Prevalence and concentrations of major liver abscess pathogens in the lungs of feedlot cattle: Assessment of lungs as a potential source for liver abscesses

Abstract

Liver abscesses are an economically significant disease in feedlot cattle, which are estimated to cost $250 million annually because of decreased weight gain, feed efficiency, and carcass weight and liver and viscera condemnations. The incidence of liver abscesses in feedlot cattle ranges from 12 to 32%. The primary pathogens are the two subspecies of Fusobacterium necrophorum (subsp. necrophorum and funduliforme), which are part of bacterial community of the rumen and hindgut of cattle, and two other pathogens, Trueperella pyogenes and Salmonella enterica are considered as secondary pathogens. Recent studies have also found Fusobacterium varium present in the rumen epithelium and contents, indicating a possible role in the development of liver abscesses. The widely accepted pathogenesis is that the pathogens translocate across the gut epithelium to enter portal circulation to reach the liver to cause abscesses. The translocation is facilitated by chronic ruminal acidosis and subsequent rumenitis in the rumen. Rumenitis and acidosis are the consequences of feeding feedlot cattle a high-grain diet, which because of digestion of rapidly fermentable carbohydrates leads to increased acidity and decreased pH of the rumen. The Fusobacterium in the ruminal contents decreases but the bacteria adhered to the epithelium escape destruction from the acidity which eventually breaks down the ruminal wall and allows for the translocation of the bacteria to enter the portal circulation. In addition to the gastrointestinal tract, liver abscesses pathogens are also prevalent in the respiratory tract and there is evidence of a potential positive association between liver abscesses and pulmonary lesions. We hypothesized that lungs harbor liver abscess pathogens and serve as a source to enter peripheral circulation and seed the liver to cause liver abscesses. Therefore, a study was conducted to determine prevalence of the major liver abscess pathogens, the two subspecies of F. necrophorum, subsp. necrophorum and funduliforme, T. pyogenes and S. enterica in lungs of feedlot cattle and determine association between prevalence of pathogens in lungs to liver abscesses. A total of 157 healthy liver tissues and 120 liver abscesses, each with a matched lung tissue, were collected from feedlot cattle at slaughter. Samples were homogenized and subjected to culture methods, direct and enriched, to detect and isolate, and quantitative PCR (qPCR) to detect and quantify F. necrophorum subsp. necrophorum and funduliforme, F. varium, T. pyogenes, and S. enterica. Data were analyzed using R statistical software with tissue type (lungs, healthy liver and liver abscesses) serving as a fixed effect. Fisher’s exact test was used to assess the difference in prevalence and ANOVA was used for the analysis of the total concentrations after qPCR confirmation. P ≤ 0.05 was considered as significant, while meaningful tendencies were discussed when P was > 0.05 and ≤ 0.10. The qPCR assay detected a greater number of lung samples as positive for F. necrophorum, either of the two subspecies, subsp. necrophorum, and subsp. funduliforme than the culture method (43.1 vs 27.7%; 29.2 vs 14.7%; 23.2 vs 15.0%, respectively; P < 0.01). The qPCR assay detected greater prevalence of F. varium than the culture method (22.7 vs 5.8%; P < 0.01). None of the 277 lung tissues were positive for T. pyogenes by either the culture or qPCR method. The prevalence of S. enterica in lung tissue was also greater by the qPCR method than the culture method (21.3 vs. 18.8%; P = 0.04). Liver abscesses were positive for the two subspecies of F. necrophorum, T. pyogenes, and S. enterica by both detection methods with subsp. necrophorum being the most dominant and prevalent at high concentrations (6 to 7 log10 CFU per g). Healthy liver tissues were also positive for the four pathogens, but at a much lower prevalence rates than the liver abscesses. There was a significantly greater prevalence of F. necrophorum in the lung tissues of cattle with liver abscesses than cattle that had healthy livers by both culture method and qPCR assay (34.6 vs 22.6%, P = 0.02; 50.2 vs 37.1%; P = 0.03). A positive association between the prevalence of F. necrophorum in the lungs of cattle with livers abscesses versus cattle with healthy livers suggests that lungs may serve as a source. These findings have implications in the development of novel interventions towards the pathogenesis of liver abscesses.