4-month-old male Holstein calf, Bos taurusThis was an experimental calf. Soon after birth it was placed in an arthropod-free containment facility. The calf was inoculated with Anaplasma marginal when it was a two-month-old. At the end of the experimental protocol, prior to release into the field, the calf was subjected to premunition with Babesia bigemina and B. bovis, after which it developed apathy and quickly progressed to death.
The calf was emaciated with moderate anemia and mild icterus. There was mild hydrothorax and hydroperitoneum. In the lung, there were multifocal mildly consolidated areas ranging from 0.3 to 0.5 cm in diameter distributed in all pulmonary lobes. The liver was markedly enlarged yellowish with focally extensive dark-red areas. There were multiple miliary white nodules in the parenchyma. There was also small amount of purulent exudate draining on the cut surface, and small thrombi were grossly observed. The spleen was moderately enlarged. Gross findings did not support babesiosis as a cause of death.
In the liver there was multifocal random degeneration and necrosis associated with an intense neutrophilic infiltrate with some lymphocytes and macrophages. There were several intralesional bacterial colonies. A lymphoplasmacytic and histiocytic infiltrate predominates in portal areas. Several blood vessels had fibrinoid necrosis in the vascular wall associated with thrombosis, particularly in centrilobular veins. Some veins are completely obliterated by thrombosis.
In the spleen (section not submitted) there were numerous hemosiderin-laden macrophages and increased erythrophagocytosis, associated with moderate lymphoid depletion.
In the lungs (sections not submitted), there was an interstitial pneumonia characterized by marked thickening of the alveolar wall with interstitial infiltration of macrophages, lymphocytes and a few neutrophils. There was multifocal intra-alveolar accumulation of fibrin and a few neutrophils.
Liver: Hepatitis, necrotizing, histiocytic and neutrophilic, multifocal, random, acute, associated with thrombosis and intralesional bacterial colonies.
Microbiology: Salmonella sp. was isolated from the liver.
Salmonella enteric comprises more than 2,000 serotypes, including serotypes Dublin and Typhimurium that are often isolated from cattle. The most common clinical manifestation of Salmonella infection in cattle is an enteric disease characterized by diarrhea that is usually associated with acute fibrino-necrotizing enteritis.(9-11) However, systemic salmonellosis in cattle may occur in the absence of enteric disease,(5) as observed in the present case.
Salmonella is highly invasive, crossing the intestinal epithelial barrier inducing ruffling of the apical surface of the epithelial layer, which results in internalization of the organism within a cytosolic membrane bound vacuole. This process is dependent on the expression of several effector bacterial proteins that are translocated into the host cells through a type III secretion system encoded by the Salmonella pathogenicity island 1 (SPI-1).(9-12) Once the organism crosses the epithelial layer, it quickly localizes in the lamina propria, where it is found mostly within macrophages. Salmonella processes another type III secretion system encoded by SPI-2 that is activated as soon as the bacteria localizes in the phagosome. This SPI-2-encoded type III secretion system is essential for intracellular survival of the organism in macrophages and, therefore, systemic dissemination of the infection.(8)
In our empirical experience, Salmonella is usually not observed in HE-stained section, both in enteric and systemic sites of infection. Interestingly, in the present case there were abundant intralesional bacterial colonies, which likely represent postmortem overgrowth of the organism.
Liver: Hepatitis, necrotizing, multifocal, random, with vasculitis, thrombosis, and rare paratyphoid nodules.
The accumulations of mixed mononuclear inflammatory cells multifocally scattered throughout the liver in this case are called paratyphoid nodules and are characteristic for the typhoidal form of salmonellosis.(3) Paratyphoid nodules or granulomas may also be found in the kidney, spleen, lymph nodes and bone marrow in cases of septicemic salmonellosis.(1) These contribute to the gross miliary pattern in affected organs, most often in the liver and spleen.(1) It is unclear whether these nodules are the result of a cell-mediated immune response or accumulate due to bacterial replication within macrophages.(1,7)
Typhoidal serotypes of Salmonella spp. are less common than the nontyphoidal forms in most species, including humans with the exception being in mice. Mice regularly develop septicemia with oral inoculation of Salmonella isolates, often without exhibiting gross and microscopic changes in the alimentary tract.(2,7)
The nontyphoidal or enteric form of salmonellosis, characterized by self-limiting enterocolitis or diarrhea in the absence of systemic disease, is a major cause of morbidity and mortality in calves and often over one billion human cases per year. While both are incriminated in cattle, Salmonella enterica serovar Typhimurium infects all species and is the most common isolate in people, while S. enterica serovar Dublin is more specific to cattle.(1,6) The similarity in enteric disease manifestation between cattle and humans and has led to their use as an experimental model for human infections.(6,11) Characteristic histopathology of nontyphoidal salmonellosis is the massive influx of neutrophils, which is also suggested as the major triggering event of gastrointestinal necrosis and diarrhea.(6) Vasculitis and thrombosis is also prominent, and in severe cases can lead to rectal strictures in pigs who have poor collateral circulation.(4) Fibrinous cholecystitis is pathognomonic for acute enteric salmonellosis in calves.(4) Chronic enteric salmonellosis is characterized by discrete foci of necrosis and ulceration called button ulcers and seen most often in pigs but also cattle and horses.(4)
The colonies of coccobacilli and large numbers of rods found in some areas of the slide are both gram-positive, leading conference participants to conclude these are postmortem bacterial overgrowth not related to the Salmonella infection in this case.
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