonary artery. C—There is an 8-mm-long ulcer (arrow) on the dorsal surface of the tongue.

Citation: Journal of the American Veterinary Medical Association 257, 12; 10.2460/javma.257.12.1245

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Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page→

Histopathologic and Microbiological Findings

Within the lungs, alveolar capillaries were diffusely congested (Figure 2). Bacterial emboli, characterized by colonies of bacterial coccobacilli embedded within fibrin and admixed with degenerate leukocytes, were scattered within alveolar capillaries throughout the lungs. The alveoli contained fluid (edema).

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Photomicrographs of a section of lung tissue from the lamb in Figure 1. A—There is diffuse congestion of alveolar capillaries, and edema is present within some alveoli. Also, bacterial emboli are present within alveolar capillaries (arrows). H&E stain; bar = 100 μm. B—Higher-magnification view of bacterial emboli (arrow). H&E stain; bar = 50 μm.

Citation: Journal of the American Veterinary Medical Association 257, 12; 10.2460/javma.257.12.1245

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Throughout the liver, bacterial emboli were scattered randomly within the hepatic sinusoids, central veins, and hepatic veins (Figure 3). Several bacterial emboli in the liver were surrounded by degenerating hepatocytes and a few necrotic leukocytes. Bacterial emboli were also observed within blood vessels and subcapsular sinusoids in a retropharyngeal lymph node and rarely within blood vessels in the kidneys. Within the myocardium of the heart, there were scattered foci of swollen, hypereosinophilic cardiac myocytes with a lack of cross striations. On the section of tongue examined histologically, there was a well-demarcated area of ulcerated glossal epithelium with underlying necrotic tissue partially covered with bacteria. The tissue deep to the ulcer was lightly infiltrated with macrophages and lymphocytes.

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Photomicrographs of a section of liver tissue. A—There are foci of intralesional bacteria within and occluding hepatic sinusoids (outlined areas). H&E stain; bar = 200 μm. B—Higher-magnification view of bacterial emboli (arrow) within the hepatic sinusoids. H&E stain; bar = 50 μm.

Citation: Journal of the American Veterinary Medical Association 257, 12; 10.2460/javma.257.12.1245

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Aerobic bacterial culture of specimens of lung tissue yielded moderate to heavy growth of Bibersteinia (Pasteurella) trehalosi. Lung and spleen tissue specimens were analyzed for epizootic hemorrhagic disease virus, bluetongue virus, parainfluenza 3 virus (only lung tissue was tested), bovine herpesvirus 4, and bovine viral diarrhea virus; none of the viruses were isolated. For attempted isolation of each virus, lung or spleen tissue homogenates were added to appropriate cell lines, as follows: a calf pulmonary artery endothelial cell line was used for epizootic hemorrhagic disease virus, a Vero cell line was used for bluetongue virus, and a bovine turbinate cell line was used for parainfluenza 3, bovine herpesvirus 4, and bovine viral diarrhea virus. Cytopathic effects were not observed in any of the cultures. A sample of cells from each test was stained with virus-specific antibodies to check for low-level infection and to rule out noncytopathic strains of bovine viral diarrhea virus. Fluorescent antibody testing of lung and spleen specimens for bluetongue virus was also conducted, and results were negative.

Morphologic Diagnosis and Case Summary

Morphologic diagnosis: acute pulmonary congestion and edema with intravascular bacterial emboli; acute multifocal hepatic degeneration with intralesional bacterial emboli; intravascular bacterial emboli in the lymph node and kidneys; mild, acute, multifocal myocardial degeneration; and acute, focal ulcerative glossitis.

Case summary: septicemic pasteurellosis (B trehalosi infection) in a lamb.

Comments

For the lamb of the present report, histologic lesions of bacterial emboli within vessels of the lungs and liver, coupled with degeneration of surrounding tissue in the liver, were suggestive of bacterial septicemia. In light of the aforementioned histologic changes and clinical signs, the moderate to heavy growth of B trehalosi from lung tissue specimens in culture indicated that this species of bacteria was the likely causative agent of disease in the lamb. Two other lambs from the same flock were also submitted for postmortem examination and had similar lesions of systemic vascular compromise (intralesional bacterial emboli within multiple organ systems), and cultures of lung tissue specimens yielded growth of B trehalosi. All 3 examined lambs had pulmonary edema and congestion as well as multifocal hemorrhages scattered in various tissues. Bacterial emboli were observed within vessels in the lungs, liver, lymph nodes, and, rarely, kidneys in 2 of the 3 lambs. Ulcerative glossitis with intralesional bacterial emboli was observed in one lamb, and necrotizing colitis with intralesional bacterial emboli was present in another lamb.

A major differential diagnosis in the case described in the present report was bluetongue virus infection because the observed gross lesions were very similar to those that commonly develop with that viral infection, especially the presence of hemorrhages on the adventitia of the pulmonary artery.¹ However, in instances of bluetongue virus infection, the histologic lesions are those of vasculitis in which small blood vessels are cuffed by inflammatory cells and hypertrophy of the vascular endothelium.¹ For the lamb of the present report, lesions were characterized by intravascular bacterial emboli with a profound lack of visible inflammatory response surrounding the vessels.

