Objective—To determine whether a deficiency in
systemic or local (pars intermedia) antioxidant capacity
is associated with pituitary pars intermedia oxidative
stress and pituitary pars intermedia dysfunction
(PPID) in horses.
Sample Population—Blood samples from 20 horses
with PPID and 20 healthy client-owned horses,
archived paraffin-embedded adrenal gland and substantia
nigra tissues from 20 horses, and pituitary
gland tissue from 16 horses.
Procedures—Total glutathione, superoxide dismutase,
and glutathione peroxidase activities were
determined in RBCs. Accumulation of a systemic
marker of oxidative stress (3-nitrotyrosine) was
assessed in plasma and formalin-fixed, paraffinembedded
adrenal gland and substantia nigra tissues.
Local antioxidants (total and manganese
superoxide dismutase, glutathione peroxidase, and
total glutathione) were measured in pars intermedia
Results—No significant differences existed in systemic
antioxidant enzyme activity or accumulation of
3-nitrotyrosine between horses with PPID and control
horses. In pituitary gland tissues, glutathione peroxidase
activity was increased in horses with oxidative
stress, whereas total glutathione concentration and
superoxide dismutase activity remained unchanged.
There was an age-associated decrease in manganese
superoxide dismutase activity in the pars intermedia.
Conclusions and Clinical Relevance—There was
no evidence of systemic accumulation of oxidative
stress markers or deficiencies in antioxidant capacity
in horses with PPID, suggesting that these are
unlikely to be major predisposing factors in the
development of PPID. Manganese superoxide dismutase
activity in the pars intermedia decreased
significantly with increasing age. Role of an ageassociated
decrease in antioxidant capacity for the
pars intermedia in the development of PPID in horses
warrants further investigation. (Am J Vet Res
Objective—To characterize age-associated changes
in lymphocyte population subsets and immunoglobulin
Animals—30 healthy young light-breed horses (5 to
12 years old) and 30 healthy aged light-breed horses
(> 20 years old).
Procedure—Lymphocyte subset populations were
identified, using monoclonal antibodies to cell surface
markers CD5, CD4, CD8, and IgG. Subset populations
were quantitated by use of flow cytometric analysis
of antibody-stained cells. Serum immunoglobulin concentration
was determined using single radial immunodiffusion.
Results—Absolute cell counts of total lymphocytes, T
cells, CD4+ and CD8+ T cells, and B cells were
decreased in aged horses, compared with young
horses. There was a significant decrease in the percentage
of CD8+ cells and an increase in the CD4+-to-
CD8+ cell ratio in the aged population, compared with
young horses. However, serum concentration of IgG,
IgG(T), IgM, or IgA did not differ with age.
Conclusions and Clinical Relevance—In horses,
total lymphocyte count and lymphocyte subset cell
counts decrease with age. Age-matched control values
are necessary for optimal evaluation of hematologic
variables in aged horses. The decrease in lymphocyte
subset cell counts in healthy aged horses
mimics that seen in other species and may contribute
to an age-associated decrease in immunocompetency.
( Am J Vet Res 2001;62:1413–1417)
Objective—To investigate effects of sample handling,
storage, and collection time and season on plasma α-melanocyte-stimulating hormone (α-MSH) concentration
in healthy equids.
Animals—11 healthy Standardbreds and 13 healthy
Procedure—Plasma α-MSH concentration was measured
by use of radioimmunoassay. Effects of delayed
processing were accessed by comparing α-MSH concentrations
in plasma immediately separated with that of
plasma obtained from blood samples that were stored at
4oC for 8 or 48 hours before plasma was separated.
Effects of suboptimal handling were accessed by comparing
α-MSH concentrations in plasma immediately
stored at -80°C with plasma that was stored at 25°C for 24
hours, 4oC for 48 hours or 7 days, and –20°C for 30 days
prior to freezing at –80°C. Plasma α-MSH concentrations
were compared among blood samples collected at 8:00
AM, 12 noon, and 4:00 PM. Plasma α-MSH concentrations
were compared among blood samples collected in
January, March, April, June, September, and November
from horses and in September and May from ponies.
Results—Storage of blood samples at 4°C for 48
hours before plasma was separated and storage of
plasma samples at 4°C for 7 days prior to freezing at
–80°C resulted in significant decreases in plasma α-MSH concentrations. A significantly greater plasma α-MSH concentration was found in September in
ponies (11-fold) and horses (2-fold), compared with
plasma α-MSH concentrations in spring.
