Objective— To evaluate effects of polymyxin B sulfate
(PMB) on response of horses to endotoxin, using
an ex vivo model.
Animals—8 healthy horses.
Procedure—In a crossover design, 3 doses of PMB
(100, 1,000, and 10,000 U/kg of body weight) and
physiologic saline solution (control) were evaluated.
Prior to and for 24 hours after administration of PMB,
blood samples were collected into heparinized tubes
for use in 2 assays. For the endotoxin-induced tumor
necrosis factor (TNF) assay, blood samples were incubated
(37 C for 4 h) with 1 ng of Escherichia coli or
Salmonella Typhimurium endotoxin/ml of blood.
Plasma was harvested and assayed. For the residual
endotoxin activity assay, plasma was collected into
sterile endotoxin-free borosilicate tubes, diluted 1:10
with pyrogen-free water, and incubated for 10 minutes
at 70 C. Escherichia coli endotoxin (0.1 or 1
ng/ml of plasma) was added to the thawed samples
prior to performing the limulus ameobocyte lysate
assay. Serum creatinine concentrations were monitored
for 1 week.
Results—Compared with baseline values, PMB
caused a significant dose- and time- dependent
decrease in endotoxin-induced TNF activity.
Compared with baseline values, residual endotoxin
activity was significantly reduced after administration
of 10,000 U of PMB/kg. Compared with baseline values,
1,000 and 5,000 U of PMB/kg should inhibit 75%
of endotoxin-induced TNF activity for 3 and 12 hours,
respectively. Serum creatinine concentrations
remained within the reference range.
Conclusion and Clinical Relevance—Results of the
study suggest that PMB is a safe, effective inhibitor of
endotoxin-induced inflammation in healthy horses.
( Am J Vet Res 2001; 62:72–76)
Objective—To determine plasma endotoxin concentration
in horses competing in a 48-, 83-, or 159-km
endurance race and its importance with regard to
physical, hematologic, or serum and plasma biochemical
Procedure—Weight and rectal temperature measurements
and blood samples were obtained before, during,
and after exercise. Blood samples were analyzed
for plasma endotoxin concentration; serum antiendotoxin
antibody titers; thromboxane B2 (TxB2) and 6-
keto-prostaglandin F1α (PGF1α) concentrations; tumor
necrosis factor alpha (TNFα) and interleukin-6 (IL-6)
activities; WBC, plasma protein, lactate, serum electrolyte,
and calcium concentrations; PCV; and creatine
Results—Detection of plasma endotoxin increased
during exercise for horses competing at all distances
but occurred more frequently in the 48- and 83-km
groups. Plasma lactate concentration was significantly
greater when endotoxin was concurrently detected.
Endotoxin in plasma was not significantly associated
with success of race completion. Plasma TxB2
and PGF1α concentrations and serum IL-6 activity significantly
increased with exercise. Horses that had an
excellent fitness level (as perceived by their owners)
had greater decreases in serum antiendotoxin antibody
titers during exercise than did horses perceived
as less fit. In horses with better finish times, TxB2 and
PGF1α concentrations were significantly greater and
TNFα activity was significantly less than that of slower
Conclusions and Clinical Relevance—Endotoxemia
developed during endurance racing, but was significantly
correlated with increased plasma lactate concentration
and not with other variables indicative of
endotoxemia. Plasma TxB2 and PGF1α concentrations
and serum TNFα activity may be associated with performance
success. (Am J Vet Res 2003;64:754–761)
Objective—To compare physiologic, hematologic,
and selected serum and plasma biochemical variables
obtained from horses competing in 48-, 83-, or 159-
km endurance rides before competition and at the
same cumulative distance points.
Procedure—Weight and rectal temperature measurements
and blood samples were obtained from horses
before, during, and after 1 of 3 rides conducted on the
same day. Plasma protein (PP), lactate, WBC, serum
electrolyte, and calcium concentrations; PCV; and
creatine kinase (CK) activity were determined.
Assessments were made to determine whether any
differences among groups, with respect to total distance
competed, could be explained by differences in
lap speed or conditioning and to investigate the effect
of time in transit or on-site prior to competition on
results of blood analyses or competition outcome.
Results—Horses in the 159-km distance group had
the lowest preride serum sodium, chloride, bicarbonate,
and calcium concentrations. As hours in transit
increased, preride PP concentration was significantly
greater; serum sodium, chloride, and bicarbonate concentrations
were lower; CK activity at 159 km was
greater; and horses were more likely to be eliminated.
The preride sodium was significantly greater in horses
that completed the ride, compared with those
Conclusions and Clinical Relevance—Among distance
groups, distance ridden, speed, level of fitness,
and years of experience of horses had little effect on
the variables examined. Electrolyte and water supplementation
and earlier arrival at the event may be beneficial
for horses that are transported long distances
to endurance competition. (Am J Vet Res 2003;64:746–753)
Objective—To determine which antimicrobials that
are used to treat neonatal foals with septicemia attributable
to Escherichia coli will minimize endotoxin
release from bacteria and subsequent activity of
inflammatory mediators while maintaining bactericidal
Sample Population—Blood samples from 10 healthy
Procedure—Escherichia coli isolates A and B were
isolated from 2 septicemic foals, and minimal
inhibitory concentrations (MIC) were determined for
9 antimicrobials. Five of these antimicrobials were
tested in vitro at 2 and 20 times their respective
MIC. Whole blood or mononuclear cells grown in tissue-
culture media were incubated with 105 colonyforming
units of E coli and each antimicrobial or
saline (0.9% NaCl) solution. After 6 hours, number
of viable bacteria remaining was determined, and
supernatant was tested for endotoxin and tumor
Results—Testing in whole blood was compromised
by bactericidal effects of the blood itself. In mononuclear
cell suspensions, each antimicrobial significantly
reduced the number of viable bacteria to low or undetectable
amounts. Antimicrobials did not differ significantly
in efficacy of bacterial killing. Amikacin used
alone or in combination with ampicillin resulted in significantly
less endotoxin activity than did ampicillin,
imipenem, or ceftiofur alone. There was a correlation
between TNF-α and endotoxin activity.
