Objective—To determine the pharmacokinetics of
acetazolamide administered IV and orally to horses.
Animals—6 clinically normal adult horses.
Procedure—Horses received 2 doses of acetazolamide
(4 mg/kg of body weight, IV; 8 mg/kg, PO), and
blood samples were collected at regular intervals
before and after administration. Samples were
assayed for acetazolamide concentration by high-performance
liquid chromatography, and concentrationtime
data were analyzed.
Results—After IV administration of acetazolamide,
data analysis revealed a median mean residence time
of 1.71 ± 0.90 hours and median total body clearance
of 263 ± 38 ml/kg/h. Median steady-state volume of
distribution was 433 ± 218 ml/kg. After oral administration,
mean peak plasma concentration was 1.90 ±
1.09 µg/ml. Mean time to peak plasma concentration
was 1.61 ± 1.24 hours. Median oral bioavailability was
25 ± 6%.
Conclusions and Clinical Relevance—Oral pharmacokinetic
disposition of acetazolamide in horses was characterized
by rapid absorption, low bioavailability, and
slower elimination than observed initially after IV administration.
Pharmacokinetic data generated by this study
should facilitate estimation of appropriate dosages for
acetazolamide use in horses with hyperkalemic periodic
paralysis. (Am J Vet Res 2000;61:965–968)
Objective—To use force plate analysis to evaluate the
analgesic efficacies of flunixin meglumine and
phenylbutazone administered IV at typical clinical
doses in horses with navicular syndrome.
Animals—12 horses with navicular syndrome that
were otherwise clinically normal.
Procedure—Horses received flunixin (1.1 mg/kg),
phenylbutazone (4.4 mg/kg), or physiologic saline
(0.9% NaCl; 1 mL/45 kg) solution administered IV
once daily for 4 days with a 14-day washout period
between treatments (3 treatments/horse). Before
beginning treatment (baseline) and 6, 12, 24, and 30
hours after the fourth dose of each treatment, horses
were evaluated by use of the American Association of
Equine Practitioners lameness scoring system (half
scores permitted) and peak vertical force of the forelimbs
was measured via a force plate.
Results—At 6, 12, and 24 hours after the fourth treatment,
subjective lameness evaluations and force
plate data indicated significant improvement in lameness
from baseline values in horses treated with flunixin
or phenylbutazone, compared with control horses;
at those time points, the assessed variables in flunixin-
or phenylbutazone-treated horses were not significantly
Conclusions and Clinical Relevance—In horses
with navicular syndrome treated once daily for 4 days,
typical clinical doses of flunixin and phenylbutazone
resulted in similar significant improvement in lameness
at 6, 12, and 24 hours after the final dose, compared
with findings in horses treated with saline solution.
The effect of flunixin or phenylbutazone was
maintained for at least 24 hours. Flunixin meglumine
and phenylbutazone appear to have similar analgesic
effects in horses with navicular syndrome. (Am J Vet
Objective—To develop and validate an ex vivo model
for study of adherence of Mannheimia haemolytica
(formerly Pasteurella haemolytica) to respiratory tract
mucosa of cattle and to use this model to confirm
adherence of M haemolytica serovar 1 (Mh1) to several
relevant respiratory mucosal surfaces.
Sample Population—Excised nasal, nasopharyngeal,
turbinate, and tonsillar mucosal tissue from the
bovine upper respiratory tract.
Procedure—Mh1 was radiolabeled by use of tritiated
leucine. Various concentrations of labeled bacteria
were incubated with bovine upper respiratory tract
tissues for various times. Tissue was washed to
remove nonadherent bacteria, and percentage of bacteria
adhered (percentage of adherence) was estimated
using radioactivity. Using an optimal inoculum
concentration and incubation time, percentage of
Mh1 adherence was compared on nasal, nasopharyngeal,
turbinate, and tonsillar mucosal tissue, and
adherence to nasopharyngeal tissue was confirmed
by scanning and transmission electron microscopy.
Results—The optimal Mh1 inoculum concentration
was 1 × 107 colony forming units/ml and incubation
time was 3 hours. Percentage of adherence of Mh1
to nasopharyngeal tissue was greater than adherence
to other tissue types.