Conclusions and Clinical Relevance—Handling and
storage conditions minimally affected plasma α-MSH
concentrations. Seasonal variation in plasma α-MSH
concentrations must be considered when evaluating
pituitary pars intermedia dysfunction in equids. (Am J Vet Res 2004;65:1463–1468)
Objective—To evaluate the correlation between plasma
α-melanocyte-stimulating hormone (α-MSH) concentration
and body mass index (BMI) in healthy horses.
Animals—82 healthy horses.
Procedure—Plasma α-MSH concentration was determined
by radioimmunoassay. At the time blood samples
were collected, body condition scores (BCS)
were determined and measurements of girth circumference,
body length, and height were obtained.
Weight was estimated by use of the following
formula: estimated weight (kg) = [girth (cm)2 × length
(cm)]/11,877. Body mass index was calculated as estimated
weight (kg)/height (m)2.
Results—A correlation was found between BMI and
BCS (rs = 0.60 [95% confidence interval (CI), 0.44 to
0.73]). A weak correlation was found between plasma
α-MSH concentration and BMI (rs = 0.25 [95% CI,
0.03 to 0.45]) and BCS (rs = 0.26 [95% CI, 0.04 to
0.46]). A correlation was found between plasma
α-MSH concentration and BMI in horses ≥ 10 years
old (rs = 0.49 [95% CI, 0.20 to 0.69]) but not in horses
< 10 years old (rs = –0.04). Horses in the upper
quartile of BMI had significantly greater plasma α-MSH concentration (median, 9.1 pmol/L; range, 2.0 to
95.3 pmol/L) than horses in the lowest quartile of BMI
(median, 7.0 pmol/L; range, 3.6 to 15.7 pmol/L).
Conclusions and Clinical Relevance—A correlation
exists between plasma α-MSH concentration and
BMI in horses. Further study is required to determine
whether melanocortin receptor defects underlie this
correlation or, alternately, whether plasma α-MSH
concentration is simply a correlate of adiposity. (Am J Vet Res 2004;65:1469–1473)
Objective—To determine effects of pituitary pars intermedia dysfunction (Cushing's disease) and age on fecal egg count and time to egg reappearance after anthelmintic treatment in horses residing in similar environments.
Animals—29 healthy horses (4 to 35 years old) and 13 horses with PPID (13 to 33 years old).
Procedures—Fecal egg counts were performed by use of a modified Wisconsin flotation method at 2-week intervals before and after ivermectin treatment.
Results—Horses with PPID had higher fecal egg counts before and 8, 10, and 12 weeks after ivermectin treatment, compared with counts for site-matched healthy horses. There was no difference in the period for < 90% reduction in fecal egg counts between the 2 groups. Age did not affect fecal egg counts at any time point.
Conclusions and Clinical Relevance—For similar environmental conditions, horses with PPID were more likely to have higher fecal egg counts than were healthy horses. Therefore, horses with PPID may need to have a more aggressive parasite prevention program than do healthy horses. Age did not affect fecal egg counts or time to egg reappearance after anthelmintic treatment, which suggested age alone does not likely require special consideration when designing a parasite control program for adult horses.
Objective—To measure plasma ACTH, D-melanocyte–stimulating hormone (D-MSH), and insulin concentrations during various photoperiods between February and October in horses and ponies with and without pituitary pars intermedia dysfunction (PPID).
Animals—13 clinically normal (control) ponies, 14 clinically normal (control) horses, 7 ponies with PPID, and 8 horses with PPID.
Procedures—Blood samples were collected from February through October during 8 photoperiods: 1, February 13 through March 2; 2, April 4 through 6; 3, June 19 through 22; 4, August 6 through 7; 5, August 14 through 17; 6, September 4 through 6; 7, September 26 through 28; and 8, October 16 through 18. Plasma ACTH, D-MSH, and insulin concentrations at each photoperiod were compared among groups.
Results—Log ACTH concentration was increased during photoperiod 4 through 8, compared with photoperiod 1 through 3, in all groups. In photoperiod 3 through 7, log ACTH concentrations were higher in horses and ponies with PPID, compared with values for control horses and ponies. D-Melanocyte–stimulating hormone (log and raw value) concentration was higher in photoperiod 2 through 8, compared with photoperiod 1, in control horses and ponies. In horses and ponies with PPID, log D-MSH concentration was higher in photoperiod 3 through 8, and D-MSH concentration was higher in photoperiod 4 through 8, compared with photoperiod 1. In control horses and ponies, plasma insulin concentration was lower in photoperiod 3 than in photoperiod 1.
Conclusions and Clinical Relevance—Plasma D-MSH and ACTH concentrations increased as daylight decreased from summer solstice (maximum daylight hours) to 12 hours of daylight.
