Subacute Ruminal Acidosis in Dairy Herds - University of Wisconsin ...

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Preconvention Seminar 7A: Dairy Herd Problem Investigation Strategies:Lameness, Cow Comfort, and Ruminal Acidosis(Holstein steer)8.07.57.0Feed and waterre-introducedRuminal pH6.56.05.55.0Accidentallylocked away fromfeed and water4.5-12 -6 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84Time from Onset, HoursFigure 6. Ruminal pH after feed deprivation in a Holstein steer. Unpublished data, PrenticeSchaefer, and Oetzel, 2000.Low ruminal pH during SARA reduces the number of species of bacteria in the rumen,although the metabolic activity of the bacteria that remain is very high (Garry, 1996). Protozoalpopulations are also limited as ruminal pH approaches 5.0. When fewer species of bacteria andprotozoa are present, the ruminal microflora are less stable and less able to maintain normalruminal pH during periods of sudden dietary changes (Garry, 1996). Thus, pre-existing SARAcould increase the risk for acute ruminal acidosis in the event of accidental ingestion of excessiveamounts of grain.Ruminal atony. Additional drops in ruminal pH may result in complete ruminal atony. Thisresponse decreases absorption of acids by keeping them away from the ruminal wall. In essence,the animal now senses that systemic acid-base balance is being jeopardized by the absorption oflactic acid, so it seeks to preserve the whole animal at the expense of the rumen. Ruminal pHwill drop even faster now - to 5.0 or below. At this point, the animal is in acute ruminal acidosis,which can be life-threatening. The cow is “betting her life” that the rumen will run out of itssupply of fermentable carbohydrates before death occurs. This may or may not be true,depending on the load of carbohydrates consumed before she received the signal to stop eating.Pathophysiology of Ruminal Acidosis in Dairy CattleThe ruminal epithelial cells are not protected by mucus (as abomasal cells are), so they arevulnerable to the chemical damage by acids. Thus, low ruminal pH leads to rumenitis, erosion,and ulceration of the ruminal epithelium. Rumenitis is the fundamental lesion of SARA, and itleads to chronic health problems. In contrast, systemic acidemia is the fundamental lesion ofacute ruminal acidosis, and acute death is its main health risk.Clinical signs from the health effects of SARA are delayed in onset from the time of the lowruminal pH insult. These signs are the result of a pathophysiological cascade of events thatbegins with rumenitis. Once the ruminal epithelium is inflamed, bacteria may colonize theUniversity of Wisconsin, School of Veterinary Medicine, 2015 Linden Drive, Madison, WI 5370696
Preconvention Seminar 7A: Dairy Herd Problem Investigation Strategies:Lameness, Cow Comfort, and Ruminal Acidosispapillae and leak into portal circulation. These bacteria may cause liver abscesses, whichsometimes cause peritonitis around the site of the abscess. If the ruminal bacteria clear the liver(or if bacteria from liver infections are released into circulation), they may colonize the lungs,heart valves, kidneys, or joints. The resulting pneumonia, endocarditis, pyelonephritis, andarthritis are all chronic inflammatory diseases that are difficult to diagnose ante-mortem. Postmortemmonitoring of these conditions in cull cows or cows that die on the dairy could be verybeneficial, but has not been described.There are few research reports that directly attribute specific health problems with lowruminal pH. In one study, we were able to document a rise in serum haptoglobins (acute-phaseinflammatory proteins) and an increase in the prevalence of ruminal biopsy samples withhistological evidence of rumenitis when Holstein steers were fed to a low target ruminal pH(Table 4).Table 4. Effect of target ruminal pH on serum haptoglobin concentrationsand ruminal biopsy results in Holstein steers.OutcomeHaptoglobin, mg/dl,meanHaptoglobin, % > 20mg/dlBiopsies, % withrumenitisExperiment 1Target pH = 6.0Experiment 2Target pH = 5.516.7 25.811.5 22.537.5 87.5Data adapted from Prentice et al., 2000.Caudal vena cava syndrome can cause hemoptysis and peracute deaths due to massivepulmonary hemorrhage in cows that are affected with SARA (Nordlund et al., 1995). In thesecases, septic emboli from liver abscesses lead to lung infections which ultimately invadepulmonary vessels and cause their rupture (Radostits et al., 1994; Rebhun, 1995).SARA may also be associated with laminitis and subsequent hoof overgrowth, soleabscesses, and sole ulcers. These foot problems generally do not appear until weeks or monthsafter the bout of ruminal acidosis that caused them. The mechanism by which SARA increasesthe risk for laminitis has not been characterized. New information from equine studies suggeststhat metalloproteinase enzymes produced by Strep. bovis in the hindgut may be the incitingcause for laminitis (Pollitt, 1999). If these enzymes are indeed the culprits in dairy cattlelaminitis, then they may be of hindgut origin as well. This hypothesis has not been tested incattle, but some research data support the general concept that laminitis is the result of abnormalhindgut fermentation of carbohydrates instead of ruminal acidosis per se.Effect of SARA on Milk Fat PercentageThe relationship between SARA and milk fat depression is inconsistent and complex.Sometimes veterinary practitioners pursue milk fat test depression problems that are in fact notproblems at all. It is surprisingly difficult, at times, to determine if a herd really has a low milkfat test or not. Normal milk fat percentage depends greatly on breed, season, and days in milk.University of Wisconsin, School of Veterinary Medicine, 2015 Linden Drive, Madison, WI 5370697
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