Thirty-seven cases of blackleg were identified. The presence of Clostridium chauvoei in the lesions was confirmed by immunohistochemistry [1] in 25 cases (67.6%), by culture and immunofluorescence in 11 cases (29.7%), and by polymerase chain reaction in 1 case (2.7%). In 32 cases (86.5%), affected skeletal muscle (n=6), heart (n=6), or both (n=20) were examined microscopically, and, when present, the lesions were consistent with blackleg in all of these cases. The remaining 5 cases were diagnosed based on the gross findings in conjunction with culture and immunofluorescence.
The affected muscles were grouped as limbs (any), cervical, thoracic (shoulder, pectorals), lumbar (hypaxial and epaxials), pelvic (gluteals), head, tongue, and diaphragm. Of the 37 cases, 33 (89.2%) had skeletal muscle involvement. Most animals (43.2%) had only 1 skeletal muscle group affected, 12 (32.4%) had 2 groups affected, 3 (8.1%) had 3 groups affected, and 1 (2.7%) had 4 groups affected. The limbs were most commonly affected, with 19 affected animals (51.4%). The involvement of other skeletal muscle groups included thoracic (n=12, 32.4%), neck (n=10, 27.0%), pelvic (n=4, 10.8%), tongue (n=3, 8.1%), diaphragm (n=3, 8.1%), lumbar (n=2, 5.4%), head (n=1, 2.7%).
Cardiac lesions were found in 26 of the 37 animals (70.3%); no cardiac lesions were identified in the remaining 11 cases. Of the 26 cases with cardiac lesions, 24 cases had gross lesions, while 2 cases were identified only by microscopic examination. Four animals (10.8%) had cardiac lesions without skeletal muscle involvement. Of the 26 animals with cardiac lesions, 2 (7.7%) had only necrotizing myocarditis; 5 (19.2%) had only fibrinous or fibrinosuppurative pericarditis, epicarditis, or endocarditis; and 19 (73.0%) had a combination of myocarditis and pericarditis, epicarditis, or endocarditis (Table 1, Figures 1, 2).
Table 1. Prevalence of cardiac lesions and proportional morbidity of bovine clostridial myositis due to cardiac lesions
Cardiac lesions
|
Number
|
Prevalence (%)
|
Proportional morbidity due to a cardiac lesion (%)
|
Necrotizing myocarditis only
|
2
|
5.4
|
7.7
|
Fibrinous to fibrinosuppurative peri-, epi-, or endocarditis only
|
5
|
13.5
|
19.2
|
Both necrotizing myocarditis and fibrinous to fibrinosuppurative peri-, epi-, or endocarditis
|
19
|
51.4
|
73.1
|
No cardiac lesions found
|
11
|
29.7
|
N/A
|
Total
|
37
|
100
|
100
|
The anatomic localization of the cardiac lesions was not always specified in the necropsy reports or able to be determined microscopically. Based on the available data, cardiac myositis primarily involved the left ventricle, whereas pericarditis, epicarditis, or endocarditis more commonly affected the right side of the heart.
In 6 cases (23.1% of cardiac associated cases; 16.2% of all cases), valvular endocarditis was noted grossly or microscopically. In 4 cases, it affected the pulmonic valve; in 2 of those cases, the left atrioventricular valve was also affected.
The 70.3% prevalence of cardiac lesions in Tennessee cattle with blackleg is similar to the 69% that was recently reported in California.[1] Of the 37 animals with blackleg, only 11 (29.7%) did not have heart involvement; 4 (10.8%) had involvement of only the heart; and 22 (59.5%) cases had involvement of the heart in addition to the skeletal muscle. Of the 26 cases with heart involvement, 92.3% had a fibrinous to fibrinosuppurative pericarditis, epicarditis, or endocarditis, and 80.8% had a myocarditis. The pericarditis, epicarditis, or endocarditis tended to involve the right side of the heart, while myocarditis tended to involve the left ventricle. Less commonly, there was valvular endocarditis, which most commonly involved the pulmonic valve. The presence of pulmonic valvular endocarditis is in conflict to previous literature indicating that the right atrioventricular valve is most commonly affected;[3] given the rarity of valvular endocarditis in cases of blackleg, the significance of this discrepancy is unclear.
The pathogenesis of clostridial myositis has been recently and thoroughly reviewed.[1,2,7] Cardiac lesions may be the result of hematogenous dissemination of the bacteria and/or toxins to the heart. In rare cases where there is only myocarditis, determining the inciting cause for the decreased oxygen tension required to cause vegetation of the spores is challenging. Some have proposed that concurrent toxicities (ionophore, gossypol) or nutritional deficiencies (Vitamin A, selenium) may precipitate cardiac hypoxia, but to date, supporting evidence of these theories is lacking.[10] Stress of handling and increased cortisol may also allow for germination of latent spores.[10] Interestingly, outbreaks of clostridial myocarditis have been associated with high rainfall in cattle[9,10] and lambs.[7] The wet conditions may create an anaerobic environment in the soil, which favors the proliferation of C. chauvoei, or may facilitate the dissemination of spores.[2] It is unclear, however, how the wet conditions would favor activation of the spores in the heart. Given the nature of this study, it was not possible to make associations with weather conditions.
Involvement of the heart in cases of blackleg is common; however, clostridial myocarditis in the absence of skeletal muscle lesions is uncommon.