Conclusions and Clinical Relevance—Aminoglycosides
appear less likely to induce endotoxemia
and TNF-α synthesis during bactericidal treatment of E
coli septicemia, compared with β-lactam antimicrobials.
Use of ampicillin, imipenem, or ceftiofur in the
treatment of septicemic neonatal foals should be
accompanied by appropriate treatment for endotoxemia.
(Am J Vet Res 2002;63:660–668)
Objective—To compare daily endogenous cortisol production rate and the pharmacokinetics of an IV bolus of hydrocortisone between neonatal foals and adult horses.
Animals—10 healthy full-term 2- to 4-day-old foals and 7 healthy adult horses.
Procedures—Blood samples were collected from each horse every 15 to 20 minutes for 24 hours for determination of 24-hour mean cortisol concentration. Afterward, dexamethasone (0.08 mg/kg) was administered IV to suppress endogenous cortisol production. Twelve hours afterward, hydrocortisone sodium succinate (1.0 mg/kg) was administered as a rapid IV bolus and serial blood samples were collected to determine hydrocortisone pharmacokinetics. Cortisol concentrations, daily cortisol production rate, and hydrocortisone pharmacokinetics were determined, and results were compared between adult horses and foals.
Results—The mean ± SD 24-hour cortisol concentration was significantly lower in foals (20 ± 4 ng/mL) than in horses (26 ± 6 ng/mL), but the daily cortisol production rate was significantly greater in foals (6,710 ± 320 ng/kg/d) than in horses (2,140 ± 400 ng/kg/d). For hydrocortisone, foals had a significantly greater volume of distribution at steady state (1.92 ± 1.11 L/kg) and total body clearance (1.39 ± 0.108 L/kg/h) and significantly lower peak plasma concentration (1,051 ± 343 ng/mL) than did horses (0.58 ± 0.15 L/kg, 0.349 ± 0.065 L/kg/h, and 8,934 ± 3,843 ng/mL, respectively).
Conclusions and Clinical Relevance—Important differences were detected in cortisol production and metabolism between neonatal foals and adult horses consistent with lower plasma protein binding of cortisol in foals. This decrease may contribute to cortisol insufficiency during prolonged critical illness in neonatal foals.
OBJECTIVE To compare tear cortisol concentrations between horses and ponies with pituitary pars intermedia dysfunction (PPID) and healthy nonaged (≤ 15 years old) and aged (≥ 20 years old) horses and to determine whether serum and tear cortisol concentrations were correlated.
ANIMALS 11 horses and ponies with PPID and 20 healthy control horses and ponies (11 nonaged and 9 aged).
PROCEDURES Paired tear and serum samples were obtained from PPID and control animals. All animals were free of active ocular disease. Tear and serum cortisol concentrations were measured with an ELISA and chemiluminescent assay, respectively. Groups were compared with Kruskal-Wallis and Mann-Whitney U tests, and Spearman correlation analysis was used to examine relationships between tear and serum cortisol concentrations within groups.
RESULTS Median tear cortisol concentration was significantly higher in PPID animals than in aged control animals, despite comparable serum cortisol concentrations in PPID and aged control animals. Median tear-to-serum cortisol concentration ratios were also significantly higher in PPID animals than in aged control animals. Serum and tear cortisol concentrations were not significantly correlated in PPID or control animals.
CONCLUSIONS AND CLINICAL RELEVANCE Some horses and ponies with PPID had increased tear cortisol concentrations, compared with concentrations in healthy aged animals. Localized cortisol production in the tear film or altered cortisol binding dynamics could have contributed to this increase. Further studies are warranted to evaluate these mechanisms and to determine whether increased tear cortisol concentrations are associated with delays in corneal wound healing in horses and ponies with and without PPID.
Objective—To evaluate the effects of a standardized exercise test to exhaustion in horses on leukocyte function ex vivo.
Animals—6 Thoroughbred geldings.
Procedures—Blood samples were obtained from each horse before exercise; at exhaustion (termed failure); and at 2, 6, 24, 48, and 72 hours after exercise to evaluate hematologic changes, rate of leukocyte apoptosis, and leukocyte production of reactive oxygen species (ROS) ex vivo. To assess leukocyte function, leukocyte ROS production in response to stimulation with lipopolysaccharide, peptidoglycan, zymosan, and phorbol myristate acetate was evaluated. Apoptosis was evaluated via assessment of caspase activity in leukocyte lysates.
Results—In response to lipopolysaccharide, production of ROS by leukocytes was significantly increased at 2 hours and remained increased (albeit not significantly) at 6 hours after exercise, compared with the preexercise value. In the absence of any stimulus, leukocyte ROS production was significantly increased at 6 and 24 hours after exercise. In contrast, ROS production in response to phorbol myristate acetate was significantly decreased at 6, 24, and 72 hours after exercise. Leukocyte ROS production induced by zymosan or peptidoglycan was not altered by exercise. Leukocytosis was evident for 24 hours after exercise, and neutrophilia was detected during the first 6 hours. A significant increase in the rate of leukocyte apoptosis was detected at failure and 72 hours after exercise.
Conclusions and Clinical Relevance—Results indicated that strenuous exercise undertaken by horses causes alterations in innate immune system functions, some of which persist for as long as 72 hours after exercise.