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 the pharmacokinetics of
azithromycin and its concentration in body fluids and
bronchoalveolar lavage cells in foals.
Animals—6 healthy 6- to 10-week-old foals.
Procedure—Azithromycin (10 mg/kg of body weight)
was administered to each foal via IV and intragastric
(IG) routes in a crossover design. After the first IG
dose, 4 additional IG doses were administered at 24-hour intervals. A microbiologic assay was used to
measure azithromycin concentrations in serum, peritoneal
fluid, synovial fluid, pulmonary epithelial lining
fluid (PELF), and bronchoalveolar (BAL) cells.
Results—Azithromycin elimination half-life was 20.3
hours, body clearance was 10.4 ml/min·kg, and apparent
volume of distribution at steady state was 18.6
L/kg. After IG administration, time to peak serum concentration
was 1.8 hours and bioavailability was 56%.
After repeated IG administration, peak serum concentration
was 0.63 ± 0.10 µg/ml. Peritoneal and synovial
fluid concentrations were similar to serum concentrations.
Bronchoalveolar cell and PELF concentrations
were 15- to 170-fold and 1- to 16-fold higher than concurrent
serum concentrations, respectively. No
adverse reactions were detected after repeated IG
Conclusions and Clinical Relevance—On the basis
of pharmacokinetic values, minimum inhibitory concentrations
of Rhodococcus equi isolates, and drug
concentrations in PELF and bronchoalveolar cells, a
single daily oral dose of 10 mg/kg may be appropriate
for treatment of R equi infections in foals. Persistence
of high azithromycin concentrations in PELF and bronchoalveolar
cells 48 hours after discontinuation of
administration suggests that after 5 daily doses, oral
administration at 48-hour intervals may be adequate.
(Am J Vet Res 2001;62:1870–1875)
Objective—To determine the effects of dexamethasone
on development of IgG subclass responses following
vaccination of healthy horses.
Animals—11 mature Thoroughbreds.
Procedure—Horses received 2 IM injections at 2-
week intervals of a vaccine containing inactivated
infectious bovine rhinotracheitis, bovine viral diarrhea,
and parainfluenza-3 viral antigens and were then randomly
assigned to 2 groups. Six horses received dexamethasone
(0.2 mg/kg of body weight, IM) twice
weekly for 8 weeks starting the day of the first vaccination.
Five control horses received an equivalent volume
of saline (0.9% NaCl) solution. Antigen-specific
serum IgG subclass titers were determined weekly
after vaccination by use of an ELISA.
Results—Vaccination resulted in similar antigen-specific
serum IgG(T) titers in dexamethasone-treated
and control horses. In contrast, although control horses
developed IgGa and IgGb responses after vaccination,
corticosteroid administration completely inhibited
these responses in treated horses.
Conclusions and Clinical Relevance—Cortico
steroids can have profound effects on primary
immune responses in horses and can significantly
affect IgG responses to inactivated vaccines.
Corticosteroid treatment regimens commonly used
to treat diseases in horses may result induction of a
nonprotective IgG subclass response, leaving treated
horses susceptible to disease. Additionally, mechanisms
regulating IgGa and IgGb responses appear to
differ from those regulating IgG(T) responses. Further
defining these mechanisms is a critical step in designing
effective vaccines, and corticosteroid-induced
immunomodulation may be a valuable tool for studying
immune responses in horses. (Am J Vet Res
Objective—To determine signalment, clinical findings,
results of diagnostic testing, outcome, and postmortem
findings in horses with West Nile virus
Animals—46 horses with WNV encephalomyelitis.
Procedure—Clinical data were extracted from medical
records of affected horses.
Results—On the basis of clinical signs and results of
serologic testing, WNV encephalomyelitis was diagnosed
in 46 of 56 horses with CNS signs. Significantly
more males than females were affected. Increased
rectal temperature, weakness or ataxia, and muscle
fasciculations were the most common clinical signs.
Paresis was more common than ataxia, although both
could be asymmetrical and multifocal. Supportive
treatment included anti-inflammatory medications,
fluids, antimicrobials, and slinging of recumbent horses.
Results of the IgM capture ELISA and the plaque
reduction neutralization test provided a diagnosis in
43 horses, and only results of the plaque reduction
neutralization test were positive in 3 horses. Mortality
rate was 30%, and 71% of recumbent horses were
euthanatized. One horse that had received 2 vaccinations
for WNV developed the disease and was euthanatized.
Follow-up communications with 19 owners
revealed that most horses had residual deficits at 1
month after release from the hospital; abnormalities
were resolved in all but 2 horses by 12 months after
Conclusions and Clinical Relevance—Our findings
were similar to those of previous WNV outbreaks in
horses but provided additional clinical details from
monitored hospitalized horses. Diagnostic testing is
essential to diagnosis, treatment is supportive, and
recovery rate of discharged ambulatory horses is
< 100%. (J Am Vet Med Assoc 2003;222:1241–1247)