Post-Mortem of a Heifer on Feed: Answer

Fig. 1. The open left ventricle view shows acute myocardial necrosis of the papillary muscles. ( Feedlot Health Management Services )

Fig. 2. The open left ventricle view shows chronic sequestra in the papillary muscles.

Fig. 3. The open-chest view shows cardiogenic pulmonary edema.

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The team diagnosed this case as “Myocarditis” most likely caused by Histophilus somni, a component of the bovine respiratory disease (BRD) complex; however, the septicemic form can cause myocarditis, pericarditis, pleuritis, arthritis, and infectious thromboembolic meningoencephalitis. 

Pathogenesis

  • Generally considered a normal inhabitant of the nasopharynx that gains access to the bloodstream (potentially due to immune stressors and/or respiratory tract infection)
  • Predilection for vascular endothelium leads to adherence and thrombus formation followed by ischemia, necrosis, and eventually sequestration in specific organs.

Epidemiology

  • Most commonly observed in calves placed in late fall and early winter, but the seasonal effect is confounded by the high number of calf placements in these seasons  
  • Mortalities caused by Histophilus somni predominately occur between 30 and 60 DOF

Ante-Mortem Clinical Signs

  • Animals may exhibit non-specific clinical signs such as depressed mentation, fever, anorexia, and/or lethargy 
  • In the acute stage of infection, it is difficult to clinically differentiate between H. somni caused myocarditis and BRD; therefore, ante-mortem cases of both disease syndromes are most accurately categorized as “undifferentiated fever”
  • Compared to BRD, H. somni caused myocarditis is approximately 2 times more likely to be missed by stock attendants and animals are commonly found dead in the pen without prior treatment history

Management

  • In populations at high risk of BRD, metaphylaxis with tulathromycin has been shown to reduce H. somni specific mortality.
  • Vaccination with an H. somni bacterin has been evaluated with mixed results; however, studies have generally used commingled experimental designs with vaccinates and non-vaccinates in the same pens
  • In an outbreak scenario, H. somni is generally susceptible to tetracycline antimicrobials. Due to the difficult nature of identifying H. somni caused disease, mass medication may be warranted.

Post-Mortem Lesions

  • Upon opening the left ventricle of the heart and incising the papillary muscles, acute myocardial necrosis appears as a circumscribed area that is darker in color (purple) than the surrounding tissue located within the papillary muscles (see Fig. 1).
  • If the animal survives the acute phase of disease, a sequestra often forms in the papillary muscles of the left ventricle. Figure 2 shows the open left ventricle of a steer calf that died at 77 DOF and had previously been treated twice for undifferentiated fever.
  • Upon observation of the open chest, cardiogenic pulmonary edema may be apparent with all stages of disease; however, it is most commonly observed in later stages of the disease (Fig. 3).

Post-Mortem Series presented in partnership with Feedlot Health management Services, Okotoks, Alberta. For more information, visit their website at www.feedlothealth.com

Working with crews at client operations, Feedlot Health conducts post-mortem exams on all feedlot and calf-grower mortalities, using a standard protocol for recording the animal’s history, digital images, and post-mortem findings. The group compiles images and post-mortem findings in a central database, for review by the professional team, as an educational tool and to track disease trends within an operation or across their client base.

More from the Post-Mortem series: 

Post-Mortem in a Feedlot Heifer

Post-Mortem of a Steer Calf on Feed

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