Conclusions and Clinical Relevance—The ex vivo
model maintained the functional and structural integrity
of bovine upper respiratory tract mucosa, as confirmed
by light and electron microscopy. Electron
microscopy revealed participation of epithelial cell cilia
and surface mucus in adherence of Mh1 to nasopharyngeal
tissue. Adherence of Mh1 was confirmed in
repeated assays, indicating that this organism
adheres to upper respiratory tract mucosa of cattle.
(Am J Vet Res 2001;62:805–811)
Objective—To investigate the concentration-dependent
effects of Mannheimia haemolytica (formerly
Pasteurella haemolytica) leukotoxin (LKT) on apoptosis
and oncosis in bovine neutrophils and to examine the
role of calcium ions (Ca2+) in LKT-induced apoptosis.
Sample Population—Neutrophils isolated from
blood samples obtained from healthy calves.
Procedure—Neutrophil suspensions were exposed
to lytic or sublytic dilutions of LKT and then examined
by use of transmission electron microscopy (TEM) or
gel electrophoresis. Contribution of extracellular Ca2+
to LKT-induced apoptosis was investigated by incubating
neutrophils with LKT or control solutions in
buffer containing 1 mM CaCl2 or in Ca2+-free buffer
containing 1 mM ethylene glycol-bis (b-aminoethyl
ether)- N,N-tetraacetic acid (EGTA) prior to diphenyl
Results—Examination by TEM revealed that bovine
neutrophils exposed to lytic dilutions of LKT had
changes consistent with oncosis, whereas neutrophils
exposed to sublytic dilutions of LKT and staurosporin,
an inducer of apoptosis, had changes consistent
with apoptosis. Effects of sublytic dilutions of
LKT on apoptosis were confirmed by gel electrophoresis.
Replacement of extracellular Ca2+ with
EGTA, a Ca2+ chelator, reduced apoptosis attributable
to the calcium ionophore A23187, but it did not have
significant effects on apoptosis induced by LKT or
Conclusions and Clinical Relevance—The ability of
LKT to cause apoptosis instead of oncosis is concentration-
dependent, suggesting that both processes of
cell death contribute to an ineffective host-defense
response, depending on the LKT concentration in
pneumonic lesions. Furthermore, although Ca2+ promotes
A23187-induced apoptosis, it is apparently not
an essential second messenger for LKT-induced apoptosis.
( Am J Vet Res 2001;62:136–141)
Objective—To characterize ultrastructural changes of
bovine lymphocytes exposed to Pasteurella haemolytica
Sample Population—Partially purified LKT from a wild
type P haemolytica A1 strain and inactive pro-LKT from
an isogeneic mutant P haemolytica strain. Isolated bovine
lymphocytes were obtained from 2 healthy calves.
Procedure—Isolated bovine lymphocytes were incubated
with various concentrations of LKT and pro-LKT
for 3 hours at 37 C and examined by use of transmission
electron microscopy. A cytochemical Klenow
DNA fragmentation assay was used to examine lymphocytes
for DNA fragmentation.
Results—Lymphocytes incubated with LKT at a high
concentration (1.0 toxic U/ml) had ultrastructural evidence
of cytoplasmic and nuclear membrane rupture
and swelling or lysis of mitochondria. Low concentrations
of leukotoxin (0.1 toxic U/ml) induced DNA
fragmentation in 80% of lymphocytes. Ultrastructurally,
these cells had nuclear membrane blebbing,
cytoplasmic vaculation, chromatin condensation,
nuclear fragmentation, and membrane-bound
apoptotic bodies. Incubation of lymphocytes with
LKT at extremely low concentrations (0.001 toxic
U/ml) or with pro-LKT did not alter their ultrastructure.
Inclusion of 0.5 mM ZnCl2
in the medium
blocked leukotoxin-induced ultrastructural changes in
Conclusions and Clinical Relevance—Low concentrations
of LKT induce apoptosis and high concentrations
induce oncotic cell lysis in bovine lymphocytes.
The ability of low LKT concentrations to induce apoptosis
in host leukocytes may allow bacteria to escape
host immune surveillance and colonize the host.
(Am J Vet Res 2000;61:51–56)