Eight-month-old, Dorset ewe, (Ovis aries).This ewe was
enrolled in a research study in which it underwent surgery and MRI the
following day, both under general anesthesia. The ewe initially recovered
normally, displaying brief weakness in the hindlimbs immediately after
surgery. However, over the next two weeks, it gradually became dull and
lethargic, dehydrated, inappetent, and weakened despite supportive care. The
animal was found recumbent, hypothermic, tachycardic, and dyspneic 13 days
after surgery and died before it was able to be euthanized.
major gross lesion was in the lungs. The right cranial lung lobe was half
normal size, mottled dark red to black, and firm on palpation. The cranial
aspect of the right middle lung lobe appeared similarly. Dissection of the
bronchial tree did not reveal the presence of foreign material. Additionally,
several ulcers, up to 1 x 2 cm, were present in the trachea.
lung is markedly hypercellular, owing to filling of alveoli by degenerate
inflammatory cells, amorphous and fibrillar eosinophilic material (edema and
fibrin, respectively), basophilic streaming nuclear and eosinophilic
cytoplasmic debris, and myriad gram-positive bacterial rods. In some regions,
well-defined areas with architectural preservation without cellular detail
(coagulative necrosis) are present. Bronchi and bronchiole lumina frequently
contain abundant bacteria and sloughed bronchial epithelium. Interlobular septa
are thickened up to 2mm wide by fibrin and edema, and blood vessels were
congested and contained bacteria. The pleura is expanded up to 3x normal with
Broncho-pneumonia, multifocal, subacute, severe,
culture of the lung yielded a pure culture of Trueperella pyogenes.
All findings in
this case were consistent with bronchopneumonia caused by Trueperella
pyogenes (formerly Arcanobacterium pyogenes), a gram-positive
non-motile, non-sporeforming, short, rod-shaped bacterium.6 Trueperella
pyogenes is one of the most common opportunistic pathogens in domestic
livestock and is often commensal flora in the mammary gland, upper respiratory,
urogenital, and gastrointestinal tracts.6,10 Although T. pyogenes
can act as a primary pulmonary pathogen, infection usually follows physical or
microbial trauma which overcomes the pulmonary defense mechanisms, allowing for
colonization of the lungs.1,8 The pneumonia was initially suspected
to be due to aspiration shortly after recovery from anesthesia, but foodstuff
was not present in the respiratory tract. The reason for the infection was
likely due to a combination of stressors, including what was interpreted as
intubation-associated ulceration of the trachea. Trueperella
and utilizes a variety of virulence factors to colonize, damage, and persist
within a variety of tissues in the host. The most important factor is pyolysin
(PLO), a cytolysin, which is able to bind to and create pores in the cell
membranes of erythrocytes, polymorphonuclear cells, and macrophages resulting
in cell lysis.6 Trueperella pyogenes mutants lacking the PLO
gene were unable to cause an intraperitoneal infection in mice injected with 108
bacteria, whereas replacement of the PLO gene to the T. pyogenes
mutants restored full virulence.5 Additional virulence factors used
by T. pyogenes include fimbriae13, extracellular matrix
binding proteins specific for collagen4, fibrinogen, fibronectin5,
and exoenzymes including DNases11, proteases12, and
neuraminidases7 which degrade host nucleic acids, proteins, and acid residues, respectively.
Note: Multiple different tissue sections of lung were used for the slides submission; therefore, not all the participants will receive similar serial microslide sections.
Lung: Bronchopneumonia, necrotizing and fibrinosuppurative,
multifocal to coalescing, severe, with marked alveolar and septal edema and
numerous large colonies of bacilli, Dorset sheep, Ovis aries.
This case demonstrates the
characteristic gross and histologic lesions associated with bacterial
bronchopneumonia. Conference participants described the suppurative
inflammation filling bronchi and bronchioles surrounded by multifocal to
coalescing areas of necrosis, and readily identified numerous large colonies of
bacilli within the areas of inflammation. Participants discussed the
differential diagnosis for large colony forming bacteria in tissue section to
include: Yersinia sp, Actinomyces sp., Actinobacillus sp.,
Corynebacterium sp, Staphylococcus sp., Streptococcus sp.,
and T. pyogenes. Trueperella pyogenes is one of the most common
opportunistic pathogens present on the mucosal surfaces of domestic animals.1,2,5,8
The bacterium induces suppurative inflammation within a wide variety of organs
and is an important cause of abortion, arthritis, endocarditis, mastitis,
osteomyelitis, and pneumonia resulting in significant losses in production
animals.2,4 Additionally, T. pyogenes is widespread in the
wild animal population and has been reported as an important cause of cerebral
abscesses in young male white-tailed deer as a consequence of antler
development and conspecific aggression between bucks. Mortality can reach up to
35% in affected free-ranging adult male deer.3
As mentioned by the contributor, while T. pyogenes can be a primary pathogen, it is usually associated with physiologic trauma to a mucosal membrane, concurrent primary infection, or immune suppression.1,2,3,8 As postulated by the contributor in this case, there well may be an association with placement of the endotracheal tube. Other common commensal organisms of the ruminant upper respiratory tract that can cause opportunistic bronchopneumonia include Mannheimia haemolyica, Pasteurella multocida, and Bibersteinia trehalosi. Causes of primary infectious pneumonia in sheep include parainfluenza virus 3, respiratory syncytial virus, and Bordetella parapertussis, which can predispose sheep to secondary infection by the commensal bacteria mentioned above.1,3,8 Mycoplasma ovipneumoniae is another important primary etiologic agent involved in chronic enzootic pneumonia; also known as chronic non-progressive pneumonia, it is a multi-factorial disease complex affecting lambs less than one-year-old. Mycoplasma ovipneumoniae usually causes a mild, subclinical infection that results in poor growth unless complicated by stressors such as over-crowding, inclement weather, or poor air quality.1,2,3,8
Another primary cause of pneumonia in sheep includes the maedi-visna virus, which results in lifelong persistent viral infection and leads to ovine progressive pneumonia (OPP), encephalitis, arthritis, and mastitis in sheep. This lentivirus is a member of the family Retroviridae, and is closely related to caprine arthritis-encephalitis virus. First reported in Iceland, the maedi (respiratory form) occurs in sheep older than three years, while the visna (neurologic form) occurs in younger sheep.1,9 The respiratory form is characterized by interstitial pneumonia with prominent perivascular and peribronchial lymphoid nodules. This slowly progressive pneumonia is often complicated by secondary bacterial infection, especially T. pyogenes.1 Peste des petits ruminants (PPRV), a morbillivirus, has also been reported to cause primary respiratory disease in small ruminants in Africa and parts of Asia. This virus primarily affects the cranioventral lung lobes and causes a bronchointerstitial pneumonia.1,8,9
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