History
An approximately 1-year-old 70-kg (154-lb) Lleyn cross ewe was found dead in a lambing pen. Thirteen days earlier, the ewe had given birth to twin lambs unassisted. No other clinical history was available.
The ewe underwent necropsy examination. For histologic analysis, tissue samples from all major organs were fixed in neutral-buffered 10% formalin solution.
Clinical and Gross Findings
Gross examination of the udder revealed that both mammae were diffusely and markedly enlarged, the skin overlying the glands was multifocally dark red to purple with black around the teats, and serosanguineous fluid exuded from the skin and teats. On transverse section, the mammae were multifocally pink to red, with scarce small cream-colored foci of normal parenchyma remaining. Multifocal, poorly defined, red, soft, cavitated foci were present in affected tissue. In a locally extensive area, the subcutis of the mammae and the abdomen cranial to the udder was markedly expanded by accumulations of translucent red gelatinous material (hemorrhage and edema) admixed with cream-colored flecks (fibrinopurulent cellulitis). Multiple subcutaneous blood vessels, including the left subcutaneous abdominal vein (mammary vein or so-called milk vein1), were segmentally occluded by dark red, firm, slightly granular material that was adherent to the tunica intima (venous thrombosis; Figure 1). The mammary lymph nodes were 3 times the size expected in a healthy sheep, and each had a diffusely cream-colored, soft, homogeneous cortex and medulla. No other notable macroscopic abnormalities were detected.
Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page→
Histopathologic and Microbiological Findings
Standard protocols were followed in the preparation of histologic sections. Examination of tissue sections from the mammary glands revealed diffuse necrosis of mammary alveolar epithelial cells. The stroma and interlobular connective tissue were expanded by hemorrhage, edema, and fibrin (Figure 2), and there were multifocal groupings of large numbers of degenerated neutrophils admixed with prominent gram-positive cocci that formed large aggregates or microcolonies (Figure 3). Microbiological culture of mammary gland specimens yielded heavy growth of Staphylococcus aureus. Histologic examination confirmed thrombosis of subcutaneous veins, including the left subcutaneous abdominal vein, in a locally extensive area cranial to the left mamma. The mammary lymph nodes had evidence of sinus histiocytosis.
Morphologic Diagnosis and Case Summary
Morphologic diagnosis: acute necrotizing (gangrenous) mastitis with numerous intralesional gram-positive bacterial cocci, multifocal venous thrombosis, and locally extensive fibrinosuppurative cellulitis.
Case summary: necrotizing (gangrenous) mastitis in a Lleyn cross ewe caused by infection with S aureus.
Comments
For the ewe of the present report, sudden death in early lactation and the macroscopic appearance of the mammary glands were both highly suggestive of necrotizing (gangrenous) mastitis. In particular, the skin discoloration, subcutaneous edema, and serosanguineous exudate were characteristic of necrotizing mastitis; historically, such features gave rise to the colloquial name of this condition, black bag. Thrombosis of the subcutaneous abdominal veins has also been previously documented as a sequela of this condition.2 Necrotizing mastitis frequently affects ewes approximately 2 to 4 weeks after lambing,3 as illustrated by the case described in the present report, which may in part reflect a degree of immunologic stress in ewes at this stage of the production cycle.4
Ovine necrotizing mastitis is typically caused by infection with S aureus or Mannheimia haemolytica.5 Less commonly implicated pathogens include Escherichia coli, Clostridium perfringens, and Pseudomonas aeruginosa.3,6 Results of a large study6 suggest that S aureus infection is the most common cause of gangrenous mastitis in Norwegian sheep, but the proportions of cases caused by infections with S aureus or M haemolytica may vary in different countries. Ovine mastitis as a result of S aureus infection may be acute and necrotizing, as it was for the ewe of the present report, but may also be more chronic with fibrosis and abscess formation.7 Staphylococcus aureus infection may also cause acute and chronic mastitis in cows and rabbits, and in these species, the peracute and acute forms may be similarly necrotizing.8
For the ewe of the present report, histologic examination of mammary gland tissue revealed changes consistent with the gross pathological findings, including diffuse necrosis of the mammary alveolar epithelial cells; expansion of the stroma with large amounts of hemorrhage, edema, and fibrin; and a multifocal neutrophilic infiltrate.9 Prominent colonies of gram-positive bacterial cocci arranged in clusters and multifocally in larger microcolonies were consistent with the microbiological diagnosis of S aureus infection. Staphylococcus aureus is one of several bacterial species that may form histologically detectable microcolonies in various species.10 Other bacteria with the potential to form large colonies include Yersinia spp,11 Corynebacterium spp, Actinomyces spp,12 Actinobacillus spp, and Streptococcus spp.10 Staphylococcus aureus has properties and virulence factors that are considered to aid tissue colonization and augment pathogenicity, including the ability to form biofilms and to produce leukotoxins that destroy neutrophils.4
There are multiple predisposing factors for necrotizing mastitis in sheep, including teat lesions13 resulting from direct trauma, disease processes such as parapoxvirus infection (contagious ecthyma [orf]), or lamb-induced damage.5 In the ewe of the present report, there was no evidence of parapoxvirus infection in the flock in general or in this animal or its lambs in particular; thus, lesions related to orf were unlikely to be a factor in this case. Supernumerary teats may also contribute to milk accumulation and thus predispose to mastitis,14 but supernumerary teats were not present in this ewe. In sheep, recrudescence of infection acquired during the preceding lactation period or nonlactation period may also be a predisposing factor for ovine gangrenous mastitis.15 Genetic influences may be important in some situations,5 although perhaps less likely in the crossbred ewe of the present report. Poor hygiene, particularly in lambing pens, has been implicated in disease pathogenesis in some instances,2,4 and this can be exacerbated by lambs feeding from multiple ewes, thereby facilitating horizontal transfer of microorganisms. Milk stasis from any cause may also result in an accumulation of milk, which potentially provides a medium for bacterial proliferation; such proliferation is favored if coupled with a ewe's compromised innate immune defenses, such as those associated with mechanical injury to the teat canal inflicted by lambs.8
In ewes that survive acute necrotizing mastitis, the necrotic mammary parenchyma and overlying skin will eventually slough and healing may occur by second intention, resulting in fibrosis and potentially a markedly misshapen udder.3 Second-intention healing may involve extensive granulation tissue formation; as this process progresses coincidently with the spring and summer months, the locally extensive granulation tissue may be subject to myiasis, adding a further clinical challenge to the management of sheep that survive beyond the acute phase of necrotizing mastitis.2 In the case described in the present report, the ewe died in the early period of infection, and death was attributed to the systemic effects of toxemia. Death secondary to toxemia is not uncommon in cases of necrotizing mastitis; acute bacterial mastitis is well recognized clinically as a cause of sudden death in sheep,16 with an estimated 30% to 40% fatality rate among untreated sheep.3
Acknowledgments
Dr. Hughes’ research on the ruminant mammary gland and mastitis is funded by the British Veterinary Association Animal Welfare Foundation Norman Hayward Fund (NHF_2016_03_KH).
The authors thank Venetia Owenson for technical assistance.
References
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