Blood ionized calcium (Ca2+) and pH; plasma lactate concentrations; and total protein, total calcium (CaT), albumin, and phosphorus concentrations in serum were determined in 40 healthy horses before (T1), at the finish line (T2), and 10 minutes after the finish (T3) of the cross-country phase of a 3-dayevent competition. Mean ( +/- SEM) Ca2+ concentrations decreased from 6.22 +/- 0.04 mg/dl at T1 to 5.04 +/- 0.07 mg/dl at T2 (P less than or equal to 0.05). This decrease was accompanied by a nonsignificant increase in CaT between T1 and T2. The mean (+/- SEM) percent ionization of calcium decreased significantly (P less than or equal to 0.05), from 50.9 +/- 2.75% at T1 to 40.3 +/- 3.58% at T2. Significant increases in mean albumin, total protein, phosphorus, and lactate concentrations and a significant decrease in mean pH were observed at T2 (P less than or equal to 0.05). At T3, mean Ca2+ and percent ionization had increased, but remained significantly less than resting values. Mean CaT was significantly decreased at T3, compared with values at T1 and T2. Correlation of mean Ca2+ concentration with all other measured variables at each time was evaluated; correlation coefficients between mean Ca2+ and all other variables were low (r2 less than or equal to 0.38), indicating low biological significance.

Fifteen isolates of Escherichia coli obtained from the blood and tissues of septic foals had plasmid DNA of size ranging from 2.5 to 93 megadaltons. These isolates grew in normal equine serum (serum resistant), a trait previously documented to be expressed by isolates obtained from blood and tissues of septic foals, but not by isolates obtained from the feces of clinically normal horses. Of these isolates, 3 contained conjugal plasmids that encoded resistance to multiple antimicrobial agents linked to serum resistance and, in 1 isolate, to production of aerobactin as well. Serum resistance and production of aerobactin are related to virulence of septicemic E coli from non-equine sources.

Hematologic and rheologic changes were examined in 49 Thoroughbreds before and after competitive racing. Mean postrace values for RBC count, hemoglobin concentration, and PCV increased by 58 to 61%, whereas blood viscosity increased 2 to 3 times. Postrace echinocyte numbers were 162% greater than prerace values. Smaller, but statistically significant, changes were found for mean corpuscular hemoglobin concentration, red cell distribution width, plasma total protein concentration, total WBC count, neutrophil count, and lymphocyte count. Variables measured did not predict whether a horse was a bleeder not treated with furosemide, a bleeder treated with furosemide, or a nonbleeder.