Bibersteinia trehalosi, formerly known as Pasteurella trehalosi or Pasteurella hemolytica biotype T, is a gram-negative bacterium that causes septicemic pasteurellosis in domestic sheep, especially weaned lambs (6 to 9 months old), in the fall months.^2–4 Bibersteinia trehalosi infection has also been known to result in fatal pneumonia in bighorn sheep and acute pneumonia and septicemia in cattle.^5,6 The pathogenesis of septicemic pasteurellosis is not fully understood. Bibersteinia trehalosi is known to be a commensal organism often found in the oropharynx and tonsils of healthy sheep.^7,8 Changes in dietary or environmental factors, such as a change in pasture type, may lead to the development of ulcerative lesions in the pharynx and upper portion of the gastrointestinal tract, which then allow bacteria entry into local blood and lymphatic vessels.^2,3 The bacteria travel through the general circulation to the lungs, liver, and other organs where they further multiply and release endotoxin, resulting in sudden death.² An alternative suggested mechanism involves the development of primary ulcerative lesions in the forestomach and intestines that allow bacteria to gain access to the portal circulation and travel to the liver. From the liver, the bacteria can spread to other organs, such as the lungs, via blood circulation.²

Several research studies^5,6,8 have identified leukotoxin as a critical virulence factor in the pathogenesis of B trehalosi. In fact, in bighorn sheep, infections with leukotoxin-positive but not leukotoxin-negative strains of B trehalosi result in fatal pneumonia.⁶ Furthermore, approximately 90% of the isolates of B trehalosi from the oropharynx of healthy bighorn sheep are leukotoxin negative.^6,8,9 Therefore, infections with only specific strains of B trehalosi, which may differ from those observed as commensal organisms, are thought to cause disease.

Clinical signs of septicemic pasteurellosis in sheep are often absent, and infected sheep may be found dead before outward signs of illness become apparent,^2,3 as highlighted by the case described in the present report. In general, the mortality rate associated with septicemic pasteurellosis seldom exceeds 5% to 10% of at-risk sheep in the flock.^2,3 However, in the situation described in the present report, the farmer lost approximately 19% of his flock of 36 sheep. Because only 3 of the 7 lambs that died were examined, it is not known whether all lamb deaths were attributable to septicemic pasteurellosis, but it seemed likely given that the 3 lambs that were examined had similar lesions and the same final diagnosis. One possible reason for the higher mortality rate in this flock may be that there were other environmental stressors (eg, sudden decrease in ambient temperature or inadequate nutrition) that made these sheep more susceptible to disease. Another possibility was that the sheep may have been infected with a comparatively more pathogenic strain of B trehalosi.

Clinical signs that have been observed in sheep with septicemic pasteurellosis include frothing at the mouth, dyspnea, signs of extreme depression, and recumbency.² With regard to the case described in the present report, some of the affected sheep in the flock were noted to have ptyalism and diarrhea. Ptyalism may have been a result of ulcerative lesions in the mouth, such as those observed grossly in the lamb of the present report. Diarrhea may have been a result of necrotizing colitis, as was observed in 1 of the 3 examined lambs. Alternatively, whipworm infection, which was present in 1 lamb, may have contributed to this clinical sign. Gross lesions typical of septicemic pasteurellosis include ulcerative lesions on the tongue, tonsillar crypts, and pharynx and hemorrhagic lesions in the neck, thorax, lungs, and epicardium^2,3; the lamb of the present report had ulcerative lesions on the tongue and widespread hemorrhages in the thorax, lungs, and epicardium. A key histologic feature of this disease is widespread bacterial emboli, especially in the lungs, liver, spleen, and adrenal glands^2,3; this lamb had bacterial emboli in the vasculature of the lungs and liver. In infected animals, the lack of a visible inflammatory response to the bacteria is likely due to the short course of disease or the effect of bacterial leukotoxin on leukocytes.³

Because of the rapid course of septicemic pasteurellosis, sheep in the early stages of disease are often not identified.² Therefore, treatment with antimicrobials is seldom possible, and control of infection and disease is best achieved by prevention.^2,5 Prophylactic antimicrobial treatment before periods of increased environmental stress has been proposed as a means of potentially decreasing the severity of disease in bighorn sheep, but the efficacy of such treatments has not been proven.⁹ Vaccination may be a possible means of decreasing the incidence and severity of disease caused by B trehalosi infection in ruminants.^2,5 In a study⁵ of the use of vaccines against B trehalosi in ruminants, variable results with regard to protective efficacy were reported, but some vaccines have been shown to reduce the severity of disease and mortality rate in sheep and cattle. Further development of these vaccines may eventually lead to stronger protection against diseases caused by B trehalosi in a variety of ruminant species.