Diagnostic Exercise: Myocarditis Due to Histophilus somni in Feedlot and Backgrounded Cattle

Abstract

A backgrounding operation for calves in Wyoming identified a disease syndrome presenting as lethargy, fever, and death between November and January each year. An unfixed heart was submitted for examination, along with samples of lung. There was focal red discoloration in papillary muscle of the left ventricular myocardium. Histologically, the lesion corresponded to acute necrotizing myocarditis with myriad intravascular and intralesional Gram-negative coccobacilli. Histophilus somni was detected by bacterial culture and immunohistochemistry. Focal myocarditis due to H. somni occurs in fall-placed cattle in western provinces and states of North America, and it can be an appreciable source of death loss. Gross lesions are readily detected in affected hearts. The presence of such changes in papillary muscles of left ventricular myocardium in feedlot or backgrounded cattle should prompt a differential diagnosis of H. somni myocarditis.

This exercise describes a disease syndrome that is common in feedlots and backgrounding operations in colder climates of North America. It is well documented in Canada's western provinces. ^5,6,8,9

History, Clinical Findings, and Gross Lesions

Myocardial disease was recognized as a recurring annual problem in 7- to 9-month-old beef cattle in a backgrounding operation in Wyoming. Total annual death loss of all causes was low (<1%). Affected cattle were depressed, febrile, and lethargic. Some responded to antibiotic treatment. Losses peaked between late October and the end of January. Treated and untreated cattle with more chronic clinical signs died in small groups after harsh winter weather. Myocardial disease accounted for 42% of deaths in 2007, based on clinical evaluation and postmortem assessment of incised hearts. Gross lesions consisted of 1- to 3-cm foci, typically in papillary muscles of left ventricular myocardium. A series of hearts was submitted for evaluation to corroborate the gross diagnosis. A heart from animal #13317 with a representative lesion was submitted in November (Fig. 1). There was a discreet 1.8 × 1.8 cm area of red-purple discoloration in papillary muscle of the left ventricular myocardium (Fig. 1).

Fig. 1.

Heart, steer. Discreet 18 × 18 mm red-purple discolored area (between arrowheads) in 1 of 2 papillary muscles of the left ventricular myocardium. Bar  =  1.5 cm.

Differential Diagnoses

Diagnostic considerations for this sort of discreet cardiac lesion in feedlot cattle with a clinical history of lethargy, fever, and death are Histophilus somni, embolic pyogenic bacterial infections, and clostridial myocarditis. The clinical signs are also consistent with bovine respiratory disease complex.

Microscopic Findings

Cardiac lesions comprised acute necrotizing myocarditis (Fig. 2). Numerous smaller lesions were present throughout the heart. Many capillaries, venules, and small veins were plugged with Gram-negative bacteria that were identified as H. somni by immunohistochemistry. H. somni formed biofilm-like aggregates adherent to venous endothelium (Figs. 3,4). It was difficult to determine whether bacteria were intracellular in endothelium, but it was clear that endothelial cells of many capillaries and small vessels were full of H. somni antigen (Fig. 3). There was mild disseminated congestion and intra-alveolar exudation of fibrin into pulmonary alveoli, associated with pneumocyte hypertrophy and intra-alveolar neutrophils (Fig. 5).

Fig. 2.

Heart, steer. Focal acute necrotizing myocarditis in left ventricular myocardium. HE.

Fig. 3.

Heart, steer. Disseminated intravascular bacterial colonies (arrow) in cardiac capillaries. Immunohistochemistry with rabbit antibody to formalin-killed Histophilus somni. ⁴

Fig. 4.

Heart, steer. Intraluminal colonies of Gram-negative coccobacilli. There is acute phlebitis. The mound-shaped aggregates have features of a biofilm. Gram's stain.

Fig. 5.

Lung, steer. Exudation of fibrin and neutrophils into pulmonary alveoli in steer with bacterial myocarditis. HE.

Laboratory Findings

H. somni was isolated in pure culture from the cardiac lesion, left atrioventricular valves, and lung. Immunohistochemical staining for H. somni ⁴ was positive (Fig. 3).

Diagnosis

H. somni septicemia with acute necrotizing myocarditis was the diagnosis.

Discussion

Bacterial myocarditis is one of several fatal syndromes associated with H. somni infection in cattle. ^2,7 Histophilosis occurs as a component of bovine respiratory disease complex and as septicemia, thrombotic meningoencephalitis, otitis, arthritis, and abortion. The basis for cardiac localization in outbreaks of histophilosis is unclear. The involvement of papillary muscles of left ventricular myocardium is a distinctive but not invariable feature of the syndrome, often signaled by purple discoloration of overlying endocardium.

There is a disparity between Canada and the USA in the importance of cardiac histophilosis in feedlots. It has been repeatedly demonstrated to be important as a cause of illness and death in fall-placed calves in western Canadian provinces, ^5,6,9 with an increased incidence since the late 1980s. ⁸ This coincided with a decline in the number of cases of thrombotic meningoencephalitis, possibly as a result of changes in vaccination or mass treatment practices or the emergence of strains with different virulence factors. ⁹ Cardiac disease due to H. somni is regarded as rare and sporadic in feedlots in the USA. Yet, as shown here, it can be an important source of mortality on individual properties.

The disease can be overlooked. Acute, subacute, and chronic fatal H. somni myocarditis tends to gives rise to singleton losses over time, rather than heavy death loss in a short period. Sporadic losses are less likely to be investigated in feedlots. Pulmonary changes of H. somni septicemia, resulting in pulmonary congestion, can be mistaken for pneumonia by lay personnel who do a high proportion of necropsies in feedlots. Cardiac lesions will be missed when hearts are not opened, particularly when cranial and caudal papillary muscles of left ventricular myocardium are not incised. Some feedlot veterinarians are adept at recognizing these lesions by palpating intact affected hearts. H. somni is a relatively slow-growing, fastidious organism. Isolation can be hampered due to overgrowth by faster growing bacteria. Failure to isolate H. somni occurs in approximately 50% of immunohistochemically confirmed cases at the Wyoming authors' laboratory, particularly those that are of longer duration and/or after antibiotic treatment. Immunohistochemistry is useful for confirmatory purposes when bacteria are not isolated yet intralesional bacteria are seen.

Samples of lung provide a useful nonspecific clue to cardiac histophilosis, even when the heart is not available for examination. Grossly, lungs are congested and described as “wet” by feedlot personnel but are not consolidated. There is histologic evidence of intra-alveolar fibrin with or without neutrophilic alveolitis. Lesions typical of fulminant acute or chronic H. somni pneumonia ¹ are often absent in acute cardiac histophilosis, even when the organism is cultured from lung. In the event of a diagnostic pathologist's being confronted with a disease episode in which calves present as febrile and depressed and with gross lesions in lung that suggest endotoxemia, ³ feedlot staff should be asked to submit intact fresh hearts to the laboratory for examination. The disease typically occurs in late fall and early winter in North America. It may recur on properties over successive years.