Feeding Saccharomyces cerevisiae fermentation products lessens the severity of a viral-bacterial coinfection in preweaned calves

Abstract

We have previously reported that supplementation with Saccharomyces cerevisiae fermentation products (SCFP) ameliorates clinical signs and lung pathology following experimental bovine respiratory syncytial virus (BRSV) infection in preweaned dairy calves. The objectives of this study were to determine the effect of SCFP supplementation on the metabolic and endocrine responses, and disease outcome of a viral-bacterial coinfection in preweaned calves. Twenty-seven, 1- to 2-d-old Holstein-Angus cross calves were enrolled in the study; one SCFP calf was removed from the trial during the pre-challenge phase due to complications from nephritis. Calves were assigned to two treatment groups: control or SCFP-treated, base milk replacer with 1 g/d SCFP (Smartcare, soluble formula) and calf starter top dressed with 5 g/d SCFP (NutriTek, insoluble formula). Calves were infected with BRSV on day 21, followed 6 d later by intratracheal inoculation with Pasteurella multocida (PM). Calves were euthanized on day 10 post-viral infection. Calves receiving SCFP had reduced thoracic ultrasonography scores on day 7 post-viral infection (P = 0.03) and a tendency toward reduced scores on day 10 post-viral infection (P = 0.09). Calves receiving SCFP also had less severe lung pathology scores at necropsy (P = 0.06). No differences between treatments were observed in lung viral loads (P = 0.48) or bacterial lung recovery (P = 0.34); however, there was a distinction in the lung location for PM recovery, with PM isolated more frequently from the cranial lobes in SCFP-treated calves, but more frequently from the caudal lobes of control calves. Calves treated with SCFP tended (P = 0.07) to have higher serum IL-6 concentrations following the coinfection. Calves treated with SCFP had lower concentrations of serum nonesterified fatty acids and beta-hydroxybutyric acid compared with controls following experimental challenge (P = 0.03 and P = 0.08, respectively), suggesting metabolic changes favoring growth and development. There were no differences between groups in gene expression of insulin receptor, insulin-like growth factor 1 (IGF-1), IGF-1 receptor (IGF-1R), growth hormone receptor, or haptoglobin in the liver. Results from this study suggest that supplementing with SCFP may moderate the impact of a respiratory viral-bacterial coinfection on preweaned calves through metabolic and immune modifications.

Keywords: Saccharomyces cerevisiae fermentation products; bovine respiratory disease; infection; innate immunity; metabolism; preweaned calves.

Figure 1.

Effects of SCFP supplementation and viral–bacterial coinfection on calf starter intake on days 1 to 30. Calf starter intake was monitored daily in control (n = 13) and SCFP-treated calves (1 g/d SmartCare, 5 g/d NutriTek, n = 14). One SCFP calf was removed from the study on day 19. The data for this calf are included until the day that the animal was removed. All remaining calves (n = 13 controls, n = 13 SCFP-treated) were challenged via aerosol inoculation with bovine respiratory syncytial viral strain 375 on days 20 to 22. Calves were then challenged via intratracheal inoculation with PM on days 26 to 28. One control calf was euthanized after reaching clinical endpoint on day 9 post-viral infection (day 29). The data for this calf are included until the animal was removed. Data are presented as means ± SEM.

Figure 2.

Clinical disease and thoracic ultrasonography (TUS) scores following bovine respiratory syncytial virus (BRSV)-PM coinfection in control and SCFP-treated calves. Following BRSV-PM coinfection, clinical signs were monitored by a single trained observer and scores were assigned daily using the system described in Materials and Methods. Parameters include rectal temperature, expiration effort, respiration rate, ocular and/or nasal discharge, cough, and lung auscultation. n = 13 SCFP-treated calves. n = 13 control calves. One control calf reached a humane endpoint and was euthanized on day 9 post-viral infection (day 29) (A). Lung disease progression was evaluated using TUS on days 7 and 10 post-viral infection. Scores were assigned as described in Materials and Methods. On day 7 after viral infection, SCFP-treated calves (1 g/d SmartCare, 5 g/d NutriTek) had lower TUS scores (P = 0.03) than control calves (n = 13 controls; n = 13 SCFP-treated). On day 10 after viral infection, both groups experienced lower scores, with SCFP-treated calves experiencing tendency to lower scores (P = 0.09) than the controls (n = 13 controls; n = 13 SCFP-treated). One calf was removed from the study on day 9 after infection. TUS data were collected from this animal prior to euthanasia and are included in the day 10 results (B). Plus symbol indicates the score of the animal analyzed on day 9. Data are presented as means ± SEM.

