WRANGLING ON THE RANGE #161
Long Coat; Night Vision
Rule out Cushing's disease if you horse has a dull, hairy coat.
Plus, how well can horses see in the dark?
Question
My 13-year-old Paint gelding has a longer, duller coat than the
other horses where I board. I blanket him in winter and work him
twice a week in summer, but even in summer his coat seems
unnaturally long. Also, he never gets shiny, whereas my retired
26-year-old Quarter Horse gelding is glossy year-round. Is there
any inexpensive way to make my Paint's coat shinier-such as
adding something to his feed?
SOFIA PRINCE, Nevada
Answer
To address a horse's abnormally long haircoat (called hirsutism),
I'd investigate underlying causes before simply adding something
to his feed. Three common causes for a long, dull coat are
pituitary pars intermedia dysfunction (PPID, also known as equine
Cushing's disease), subclinical illness, and parasites.
Given that your gelding is 13, it's unlikely but still possible
for him to have PPID. Cushing's is usually found in horses 15 or
older, but I've seen it in a horse as young as 9. So a PPID test
or a combination of tests is warranted for your gelding. There's
no single perfect test, but several testing methods are
available, including measuring plasma ACTH (adrenocorticotropic
hormone) and a low-dose dexamethasone-suppression test. Your
veterinarian can advise you on which to use.
Be aware there's a strong seasonal influence on PPID test
results. Shortening days complicate the interpretation of
results, and false-positive results aren't uncommon if horses are
tested in fall. Work with your veterinarian, and try to avoid
testing from August through Ocober. The optimal time for PPID
testing s early spring through early summer, as lays lengthen.
Typically, the longer PPID exists in horse, the less effective
treatment is. Pergolide mesylate can help normalize his hormonal
balance. It must be arefully prepared in a compounding pharmacy,
and its shelf life is limited, o work closely with your vet on a
treat-lent plan.
If you rule out PPID, then order a general veterinary evaluation,
including baseline blood work (a complete blood count and serum
chemistry analysis). This checks for a simmering systemic
illness-something happening in your horse's body that's not
causing a major problem, but is keeping him from blooming. His
teeth, heart, lungs, and other systems should be evaluated as
potential causes of a less-than-thriving appearance.
Also, a quantitative and qualitative fecal test will tell if
parasites are present in your horse's system and, if they are,
how heavy the load is. Horses with a large parasite burden
typically improve markedly in appearance after appropriate
treatment.
BETH DAVIS, DVM, Ph.D., DACVIM Associate Professor, Equine
Internal Medicine Kansas State University Manhattan, KS
For more information on Cushing's disease, visit
HorseandRider.com.
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Question
I like to trail ride after work, but get nervous if my horse and
I are out after dark. I'm afraid my horse will bump into a tree
or trip over something. I've heard different opinions about how
well horses see in the dark and would like to know if there's any
research on this.
CHRIS LARSON, Alabama
Answer
Horses graze, interact, and wander about during the night as much
as they do in the day. I've even witnessed wild mustangs
negotiating sagebrush, rocks, hills, and gullies at a full gallop
over rough terrain-with only starlight to guide them. So it seems
horses do see pretty well at night, but until recently there was
no behavioral research on the subject.
Now, physiological studies have shown that horses possess
features indicating functional scotopic (dim light) vision. These
include a retina with considerably more rods than cones and a
reflective tapetum lucidum that increases lightgathering
properties. Despite increasing the eye's sensitivity to dim
light, however, the tapetum lucidum is thought to reduce visual
discrimination. In other words, horses may see better in the dark
than we do, but their ability to tell objects apart may still be
significantly diminished.
So, how can we really know how the world looks to a horse at
night? Examining physiological features gives us some answers,
but in order to gain a complete understanding of a horse's
capabilities, it's also important to consider how it behaves in
specific situations.
Toward that end, the Equine Research Foundation in Aptos,
California, designed a study in which horses were tested for
object discrimination under various light levels. In a
windowless, enclosed building, some horses were trained through
positive reinforcement to select a black circle when it was
presented with a black triangle (both on white backgrounds),
while others learned to select the triangle instead of the
circle. Once the horses reliably and repeatedly chose the correct
shapes under normal lighting conditions, they were tested in
ever-diminishing light levels.
With the assistance of our learningvacation participants, we
found that horses could easily discriminate these shapes (as well
as others) in almost all light levels-comparable to moonlight;
starlight; and even dark, moonless nights in wooded areas. Only
when light levels dropped to nearly pitch black did the horses
lose the ability to discern shapes. Yet, even in these extremely
low light levels, they retained the ability to walk around within
the enclosure and avoided bumping into nearby objects.
This research showed, for the first time, how capable horses are
at visually discriminating medium- or larger-sized objects in
very dim light. So, while there could be other reasons not to
ride after dark, seeing well at night is probably not an issue
for horses with normal vision.
EVELYN B. HANGGI, M.S., Ph.D. Equine Cognition/Perception
Researcher, Behaviorist Equine Research Foundation; Aptos, CA
equineresearch.org
Send horse health and behavior questions to jfmeyer@aimmedia.com.
Include horse's age, breed, gender.
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From "Horse and Rider" - October 2011
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