By:
Jenna Bayne, DVM
Anhidrosis
in the horse, more commonly known as a dry horse or a non-sweater, is a medical
condition typically seen in horses residing in hot and humid environments. The
inability of affected horses to properly regulate their body temperatures
through sweating can have dire consequences and is of major importance from an
animal welfare perspective.
The
mechanisms involved in the development of anhidrosis are not completely
understood. A probable etiology is a combination of environmental factors and
multiple pathophysiological responses in the sweat gland of the horse (McEwan
Jenkinson et al., European Society of Veterinary Dermatology, 2006 and 2007).
High climatic temperatures and humidity can lead to a continuous release of
adrenaline into the horse’s bloodstream. A state of adrenaline-driven
hyperactivity occurs, which leads to changes in the secretory cells that make
up the sweat glands. Adrenaline normally binds to the β2-adrenergic
receptors of horse sweat gland cells, which sets of a cascade of intracellular
events, culminating in the release of sweat onto the body’s surface. Continuous
stimulation of these adrenergic receptors eventually leads to their
desensitization – higher concentrations of adrenaline are needed to produce a
normal response. Eventually the response cannot be elicited despite the high
concentration of adrenaline delivered to the sweat gland cells. As well, over
stimulation leads to changes within the sweat gland cells themselves, including
down-regulation and a decreased rate of adrenergic receptor production by the
cell. The sweat gland cells have fewer receptors which are less responsive than
normal. Irreversible glandular degeneration can be seen in extreme cases.
Overall, desensitization and down-regulation is seen clinically as the
inability to produce normal amounts of sweat in horses with anhidrosis.
A
recent study on the epidemiology of anhidrosis in horses residing in Florida
was carried out by Johnson and colleagues (JAVMA, 2010, 236(10): 1091-1097). It
was shown that at the farm level, horses located in central and southern
Florida were 2.13 and 4.40 times more likely respectively, to develop the
condition compared to their counterparts in northern Florida. As well, the type
of horse operation was significant in the occurrence of the disease – horses
used in riding instruction and showing were 15.40 and 5.26 times more likely to
develop anhidrosis, respectively, compared to ranch operations. Animal factors,
which include breed, use, place of foaling, as well as a family history of
anhidrosis were found to be significant. Horses originating from more temperate
climates are at a greater risk of developing anhidrosis when transported to hot,
humid climates for racing or exercise compared to horses native to Florida.
Thoroughbreds and Warmbloods develop the condition more often than Quarter
horses and other breeds, and a strong association between family history of
anhidrosis and risk of this condition exists.
There
are variable ways in which anhidrosis can present (Hubert et al., Vet Clin
Equine, 2002, 18: 355-369). The onset of the inability to sweat can be sudden,
which usually occurs following a period of profuse sweating, or it can be insidious
and progressively worsen over time. The most common clinical sign seen in
horses is a compensatory increase in respiration rate – in the range of 60 –
120 breaths per minute. Other variable signs may be an increase in body
temperature and heart rate. The demonstration of an inability to sweat can be
variable in its degree and course of onset. Though complete anhidrosis can be
seen, partial anhidrosis may limit the areas of the horse that retain their
ability to sweat to include the mane, the saddle and halter areas, and the
axillary, perineal and inguinal regions. Clinical signs of long standing
anhidrosis may include dry, flaky skin, hair loss, a reduced appetite and a
decreased water intake. A tentative diagnosis of anhidrosis may be made on history
and clinical signs. A semiquantitative test, involving the intradermal
injection of β2 agonists (salbutamol sulphate, adrenaline bitartrate
and terbutaline sulphate), may be used to identify partial and complete
anhidrosis.
Many
medical treatments available for anhidrosis are largely based on anecdotal
reports and impressions rather than scientific evidence. It should be stressed
that sound environmental management continues to be a very important aspect of
treatment for anhidrosis. Reported medical therapies include nutritional
supplementation of L-tyrosine, ascorbic acid, niacin and cobalt (commercial
preparation available); use of drugs that decrease sympathetic drive (ie.
methyl dopa), have been used by some practitioners with reported success, though
no control studies support these claims of efficacy. Use of acupuncture and a
homeopathic combination of sulphur and lycopodium have been reported. Other
claims of treatment include: vitamin E therapy, adrenocorticotropic hormone
injection, antidiuretics, electrolyte supplementation and antihistamine
preparations. Results at best are equivocal for the above medical therapies.
The
most reliable treatment for anhidrosis is removal of the severe climatic
stress. The ability to sweat can be restored in early stages of anhidrosis if
the horse is removed to a cooler, drier environment, though longstanding cases
may fail to respond to such changes. Maintenance of horses with air
conditioning may be a means of treating as well as preventing the onset of the
condition, by minimizing exposure to extreme environmental conditions. Short of
having the ability to move horses, or keep them in climate-controlled stalls, a
reduction in aggravating factors may provide practical ways to reduce the
development or progression of anhidrosis in susceptible or afflicted animals.
Factors that increase the basal metabolic rate, such as high protein feed,
disease, exercise or those that increase water loss or influence heat loss
should be avoided. Management
changes may include feeding less grain to reduce metabolism and therefore heat
production, exercising the horse during the coolest part of the day, and the
use of misting fans and cooling the roof of the barn by running water on it
will help maintain a cooler environment. Sound environmental practices at this
time remain the most effective ways in which to address anhidrosis in horses.
For more information you may contact Surgi-Care
Center for Horses at 813-643-7177 or by email lkuebelbeck@surgi-carecenter.com.
This article was originally published in Horse & Pony magazine and is reprinted with their permission.