Figure 3.

Calves receiving SCFP supplementation tend to develop less bovine respiratory syncytial virus-PM induced lung pathology and less neutrophil infiltration into the airways. On day 10 post-viral infection, the calves were humanely euthanized and necropsied. The extent of gross pneumonic consolidation was evaluated based upon the percentage of lung affected (0 = free of lesions; 1 = 1% to 5% affected; 2 = 6% to 15% affected; 3 = 16% to 30% affected; 4 = 31% to 50% affected; 5 = >50% affected). Representative gross lung lesions from three individual calves in the control group (top) or SCFP-treated group (1 g/d SmartCare, 5 g/d NutriTek; bottom) (A). Aggregate gross pathology results from all calves (B). Plus symbol indicates the score of the control calf that was euthanized on day 9. BAL samples were collected from all calves (SCFP-treated, n = 13; and controls, n = 12) on day 10 post-viral infection (C). Cytospin preparations were made and differentially stained with Modified Wrights Stain. Relative numbers of neutrophils, macrophages, lymphocytes, eosinophils, and other cell types were determined by microscopy. The relative numbers of macrophages (*P = 0.07) and neutrophils (**P = 0.04) present in the lung airways differed between SCFP and control calves. Data are presented as means ± SEM. The lung location and percentage of lung lobes positive for PM bacterial recovery in each treatment group (D).

Figure 4.

Calves supplemented with SCFP tended to have reduced viral shedding compared with control calves. Nasal swabs were collected from all animals on days 0, 2, 4, 6, and 8 post-viral infection (SCFP-treated, 1 g/d SmartCare, 5 g/d NutriTek), n = 13; and controls, n = 13) and from all remaining animals on day 10 post-viral infection (SCFP-treated, 1 g/d SmartCare, 5 g/d NutriTek), n = 13; and controls, n = 12). Swabs were snap-frozen at −80 °C until later analysis. Virus shedding was quantified by qPCR for the bovine respiratory syncytial virus NS2 gene. NS2 mRNA was undetectable in all animals on days 0 and 2 post-viral infection. Data are presented as means ± SEM.

Figure 5.

Serum metabolic responses to viral–bacterial coinfection and SCFP supplementation. Serum was collected on days 0, 6, 7, and 10 post-viral infection and analyzed using commercial (A) BUN, (B) glucose, (C) insulin, (D) NEFA, and (E) BHB ELISA kits. SCFP-treated (1 g/d SmartCare, 5 g/d NutriTek), n = 13; and controls, n = 13. One control calf was removed from the study on day 9 post-viral infection and is not included in the day 10 results. Data are presented as means ± SEM.

Figure 6.

Serum and liver TNF-α, IL-6, and haptoglobin responses in SCFP-treated calves compared with controls following viral–bacterial coinfection. Liver biopsies were collected 7 d prior to bovine respiratory syncytial virus infection and again at necropsy on day 10 after viral infection. RNA was isolated from liver samples and analyzed by qPCR for expression of (A) TNF-α, (B) IL-6, and (C) haptoglobin. Serum samples were collected on days 0, 6, 7, and 10 post-viral infection and analyzed using commercial ELISA kit for concentrations of (D) IL-6 and (E) haptoglobin. SCFP-treated (1 g/d SmartCare, 5 g/d NutriTek), n = 13; and controls, n = 13. One control calf was removed from the study on day 9 post-viral infection and is not included in the day 10 results. Data are presented as means ± SEM.