Mannheimia haemolytica vegetative endocarditis in a Suffolk wether

Abstract

A 12-week-old Suffolk wether was diagnosed with bacterial endocarditis associated with Mannheimia haemolytica. The wether had shown signs of lethargy, inappetance, fever, and a grade 5 of 6 holosystolic murmur. Mannheimia haemolytica was cultured from blood premortem and the valvular lesion postmortem.

Case description

A 12-week-old, weaned, 37-kg Suffolk wether was presented for ill thrift of several weeks duration in late July 2013. The wether was one of a set of twins born to a multiparous ewe. The wether’s twin was considered normal. The lamb was 1 of 160 lambs born in the spring of 2013. The flock is maintained year round on pastures with protective sheds and pens available. Ewes are brought in to the pens at night for predator control. Ewes were fed a concentrate containing lasalocid for coccidia control shortly before lambing. They have continuous access to a mineral mix containing selenium and are fed alfalfa hay during the winter and early spring. A traditional claiming pen system is utilized during lambing. While in the pens the lambs are castrated using an elastrator band, tail docked with a hot iron, navels are dipped in 2% iodine. Lambs that are not nursing within several hours of birth are assisted to ensure passive transfer. Ewes are vaccinated approximately 3 wk before lambing for Clostridium perfringens Types C and D and tetanus. The lambs are vaccinated at 8 to 10 wk just prior to weaning, with a subsequent dose given at least 3 wk later.

At the time the wether was presented, the weaned lambs had been returned to pasture and a creep feed containing lasalocid was available. No other illnesses in weaned lambs or during the lambing season were reported by the herdsman. The wether was first brought to the herd manager’s attention for failure to stay with the flock. The animal had been treated with long-acting oxytetracycline once, at 20 mg/kg body weight (BW) subcutaneously, by the herdsman in the week before presentation, but continued to be lethargic with a poor appetite. On physical examination the wether was in poor body condition (BCS 1.5/5), was ambulatory but febrile at 39.6°C, and appeared weak. Bruxism interpreted as internal pain was apparent. Breath sounds were mildly reduced bilaterally and rumen motility was decreased with a poor fiber mat present. On cardiac auscultation, a grade 5/6 (1) holosystolic murmur was heard with the point of maximal intensity at the left heart base; the heart rate was 108 beats/min. No other significant clinical signs were noticed.

An echocardiogram revealed that the noncoronary and right coronary cusps of the aortic valve were severely thickened, irregular, and hyperechoic with incomplete coaptation. This is consistent with a vegetative lesion. There was moderate to severe aortic regurgitation and outflow turbulence. The vegetative lesions appeared to extend to the mitral valve, which was also thickened. Three blood cultures were all positive for Mannheimia haemolytica.

Due to a poor prognosis based on physical examination and echocardiographic findings, the animal was euthanized. Necropsy confirmed a 1-cm³ rough, red mass on the septal aortic leaflet that extended into the right leaflet. On histology there was an inflammatory infiltrate extending into the myocardium of the right ventricle. In addition there was mild diffuse fibrinous pericarditis and peritonitis, indicating bacterial embolization. Although breath sounds were reduced on physical examination, the lungs were within normal limits with no signs of pathology. Culture of the vegetative lesion grew M. haemolytica, confirming the earlier diagnosis. Histopathology confirmed chronic severe valvular endocarditis with valvular fibrosis and intralesional bacteria. Histopathology also revealed moderate myocarditis and choroiditis with fibrosis. The kidneys showed a moderate lymphocytic infiltrate and the liver was within normal limits. These findings suggest that the bacterial endocarditis was secondary to bacteremia associated with M. haemolytica.

Discussion

Bacterial endocarditis is a rare disease in sheep with few cases reported. Most published reports of bacterial endocarditis in ruminants involve dairy cattle, in which it is the most common valvular lesion seen (2). In cattle the most common bacterial isolate is Trueperellla pyogenes (3,4), with many other bacteria reported, especially Streptococcus spp. (2). Reported causes of bacterial endocarditis in adult sheep include Listeria, Erysipelothrix rhusiopathiae, and Streptococcus (5–7). One source reported incidental finding of vegetative endocarditis in 6/39 sheep euthanized for septic joints; however, the lesions were not cultured (8).

To our knowledge this is the first report of M. haemolytica associated bacterial endocarditis in sheep. In young lambs M. haemolytica is associated with septicemia and pneumonia (9,10). However it has been isolated from endocarditis in a tiger (11). Mannheimia haemolytica consists of 2 major biotypes, A and T (9). Biotype T has now been reclassified as Bibersteinia trehalosi, and often causes septicemia in young sheep without signs of pneumonia along with several biotype A serotypes of M. haemolytica (12,13). The isolate in this case was not typed but was differentiated from B. trehalosi.

Bacterial endocarditis may be initiated by the hematogenous spread of bacteria and endocardial involvement (14). Any bacterium associated with bacteremia has the potential to cause vegetative endocarditis given the appropriate circumstances. While pre-existing heart or valvular lesions will predispose the animal to bacterial colonization (15), bacterial endocarditis has been experimentally produced by injection of bacteria into the bloodstream in the absence of any valvular lesions (2). Endocarditis occurring without any predisposing lesions may be due to microscopic damage or highly pathogenic organisms (16,17). The most common clinical signs in cattle include fever, tachycardia, systolic murmur, tachypnea, and lameness with weight loss common in chronic cases (17). All these signs, with the exception of lameness, were seen in this lamb, although generalized weakness that could have been interpreted as lameness was noted.

The endocarditis in this lamb was most likely the result of an initial septic event with secondary aortic valve colonization. Endocarditis is considered to occur sporadically in ruminants and is difficult to prevent. Controlling risk factors by ensuring passive transfer, providing good hygiene and disease control, and limiting stress that can lead to immunocompromise is important to limit bacterial infections and the possibility of endocarditis.