Epidemiological and clinicopathological findings in 15 fatal outbreaks of salmonellosis in dairy calves and virulence genes in the causative Salmonella enterica Typhimurium and Dublin strains

Abstract

Salmonella enterica is a major food-borne pathogen that affects cattle-rearing systems worldwide. Little information is available on the epidemiology and pathology of salmonellosis and the virulence genes (VGs) carried by Salmonella in spontaneous outbreaks in cattle. We describe epidemiological findings in 15 fatal outbreaks of salmonellosis in Uruguayan dairy farms and the age, clinical signs, and pathology in 20 affected calves. We also describe the serotypes and frequencies of 17 VGs in the causative Salmonella strains and explore their associations with epidemiological, clinical, and pathological findings. Salmonella Typhimurium and Dublin were identified in 11/15 and 4/15 outbreaks, respectively. The most frequent reason for consultation was digestive disease (8 outbreaks caused by S. Typhimurium), followed by sudden death (4 outbreaks, 3 caused by S. Dublin). Morbidity, mortality, and lethality ranged 4.8-100%, 3.8-78.9%, and 10-100%, without significant differences between serotypes. Diarrhea, the most common clinical sign (14 cases), was associated with the Typhimurium serotype (OR = 26.95), especially in ≤ 30-day-old calves with fibrinous enteritis as the main autopsy finding. The Dublin serotype affected ≥ 50-day-old calves and was associated with fibrinosuppurative splenitis (p = 0.01) and tubulointerstitial nephritis (OR = 48.95). The chances of the Dublin serotype increased significantly with age. There was low variability of VG across serotypes. The pefA gene was associated with the Typhimurium serotype (OR = 21.95), macroscopic enteritis (p = 0.03), and microscopic fibrinosuppurative splenitis (p = 0.04). Understanding the epidemiology, pathology, and virulence of S. enterica at the farm level is key to delineating prevention and control strategies to mitigate its impact on animal and human health.

Keywords: Calves; Epidemiology; Pathology; Salmonella enterica; Uruguay; Virulence genes.

Fig. 1

Boxplot distribution of the age of the calves with salmonellosis caused by S. Dublin (average = 55 days, n = 5 calves) and S. Typhimurium (average = 23.9 days, n = 15 calves)

Fig. 2

Boxplot distribution of the age of the calves with salmonellosis without (average = 53.3 days, n = 6, left plot) or with (average = 22.4, n = 14 calves, right plot) manifestation of diarrhea as a clinical sign before death

Fig. 3

Gross lesions in dairy calves with salmonellosis. (a) Calf 1, outbreak XIII. Segment of jejunum with abundant yellowish fibrillar material (fibrin) adhered to the mucosa (severe fibrinous enteritis); the mesenteric lymph nodes are enlarged and swollen (mesenteric lymphadenitis). (b) Calf 2, outbreak XI. The gallbladder is distended and contains bile admixed with abundant fibrin (fibrinous cholecystitis). (c) Calf 2, outbreak XI. Left side view of abdominal and thoracic organs in situ. The spleen is enlarged (splenomegaly), and there is yellow discoloration of the omentum, serosal surfaces of the forestomachs and adipose tissue (jaundice). The lung failed to collapse and showed rib imprints on the pleural surface and dark red discoloration of multiple lobules (interstitial pneumonia). (d) Calf 1, outbreak IV. The urinary bladder contains abundant urine with granular to fibrillar yellowish clots (fibrinous urocystitis)

Fig. 4

Microscopic lesions in dairy calves with salmonellosis. (a) Calf 1, outbreak IV. The small intestinal histoarchitecture is effaced, and the lamina propria is expanded by degenerate neutrophils, proteinaceous material, and necrotic debris (inset). (b) Calf 1, outbreak IV. Multifocal random necrotizing hepatitis, characterized by infiltration of macrophages and neutrophils admixed with necrotic hepatocytes (inset). (c) Calf 1, outbreak XII. There is multifocal extravasation of fibrin entrapping macrophages and neutrophils (inset) in the red pulp of the spleen (splenitis). Hematoxylin and eosin stain. Bars = 100 μm