Mannheimia haemolytica may be the most significant pathogen of cattle and sheep worldwide. It is the most important cause of mortality from bronchopneumonia in cattle but also causes mastitis in cattle and pneumonia and mastitis in sheep.
The owner of a mixed farm in central west NSW reported five dead and one sick ewe from two mobs of 200 and 170 Merino ewes with lambs at foot. Two ewes were initially found dead in the paddock but the owner also saw two ewes that were dull and separate from the mob. When approached these ewes bolted towards the mob but then collapsed and died. The owner also noticed two more dull ewes.
The ewes joined to Merinos (mob 1) and Poll Dorsets (mob 2) commenced lambing on the 10th September 2007 and the lambs were marked towards the end of October. The owner reported that the ewes were generally in fat condition as attested by marking 143% of lambs from mob 1 and 125% n mob 2. Both mobs of ewes were running on mixed improved pastures that included summer active perennial grasses.
The owner presented two ewes for examination on 29th December 2007. The first was dull but alive and the second was one of the ewes that died after bolting.
The first ewe was in lateral recumbency and was alert and responsive but trembling and weak. This ewe was in poor condition (fat score 1.5), febrile (T 40.8) with a rapid pounding heart beat (120 per minute) and rapid respiration (72 per minute) however lung sounds were relatively normal. Mucous membranes were normal. The left udder of this ewe was flaccid and empty while the right udder was firm, meaty, swollen and painful and yielded a thick pink discharge. The skin over the udder was normal.
The dead ewe was fat. The most remarkable post-mortem finding was a marked fibrinous pneumonia. The body fat was also slightly yellow. Both halves of the udder were swollen but were also filled with thick yellow milk and were without the meaty consistency of the first case. The teat of the right udder was circled by a slight purple discoloration.
Cultures of milk samples from both ewes and from the lung of the dead ewe all grew a profuse pure growth of Mannheimia haemolytica sensitive to ampicillin and tetracycline. On histopathology the lung showed severe diffuse fibrinosuppurative pneumonia with bacterial colonies evident. The mammary gland also showed severe diffuse fibrinonecrotising mastitis with bacterial colonies.
Mannheimia haemolytica is a common cause of mastitis in Australian sheep especially in British breeds such as Dorsets (Barber et al, 2006). I have seen it both in lactating Dorsets and also Merinos. Mannheimia haemolytica has been isolated from the mouths of ewes and lambs and also from the skin of the udders of ewes with lambs at foot. It has therefore been theorized that suckling may allow Mannheimia bacteria to enter the teat canal and cause mastitis (Scott and Jones, 1998). In my experience concurrent mastitis and pneumonia is unusual.