Objective—To compare endogenous ACTH and α-melanocyte-stimulating hormone (α-MSH) concentrations after administration of thyrotropin-releasing hormone (TRH) and to compare ACTH concentrations after TRH administration with those following domperidone administration in healthy horses and horses with pituitary pars intermedia dysfunction (PPID).
Design—Prospective case series.
Animals—69 clinically normal horses and 47 horses with or suspected to have PPID.
Procedures—ACTH concentrations were measured during 108 TRH stimulation tests in 88 horses, and α-MSH concentrations were measured during 56 TRH stimulation tests in 50 horses. In 28 of these horses, ACTH concentrations after domperidone administration were measured and test results were compared. The pituitary gland was histologically examined in all horses that were euthanatized.
Results—ACTH and α-MSH concentrations increased in all horses afterTRH administration, with a greater and more prolonged increase in horses with PPID. Percentage increase was significantly greater for α-MSH concentration than for ACTH concentration. The change in ACTH concentration after domperidone administration was less consistent in differentiating clinically normal horses from those with PPID than was the response to TRH.
Conclusions and Clinical Relevance—Results suggested that ACTH concentration in response to TRH administration was useful for the diagnosis of PPID in horses and appeared more accurate than response to domperidone administration. Use of an α-MSH concentration ≥ 30 or 50 pmol/L did not appear superior to use of an ACTH concentration ≥ 36 pg/mL for the diagnosis of PPID, either before or 30 minutes after TRH administration.
Objective—To evaluate concordance among veterinary
pathologists in the assessment of histologic findings
in the pars intermedia of pituitary gland sections
from aged horses with mild signs suggestive of pituitary
pars intermedia dysfunction (PPID).
Sample Population—10 pituitary glands from aged
Procedure—7 pathologists were provided with signalment,
clinical signs, and a single H&E-stained pituitary
gland section from 10 aged horses with mild
signs suggestive of PPID. Pathologists described histologic
findings for each section and stated whether
findings were consistent with PPID. Agreement
among pathologists and with antemortem diagnostic
test results was calculated.
Results—Overall, only fair agreement was found
among the pathologists as to which horses had histologic
findings consistent with disease (mean ± SE
kappa value, 0.34 ± 0.069). Interpretation of individual
sections varied, with minimal agreement (4 or 5/7
pathologists) for 5 of 10 sections evaluated.
Postmortem assessment was in agreement with an
antemortem endocrine diagnostic test result 79% of
Conclusions and Clinical Relevance—Validation of
antemortem diagnostic testing for PPID in horses
often relies on the results of postmortem histologic
evaluation. The lack of consensus in histologic interpretation
of pituitary glands from aged horses with
mild clinical signs in our study indicates that postmortem
histologic evaluation of pituitary glands is an
inappropriate standard in validation of antemortem
diagnostic tests for detection of early PPID. Caution
should be used when interpreting diagnostic test
results in horses in which early PPID is suspected.
(Am J Vet Res 2005;66:2055–2059)
OBJECTIVE To determine whether prophylactic administration of valacyclovir hydrochloride versus initiation of treatment at the onset of fever would differentially protect horses from viral replication and clinical disease attributable to equine herpesvirus type-1 (EHV-1) infection.
ANIMALS 18 aged mares.
PROCEDURES Horses were randomly assigned to receive an oral placebo (control), treatment at detection of fever, or prophylactic treatment (initiated 1 day prior to viral challenge) and then inoculated intranasally with a neuropathogenic strain of EHV-1. Placebo or valacyclovir was administered orally for 7 or 14 days after EHV-1 inoculation or detection of fever (3 horses/group). Effects of treatment on viral replication and clinical disease were evaluated. Plasma acyclovir concentrations and viremia were assessed to determine inhibitory concentrations of valacyclovir.
RESULTS Valacyclovir administration decreased shedding of virus and viremia, compared with findings for control horses. Rectal temperatures and clinical disease scores in horses that received valacyclovir prophylactically for 2 weeks were lower than those in control horses. The severity of but not the risk for ataxia was decreased by valacyclovir administration. Viremia was decreased when steady-state trough plasma acyclovir concentrations were > 0.8 μg/mL, supporting the time-dependent activity of acyclovir.
CONCLUSIONS AND CLINICAL RELEVANCE Valacyclovir treatment significantly decreased viral replication and signs of disease in EHV-1–infected horses; effects were greatest when treatment was initiated before viral inoculation, but treatment was also effective when initiated as late as 2 days after inoculation. During an outbreak of equine herpesvirus myeloencephalopathy, antiviral treatment may be initiated in horses at various stages of infection, including horses that have not yet developed signs of viral disease.